Noise Control 101

The wooden box story has been told for many years to illustrate the problems faced when attempting to reduce noise. The story begins with a noise source, let’s pretend it’s a generator in your backyard. Neighbors are complaining about the noise so you buy some plywood to build a box around the compressor so it will be quieter. Before the box is in place, the noise level is 90 dB(A). After you have constructed 4 sides and a top from the plywood and placed it around the generator, the noise level is 94 dB(A). “Hey”, you ask, “What happened here? How can the noise be louder when the generator is inside the box?”

There are two basic types of materials used to reduce noise: an absorber and a barrier. The absorber is soft, flexible material such as fiberglass or foam. The barrier can be flexible or rigid and incorporates mass, typically at least 1 pound per square foot. We can use these materials separately but often use them in a composite to achieve best results. The absorber reduces reverberation (echo). A common absorber is the acoustic ceiling tile in your office. It reduces the echo in your office but allows the person on the other side of the wall to hear everything you say. (You didn’t know that?) The barrier reduces noise transmission (energy traveling from one area to another). If you place the proper barrier on top of the acoustic ceiling tiles, sound transmission will be reduced. Your office mate will no longer be able to hear your conversations. The reason we use these two materials in a composite is that used separately, unintended consequences sometimes happen. It you put a barrier without an absorber around a noise source, the reverberation increases as the sound hits the reflective barrier and the noise level increases. If you use the absorber without a barrier, your office is quieter but adjacent offices can hear your voice.

There is a happy ending to the wooden box story. You have lined the box with a well chosen acoustic absorber and reduced the noise level to 70 dB(A). The ending could have been happier if you had chosen a flexible barrier/absorber composite but that’s a lesson for another day.