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Volume from a speaker

  1. Oct 11, 2011 #1


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    Now I have an absolutely horrible question for someone with their MS in physics to be asking. Let's say you want to setup a 500hz tone out of a speaker. To do this, the driver in the speaker oscillates at 500hz. The thing I am wondering is what exactly causes something to be louder? Is it how far out the driver goes when it's generating the sound wave?

    I can't believe I've never learned this
  2. jcsd
  3. Oct 11, 2011 #2


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    The speaker drivers output depends on following factors:
    BL product
    Moving mass incl. air
    Cone area
    Resonance frequency
    Losses in spider and surround roll.

    At 100% efficiency the SPL is 112dB at 1W input. This is however measured at 1m distance under given conditions.

    The more the cone moves, the louder it sounds. This also means that the more power input you feed, the louder it sounds.

    Speakers are mechanical devices with their weak points. At high power inputs the cone excursion will cause the voice coil to travel out of the magnetic field. This will generate harmonic distortion. Much of the 500Hz input will therfor produce a range of harmonics from the speaker itself and supress the 500Hz tone. Instead you get 1000, 1500, 2000, 2500 etc Hz from the speaker as well. There are more factors that cause harmonic distortion, like increasing heat, cone material, over all assambly. The magnetsystem and the coil alignment is one important reason.

    It is also possible to use the back side of the cone to increase SPL. By making an enclosure with a short port, the port can be tuned to 500Hz as well.

    To give you a general answer to your question: Yes


  4. Oct 11, 2011 #3

    I like Serena

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    When the driver goes out more (of course with the same frequency) the air is displaced more.
    A sound wave is a wave of dense air.
    Making the driver go out more means the air becomes more dense and it holds more energy.
  5. Oct 11, 2011 #4


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    The usual way to measure the amplitude of a sound wave (which is not quite the same as the subjective measure of "loudness", but let's ignore that detail) is by the pressure ampltude of the wave.

    Sound waves in the "normal" range of human hearing, and not loud enough to cause rapid hearing damage, can be considered to be linear adabatic waves in an ideal gas, to a good approximation. It follows that for a sinusoidal wave, the pressure amplitude is proportional to the velocity amplitude.

    The boundary conditions at the interface between the speaker cone and the air mean the normal velocities of the cone and the air are equal.

    And assuming the speaker cone in performing simple harmonic motion, at a fixed frequency the maximum cone velocity is proprtional to the maximum displacement.

    Or to summarize all that, "yes".
  6. Oct 11, 2011 #5


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    Ignoring all of the factors involved in actually making the cone move, there are only two ways to make the sound louder (at the actual cone level). You could increase the cone's amplitude of oscillation, or you could increase the size of the cone. Increasing the size will increase the loudness for a given displacement, and increasing the displacement will increase the loudness for a given cone size.
  7. Oct 11, 2011 #6


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    Indeed. Of course at the level of the "speaker" (as opposed to the "driver" cone) it gets a little more complicated.
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