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Sound of symphony from single source

  1. Jul 28, 2011 #1
    Hi,

    I'm sure the question is quite naive but I'm having a hard time getting around the fact that the sound of a symphony orchestra can be emitted from a speaker. That a large collection of sounds of vastly different tonality, playing simultaneously, can be reproduced by a single sound source.

    I have a vague idea that the combination of sounds forms a composite waveform and then that is reproduced by the speaker-cone but it escapes me how all those timbres can be expressed at the same time just by a speaker cone moving back and forth with a certain amplitude and frequency (albeit constantly changing). It strikes me that a humble speaker just doesn't have what it takes to pull off such a feat, but yet the sound of the orchestra pours forth.

    Could anybody shine some light on the matter? I find it hard to move down from my tree house and rejoin society while this question haunts me.
     
  2. jcsd
  3. Jul 28, 2011 #2

    DaveC426913

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    Here's a quick read on adding waveforms:
    http://clas.mq.edu.au/acoustics/waveforms/adding_waveforms.html

    The only real question is: how fast can a speaker membrane respond to a change in amplitude? As long as it can physically respond as fast as the changes in the magnet, which is controlled by the signal, then the speaker will vibrate with acceptable fidelity.
     
  4. Jul 28, 2011 #3

    Andy Resnick

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    You are touching on a lot of interesting concepts. First, as you already know, the sound emitted by a speaker does not truly reproduce the original sound- and that's not accounting for the acoustic properties of the room the sound was recorded in vs. the acoustic properties of the room you are listening in.

    One way to consider this is that your brain only can 'hear' as well as your ear (the whole structure, including the cochlea) is able to translate moving air into electrical impulses. From your brain's perspective, it does not matter if the air was set in motion by an actual orchestra playing in a concert hall or if was set in motion by a speaker (plus electronics) set up to approximate the original sound. At some level of approximation, your brain will not be able to distinguish between "real or memorex"- what that level is is subject to often emotional debate (see, for example, analog vs. digital storage media).

    Speaker designers make every possible effort to produce a device that faithfully converts electrical signals into audio signals- based on white or pink noise, tone generators, etc. Concert hall designers account for the way sound propagates and attenuates to manipulate the overall tonality and reverb characteristics.
     
  5. Jul 28, 2011 #4

    DaveC426913

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    See, you've taken it to a different place. Leaving aside the whole problem of perception - I assumed he was simply asking how the one dimensional movement of a speaker could reproduce every harmonic of every instrument in a 100-piece orchestra simultaneously.

    Kind of like sketching a picket fence with a crayon. At 10cm per board, that's pretty easy to accurately draw with a crayon. But sandwich 100 picket fences together, all of different patterns and all staggered, and the resolution required to represent them all drops to a fraction of a millimeter - far less than can be rendered with a fat crayon.
     
  6. Jul 28, 2011 #5

    Andy Resnick

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    I apologize, I was unaware that you are in charge here.
     
  7. Aug 1, 2011 #6

    DaveC426913

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    I hope you're kidding. :wink:

    I was pleased to see a viewpoint other than my own. Multiple viewpoints are what make it interesting around here.
     
  8. Aug 2, 2011 #7
    If you look at what adding the waves together looks like in the link posted, in my understanding they literally physically add themselves together before they reach your ears. Also, your brain probably adds the waves together as if they were physically added together, in a situation where they don't physically add together. This for example makes possible binaural beats, where the waves can't interfere with each other but your brain makes them, for example when you play a tone in headphones on one side and a tone a couple hz different in the other side, they can't add up before reaching your ears, but your brains adds them up and you still hear beats as if they interfered with each other. Basically the waves show changes in pressure over time and your ear detects them. A symphony playing creates a complex wave and your brain dissects it, if that exact same wave is created by a loudspeaker, it has no way of distinguishing.

    If you use a speaker to generate a perfect triangular wave, just by constantly increasing pressure, then suddenly dropping the other way etc, you will hear the harmonics, despite the fact that nothing is actually resonating at the frequency of the harmonics. Your brain just dissects them into sine waves in a similar way to Fourier Transform, because that way the brain can analyze multiple sounds at the same time, analyze harmonics to distinguish timbers, vowels etc.
     
  9. Aug 2, 2011 #8

    Andy Resnick

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    Let's just chalk it up to the (context) limitations of text conversations :)
     
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