Eli5. What is the 3D shape of a sound wave.



I always have seen sound waves shown as sine waves in textbooks and basically every other context in which you learn about sound, but what’s the actual three dimensional shape it makes. Are they just compressional waves?

In: Physics

Yep, they’re compression waves. It depends on what’s making the sound, and what the sound is travelling through. I usually describe them as being like a stretched slinky. If you give it a push and pull, that compression will travel along the length of the slinky.

If you had an ideal omnidirectional source of sound (i.e. something that sends out sound in every direction equally), the wave would be like a bubble of squashed air that expands, but gets weaker as it goes further.

Depends on the shape of what made the sound.

If you have something small, the pressure fronts will look like spherical shells. For something like a tuning fork, you’ll get waves that look like the sides of the fork that expand as they go outwards. The sound from a trumpet goes out mostly forward in a cone. Whatever it started with though, it will tend to get more spherical as it goes. If you’re far away from something, the difference in that thing’s shape is pretty small from your perspective.

For a visual representation, go look up the Mythbusters clip where they blew up a cement truck. The explosive shock front is quite visible, and it is simply an instantaneous sound wave.

When you look at a [2D picture that represents a sound wave](https://www.researchgate.net/figure/1-Sound-wave-http-wwwmediacollegecom-audio-01-sound-waveshtml-This-image-is_fig2_329027353) it tries to represent several characteristics of the sound. For simplicity they often picture a (somewhat mythical or ideal) pure tone from a tuning fork.

They try to show . . .

Wavelength – The distance from one wave to the start of the next wave- λ

Frequency – how often the wave occurs in “cycles per second”

Amplitude – The height of the wave

Also important is the speed of propagation or velocity of the wave.

Where it gets tricky is that sounds can be described as different types of waves.

– Sound is a Mechanical Wave
– Sound is a Longitudinal Wave
– Sound is a Pressure Wave

Each of these ways of describing sound include different ways of trying to picture or portray sound.

Then it gets more complex when we realize that a simple “tuning fork” simple tone is seldom what we hear, and that the complex sounds that hit our ear drums (some we perceive and others are outside our range of hearing and sensitivity) make the question even more complex.

The best 3D visualization of sounds might be the behaviour of “non Newtonian fluids” on a speaker [like this](https://www.youtube.com/watch?v=3zoTKXXNQIU) or here in [slow motion](https://www.youtube.com/watch?v=sWu9CyDwuFs)