COMMON ACOUSTICAL DEFECTS
Acoustical conditions in a big room, ball or auditorium etc`. are achieved when there is clarity of sound in every part of me occupied space. For this, the sound should rise to suitable intensity everywhere with no echoes or near echoes or distortion of the original sound ; with correct reverberation time.
COMMON DEFECTS : (1) Reverberation (2) Formation of echoes (3) Sound foci (4) Dead spots (5) Insufficient loudness (6)External noise
1)
REVERBERATION:
It is persistence of sound in the enclosed space, after the source of sound has stopped. Reverberant sound is the reflected sound as a result of improper absorption. Excessive reverberation is one of the most common defect, with the result that sound once created longs for a longer duration resulting in confusion with the sound created next. However, some reverberation is essential for improving quality of sound.
Thus, optimum clarity depends upon correct reverberation time which can be controlled by suitably installing the absorbent materials .
REVERBERATION TIME: IS THE TIME INTERVAL WITH IN WHICH ,THE INTENSITY OF SOUND PRODUCED, DIMINISHES TO ONE MILLIONTH OF ITS ORIGINAL INTENSITY. •DEPENDS ON THE SIZE OF ROOM AS IF ROOM IS SMALL REFLECTIONS WILL TAKE PLACE QUICKLY AS WAVES HAVE TO TRAVEL LESS DISTANCE, SO TIME WILL BE LESS.
•PLAYS AND IMPORTANT ROLE IN ACHIEVING DESIRABLE ACOUSTICAL CONDITIONS. •IN CASE OF CINEMA HALLS R.T SHOULD BE SHORT.
RT60 IS TIME IN SECONDS FOR REVERBERATION TO DIMINISH TO 60DB (1/1,000,000). • WITH PRACTICE THIS TEST CAN BE APPROXIMATED WITH A SINGLE HANDCLAP, IN A QUITE ROOM, AS IN THE ABOVE GRAPH. BUT WITH CONTINUOUS SOUND (MUSIC) REVERBERATION BUILDS UP AND REMAINS AT A CONSTANT LEVEL.
3) SOUND FOCI :
Reflecting concave surfaces cause concentration of reflected sound waves at certain spot, creating a sound of large intensity. These spots are called sound foci. This defect can be removed by (a) geometrical designed shapes of the interior faces, including ceilings (b) providing highly absorbent materials on focusing areas.
4) DEAD SPOTS :
This defect is an outcome of the formation of sound foci. Because of high concentration of reflected sound at sound foci, there is deficiency of reflected sound at some other points. These points are known as dead spots. where sound intensity is so low that it is insufficient for hearing. This defect can be removed by a) installation of suitable diffuser so that there is even distribution of sound in the hall .
5) INSUFFICIENT LOUDNESS :
This defect is caused due to a) Iack of sound reflecting flat surface near the sound source b) excessive sound absorption treatment in the hall. The defect can be removed by a) providing hard reflecting surface near the source, b) by adjusting the absorption of the hall so as to get optimum time of reverberation. c) When the length of the hall is more, it may be desirable to install loud speakers at proper places.
6)EXTERNAL NOISE: External noise from vehicles, traffic engines , factories, cooling plants etc. may enter the hall either through the openings (such as doors, windows, ventilators etc.) or through walls and other structural elements having improper sound insulation. This defect can be removed by a) proper planning of the hall with respect of its surroundings b) proper sound insulation of exterior walls.
ECHO: •A SOUND REFLECTED OFF A SURFACE THAT ARRIVES AT THE LISTENER AFTER THE DIRECT SOUND. SOMETIMES THOUGHT OF AS REVERBERATION, BUT AN ECHO IS VERY DISTINCT WHILE REVERBERATION IS A MIXED TOGETHER SOUND WHICH DECAYS GRADUALLY. •ECHO IS HEARD AS DISTINCT REPEAT, 100 MILLI-SECONDS (1/10 SEC) OR GREATER, FROM WALLS AND CEILING WITH PATH-LENGTHS GREATER THAN 15 METERS (45FT) APART. • DEFECTS CAN BE REMOVED BY - BY SELECTING PROPER SHAPE OF THE HALL. - PROVIDING ROUGH AND POROUS INTERIOR SURFACES.
Case Study: Lecture Room This study presents results from an acoustic analysis carried out on the proposed design for a 200 seat lecture facility. The proposed building is an earth-covered structure designed to take maximum advantage of thermal mass, passive design and ESD principles. It features a natural ventilation tower which draw air through an intake plenum beneath the seating to take advantage of ground temperature cooling in summer and heating in winter. The preference of the architect is for a slightly 'live' facility that is suitable for both unassisted speech and music production.
The acoustic analysis of the design involved two phases:
•Statistical Analysis The derivation of room-averaged values calculated using published empirical formula worked out over time from the comparison of many similar enclosures.
•Geometric Analysis Position-specific data generated directly from a computer model of the enclosure geometry.
STATISTICAL ANALYSIS •At the earliest stages in the design phase, the most important objective measure of the acoustic performance of the hall is its Reverberation Time (RT). •This refers to the time taken for generated sounds to decay away. •Too short an RT and the hall appears dead and 'lifeless'.
•Too long and the audience will experience difficulties understanding speech as the individual syllables will blend together and become almost indistinguishable. •From empirical experience with the performance of a wide range of halls and lecture theatres, there are published recommended RT values for lecture facilities and auditoria of all sizes.
•As the RT is a function of room volume and surface absorption, there are also published recommended volumes required to achieve satisfactory acoustic conditions, given standard building materials. •These are given as a volume-per-seat value, which is simply the total internal room volume divided by its total seating capacity. •These two values, the RT and the volume-per-seat can be used as a preliminary, yet quite accurate initial guide to the predicted acoustic performance of a design.
Reverberation Times •Reverberation time calculations were carried out comparing the effects of linoleum and a thin carpet as floor coverings. •The two graphs below show a comparison of the two materials for unoccupied and fully occupied conditions. The volume of the space was calculated at 1161m³.
•At mid and high frequencies, a completely carpeted floor provides close to the optimum RT of 0.83 seconds for speech. •The linoleum floor is more suited to musical performances with an optimum RT of 1.44 seconds.
•An obvious compromise is some combination of the two, or a linoleum floor with some additional absorption panels on the side or rear walls. •It is suggested that carpet be used in traffic areas within the space which are open to direct reflection of sound whilst areas beneath the audience be linoleum. •The next set of graphs shows the resulting reverberation times from an example of such a compromise. •Carpet was used on the main entry floor at the rear of the facility, down the two ramps and across the front of the first row of seating.
•The stage area and all other floor surfaces are linoleum.
•This shows that it is necessary to include some measures to reduce low frequencies within the space, between 63Hz and 250Hz. • This can be achieved using approximately 15-20m² of thin wooden panel absorbers fixed to either the side or rear walls. • The image below shows the plan view of a suggested configuration for these absorbers
Ventilation Shaft The internal surfaces of the ventilation shaft must be made as absorbent as possible. This has two effects: 1. Minimizes the effects of external noise entering the space through the upper vents. 2. Minimizes any effects of acoustic coupling between the main internal volume and the volume inside the shaft.
If the shaft volume is less reverberant than the main space there will be no problem. There are a wide range of materials that would be suitable for lining the duct, from applied foams or fibrous mats to acoustic tile. The major consideration should be particle durability and long life, such that particles do not drop into the main space as the material ages. A determining factor for the exact selection of this material will be the type of external noise penetration expected, if any.
A significant low frequency content will require more absorbing material than a spectrally even white noise source.
GEOMETRIC ANALYSIS •A preliminary acoustic ray-trace was performed for a number of points within the enclosure.
•The orientation of the side walls in the proposal, particularly toward the rear of the enclosure, did not allow any reinforcing sound reflections back into the audience plane. •A preliminary model that is rectangular in plan shows a greater amount of lateral energy arriving at all points. •This is desirable as it promotes a feeling of being surrounded and involved in the sound field, as opposed to simply observing it. •Additionally, any further first or second order reflections that can be directed towards the audience will affect a perceived increase in the direct sound level coming from the speaker.
•A comparative study of an alternative rectangular plan was then carried out. •The rectangular form retains the reflectors at the front of the facility whilst removing the alcoves at the rear, as shown in the following image.
•The results show an average 9% increase in the lateral energy fraction arriving in the first 50ms at each test point in the enclosure. •The rear side points experience a 13% increase. •A further increase is possible by angling the walls in even further, however, this reduces the available audience area and gives diminishing returns. Thus the rectangular plan as shown in the above figure is recommended.
•Diffuse Rear Ceiling •An acoustic ray-tracing analysis of the first three (3) reflections was used to determine the distribution of sound rays within the enclosure. A number of potential echo sources became obvious.
•In order to prevent cue-ball echoes off the rear section of the ceiling and back off the rear wall, three patches of either absorber or a diffusing material is required. • An absorber will reduce the level of the reflected sounds so that they are less noticeable. A diffusing material will actually break up the wave front and scatter the sound in many directions. •A diffusing material is recommended. The following image shows the areas requiring treatment, each 2m from the rear wall.
•Acoustic diffusion can be achieved a number of ways, each requiring a rough surface to scatter incident sound waves. • In this situation, the diffuser must be most effective throughout the speech band, from 500Hz to 4000Hz. •This will require a range of different spacings and sizes, from 20mm to 80mm. •These materials must be arranged in a random or pseudo-random manner. •Figures 2 and 3 show two possible arrangements, one using timber sections, the other flat plywood panels. The exact size and position of each member is not important, (as long as they are relatively evenly distributed over the 20-80mm range)
RECOMMENDATIONS From an analysis of the results of this study, the following recommendations are made: Overall, with some simple surface treatment to achieve the recommended reverberation time, the proposed design of the hall will perform adequately. The reverberation time of the hall with untreated surfaces is quite high, too high for adequate speech intelligibility. The high RT at low frequencies can be overcome using the plenum which is understood to be proposed beneath the audience seating. In consultation with the environmental consultants, it should be possible to utilise this as a bass trap without affecting its thermal or natural ventilation functions. This simple means including an amount of sound absorption material around the support columns, leaving the other internal surfaces free for thermal exchange. The volume of the hall is also quite high. Given the divergent requirements of speech and music, it is recommended that the acoustic design be primarily based on meeting the speech intelligibility requirements as it is understood that this is to be the primary function of the space.
The front 9m of the exposed concrete ceiling provides a suitable surface for first-order reflections onto the audience plane. This will function best if kept free of ribs or deep beams. If no absorption is to be applied to the ceiling in order to maximize its thermal effect, then the rear 6-7m should be made as diffusing as possible. This can be done using ribs, beams or even a more creative pattern using the concrete formwork. The required dimensions of the ribs will be provided.
