How do soundboards work?

  1. Hello,

    How do soundboards work? When I strike a tuning fork, I have to put it very close to my ear to hear it, but when I press the non-vibrating part of the fork against the top of my desk, suddenly the fork has enough energy to fill the whole room with the sound. The same is with piano soundboards and guitars and so on, just a string vibrating in air doesn't create a loud sound, but with the soundboard they create a lot louder ie more energetic waves.

    Where does the extra energy come from to make the sound several times louder?
     
  2. jcsd
  3. Chegg
     
    Last edited: May 16, 2011
  4. rcgldr

    rcgldr 7,521
    Homework Helper

    A tuning fork or guitar string is so small that it doesn't displace a lot of air so they don't generate a lot of sound. However if the tuning fork or string is attached to a soundboard, the energy goes into the soundboard, which is large enough to displace sufficient air to be heard. The harmonics of the soundboard allow it to do this with a minimal cost of energy.

    You can also place the vibrating end of a tuning fork near the opening of an aluminum drink can, and depending on pitch, the can will resonate and generate a sound, even without direct contact.
     
  5. Does it mean that the string is moving through air without displacing it? Does this also mean that the string will fade away quicker when it is attached to a soundboard, because it's energy is being used up at a faster rate to displace air? What do you mean by the harmonics of the soundboard allowing it to do it at a minimal cost of energy? Isn't the same energy always required to make a sound with the same loudness?

    I don't understand how the aluminum can can resonate the sound. Doesn't the can get resonated by the sound already made by the fork? If it is not against it, where does the can get the extra energy?
     
  6. AlephZero

    AlephZero 7,299
    Science Advisor
    Homework Helper

    The tuning fork, or the diameter of the string, is much smaller than the wavelength of the sound in air. Therefore the pressure variations form "opposite sides" of the fork or string are almost out of phase with each other, and destrictiely interfere with each other at any position a long way from the source of the vibration.

    If you hold a tuning fork close to your ear and rotate it slowly, you can hear the effects of the phase differences as changes in the sound volume. THe sound is loudest when the difference in the distances of the two prongs from your ear is greatest.

    Yes, but for a given amount of energy, making a sound lound enough to hear for say 2 seconds is more useful tham a makiing a "sound" that is too soft to hear but lasts for 2 minutes.

    I don't undestand what rgcldr meant by that.

    The speed of sound travelling through a solid object like a soundbord is much faster than the speed in air. Therefore, you can make a soundboad with a size similar to the wavelength of the sound in air (say of the order of 0.1 to 1.0m, for a tuning fork) and the whole area of the soundboard will vibrate almost in the same phase.

    This vibration is transmitted to the air and produces a wave pattern that close to a plane travelling wave, which does not reduce in amplitude or destructively inferfere with itself as it travels through the air, so the sound can be heard a a large distance from the source.

    It is the air inside the can which is resonating, not the can itself. The sound is then propagated into the air from the opening in the can. Even if the opening is small, the difference between this and a tuning fork is that there is only one source of the sound, not two sources close together and out of phase. The sound tends to propagate as a spherical wave and the amplitude is proportional to 1/r2 at distance r from the can, compared with the double source (dipole) from a tuning fork where the amplitude is proportional to 1/r4.
     
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