# How can sine waves represent both AC and sound waves?

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?"

• I believe sound waves are longitudinal waves – Bob D Oct 30 '20 at 17:57
• Because the sine function is a solution to the physically common differential equation where the restoring force is proportional to the negative displacement – Nayuki Oct 30 '20 at 18:12
• 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. – Philip Wood Oct 30 '20 at 18:16

## 2 Answers

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.

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.