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Count the channels in the noise
The exercise is called the mixer. In a coffee bar, the question is how many channels of sound can be heard, and how many individual channels are in that mix at once.
It works in a beautiful place too. At a lake, the questions become how many birds are audible, where they are, and where the ripples are.
Much of life is spent in noisy places, which makes them the easiest training ground for listening.
How does the brain split sound into sources?
Psychologist Albert Bregman of McGill University called this auditory scene analysis in his 1990 book of that name. The brain groups parts of a sound into streams by cues such as shared pitch, matching onset times, and location. Components that start and stop together tend to be heard as one source. Bregman explained the challenge with an analogy: working out what is happening on a lake by watching only the waves in two narrow channels cut into its shore.
How do two ears help in noisy rooms?
A sound off to one side reaches the nearer ear slightly before the other. People can detect these interaural time differences down to about 10 microseconds. For high pitches, the head also casts a shadow that makes the far ear's signal quieter. The largest possible delay between the ears, for a sound directly to one side, is roughly 0.6 milliseconds.
How does the brain fill gaps in noise?
In 1970, Richard Warren replaced one speech sound in a recorded sentence with a cough. Listeners reported hearing the complete word and could not say where the cough had occurred. The effect is known as phonemic restoration. It works best when the gap-filling noise is plausible in context.
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