APPLICATION OF SOUND BARRIERS
Topics covered here are:
Effectiveness in Reducing Sound Levels
Construction Materials
Example
Sound barriers are most commonly seen along interstate highways. The goal is to reduce the spread of traffic noise into adjacent communities. The same acoustic principles can the applied to the reduction of concert sound, but with limited effectiveness. The design parameters, however, are not the same since the source in amphitheaters is the full audio spectrum of music with substantial bass.
Sound barriers work on the principle of casting a “acoustic shadow” away from the source. Clearly if there is line-of-sight between source and the listener, there is no “acoustic shadow” and no noise reduction.
The depth (darkness) of the “sound shadow” can be calculated in decibels. This is often called the attenuation of the sound barrier.
Effectiveness in Reducing Sound Levels
The effectiveness of a sound barrier depends on several factors:
| a) |
When sound must travel an additional distance from the source to the listener on account of the barrier, its level is reduced. The taller the barrier the greater the additional distance, the more effective the barrier. |
| b) |
The effectiveness of the barrier depends upon the result of multiplying this additional distance with the frequency of the sound. Thus, the attenuation at low frequencies is much less than at high frequencies. This means high-energy music with a strong bass cannot be attenuated readily by the application of a practical-height noise barrier. |
| c) |
The greatest depth of the “sound shadow” is directly behind the sound barrier; further away the barrier becomes less effective. At long distances the sound attenuation of a sound barrier is negligible. |
| d) |
Stacking loudspeakers in towers raises the source height and reduces the effectiveness of the sound barrier. |
| e) | Where there is a high wall facing the sound source behind the barrier, sound can be reflected back into the “sound shadow” to partially fill it in. |
| f) |
Without a barrier, the sound may already be attenuated by open ground. Installing a barrier causes the sound to spill over the top, now at an increased height above ground level. This process negates the already beneficial effect of some of the open ground. Consequently, the addition of a barrier has less effect than anticipated. |
| g) |
The effectiveness of a barrier must be evaluated in every direction from the stage, including to the rear of the stage. |
The potential effectiveness of noise barriers can be calculated by the experienced acoustical consultant. It is no simple task, particularly when the terrain is undulating that makes the lines-of-sight varied in different directions.
In most practical situations the overall geometry of
sound barrier does not permit sound to be reduced by more than about 10
dB. This means that the if the transmission loss of the barrier is 15
dB then this is adequate to do the best possible job. Also for 10 dB of
effectiveness the sound barrier must be tall, so the wind load factors
are of major concern particularly in tornado alley of the Midwest. So
, in general, the acoustical requirements for the materials are minimal.
So, if 3/4" CDX plywood is OK for wind loads then that is OK acoustically.
But be sure there are no openings in the barrier.
Note that lining the the wall with fuzz does not increase its noise
reducing effectiveness, though it does reduce "black slap" and
focussing of sound back onto the lawn seating area. Backslap muddies
the perception of the sound, seriously interfering with its music
quality.
An example of a sound barrier is at the Tweeter Center (formerly Great Woods Center for the Performing Arts), Mansfield, MA. The sound barrier is incorporated into a bleacher section at the rear of the lawn. This design by Cavanaugh Tocci Associates allows for a tall sound barrier that also accommodates additional seating along with "back slap" sound absorption by the bleachers themselves..
