Sound waves in air

Here we discuss sound propagation through air qualitatively, contrasting ray and wave transmission. This is a background page to the multimedia chapter Sound. For a quantitative discussion of sound transmission, see The wave equation for sound.

Sound transmission through air

Here is a classic demonstration of how air is a medium for the transmission of sound. A toy monkey plays the drum in regular bursts, inside the bell jar. Each sharp peak on the sound track graph is one drum beat. The vacuum pump is turned on, making a continuous sound.

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Once nearly all of the air is pumped out, the external recording microphone receives a much weaker signal. The signal is also less bright: its high frequency components have been attenuated more strongly than the lower frequencies. I suspect that much of the remaining sound that we can hear is actually transmitted through the floor of the bell jar.

While the air is rushing in to the bell jar, we can see in the soundtrack and also hear the sound of one volley of drumming, with an ampltidue smaller than that when all of the air has entered.

Transmission and diffraction

This puzzle was posed in the multimedia chapter: when someone covers his mouth with his hand, you can no longer see the mouth, but you can still hear the voice? Why?

An argument like this probably led to one of the rare mistakes made by Isaac Newton. He knew that sound was a wave, and that it diffracted around corners: we are very familiar with hearing sound sources that we cannot see. Light, on the other hand, casts a well-defined shadow: why doesn't it diffract around a hand, or a finger? Newton concluded that it was made of particles.

Light does show the typical properties of waves, including diffraction and interference. However, light waves have extremely short wavelengths: typically 5 μm or 0.0005 mm. If you look at a light through a narrow gap between two fingers (hold them rather less than a mm apart, and 200 mm or so from your eyes), you will see interference effects.

The sound of your voice includes wavelengths from several centimetres to a few metres. Most of those diffract around the hand. Nevertheless, large objects cast substantial sound shadows: you may hear the low frequency (long wavelength) rumble of traffic from the other side of the freeway wall, but you are thankfully shielded from most of the high frequency noise.

For a quantitative discussion of sound transmission, see The wave equation for sound.