9

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?!

Widor
  • 365

5 Answers5

12

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/

Gauthier
  • 236
  • The third point is something that is especially easily noted: From music that is loud but far away, often you only hear the thumping bass. – Lagerbaer Mar 23 '12 at 15:18
5

I mainly has to do with reverberation effects. We seldom hear anything purely as direct sound, it's normally mixed with reflections from all walls/objects around. The direct sound is always the first thing we hear. If the source is nearby, it's also the loudest signal; if it's a soft noise the reverb is even softer, the direct signal still stands out signalling that the source is close to our ears. But when the source is not closer than the walls, the reverb amount it significant. The amount of reverb mainly tells us the relation room size / source distance, not either of them directly. But what we also notice is how much the reverb is delayed: in a small room, all reflections are very quick while in the open, it can take seconds for the echo to reach us.

leftaroundabout
  • 14,903
3

I don't know much about this, but I know one thing:
Lower frequences carry easier over distance than higher frequences. Specially if there are physical obstackles in between, which will stop/dampen high frequences, but not low frequences.

So 'far away' will often have less sharp and high pitches, and low pitches will somewhat dominate the accustic picture.

awe
  • 131
3

Mostly the effect is due to things other than "timbre", such as reverberation.

The one thing that does affect timbre is the fact that air absorbs higher frequency sound more than lower frequency sound.

http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html

enter image description here

So as the waves travel to you, higher frequencies will be attenuated more than lower frequencies. This affects the timbre of the sound, and is part of the reason why loud sounds from very far away are almost entirely bass.

endolith
  • 5,666
  • 1
    Though reflections have a great deal of importance indoors or in reflective city landscapes, in most natural outdoor environments this is the primary explanation for the differences in timbre distant sounds have. They will have lost much of their high-frequency energy due to molecular absorption. – Halyn Betchkal Nov 11 '17 at 20:26
  • @D.Betchkal Echos matter a lot. I was camping in the woods recently and it was very quiet, and the echos from clapping were very apparent – endolith Nov 11 '17 at 20:42
  • I suppose you're right. I guess what I mean to say is over longer distances (say hundreds of meters) that's the most important factor. – Halyn Betchkal Nov 11 '17 at 20:45
2

This is my first post to the Physics stack. With due respect to the detailed and accurate answers given above, let me try to summarize:

When a sound is close by, your ears hear the sound coming directly from its source to your ears.

When a sound is far away, you may hear the sound coming directly from its source, but mixed in with that, you will also hear the sound being bounced off surfaces and materials between the source and you, and these indirect reflections of the source will be substantially different in character. They will arrive later in time, with different tonal characteristics.

Your two ears and your brain will intuitively process the information and deduce that the relationship between the direct sound and the other reflected sounds (which may be delayed by some milliseconds, or brighter or darker in certain overtones, and attenuated or amplified to varying degrees) means that you are listening to something from farther off. (Your ears and brain, working in binaural stereo, may also give you an impression of which direction the sound is coming from, and how far away it is.)

This gets into the realm of what is called "psycho-acoustics". In other words we can use scientific instruments to measure, analyze, and mathematically model the sound arriving at your ears, but it all comes down to how your ears and brain perceive and interpret the sound.