Longitudinal Wavelength of Sound Waves

Longitudinal and Transverse Waves

Sound is a longitudinal mechanical pressure wave with the particles traveling directly toward or directly away from the vibration source.

Compression and Rarefaction

Sound waves have two phases, compression and rarefaction, or decompression.

When you look at snapshot of a pressure wave, you'll see points where compression and rarefaction are taking place. Compressions are places where the carrier medium is denser than at equilibrium. Rarefaction occurs at places in between, where the carrier medium is less dense. Note again, that the individual particles only move directly toward or directly away from the vibration source to create compression or rarefaction.

For more discussion about compression and rarefaction, see sound wave interference.

Wavelength

Wavelength is merely the distance between two compression or rarefaction cycles.

Since a pressure wave repeats its pattern once every wave cycle, the wavelength is the distance between two adjacent compression cycles, or the distance that the compression wave traveled through the medium in one cycle. In other words, the wavelength is the distance from one compression to the next, or from one rarefaction to the next, and is the distance the wave traveled through the medium in one cycle. It is interesting to note that the same vibration will have different wavelengths depending on the carrier medium.

Drawing a Pressure Wave as a Transverse Wave

It is easier to draw a pressure wave as a transverse wave. Imagine you set up a microphone and measured the instantaneous pressure as sound waves traveled by. You would be converting the pressure waves into a transverse wave diagram that is easier and more convenient to draw.

Remember, the sound is still a longitudinal pressure wave, but by looking at the microphone's pressure measurements, we can more easily draw the sound wave as a transverse wave. If we drive a speaker with the microphone's transverse wave, the speaker will convert the transverse pressure signal into an actual longitudinal pressure wave that we can hear as sound.