# Do waves have acceleration?

1. Oct 20, 2015

### Granger

I'm calculating a problem with motion equations. I need to use a motion equation for a sound wave (this is a simple problem of kinematics (fall of a rock) and I don't have any background about waves or harmonic motion)... So then I started to think... For intuition I know that sound doesn't have acceleration... But all waves don't have acceleration? Is it because they don't have mass?

2. Oct 20, 2015

### lasha1

We know that a=v-v0/t so waves have acceleration but it depends on what kind of wave it is. For example sound wave have acceleration if we change temperature when sound created.

3. Oct 20, 2015

### phinds

Yes, but I think the question being asked, really, is does it ever have a NON-ZERO acceleration.

4. Oct 20, 2015

### davenn

velocity is just a speed with direction, isn't it ? km/hr, metres/second with a given vector/direction
acceleration is an increasing speed or velocity eg acc due to gravity 9.81 m/s2

a sound wave or say a seismic wave has a constant velocity ( speed) through a constant density medium

I don't see how an object/wave having a constant velocity automatically infers it has an acceleration
which is what is being implied in posts 2 and 3

Dave

5. Oct 20, 2015

### jfizzix

Sound waves have acceleration the same way as other waves do. If nothing else, a sound wave bouncing off of a wall, means its velocity has changed through the bounce.

Also, as Lasha1 mentioned, the speed of sound depends on the material the waves are traveling through.

Here's a neat example:
Since the density of air decreases with altitude, the speed of sound decreases with altitude as well.
As a result, sounds initially propagating horizontally, also bend upward slightly as they travel further away.
This means that on a flat field sound will die off (a little) more quickly with distance than might be expected.
It also means that acoustic anomalies like "sound mirages" are theoretically possible in the same way as they work for light, though as for that, the distance scales needed might be orders of magnitude different.

Now that I think of it, you can do most anything to sound that you can do to light (polarization notwithstanding)

Since Helium is less dense than air, sound travels more slowly through it. This means a properly shaped balloon would work well as a sound lens. One could imagine an underwater sonar array as being an acoustic telescope.

The speed of sound also depends on the frequency of the sound wave. This means the right material could work as a prism for sound waves, though maybe only for a narrow range of frequencies.
Alternatively, an array of bars or holes could act as a diffraction grating for sound waves over the right range of wavelengths.

Indeed, we can even make beams of coherent sound, similar to how we can make laser light.

Last edited: Oct 20, 2015
6. Oct 20, 2015

### phinds

That sounds like an argument that zero is not a value

7. Oct 21, 2015

### cosmik debris

I'm wondering if the OP is referring to acceleration of a wave when it is formed? When wave starts to propagate does it accelerate up to it propagation velocity?