Sound is energy and the amount of energy determines the quality of the sound as well as the ability of the sound to travel through air, walls, water or other mediums. Low frequency sound waves have tremendous energy and, therefore, can be heard miles from the noise source. If you have lived in an apartment, you know the music you hear from the adjacent apartment is the bass, the low frequency sounds. High frequency sound contains small amounts of energy and can be diminished by standard sheetrock or other types of mass barrier.
To reduce noise transmission from one area to another, three methods are commonly used: treat the noise source, treat the noise path or isolate the receiver (the person who is hearing the noise). Treatment of the noise source is typically the most effective and economical. As an illustration, picture a light bulb (noise source) in the center of the room. Your goal is to keep the room dark (quiet). If you encapsulate the bulb, the room is dark. However, once the room is lit, reducing the light by adding materials to the room is not very effective (treating the noise path). If you use sunglasses on the eyes of the receiver (the person who desires darkness or quiet), you will have to provide sunglasses (ear plugs) for anyone who enters the room. The above clearly demonstrates the economy of treating the noise source or light bulb instead of the path or receiver.
The effects of noise? We often think of our ears when discussing effects of noise. Research validates the effects of noise impact our entire bodies. From Handbook of Environmental Acoustics by James P. Cowan:
- Physiological: hearing loss, hypertension, cardiac disease, ulcers, endocrine and biomedical disorders, nausea, headache, dizziness.
- Psychological: Insomnia, annoyance, fear, stress, learning disability.
- Others: compromising safety, speech interference, sleep interference, compromising privacy, lack of concentration, compromising enjoyment of leisure activities.
Let’s reduce the noise in our world.
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.