Science Questions

Does higher air pressure result in louder sound?

Sat, 6th Aug 2011

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Question

Malte Ohlsen asked:

Dear Chris,

 

I have a question I'd like to hear your opinion on.

 

Loudness of a sound decreases when air pressure decreases. No sound can travel in a vacuum.  However, does this mean that higher air pressure would result in louder noises? Do deep-sea divers in decompression chambers have to keep their voice down because sounds appear louder?

 

Love the show! As a science teacher in training, I am learning a lot of things relevant for the classroom that I would never get from academic training. 

 

Kind regards,

 

Malte from Hamburg, Germany

Answer

Dave -   I don't know about deep sea divers, but the effect related to how efficiently you can couple vibrations - so get a vibration from the musical instrument, or your voice, into the air, then from the air into your ears.  With all of these transitions, you tend to get some sound carrying on and some sound reflecting.  And the more similar the material which you're going from and to, the more energy it gets transferred, so if you're going from one bit of air to another bit of air, pretty much all of the energy is transferred from one to the other.  If you're going from air to a solid piece of steel, almost none of the energy is transferred and actually, vice versa; If you’re going from a solid bit of steel almost none of the energy is transferred into the air and all of it is reflected.  So if you have very, very high pressure, it increases the pressure of air, it’s going to increase its density and make it seem to the sound wave more like the musical instrument or more like your ear so more energy will get transferred, everything will get more efficient, and I would’ve thought you would hear things louder.

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The loudness of a sound is measured in two ways: sound pressure level (SPL) and power to per unit area. Decibells are a measure of the ratio of a the power per unit area relative to an arbitrary standard level. A common reference level is the threshold of normal human hearing; this is standardized as one picowatt per square meter (10^-12 w/m²). 1 w/m² is a 120 db louder than the reverence level, usually stated simply as 120 db.

If you shout into a kilometer-deep well whose sides reflect sound perfectly, with a loudness of 100 db, a person at the bottom will hear you at 100 db. This is necessary because of conservation of energy. To increase the power, you must have a power source.

The situation is different if you shout into one end of a kilometer-long horizontal pipe with a piston at one end to compress the air after you shout. If the piston is at your end, it will not add energy to your shout because the added pressure can't catch up with the sound already emitted. If the piston is at the far end, a microphone attached to the piston would pick up more loudness, not because of increased pressure, but because of the Doppler effect; the energy would not be increased, but it would be absorbed in a shorter time. If the piston is the whole top of the pipe, from end to end, it would increase the loudness by squeezing the constant power into a smaller area.  Phractality, Fri, 8th Jul 2011



I don't think the density of the medium affects the amplitude as much as it affects frequency. We all know the effect that inhaling helium has on the human voice, so I would think that there would be a corresponding deepening of the human voice at higher air pressures. However, I've no idea if that is actually true.

If it is true, would that mean bass singers will find it easier to hit their lowest notes at sea level rather than on top of a mountain? Geezer, Fri, 8th Jul 2011

Malte Ohlsen asked the Naked Scientists: Dear Chris, I have a question I'd like to hear your opinion on. Loudness of a sound decreases when air pressure decreases. No sound can travel in a vacuum.  However, does this mean that higher air pressure would result in louder noises? Do deep-sea divers in decompression chambers have to keep their voice down because sounds appear louder? Love the show! As a science teacher in training, I am learning a lot of things relevant for the classroom that I would never get from academic training.  Kind regards, Malte from Hamburg, Germany What do you think? Malte Ohlsen , Tue, 9th Aug 2011

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