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The ancient myths and legends of soundproofing

There is so much folklore about sound insulation, but the science and technology is easy to understand. Perhaps because it is expensive and difficult to achieve really good results, people look for 'magical' solutions...

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There is so much folklore about sound insulation, commonly known as soundproofing, but the science and technology is easy to understand. Perhaps because it is expensive and difficult to achieve really good results, people look for 'magical' solutions.

Let's suppose sound insulation had never been invented. OK, let's now invent it - what do we need to do?

We need to prevent sound from passing from one space (your studio for example) to another (your neighbor's living room). Let's think about the possible ways we could do this...

  • Absorb the sound energy and convert it into heat.
  • Create an equal but opposite sound wave that cancels the original sound out.
  • Reflect it back into the space where it was generated.

If you can think of any other possibilities, apply for a Nobel Prize now.

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The first option, absorption, sounds good in principle. However, materials that absorb sound are insufficiently effective. Light for instance is easily absorbed, so a photographic darkroom doesn't need a door - a folded passageway painted black will work just as effectively. It's a pity that nothing will work as well with sound.

This is perhaps the greatest myth of sound insulation. Absorption alone does not work, but it has its place as we shall see in a moment.

Creating an equal and opposite sound wave to cancel the original out actually does work. The problem is that it only works well for certain sound sources. For example a noisy piece of machinery can be enclosed, its sound captured, inverted and used to drive loudspeakers. This technique has also been applied to cars and aircraft. Perhaps one day it will be perfected for a wider range of sound sources, but the technology is not ready for the recording studio yet. Yes there are such things as noise canceling headphones and they work by cancellation, but the only way they could be used in the studio is for monitoring and it would be difficult to recommend working in a noisy environment and relying on them as a solution.

So, we come to the technique that actually does work - reflecting it back. It is easy, in principle, to reflect sound - simply set up a solid continuous barrier that possesses significant mass. Mass reflects sound - it's as simple as that. And every time you double the mass of a barrier, the degree of sound insulation improves by 6 decibels.

But what do you do with all that sound that is reflected? Surely that will cause a problem inside a well-insulated studio? Well yes it will, so you absorb it! This is the role of absorption - to dispose of the energy of sound that has been insulated by reflection. This is standard practice in almost every situation where good sound insulation is required. Mineral wool is a very useful material for this, and also is normally incorporated into a double-leaf partition to absorb sound that would otherwise reflect back and forth between the panels.

One last point - reflecting low frequencies is difficult as mass loses effectiveness by 6 dB for every halving of frequency. However, if the barrier can be made flexible as well as massive, or a flexible membrane incorporated, then low frequencies will be absorbed by the barrier bending. Here absorption does produce useful additional insulation.

By David Mellor Wednesday September 25, 2019
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