By Cliff Kaminsky
Vibro-Acoustic Sciences, Inc.
To prevent noise from getting out:
In the noise control business we talk about TL, the transmission loss of the wall. The higher the TL, the better. TL is a function of frequency. It is very difficult to stop low frequencies and quite easy to stop high frequencies, so TL is low at low frequencies and high at high frequencies.
Your TL at low frequencies is determined entirely by surface density (pounds per square foot or kilograms per square meter) of the wall. The heavier the wall, the better your TL will be. For each doubling of surface density, you will get a 6 dB increase in TL. Generally, 3 dB is considered an audible difference. Popular materials for adding mass are heavy rubber mats, sand, concrete, steel sheets, or lead-lined vinyl.
TL at higher frequencies is determined by both mass and the number of layers. This is where the double-layer system becomes effective. With a single layer system you get about 6 dB increase in TL for each octave. That is, if you have 30 dB at 1000 Hz, you will have 36 dB at 2000 Hz. Each additional layer (separated by an air space) will give you another 6 dB per octave. 2 layers is 12 dB per octave, 3 layers is 18 dB per octave, etc. However, you have to be careful because each time you put an air space (or insulation material) between two wall panels, you create a resonant system and the resonance will decrease your TL at the resonant frequency. Unfortunately, calculating the resonant frequency can be complicated, but it usually falls in the high 10s to low 100s of Hz.
One thing you can do to improve the TL of the wall is to build it in two separate sections (double-stud construction). The exterior panels are connected to one set of studs and the interior panels connected to a different set of studs and the two sets of studs and their respective frames are not attached to each other. That will be a better wall than the standard construction. Also, any open air leak, such as vent holes, will almost certainly be the weak point in your design. Don't spend thousands of dollars fixing the walls without addressing the ventilation system and any other possible leak paths.
Interior sound quality:
You probably already know that there is an ideal amount of reverberation , or reverb, for a room. You don't want too much or too little. Reverberation is related to the absorption characteristics of each of the room's surfaces. Absorption is also a function of frequency and we treat low frequencies and high frequencies differently.
For low frequencies, we generally use resonant absorbers. For example, when you blow across a Coke bottle, it hums at a certain frequency. If you place that Coke bottle in your studio, it will absorb sound at that frequency. You can build tuned absorbers to absorb at whatever frequency you want. Low frequencies is where we find "room modes." That is, the room's resonant frequencies. Resonances cause you to have greatly distorted response depending on where you are standing in the room. For this reason, resonances are bad both in the recording room and in the control room. Resonances will be especially bad in rooms that are nearly square. For any rectangular room it is easy to calculate the room's resonant frequencies. Then you can build or buy absorbers to damp those resonances.
For high frequencies, we use porous materials. Fiberglass, foam wedges, acoustic tile, and carpet are all examples of acoustically absorptive material. It is a myth that anything shaped like an egg crate will absorb sound. Hanging egg crates on your walls will hardly affect the room at all. The material must be porous to absorb sound. The common "egg crate" shape you often see is just an optimization. An easy way to see if it's porous is to try to blow air through it.
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