11
$\begingroup$

Water waves are dispersive ( longer wavelengths travel faster ) but sound waves in air are not, otherwise we would listen first the high frequencies and the low frequencies after.

What decides if a wave will be dispersive or not?

This question has been asked again. I am looking for an answer or a comment that explains the physical reasons behind the mathematics.

$\endgroup$
  • 6
    $\begingroup$ The physical reason is that you have not paid attention: sound most certainly disperses in air. Listen to a thunderclap: first you hear a high crack, then later on a low rumbling. $\endgroup$ – Carl Witthoft Apr 13 '16 at 19:33
  • 1
    $\begingroup$ I'm voting to close this question as off-topic because its assumptions are wrong $\endgroup$ – Carl Witthoft Apr 13 '16 at 19:33
  • 2
    $\begingroup$ @CarlWitthoft Why not, you are right. But the essence of the question is why waves are dispersive $\endgroup$ – veronika Apr 13 '16 at 20:29
  • 1
    $\begingroup$ "What makes a medium dispersive?" seems to be the essence of the original question. As discussed below, a medium can be dispersive if it has some natural frequency that governs the medium's response to incident frequencies. The natural frequency is determined by the physical constraints on the medium's constituent bodies. $\endgroup$ – curiousStudent Apr 13 '16 at 20:54
  • 7
    $\begingroup$ Closing questions because the assumptions are wrong? Why not correct those assumptions with an answer instead? $\endgroup$ – Kyle Kanos Apr 14 '16 at 10:29
16
$\begingroup$

Dispersion of sound in air, with constant temperature and pressure, is very slight, increasing for very short wavelengths, and for very loud noises. Why? Because the rapid sequence of weak compression/decompression steps as the sound propagates are adiabatic, or energy-conserving, for the normal ranges of sound. This leaves the local pressure, temperature and density unchanged.

As a result, the equation for the speed of sound in an ideal gas is $c^2 = \gamma P/\rho$, with $c$ the speed of sound, $\gamma$ is the adiabatic constant for the gas, $P$ the gas pressure, and $\rho$ the gas density; other formulas are equivalent. Note that intensity and frequency do not appear in this equation. Hence sound is non-dispersive over wide ranges, given stable atmospheric conditions.

Sometimes thunder is given as a counter-example, where a variety of sounds are heard following a lightning strike but this is not due to dispersion; rather it is the multiple branches of the pre-strike, the main strike, and the extended distances covered by the lightning, plus, sometimes, echos.

Light is similarly non-dispersive in the ordinary atmosphere, but changes in pressure, temperature, and humidity change that, hence mirages.

$\endgroup$
8
$\begingroup$

Take a look at Griffiths Introduction to Electrodynamics, particularly the section called "The Frequency Dependence of Permittivity".

Dispersion can arise from the constraints, or bound nature, of the constituent particles in a given medium. For the example of optical dispersion in a dielectric medium, we could picture the electrons as bound, damped oscillators to which a passing light wave applies a driving force. These oscillators have a natural frequency and, depending on the closeness of the light wave's frequency to the natural frequency, the response of the system will vary-- for instance, more or less energy will be absorbed from the passing light wave.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.