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Sound Isolation Principles |
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Your Home Studio |
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Copyright 2006
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LGM Productions |
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Principle 1 - Mass The first principle of sound isolation is mass. Mass impedes the transmission of sound in a simple way - it’s harder for the sound to shake a very heavy thing than a very light thing, no different than saying it’s harder to push a shopping cart full of lead bricks than an empty cart. However, to make large changes in performance you have to make very large changes in mass. In theory doubling the mass of a panel without an air cavity will improve things by 6dB. Typically, on the common single wood stud wall, doubling the number of drywall layers yields 4-5dB of improvement. As this series of tests show, just adding layers of drywall to the common wood stud wall or wood joist ceiling yields only a small benefit. To really improve your wall, you have to not only add mass, but also improve some of the other 4 principles. Simply adding layers of drywall is perhaps the least efficient way to improve sound isolation as illustrated below.
Principle 2 - Mechanical de-coupling, or mechanical isolation. One of the most familiar of the 5 basic principles is mechanical decoupling. Sound clips, resilient channel, staggered studs and double stud walls. All of these function by inhibiting the movement of sound from one side of the wall to the other through mechanical paths (like studs or joists). Instead, the vibration has to pass through the air cavity in the wall, where some of it will be lost, and through the insulation/absorbing material, where (at some frequencies) much of it will be lost. What most people don’t understand about mechanical decoupling is that it is frequency-dependent. When you decouple, for example, two pieces of drywall, you create a resonance, and only well above that resonance does the decoupling help you – below about 1/2 of an octave above that resonance it actually makes things worse. De-coupling is a very powerful tool, but one must plan around this resonance and the low-frequency performance problems it can cause.
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