Sound proofing by closing a door

By Marcus Wilson 27/10/2010

We’ve had a couple of fire alarms in the last week. Both false alarms, which is good. We still however pile out onto the grass in front of the Faculty of Science and Engineering and time how long it takes for the fire engines to arrive and bet on whether the Hamilton City-based crew will beat the Chartwell-based crew to the scene.

We have five small buildings joined together – and an alarm in one will trigger the alarms in the other. Yesterday, when the alarms were switched off, the alarms in just one of the buildings carried on for a while. As I walked through that building to get to mine, the noise was pretty intense. But after going through a couple of doors and into my office, I could hardly hear it.

Conclusion: doors make pretty good sound proofing. But why?

Sound moves through air as a compression waves. That means the air molecules move backwards and forwards as the wave passes, causing a fluctuation in pressure. It’s this pressure change that your ear picks up and your brain interprets as sound. Sound also travels through doors as a compression wave. And like all waves, when the medium in travels in changes, there can be reflections.

Light is an obvious example. When light hits glass, some of the light gets reflected. We can see faint reflections, even though most of the light is transmitted through. Just how much light is reflected depends on a quantity called the impedance of the glass. Or, more precisely, the relative difference between the impedance of the two media – the air and the glass.

Similarly, there are acoustic impedances. Where two materials have very different acoustic impedances, there will be a lot of sound reflected, and very little transmitted. Acoustic impedance is strongly related to the density of the medium. A door is way more dense than air, and so most of the power incident on the door will be reflected. Relatively little gets through, and so the noise is much quieter on the other side.

This is one reason why things are typically very quiet underwater even if you are in a noisy swimming pool. The noise from a hundred screaming children travels very nicely in the air, but most gets reflected off the surface of the water and little actually makes it underwater. Also, your ear is designed to hear in air, not water, and so again much of the power hitting your eardrum gets reflected and fails to transmit through to the inner ear.  Everything appears quiet.

It’s the Thesis in Three final tonight – I’m really looking forward to it.

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