Sound

Sound

a longitudinal pressure wave

Sound squashes and stretches the air along its travel; we graph the pressure against position, not a sideways string.

waves The curve is pressure versus position, an honest stand-in for a longitudinal wave.

Example

Sound squashes and stretches the air along its travel; we graph the pressure against position, not a sideways string. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Sound squashes the air along its travel

Sound is a longitudinal wave: the air does not bob sideways like a string — it squashes and stretches back and forth along the same direction the sound travels, making regions of high and low pressure that move outward.

sound: compressions and rarefactions along the travel\text{sound: compressions and rarefactions along the travel}
Pressure rises and falls along the pathA curve showing air pressure plotted against position, high at the compressions and low at the rarefactions.pressure

We graph the pressure, not a sideways string

To use our wave tools we plot the pressure against position. Read this carefully: the up-and-down of this curve is PRESSURE, not the air moving sideways. The crests are compressions, the troughs are rarefactions. With that picture, wavelength and frequency mean the same as for any wave.

the curve is pressure vs position, not a transverse string\text{the curve is pressure vs position, not a transverse string}

Louder sound means a larger pressure swing

A compression is pressure above the usual air pressure; a rarefaction is pressure below it. In this simplified picture, a larger pressure swing means a louder sound, but it is still pressure changing along the travel direction, not air moving sideways.

Δpmax⁡compressionrarefaction1 Pa1 Pa−1 Pa2 Pa2 Pa−2 Pa3 Pa3 Pa−3 Pa\begin{array}{c|c|c}\Delta p_{\max} & \text{compression} & \text{rarefaction} \\ \hline 1\ \text{Pa} & 1\ \text{Pa} & -1\ \text{Pa} \\ 2\ \text{Pa} & 2\ \text{Pa} & -2\ \text{Pa} \\ 3\ \text{Pa} & 3\ \text{Pa} & -3\ \text{Pa}\end{array}