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Thread: How is it that we're able to hear multiple sounds at any one time?

  1. #1 How is it that we're able to hear multiple sounds at any one time? 
    DCC
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    Some time ago I read that, when sound waves reach the ear drum, they cause a series of bones -- one placed after another -- to vibrate, and these vibrations ultimately reach the nerves that lead to the brain. Correct me if I'm wrong, but I assume that only one vibration can move through these bones at any one time. If that is so, then when two or more sound waves reach the ear drum -- and thus the bones -- at the same time, they combine together into one vibration that proceeds to move through the bones to the nerves. Yet somehow we're able to hear two or more sounds simultaneously. Does anyone know how this works?


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    You are correct. The only thing that reaches our ear is a sound pressure signal which varies in amplitude over time. Somehow, our ears and brain are able to make sense of it, which is remarkable.
    Something that may help to understand it, is the superposition principle.
    Superposition principle - Wikipedia, the free encyclopedia
    The pressure that reaches our ear is the sum of the pressure that would be produced by the individual sources of sound. Any complex signal can be mathematically analyzed as the superposition of a number of sine waves of different frequency and amplitude. One way of doing this is by Fourier analysis.
    Fourier analysis - Wikipedia, the free encyclopedia
    This doesn't totally explain what is happening when we hear sounds, but might give some idea of it.


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    DCC
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    Thanks. So our brains are able to differentiate between signals that are caused by only one sound wave, and those that are caused by more sound waves? And if the signal is actually a sum, then the brain is able to break it down into the individual signals that make it up?
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    What follows is exactly what happens when we hear, except that the "other piano" is the cochlea, with its Organ of Corti, that's contained within each of our ears.

    Alexander Graham Bell grew up in a house with two pianos in two rooms. He found that when a note was played on a piano in one room, the sound energy radiated through the air, and excited the wires* (and only those wires*) for the same note in the other piano. He found this by having someone repeatedly play a note on one piano while he felt the wires in the other piano with his fingers. Yes, he was a technogeek. Bell also found that when a "chord" (multiple notes) was played, then the wires (and only those wires) for the same notes vibrated in the other piano.

    (It was this research that led Bell to understand the nature of sound and how it affects physical objects that led him to invent the telephone. In fact, he was trying first to invent the harmonic telegraph, which he then intended to expand into what he thought was needed to make a telephone (which would have been a monstrosity), when he discovered that such a complicated telephone wasn't necessary at all!)

    * Pianos tend to have more than one wire for each note (I think it's usually three).

    Basically, in the ear, each "wire" in the "second piano" excites a nerve cell that sends a signal to the brain. Here are a few videos from YouTube about how the ear works. The third video probably has more information than you ever wanted to know. If you look around YouTube, you'll probably find other videos on this.

    How the human ear works - YouTube

    Ear Organ of Corti - Condensed Version - YouTube

    Ear Organ of Corti (Full Version) - YouTube

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