I'd like to know what are the differences in timbre - or the acoustic properties of a sound - that allow us to differentiate between a sound which is quiet (but close-by) and one which is far away.
For example, you can tell when someone near to you is playing an instrument quietly even without looking to see where they are - they don't sound 'far away'.
Hearing a loud gig or a car stereo playing from the next street doesn't sound like it's quiet - it sounds loud, but far away.
But other times we can't differentiate - I sometimes hear a siren on TV and think it's on the street!
I thought only the amplitude (i.e. volume) of a sound wave diminished with distance - does the shape/frequency alter too? Is this ability just to do with having two ears to locate the source - surely someone who is deaf in one ear can still tell an orchestra is playing a diminuendo and not gradually getting further away?!
Answer
There are different cues to the perception of distance:
- intensity,
- direct-to-reverberant energy (D/R) ratio (decreases with distance),
- spectral balance (reverberations have more low frequencies than the source signal, so when D/R ratio decreases, basses increase. Sound going through walls and windows also lose treble),
- interaural coherence (the closer the source is, the more different the signals to left and right ears are).
These different cues (maybe more, and mainly the two first) are weighted in your brain to give a stable assessment of distance. Research shows that the weighs associated with these cues vary with the type of signal (noise or speech), and angular position of the source [2].
As you guessed, the interaural coherence is not a major cue for distance perception, especially for greater distances, see [1].
sources:
1 http://www.acoustics.org/press/160th/lavandier.html
2 http://www.mendeley.com/research/assessing-auditory-distance-perception-using-virtual-acoustics/
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