BRIEF HISTORY OF ACOUSTICAL IDEAS: Acoustics first became associated with architecture when man began to assemble in groups to hear speeches, listen to music and see and hear plays. To create a favourable setting for such activities the Greek and Roman open-air theatres and forums evolved.The typical open-air amphitheatre consists of steeply banked benches arranged in a semicircle in front of a platform. With the passage of time the platform evolved into a stage with massive rear and side walls of masonry (and sometimes a ceiling) that served the acoustical purpose of reflecting, directing and thereby re-inforcing the sound intended for the audience. ACOUSTICS Acoustics
is
the
science
of
the
generation,
propagation,
transmission,
reproduction, reception, measurement and effects of sound and of the phenomenon of hearing.Acoustics also refers to the quality of sound as heard or transmitted in a room or building ARCHITECTURAL ACOUSTICS Architectural acoustics (also known as room acoustics and building acoustics) is the science and engineering of achieving a good sound within a building and is a branch of Acoustical engineering. Architectural acoustics can be about achieving good speech intelligibility in a theatre, restaurant or railway station, enhancing the quality of music in a concert hall or recording studio, or suppressing noise to make offices and homes more productive and pleasant places to work and live in. Architectural Acoustic design is usually done by acoustic consultants. NEED TO STUDY ACOUSTICS All human beings live with sound by birth . It is a medium of communication , which includes speech and music . We spend the majority of our time indoors, so a good working environment is essential. In many environments where people are present and communicate, high sound levels are perceived as one of the most disturbing factors. The negative sound is unwanted and we call it noise. It causes stress and affect human wellbeing .Noise impairs productivity in the workplace and the classroom, it also affects the patient outcomes in hospitals and aged care facilities.
A good acoustic environment is absolutely essential to maintaining a high level satisfaction and moral health among people.A well planned sound environment improves people ability to perform. A good acoustic environment keeps noise at levels that do not interfere with activities within programmed space. It also benefits teaching and learning. It provides teachers, students and pre-school/day-care staff with the best possible environment to support their educational ambitions. Understanding of acoustics enables us to define the relevant descriptors to get the best end-result. GOOD ACOUSTICS The term 'good acoustics' can be applied in a number of different ways, with different meanings. Generally it means a balanced united action between reverberation time, background noise and sound insulation. In a room with good acoustics the required sound is emphasized, while unwanted sounds are eliminated or reduced sufficiently so as not to cause a disturbance. A good acoustic environment keeps noise at levels that do not interfere with activities
within
programmed
space.For
the
residence,
favorable
acoustic
environment refers to the sound that does not interfere with rest or sleep. WHAT IS SOUND? A sound is any vibration (wave) traveling through the air or other medium which can be heard when it reaches a persons ear. Sounds waves are: Longitudinal - oscillations parallel to propagation. Mechanical - require a medium to travel through REQUIREMENTS FOR SOUND The three basic elements for transmission and reception of sound must be present before a sound can be produced. They are (1) the source (or transmitter), (2) a medium for carrying the sound (air, water, metal, etc.), (3) the detector (or receiver).
The material through which sound waves travel is called the medium.The density of the medium determines the ease, distance, and speed of sound transmission . The higher the density of the medium, the slower sound travels through it. The detector acts as the receiver of the sound wave. Because it does not surround the source of the sound wave, the detector absorbs only part of the energy from the wave and sometimes requires an amplifier to boost the weak signal. PHYSICAL PROPERTIES OF SOUND A vibrating object produces a sequence of compressions and rarefactions in the air surrounding it. These small fluctuations in air pressure travel away from the source at relatively high speed, gradually dying off as their energy is absorbed by the medium. What we call sound is simply the sensation produced by the ear when stimulated by these vibrations. Wavelength: Is the distance between two pressure peaks or valleys, measured in meters (m) and represented with the Greek alphabet ‘l’ (lambda). Amplitude: Is the strength or power of a wave signal.It is also the "height" of a wave when viewed as a graph. Higher amplitudes are interpreted as a higher volume, hence the name "amplifier" for a device which increases amplitude Frequency: The number of times the wavelength occurs in one second. The faster the sound source vibrates, the higher the frequency. A commonly used unit for frequency is the Hertz (abbreviated Hz), where 1 Hertz = 1 vibration/second When the frequency is low, sound waves are long; when it is high, the waves are short.Higher frequencies are interpreted as a higher pitch. For example a police whistle Velocity Refers to the speed of travel of the sound wave. This varies between mediums and is also dependant on temperature. Assuming air acts as an ideal gas, its velocity (V in m/s) relates to temperature (T in °C) as follows:
V = 331.5 + (0.6 T) (m/s) In other materials, the speed of sound can vary quite substantially. The following table shows the speed of sound in a number of different materials. Sound waves are introduced into a medium by the vibration of an object.The amount of energy that is transported past a given area of the medium per unit of time is known as the intensity of the sound wave. As a sound wave carries its energy through a two-dimensional or threedimensional medium, the intensity of the sound wave decreases with increasing distance from the source. The mathematical relationship between intensity and distance is referred to as an inverse square relationship. The intensity varies inversely with the square of the distance from the source. So if the distance from the source is doubled (increased by a factor of 2), then the intensity is quartered (decreased by a factor of 4). Intensity of sound is purely a physical quantity which can be accurately measured and is independent of the ear of the listener. Sound Pressure Level (SPL): intensity of a sound relative to the threshold of hearing is SPL and is measured in dB.sound pressure level, is the result of the pressure variations in the air achieved by the sound waves. The lowest sound pressure which can be heard by humans is called the hearing threshold, the highest which can be endured is known as the pain threshold. Sound pressure at the pain threshold is a million times greater than that at the hearing threshold. ATTENUATION Sound energy lessens in intensity as it disperses over a wide area. Attenuation is the decrease in energy or pressure for each unit area of a sound wave. Attenuation occurs as the distance from the source increases as a result of absorption, scattering, or spreading in three dimensions. LOUDNESS AND THE DECIBEL SCALE One property of sound is its loudness. Loudness of a sound corresponds to the degree of sensation depending upon the intensity of sound and sensitivity of ear drums.
It may also happen that the same listener might give different judgments about the loudness of sound of the same intensity but of different frequencies as the response of the ear is found to vary with the frequency of vibration.loudness is measured in decibel. A decibel is a unit of measure. The decibel scale works logarithmically. This means that when a sound increases by 10 decibels, it is 10 times louder. QUALITY OR TIMBRE OF SOUND The quality or timbre of sound is that characteristics which enables us to distinguish between two notes of the same pitch and loudness played on two different instruments or produced by two different voices.
BEHAVIOUR OF SOUND IN AN ENCLOSURE Sound waves propagate away from the source until they encounter one of the room's boundaries . Then – some of the energy will be absorbed, some transmitted and the rest reflected back into the room. Sound arriving at a particular receiving point within a room can be considered in two distinct parts. 1.Sound that travels directly from the sound source to the receiving point itself. This is known as the direct sound field and is independent of room shape and materials, but dependent upon the distance between source and receiver. 2. After the arrival of the direct sound, reflections from room surfaces begin to arrive. These form the indirect sound field that is independent of the source/receiver distance but greatly dependent on room properties. BEHAVIOUR OF SOUND IN AN ENCLOSURE
On encountering barriers posed by the enclosure, sound waves are likely to behave in the following ways: • Reflection • Absorption • Refraction • Diffusion • Diffraction • Transmission Reflection : This occurs when the wavelength of a sound wave is smaller than the surface of an obstacle. In the case of an enclosed space, the sound waves hit every side of the enclosure continuously until the sound energy reduces to zero. The amount of waves reflected depends on the smoothness, size, and softness of the materials of enclosure. The angle of incidence of sound rays is equal to that of the reflected rays only if the surface of the reflector is flat. But when it is curved, the angles are different. Absorption When sound waves hit the surface of an obstacle, some of its energy is reflected while some is lost through its transfer to the molecules of the barrier.The lost sound energy is said to have been absorbed by the barrier. The amount of sound energy absorbed depends on the thickness and nature of the material as regards its softness and hardness Refraction: This is the bending of sound when it travels from one medium into another medium. The difference in the composition of the two different media bends the sound i.e. the angle of incidence changes into an angle of refraction as it travels into the new medium. Diffusion Diffusion is described as the process of spreading or dispersing sound energy so that it is less direct or coherent. In this process the direction of the incident ray changes when it strikes the surface of the obstacle. Sound diffusion is a very important consideration in acoustics because it minimizes the coherent reflections that cause problems. It also tends to make an enclosed space sound larger than it is. Diffusion is an excellent alternative or complement to sound absorption in acoustic treatment because it doesn't really remove much energy, which means it can be used to effectively reduce reflections while still leaving an ambient or live
sounding space. sound diffusion can effectively turn virtually any space into one that is appropriate and useful for the purpose of recording or monitoring sound with a high degree of accuracy. Diffraction Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Diffraction occurs when an object causes a wave to change direction and bend around it. Diffraction is the apparent bending of waves around small obstacles and the spreading out of waves past small openings. The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the wave is smaller than the obstacle or opening, no noticeable diffraction occurs. Transmission When a sound wave enters another medium, there is transmission of the wave in the medium. A sound wave in air can be transmitted through a wall in a house. Or sound can enter water and be transmitted in the liquid In this phenomenon, sound wave is carried by molecules of the obstacle through vibration and re-emitted at the other side irrespective of the medium. It can be structure borne, air borne or impact sound.
ACOUSTICAL DEFECTS List of acoustical defects Reverberation, Formations
of
echoes, Sound
foci, Dead
spots, Insufficient
loudness, Outdoor noises, Indoor noises Reverberation: In an enclosed environment sound can continue to reflect for a period of time after a source has stopped emitting sound. This prolongation of sound is called reverberation. Reverberant sound is the reflected sound , as a result of improper absorption.
It occurs when sound waves hits a surface and are reflected toward another surface which also reflects it. Some of the sound is absorbed with this continuous reflection which gradually reduces the energy of the sound to zero. The phenomenon can affect the audibility of sound in an enclosure,i.e Reverberation may results in confusion with the sound created next. The time during which the sound persists is called the reverberation time of sound in the hall. As per Prof. W .C. Sabins reverberation time ‘t’ is given by formula :T = 0.16V /A
where V=volume of room in cubic meters A= total absorbing power of all the surfaces of room/ hall.
Reverberation time is the primary descriptor of an acoustic environment. A space with a long reverberation time is referred to as a "live" environment. When sound dies out quickly within a space it is referred to as being an acoustically "dead" environment. An optimum reverberation time depends highly on the use of the space. For example, speech is best understood within a "dead" environment. Music can be enhanced within a "live" environment as the notes blend together. Different styles of music will also require different reverberation times Reverberation time should remain within limits as per Indian Standard Code: 2526-1963.
Echo: Not all sound that hits matter is absorbed. Some of it is reflected. That means sound bounces off the solid matter the way a tennis ball bounces off a wall. Sound reflected back to its source is an echo. An echo is produced when the reflected sound wave reaches the ear just when the original sound from the same source has been already heard. Thus there is repetition of sound.The sensation of sound persists for 1/10th of a second after the source has ceased. Thus an echo must reach after 1/10th second of the direct sound.Multiple echoes may be heard when a sound is reflected from a number of reflecting surfaces placed suitably.
SOUND FOCI Some times shape of the hall makes sound waves to concentrate in some particular areas of hall creating a sound of large quality. These spots are called sound foci or hot spots . The intensity of sound at hot spots is unnaturally high and always occurs at the expense of other listening areas. This defect can be removed by-Geometrical design shapes of the interior faces.Providing highly absorbent materials on critical areas (curved spaces). DEAD SPOTS This defect is the out come of formation of sound foci.Because of high concentration of reflected sound at sound foci , there is deficiency of related sound at some other points . These spots are known as dead spots where sound intensity is so low that it is insufficient for hearing. This defect can be removed by suitably placing diffusers and reflectors .Having right proportions of internal spaces. FLUTTER When sound waves are rapidly reflected back and forth between two parallel flat or concave surfaces, there is an effect called flutter. Flutter is a rapid succession of echoes with sufficient time between each reflection for the listener to be aware of separate, discrete signals.We perceive flutter as a buzzing or clicking sound. Flutter often occurs between shallow domes and hard flat floors.The remedy for flutter is to change the shape of the reflecting surfaces or change their parallel relationship. Flutter echo can be reduced in one of two ways, with the use of sound absorption or sound diffusion. Flutter echoes can be acoustically treated with careful placement of sound absorption materials such as foam or wall panels on the walls or ceiling tiles, baffles or banners in the ceiling.(The idea here is to absorb the sound wave at one or both surfaces and keep that sound wave from reflecting of the surface back towards the noise source.) Flutter echoes can also be acoustically treated with the use of sound diffusers.(Sound diffusers are multi-faceted, slotted or curved materials that are reflective in nature and are designed to scatter or redirect sound waves.)
The sound diffusers can break up flutter echo within a room by taking the sound waves and sending them in different directions and eliminating the repetitive reflections caused by reflective, parallel surfaces. CREEP The reflection of sound along a curved surface from a source near the surface is called creep . The sound can be heard at points along the surface but is inaudible away from the surface. A space with concave surfaces can become a whispering gallery, a room in which two people can stand at two related focal points of curved surfaces and hear each other’s whispers with startling loudness and clarity while remaining unheard by other people in the space. OUTDOOR NOISE External noises from vehicles , traffic engines , factories , machines etc. may enter the hall either through the openings or even through walls and other structural elements having improper sound insulation. This defect can be removed by proper planning of the building with respect to its surroundings and by proper sound insulation of external walls. INDOOR NOISES Indoor noises are those which are caused either in the same room or adjacent rooms. And these are due to:Conversation of people.Moving of people .Moving of furniture.Crying of babies.Playing of radios/ other musical instruments.Operations of water closets and cisterns.Noise of type writer,Banging of doors etc.
GEOMETRICAL ACOUSTICS
Wave theory is based on the study of wave motion within three-dimensional enclosures. It requires the establishment of boundary conditions which describe mathematically the acoustical properties of the walls, ceiling and other surfaces in the room.
When employing wave theory, a room is considered as a complex resonator possessing many normal modes of vibration which are excited when a sound source is introduced to the room. The acoustic energy generated by the source acts to excite these room modes with the resulting sound energy residing in the standing waves established in the room. The characteristic frequencies of these vibrations depend on the room size and shape whereas the absorption of the resulting waves depends upon the boundary conditions. Thus, every room imposes its own characteristics on to the sound source present
For simplicity we assume that sound moves in straight rays, perpendicular to the wave front, therefore we can apply the principles of light on sound waves. Geometrical acoustics or ray acoustics is the equivalent principle of geometrical optics applied in acoustics. Geometrical optics, or ray optics, describes light propagation in terms of rays. The ray in geometric acoustics is an abstraction, or instrument, which can be used to approximately model how sound will propagate in an enclosed space. In short - geometrical acoustics refer to the way in which the boundaries ‘sculpt’ the sound within a space. The ray concept is used in designing sound focusing systems. In an enclosed space the boundaries determine the direction(s) that sound travels, and when it arrives at a particular location (such as the listener). A knowledge of geometrical acoustics is therefore essential if one wants to ‘sculpt’ rooms that have particular acoustic qualities such as coming from the right direction at the right time. In geometrical acoustics, we are primarily interested in three key phenomena namely reflection, diffusion and diffraction. It is an excellent approximation, however, when the wavelength is very small compared with the size of structures with which the sound interacts. A rough way of finding if a room will echo or not is using a ray-diagram analysis. By charting sound wave rays from speaker to listener in key points around the room, one will be able to tell if a room will have a strong echo or not.
We have the direct line-of-sight sound wave ray for each listener, 12ft and 33ft. Then we have the reflected rays, which add up to a distance of (11+18 = 29ft) and (16 + 26 = 42 ft). If we find the difference of these numbers, they end up being 17ft for the closer person, and 9 ft for the far away person. We then apply those numbers to a table of sound-path difference, and we see if the 'echoes' produced are small enough that they may be neglected. Acoustic design The acoustical environment of a workspace is typically given little or no attention during project planning and design. The functionality and aesthetics of the workspace are usually the primary focus of the designer. ROLE
OF
THE
DESIGNER,ADDRESSING
INTERIOR
ACOUSTIC
DESIGN
ISSUES,BLOCKING EXTERIOR NOISE,ACOUSTICS AND BUILDING CODES,REDUCING NOISE,MASSIVE MATERIALS,REFLECTIVE MATERIALS,ACOUSTICALLY TRANSPARENT SURFACES.
ROLE OF THE DESIGNER When designing a building, the architect and interior designer must recognize potential noise problems and take steps to solve them. The acoustic design of the building should be integrated with other architectural requirements. By carefully planning the building’s siting and structure, the architect can reduce noise penetration into the building. The overall building design and function ought to be reviewed in terms of desirable acoustic qualities. Noise sources should be placed as far as possible from quiet areas. The internal acoustics of individual rooms must be reviewed. For special acoustic issues, an acoustic consultant should be brought into the process at the earliest possible time.
By limiting sources of noise, the amount of necessary acoustic treatment can be reduced. When designing for an existing building, the architect and interior designer must first define the character of the sound problem. For new buildings, they have to imagine what noise sources can be anticipated. All parts of the building and its surfaces are potential paths for sound travel. Acoustic consultants are most commonly called in for buildings where loud noise is a special problem, or where the quality of interior sound is critical. Acoustic consultants play a role in selection of materials and the detail of construction components. They also influence the selection and use of interior surface materials. Their work has direct implications for the interior designer. Acoustic consultants also design and specify sound and communications systems, and detail components for noise and vibration controls in mechanical systems. ADDRESSING INTERIOR ACOUSTIC DESIGN ISSUES Acoustic attenuation is the term used for the reduction of the magnitude of a sound signal by a variety of means. This reduction may be a result of separating a sound source from the listener, enclosing the source to isolate the sound, absorbing the sound with materials that change the sound energy to heat, or canceling sound waves by electronic means. Changes in the ways buildings are designed and built have had an effect on the amount of noise produced and transmitted indoors.How sound behaves in a given room depends on the shape, size, and proportions of the room. The amounts of sound of various frequencies that are absorbed, reflected, and diffracted from the room’s surfaces and contents also determine acoustic effects. The sounds of cars, trucks, airplanes, and trains outside the building vary with the time of day and volume of traffic. Traffic noise ranges from higher pitched horns and squealing brakes to lowfrequency truck motors. Other noise sources coming from the building’s neighborhood might include construction noise, playgrounds , industrial plants, and sports arenas. On-site noises can include children’s play areas, refuse collection, and delivery or garage areas.
Sound can also be reflected from other buildings. Changes in the ways buildings are designed and built have an effect on the amount of noise produced and transmitted indoors.
ACOUSTICS AND BUILDING CODES Long periods of exposure, such as an eight-hour work day , result in permanent hearing impairment. It is required that workers be protected from high noise levels. Continual exposure to high noise levels results in a degree of temporary deafness in the majority of people . ThereforeCity and town regulations or zoning bylaws set standards, regulations, criteria, and ordinances for noise. Building codes have recently added limits on noise as well. The Building Officials Code Administrators International (BOCA), the International Conference of Building Officials (ICBO), and Southern Building Code Congress International (SBCCI) all include acoustic standards. Federal Occupational Safety and Health Administration (OSHA) OSHA sets the safe upper limit at 85 dB. A continual 75- to 85-dB level produces or contributes to physical and psychological ailments such as headache, digestive problems, heart problems, anxiety, and nervousness. REDUCING NOISE The noise inside a building comes from the activities of the building’s occupants and the operation of building services. Additional sound comes in a building from outside the building. The first principle of noise reduction in a building is to reduce the noise at its source. This usually involves proper selection and installation of mechanical equipment. The second step is to reduce noise transmission from point to point along the transmission
path
by
selecting
appropriate
construction
materials
and
construction techniques. Finally, noise can be reduced at the listener’s end by acoustic treatment of the space. MASSIVE MATERIALS
Many of the structural materials used in building construction attenuate airborne sound very well. Heavy, dense materials prevent outdoor sound from carrying to the inside of the building REFLECTIVE MATERIALS Reflective materials bounce sound back into the space of origin. ACOUSTICALLY TRANSPARENT SURFACES Soft, porous, acoustically absorbent materials are often covered with perforated metal or other materials for protection and stiffness. These coverings are designed to be acoustically transparent except at higher frequencies. With even smaller holes, the higher frequencies can also pass through. Staggering the
holes
improves
absorption.Open
weave
fabric
is
almost
completely
transparent to sound, and provides a decorative cover on absorbent wall coverings. SOUND ABSORPTION WITHIN A SPACE If noise problem is not coming from outside the room but is a result of the sound inside the room bouncing around, you need to address noise reduction within the space. The acoustic treatment of a space starts with reducing the source of the noise as much as possible. unwanted sound reflections need to be controlled. Speech privacy is another major acoustic concern for the interior designer. Sometimes it is also necessary to decrease or increase reverberation time for sound clarity and quality. Noise is reduced within a building by intercepting the sound energy before it reaches your ears. This is accomplished by changing acoustic energy into heat energy.The contents of the space control the noise levels within the space, while the structure of the building controls the transmission of noise between spaces. In a normally constructed room without acoustical treatment , sound waves strike walls or the ceiling, which then transmit a small portion of the sound. The walls or ceiling absorb another small amount, while most of the sound is reflected back into the room. The amount of transmission to an adjoining space is determined primarily by the mass of the solid, airtight barrier between the spaces, not by the surface treatment.
The amount of sound that is reflected off the surfaces back into the room is greatly decreased by absorptive materials. Adding absorptive materials to a room changes the room’s reverberation characteristics. This is helpful in spaces with distributed noise sources, like offices, schools, and restaurants. The acoustics of a space with hard surfaces can be improved by adding absorptive materials. In spaces with concentrated noise sources, the noisy equipment should be enclosed, rather than trying to treat the entire space. SOUND ABSORPTION is defined, as the incident sound that strikes a material which is not reflected back. An open window is an excellent absorber since the sounds passing through the open window are not reflected back but makes a poor sound barrier. Painted concrete block is a good sound barrier but will reflect about 97% if the incident sound striking it. Acoustic absorption is that property of any material that changes the acoustic energy of sound waves into another form, often heat, which it to some extent retains, as opposed to that sound energy that material reflects or conducts. ABSORPTION COEFFICIENTS Materials are neither perfect reflectors nor absorbers of sound. The efficiency of the
sound
absorption
of
material
is
rated
by
the
sound
absorption
coefficient(SAC). Absorption Co-efficient:It is that portion of incident sound that the receiving surface hasn’t reflected. It depends on sound frequencies, type of materials receiving sound and their construction. Sound absorption is different from sound Isolation, many sound absorbing surfaces have poor sound isolation properties.Sound absorbing materials can reduce noise levels by 10 db. (they are relatively expensive).
SOUND ABSORBING MATERIALS All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material's sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. The coefficient can be viewed as a
percentage of sound being absorbed, where 1.00 is complete absorption (100%) and 0.01 is minimal (1%). Incident sound striking a room surface yields sound energy comprising reflected sound, absorbed sound and transmitted sound. Most good sound reflectors prevent sound transmission by forming a solid, impervious barrier. Sound reflectors tend to be impervious and massive. Most good sound absorbers readily transmit sound. Sound absorbers are generally porous, lightweight material. It is for this reason that sound transmitted between rooms is little affected by adding sound absorption to the wall surface. There are three basic categories of sound absorbers: POROUS MATERIALS commonly formed of matted or spun fibers; PANEL (MEMBRANE) ABSORBERS having an impervious surface mounted over an airspace; and RESONATORS created by holes or slots connected to an enclosed volume of trapped air. The absorptivity of each type of sound absorber is dramatically (in some cases) influenced by the mounting method employed. POROUS ABSORBERS: Common porous absorbers include carpet, draperies, spray-applied cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted or cast porous ceiling tile. Generally, all of these materials allow air to flow into a cellular structure where sound energy is converted to heat. Porous absorbers are the most commonly used sound absorbing materials. Porous/Fibrous materials are good for general sound absorption, they can be made from metallic fibres and tissues.Their ability for absorption depends on material thickness and sound frequency. With more thickness there would be more absorption especially in low frequencies.Thickness can be increased by increasing the gap between absorbing surface and the fixing Wall/ceiling. PANEL ABSORBERS:
Panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode in response to sound pressure exerted by adjacent air molecules. Common panel (membrane) absorbers include thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Panel absorbers are usually most efficient at absorbing low frequencies. This fact has been learned repeatedly on orchestra platforms where thin wood paneling traps most of the bass sound, robbing the room of "warmth." RESONATORS Absorb sound in a narrow frequency range. Perforated materials and materials that have openings (holes and slots). The classic example of a resonator is the Helmholtz resonator, which has the shape of a bottle. The resonant frequency is governed by the size of the opening, the length of the neck and the volume of air trapped in the chamber. Long narrow slots can be used to absorb low frequencies. SPACE ABSORBERS: When the regular boundary enclosures of an auditorium do not provide suitable or adequate area for conventional acoustical treatment, sound absorbing objects, called space absorbers, can be suspended as individual units from the ceiling. These are made of perforated sheets in the shape of panel, prisms, cubes, spheres, etc., are generally filled or lined with sound absorbing materials such as rock wool, glass wool, etc. their acoustical efficiency depends on their spacing. In order to achieve a reasonable amount of room absorption, it is essential that a large number of space absorbers be used within a space. VARIABLE ABSORBERS: For change in RT, various sliding, hinged, movable, and rotator panels have been constructed that can expose their absorptive or reflective surfaces. Draperies have been installed that can be spread out on walls or be pulled off into suitable pockets, thus arbitrarily increasing or reducing the effective absorptive treatment in the room. INSTALLATION OF ABSORPTIVE MATERIALS
The way materials are installed affects their ability to absorb sound. The absorption co-efficient varies widely with frequency for different mountings. Materials absorb high frequencies better than lower frequencies. The amount of absorption is not always proportional to the thickness of the material, but depends on the material and its method of installation. Sound absorbing materials should preferably be hung away from the wall ,not touching the wall ,if it is desired to increase the low frequency absorption. The best way to install acoustically absorbent material is to hang cubes or tetrahedrons from the ceiling. In general a space of 25mm to 100mm behind a material with a 15 to 50mm thickness is an effective way to improve the low frequency performance of an absorbing material. A layer of air between the absorptive material and a rigid surface works well in midrange frequencies. To get the best low-frequency absorption, a deep air space on the ceiling can be provided . For best results, treat the ceiling, floor, and wall opposite the sound source approximately equally. Beyond a certain point, added thickness does little to increase absorption, except at very low frequencies. The lowest musical frequencies can’t be absorbed efficiently by ordinary thicknesses of porous material.
ACOUSTICAL CHECKLIST BY PROJECT TYPE BALLROOMS Goal: To provide a multi-purpose space that can successfully control noise appropriate for meetings and receptions, both large and small. Recommended Reverberation Time: 1 - 1.2 seconds - Absorptive materials are needed to reduce the reverberation time, and are most appropriate when applied to the ceiling. - Be cautious when designing ceiling intricacies as they may cause unwanted reflections. - Movable partition walls covered with fabric alone do not guarantee absorption. - Once divided by movable partitions, the acoustics in the new space can change.
- Control exterior and background noise levels from HVAC. - Maintain an elegant appearance by addressing acoustical issues during the design phase.
CINEMAS Goal: To properly use absorption, working with the sound system, to provide the best acoustical environment for every moviegoer.Recommended Reverberation Time: 0.8 - 1.2 seconds - Background noise levels should be kept to a minimum. Two primary potential noise sources are mechanical equipment (HVAC) and outdoor noise. - Excessive room length should be avoided. - Walls, except those close to the screen, should be absorptive.
CONFERENCE/BOARD ROOMS Goal: To provide a space where a variety of communications styles can be effectively used.Recommended Reverberation Time: 1 second - Limit the amount of reflections to keep speech intelligibility at a maximum. - Absorptive materials are needed to reduce the reverberation time. - Avoid reflective parallel surfaces. - Control exterior and background noise level. - Ensure flexibility for future multi-media advancements. - A sound system may be necessary.
CORRIDORS Goal: To eliminate excessive traveling of noise which is common in hallways, particularly in sensitive or confidential offices such as attorneys, psychiatrists, and personnel managers.Recommended Reverberation Time: 0.8 - 1.2 seconds. - Be cautious with curved surfaces as they lead to the undesirable acoustical condition known as creep.
- If surfaces are left untreated, a corridor can act as a megaphone, transmitting conversations into nearby offices. - Surfaces should be absorptive to maintain a low reverberation time. Horizontal FABRI TRAK® panels are an excellent solution for this application.
HOME THEATERS Goal: To adapt an existing space in a typical home, to provide the best acoustical environment possible for speech and music.Recommended Reverberation Time: 0.8 - 1.2 seconds - Verify that ratio of room dimensions will not cause unwanted reflections. - Surfaces should be absorptive to maintain a low reverberation time.
LECTURE HALLS Goal: To allow all audience members to easily hear and understand the presenter.Recommended Reverberation Time: 1 Second - The front wall and ceilings can be reflective aiding sound in reaching everyone. - Absorptive material on the back and side walls will help reduce the reverberation time and unwanted reflections. - Excessive background noise levels caused by HVAC systems can greatly degrade speech intelligibility. - Splay the side walls to eliminate flutter echoes.
LIBRARIES & MUSEUMS Goal: To limit noise levels, allowing users to read and contemplate without disturbances.Recommended Reverberation Time: 0.8 - 1 second
- If domes or other concave surfaces are desired, they must be treated with absorptive material to reduce unwanted reflections. - Absorptive materials are needed to reduce the reverberation time. - Books are not very absorptive.
- If possible, place noisy equipment and activities in remote areas.
OPEN OFFICE PLANS Goal: To Provide an environment free from distractions, by reducing noise levels and
the
understanding
of
overheard,
nearby
conversations.Recommended
Reverberation Time: 0.75 seconds
- Avoid direct sound pathways between cubicles, by proper placement of wall partitions. - Without controlling reflections off the ceiling and perimeter walls, partitions can be ineffective. - Absorptive partitions, ceiling tile, and wall treatments will be necessary to control the reverberation time. - Reasonable precautions should be taken to insulate against noise from adjacent rooms, machinery, ducts, and the outside.
RECORDING STUDIOS Goal: To have complete control over all acoustical aspects of a given space.Recommended Reverberation Time: 0.5 second - Absorptive materials must be used to control the reverberation time over a wide range of frequencies. - Sound must be both absorbed and diffused. - HVAC noise must be reduced to an absolute minimum. - An unusually high degree of isolation from extraneous noise vibrations is need.
THEATERS Goal: To properly balance absorption and reflection to provide a favorable acoustical environment, similar to worship centers. One must address both the need to hear and understand speech, and the desire to have a pleasant space for music.Recommended Reverberation Time: 1.0 - 1.3 seconds - Control the reverberation time in the room by adding absorptive material.
- Control the reverberation time on the stage. - Splay or use irregular surfaces on the side walls to avoid flutter echoes. - Don't forget the ceiling. It should be faceted for better dispersion of sound. - Remember the space will be less absorptive when only half full, since the audience itself is absorptive. By using absorptive seating areas, the reverberation time will remain consistent regardless of the audience size. - Be sure openings, such as doorways, are properly sealed. - The balcony should be no deeper than twice its height.
WORSHIP CENTERS Goal: Consider and address both the need to hear and understand speech and the desire to have a pleasant space for music.Recommended Reverberation Time: 1.2 - 3.5 seconds - Address lower frequency or bass sound. - Background noise levels should be kept to a minimum. Two primary potential noise sources are mechanical equipment (HVAC) and outdoor noise. - Reflections should be carefully monitored, especially if domes or concave surfaces are incorporated in the design. - The seating area should be absorptive. This allows the acoustical environment to remain constant regardless of the number of attendees. - A sound-amplification system may be necessary. Ranges dramatically for different worship center projects. The recommendation will be influenced by the musical program type and if congregational singing is desired. Highly reverberant spaces are desirable for some type of musical programs. However, reverberation time of 1.2 - 1.5 seconds is appropriate for a number of worship centers. This allows an adequate enhancement of the music program, and can allow adequate understanding of speech with an appropriate sound system.
SOURCES OF NOISE:
Sources of noise can be classified as those originating outside and those originating inside a building. OUTSIDE NOISE: Motor traffic and airplanes are major sources of noise. The exhaust of big jet can develop 120db or more. Other sources are power lawnmowers, children playing, etc. Even the weather- the whistle of the wind and rain- can be the source of noise. INSIDE NOISE: Motor driven appliances are the principle source. These are dishwashers, refrigerators, vacuum cleaners, exhaust, Ac, radios, TV's, etc. The major sources of environmental noise are road, rail and air traffic, industries , construction and public works, in residential neighborhoods Noise is generally defined as unwanted sound .The effects of noise are seldom catastrophic (extremely harmful), and are often only transitory, but adverse effects can be cumulative with prolonged or repeated exposure. Sleep disruption, the masking of speech and television, and the inability to enjoy one's property or leisure time impair the quality of life. In addition, noise can interfere with the teaching and learning process, disrupt the performance of certain tasks, and increase the incidence of antisocial behavior. noise can adversely affect general health and wellbeing in the same manner as chronic stress. annoyance is among the most immediate and obvious effects of noise exposure on people. ANNOYANCE is defined as an unpleasant mental state that is characterized by irritation and distraction from one's conscious thinking .It can lead to emotions such as frustration and anger. The property of being easily annoyed is called petulance, and something which annoys is called a nuisance. BACKGROUND NOISE or ambient noise is any sound other than the sound being monitored.
Background noise is a form of noise pollution or interference . It is any sound element that tends to distract or in some manner interfere with the ability of the individual to hear or be heard. Examples of background noises are environmental noises such as waves, traffic noise, alarms, people talking, bio acoustic noise from animals or birds and mechanical noise from devices such as refrigerators or air conditioning, power supplies or motors. The prevention or reduction of background noise is important in the field of active noise control. Background noise is an important concept in setting noise regulations. There are many different types of background noises, ranging from those that are almost undetectable to others that are extremely irritating . In some cases, background noise is not considered unpleasant at all. For example, many restaurants utilize music playing softly in the background as a way to create a more inviting ambiance for diners. The idea is to set the volume of the music so that conversing in normal tones is possible, as the music helps to relax customers and enjoy the meal. The use of background music is sometimes complicated with the use of televisions placed around the dining area. When the volume on the sets is turned up, conversation becomes more difficult and may prove annoying to some patrons. The result is a loss of repeat business, since diners who do not enjoy themselves are less likely to return. Background noise also occurs when using various types of telephone devices. For example, a conference call involving two or more locations using a speakerphone may encounter background noise that is created from air vents blowing on the speakers, or several people tapping pencils on the conference room table. Noise is classified as: 1) AIR BORN and 2) STRUCTURE BORN OR IMPACT SOUND AIR BORN NOISE: These are the noises which are generated in air & which is transmitted in air directly to ear. Such a sound travels from one part of the building to the other, or from outside of the building to inside by
1) openings like doors, windows, ventilators, key holes, etc. 2) forced vibrations set up in walls, ceilings, etc. Air born noises processes power, continues for long duration, and is confined to places near its origin. Airborne sound originates in a space with any sound producing source, and changes to structure-borne sound when the sound waves strike the room boundaries. The noise is still considered airborne,because it is originated in the air. STRUCTURE BORN / IMPACT NOISES: Structure-borne sound is energy delivered by a source that directly vibrates or hits the structure. These are the sound, which originate and progress on the building structure. These are caused by structural vibrations originated due to impact. The common sources of this sound are: footsteps, movement of furniture, dropping of utensils, hammering, drilling, operation of machinery, etc. These are more powerful, propagate over long distances and persists for a very short duration. The difference between the air born and structure born noise is related to the origin of noise in relation to the receiver room only. In a three story building, washing of cloths on the middle floor will be heard as impact noise for the room below and air born for the above floor. SOUND TRANSMISSION BETWEEN SPACES Sound travels through other materials as well as air. It can be transmitted through steel, wood, concrete, masonry, or other rigid construction materials.In order to deal with sound control one should understand how sound travels. There are three paths by which sound travels: a direct path which is the straight line between the source and receiver; a reflected path which occurs as sound bounces off various surfaces; and a diffracted path which involves sound bending over the top and around the
sides of
partitions. The control of sound in a building requires consideration of all three paths.
In buildings sound
sound transmitted will either be air borne or structure borne. All
transmission
involves
both
airborne
and
structure-borne
sound.A
combination of air borne and structure borne approaches need to be considered for SOUND ISOLATION. what do we do if we want to isolate sound? Only two things stop sound – MASS AND SPACE. You need mass to contain the airborne sound, you need space (an air gap or similar unobstructed area) so that the structure borne sound can not be transmitted. Two aspects need to be considered for sound isolation: Keeping sound out of the listening environment, and retaining sound within the listening environment. Sound insulation is the prevention of transmission of sound or alternatively, a reduction of sound energy transmitted into an adjoining air space. Two types of sound insulation are to be dealt with in building construction. (a)Airborne Sound Insulation : the insulation against noise originating in air, e.g. voices, music, motor traffic, wind. (b) Impact Sound Insulation : the insulation against noise originating directly on a structure by blows or vibration e.g. footsteps above, furniture being moved, drilling and hammering the structure. TRANSMITTION LOSS When sound is transmitted from the source or the origin to the adjoining room/area, reduction in sound intensity takes place, this is known as transmission loss (TL) . It is numerically equal to the loss in the intensity of sound expressed in decibels. For ex. If 60dB and 40 dB are the sound levels measured on either side of a wall, the transmission loss = 60-40=20 dB 1.Transmission loss is expressed in terms of loss of sound intensity (expressed in decibles) 2.The
efficiency
of
sound
insulation
of
barrier(such
as
wall
,partition,door,floor,etc.)is expressed in terms of transmission loss of air-borne sound passing through the barrier.
3.The transmission loss (or sound insulation) offered by a structure depends upon the materials used and method of construction. 4.Transmission loss depends upon the frequency of sound . Hence transmission loss of a structure should be studied over a wide range of sound frequencies ACCEPTABLE INDOOR NOISE LEVEL Acceptable noise levels are those which will neither cause uncomfortable conditions nor damage the acoustics of the building .Acceptable noise levels depend upon (a) nature and type of noise .(b) time of fluctuations of noise.(c) background noise.(d) type and use of building Air-borne noise can be stopped in the following ways: ADDING MASS BETWEEN SPACES – a 12" concrete floor absorbs more sound energy than a 4" concrete floor. Extra layers of drywall reduce transmitted noise through both walls and ceilings. PUTTING GAPS IN THE STRUCTURE – a typical sound insulating wall between units may be composed of two sets of studs, one set for each unit. This eliminates structure-born noise transmission between the units. A flexible support system (sound isolating channels) can cut down on structure-born noise passing from a structural beam to a drywall ceiling. CLOSING AIR-HOLES BETWEEN SPACES – sound waves carry through key-holes, around pipes, between a wood floor and a brick wall, through outlets placed back to back, and over-and-under walls which are not thoroughly caulked at the top and bottom. PUTTING DAMPENING ELEMENTS IN THE VOIDS BETWEEN SPACES – fiberglass or cellulose insulation absorbs air-borne noise and also dampens vibrations in the wall surface, much as stuffing a sock in a guitar would make it quieter. The air between two sheets of drywall can’t transmit noise efficiently if it is filled with fluff.
CALMING AIR-BORN NOISE BEFORE IT ENTERS THE STRUCTURE – a radio playing in an empty apartment will transmit a lot more noise than one playing at the same volume in a unit filled with furniture, window coverings, paintings, clothing and other possessions. Hotel rooms are not crammed with upholstery, drapes and
thick carpets just for looks – all that "stuff" keeps noise from building up and simplifies the sound-deadening task of walls and floors. New buildings are "noisier" than long-occupied buildings for the same reason (although that is hard to tell a new occupant). Carpets do more than just reduce impact noise – they also absorb sound within the space. Music rehearsal spaces make use of many sound deadening technologies, including various foam products. Sound/noise can be controlled in three ways: 1.Eliminate the source 2.Isolate the source -provide a barrier between the user and the source 3.Mask the offending sound if not possible to isolate the sound, minimize its impact on the user SOUND PROOFING/BLOCKING Sound Blocking is commonly called sound proofing and is used to block sound from transmitting into adjacent areas or workstations. In general, if the source of unwanted sound is generated in another room, space, or from the outside and is transferring through a wall, ceiling, or floor into your space, then the sound has to be blocked by using sound blocking products. To stop the direct path of sound, we erect barriers (system wall panels) which stop sound from passing through. the sound blocking materials are dense and heavy. With no air spaces for sound waves to slip into and through, these products essentially cut off the direct path into adjacent areas.
WALL INSULATION: VERTICAL BARRIERS Wall construction used for sound insulation can be of three types: 1) Rigid and massive homogeneous walls: this consists of stone, brick or concrete masonry, well plastered on one or both sides. Their sound insulation depends on their weight per unit area. 2) Partition wall of porous material: these can be of rigid or non-rigid type. In the rigid partitions, insulation is 10% more. 3) Double wall partition: this consists of plasterboards or fiberboards or plaster on laths on both the faces, with sound absorbing blankets in between.
4) Cavity wall construction: this is an ideal construction from the point of view of sound insulation. The gap between two walls can be filled by air or some resilient material. FLOORS AND CEILING INSULATION: HORIZONTAL BARRIERS These act as horizontal barriers to both air-borne and impact noises. Main emphasis is given to the insulation against the impact noises. This may be done by: 1) Use of resilient material on the floor surfaces: this consists of providing thin concrete slab as the RCC floor slab, and then providing a soft floor finish material such as linoleum, cork, asphalt mastic, carpet, etc. 2) Concrete floor floating construction: in this an additional floor is constructed and isolated from the existing concrete floor. 3) Timber floor floating construction: this is done by employing mineral or glass wool quilt for isolation purposes. A further improvement in the insulation of such floors is achieved by employing a plugging or deadening material in the air gap between the wooden joists. 4)Timber floor with suspended ceiling and air space: the highest insulation can be achieved by using a very heavy ceiling, which are arranged to be independent of the floor by supporting it on resilient mountings. 5) Skirting: the larger the contact area a skirting provides between the floors and the walls, the lower would be insulation. For this the lower edge of the skirting is chamfered thus reducing the area of contact.
SOUND MASKING is a method of introducing an electronically produced sound, evenly distributed throughout a space, in an effort to overwhelm sound which cannot be blocked or absorbed by any other means. It is a soft ambient sound introduced into a room, that sounds like air-conditioning. The end result is a soft sound that masks human speech, and other related offices noises. Typical office noise ranges from human speech and conversations, to phones ringing, keyboards clicking, fax machines, filing cabinets doors slamming, people walking, computer sounds, car/truck traffic outside the window, elevators dinging, and many more.
These noises cause distractions that lower employee productivity, raise data entry errors, increase employee stress, mental fatigue, and sometime lead to turn over, especially in call centers. BENEFITS FOR SOUND MASKING Sound masking offers confidentially and speech privacy. Anywhere people need privacy, or to protect their conversations from being overheard by others. sound masking can raise productivy of employees in offices by 15–35%. sound masking can reduce employee stress, mental fatigue, burn-out, and potentially lower turnover. INSTALLATION OF SOUND MASKING SYSTEM Speakers are mounted above the ceiling tile where you can't see them and pointed upward. Audio equalizers are used to tune the sound to the specific acoustic characteristics of the office. The background sound can adjust automatically throughout the day. The level of sound is lower during non-peak hours than during periods of greater activity. Our sound masking is used in cubicle areas, exam rooms, private offices, as well as reception areas, and public spaces. Effective sound masking has the following characteristics: It isn't noticed when it's on, but is missed if turned off. It has the correct tonal qualities. A "humm" not a "hiss." It is never obtrusive - it disappears and may be thought to be the normal background sound of a well designed HVAC system. It is the correct volume - louder than what you don't want to hear but, not so loud as to interfere with the conversations you want to have. One should never feel compelled to speak over it or strain to listen. It is uniform throughout the space - no "hot spots" or "dead spots."
ENVIRONMENTAL NOISE CONTROL The objective of environmental noise control is to improve the acoustic environment in a community by reducing noise levels.
Noise from industrial operations can affect neighboring residential areas, ranging from intolerable noise levels to structural vibrations. Well-planned noise control can eliminate a major component of an industrial site’s impact on its surrounding environment. Two types of noise exist: steady noise and non-steady noise. Steady noise with audible discrete tones is called discrete frequency noise and is the most common noise found in industry. Discrete frequency noise is caused by rotating
parts
of
machines
such
as
fans,
internal
combustion
engines,
transformers and pumps. Non-steady noise consist of fluctuating noise (noise that doesn’t remain at any constant level over a given period of time), intermittent noise (noise that returns to the ambient or background level), and, more commonly, impulsive noise (sounds of short duration with high peak pressures). Peak pressures rise at least 40 dB in 0.5 seconds. In industry, the most common noise sources are described as a point source, like a gas turbine, or a line source, like a pipeline. In the free field, sound propagates outward from point sources in uniform, concentric circles and from line sources as a cylindrical wave, much like a weather front. Free field conditions exist when no obstacles block the sound path Once the noise sources are identified and assured, the next step is to attenuate the noise. The aim of attenuation is to reduce or divert the amount of sound energy reaching the receiver. The key to attenuation is to apply noise control materials and measures that are both effective and economical. BUFFERS One of the simplest attenuation methods is to place enough distance between the noise source and the noise receiver so that noise is not a concern. Establishing a buffer zone is possible when land is readily available. NATURAL BARRIERS
Shrubs, trees and berms are often used as natural noise blockers. For trees to be effective barriers, they must be in a continuous stand, 50 feet tall, 100 feet deep, have dense foliage down to the ground, and be evergreen. When only a line of deciduous trees is planted, noise easily travels through the stand, particularly during the winter when trees lose their foliage. Berms are more effective in stopping high frequency noise. Low frequency noise, with its long wavelength, can easily slip over berms. BARRIERS Barriers are free-standing walls or structures intended to block source noise. The barrier functions by absorbing a large amount of the sound energy and/or deflecting it away from the source. Barriers reduce sound levels, but work best at reducing high frequency noise. Barriers are most effective when they are at least three times larger than the wavelength of the major noise contributor. For best results, barriers should have a high transmission loss and be highly absorptive.
Barriers
made
from
a
combination
of
sound-absorbing
and
transmission loss materials give highest acoustic performance. Concrete walls are often used as barriers. As a dense material, concrete is a better sound insulator than sound absorber, so barriers made from concrete reflect sound rather than absorb it. When a barrier is wrapped around a noise source, it acts as a partial enclosure. Partial enclosures come in a variety of configurations: two-sided, three-sided with a roof, four-sided without a roof, and so on. Barriers and partial enclosures can be effective and economical noise reducers, lowering noise levels by up to 12 or 15 dB.
MUFFLERS/SILENCERS Mufflers are devices that are inserted in the path of ductwork or piping with the specific intention of reducing sound traveling through that conduit. They are often effective for controlling noise from stacks on rooftop ventilation equipment. VIBRATION ISOLATION Mechanical equipment can generate vibrations that can travel through a building’s structural members to affect remote locations within a building.
It is therefore prudent to isolate any heavy equipment from any structural members of buildings. This can be accomplished by mounting the equipment on springs, pads, or inertia blocks. MASKING Masking systems are composed of natural or electronic components that add sound to an environment to cover or mask objectionable sounds. They work best when they blend with the environment to the point at which they go unnoticed. Acceptable sounds outdoors include more natural sounds, such as running water or rustling leaves. Outdoor fountains are not only effective in masking sound, but they can also add aesthetically to an area. Any other natural masking sounds would have to be added electronically using weather-proofed loudspeakers. A number of cases exist where sound masking has been successfully installed for exterior applications, the most common target of concern being roadway noise. In one example application, a large artificial waterfall was constructed as part of the garden exterior of an urban hotel in Santa Rosa, California. The waterfall cascades down an extensive wall approximately four meters in height and functions both for sound masking and as a physical barrier to road noise.
