Listening For Change In Complex Soundscapes

Listening for change is something we do for both pleasure and survival. Unless we are in a very rarefied atmosphere, the soundscape is usually complicated. Even when we are sitting in the sun out in the country, we have multiple components to the sound scene we are listening to at any given moment. We hear the rustling of leaves in the trees and birds singing or quarreling in their branches, but the moving around of little rodents in the nearby bushes adds to the complexity.

With music on the menu, we listen to the rise and fall of volume and changes in complexity as instruments and voices move in and out of the composition of the soundscape. Sometimes it’s pleasing and sometimes it’s not.

Picking up rapidly on soundscape changes is a survival instinct that we are still fairly good with. Not so good as our dog perhaps, or that deer out in the woods.

Sudden changes in volume are important, of course, but something with large teeth creeping up on you won’t be adding much to volume. Rather it will be bringing in new sounds as it tries to approach stealthily. You turn your head, it stops moving and that sound is gone. You wait a little, shrug and continue and then you hear the sound appear again. The cat and mouse game continues until – well, that depends.

In this week’s PLoS ONE, Constantino et al have their paper in which they describe a series of experiments to see how easily people can spot changes to a sound scene as a new sound come is or an old one disappears. The scenes are non-musical and consist of groups of frequencies played together in groups of 4, 8 or 14. Experiments were carried out to eliminate problems such as changes in loudness as one sound appeared or disappeared from the mix.

The results were quite clear that sounds disappearing weren’t noticed as successfully as sounds appearing in the mix. The bigger the mix, the less the loss of one component was noticed, but even in the largest mix, the appearance of an additional sound was spotted rapidly and successfully.

Clearly our survival mechanism is dominant and our auditory senses work similarly to our visual ones where we pick up on a change as a head pops up. Just like the other animals out there, our first reaction is to get ready to run or take the safety off our gun.


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