How can sine waves be used to describe both alternating current and sound waves? In the case of alternating current, the zero crossing represents zero current, and the waveform below the zero crossing represents current with opposite polarity. What does the zero crossing represent in sound waves? Silence? When representing a sound wave, what does the waveform represent that is below the zero crossing? When representing alternating current, the waveform below the zero crossing represents "negative current." When representing a sound wave, does the waveform below the zero crossing represent "negative sound?"

  • $\begingroup$ I believe sound waves are longitudinal waves $\endgroup$
    – Bob D
    Oct 30, 2020 at 17:57
  • $\begingroup$ Because the sine function is a solution to the physically common differential equation where the restoring force is proportional to the negative displacement $\endgroup$
    – Nayuki
    Oct 30, 2020 at 18:12
  • 2
    $\begingroup$ Perceived sounds don't correspond to individual points on the pressure against time graph, rather it requires several cycles before we perceive a specific sound. $\endgroup$ Oct 30, 2020 at 18:16

2 Answers 2


There are several parameters that can be used to represent the sound wave. The most common is acoustic pressure. This is the change in pressure above and below the equilibrium pressure. So zero acoustic pressure means equilibrium pressure which may be just atmospheric pressure for sound waves in open air.


Look at this animation showing a sound wave propagating from right to left.

enter image description here
(animation from Dennis Lindwall - Marine Seismic Surveys with Vector Acoustic Sensors)

In real sound waves the oscillation is not as slow as in this animation, but much faster (between $20$ and $20000$ oscillations per second).

You see there are several kinds of oscillations occurring:

  • Every single particle oscillates between a left-most and a right-most position (see the red arrow). So its position as a function of time can be described by a sine function.
  • The density at every place oscillates between higher and lower density (see the blue marks). So the density as a function of time can be described by a sine function.

Silence (i.e. no sound) means, all particles are sitting still and also the density is constant all over the place.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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