Neil's News

Particular Sounds

26 November 2003

Recently I've been doing some fairly heavy trigonometry. It got me thinking about what the universe would be like if sound were a particle instead of a wave. What if sound were like light?

Particles fall. Thus in a universe where sound behaved as a particle, sound would move on parabolic paths. Assuming you shouted upwards at a 45° angle, the furthest your voice could travel is 12.5km [calculation available upon request]. Frankly this isn't much of a problem. There are very few sounds in human life which travel greater distances than that. Whales would be inconvenienced; they would have to swim down several kilometres in order to hear the songs of others (assuming the water surface formed an acoustic cap).

Another problem is that the speed of sound (340m/s) is smaller than Earth's escape velocity (11,000m/s). This means that what goes up, must come down. All omni-directional noises would have an echo as the upward sound reaches an apogee at 6.2km, then falls back down returning to the source 71.4 seconds later [calculation available upon request].

Earth would be a sonic hole, with a sonic horizon. The sonic horizon would be 51,400km above the surface [someone please double-check this calculation], or one and a half times as far as geosynchronous orbit. Only in deep space (outside the sonic horizon) could everyone hear you scream. That is, if sound were a particle.

Update: it occurs to me that due to Dopler shifting, an aircraft flying overhead would sound like an insect.

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