# Do sound waves carry momentum?

granpa
do sound waves carry momentum? any momentum at all (no matter how small)?

the obvious answer is that they carry energy but not momentum but i am wondering whether they don't theoretically carry some very tiny (usually negligible) amount of momentum in the same way that light, even though it can normally be thought of as being massless, theoretically does have some very tiny amount of mass.

Kurret
sound waves are vibrating molecules, and everyone of these molecules move and thus have a momentum.

Hey I got an idea, if we can get a formula for momentum for sound, ie vibrating particles, then we can use it the other way round to determine the properties of the "medium" that light eventually travels through, right? ;)

Staff Emeritus
Gold Member
sound waves are vibrating molecules, and everyone of these molecules move and thus have a momentum.

Hey I got an idea, if we can get a formula for momentum for sound, ie vibrating particles, then we can use it the other way round to determine the properties of the "medium" that light eventually travels through, right? ;)

Wrong, light does not propogate in the same manner as sound. The properties of materials which effect light transmission are pretty well understood.

NYSportsguy
Okay so do sound waves have momentum or not? I believe they do...as do light waves.

I know sound waves move a lot like a wave and light as a wave function consisting of particles (photons) thus having a wave-particle duality.

Domenicaccio
Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

However I seem to have some problem in seeing the momentum of an acoustic wave with a classical approach. :/ There are certainly cases where an acoustic wave can "push" an object, for example in the case of a "shock wave".

BryanP
or better yet, bass frequencies...

you can "feel" bass

Kurret
Wrong, light does not propogate in the same manner as sound. The properties of materials which effect light transmission are pretty well understood.
It was more like an idea IF there is a medium that light travels through (not any "material"), but more like the luminiferous aether that people thought existed a while ago. Do we really know everything about the properties of light? I doubt that.

Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

Phonons carry pseudomomentum. Summed up, the momentum of all phonons usually cancels to 0. Otherwise the center of mass of the crystal would have to move.

Sound waves have momentum and dispersion, a phase and a group velocity. That's one of the reasons why gongs, when struck with a soft mallet, have long attacks.

granpa
Uhm... sound waves in a crystal lattice are quantized and can be seen as "phonons" which definitely have a momentum (equal to hk, if I remember correctly).

However I seem to have some problem in seeing the momentum of an acoustic wave with a classical approach. :/ There are certainly cases where an acoustic wave can "push" an object, for example in the case of a "shock wave".

same here.

shamrock5585
I know sound waves move a lot like a wave and light as a wave function consisting of particles (photons) thus having a wave-particle duality.

sounds pretty dumb to say sound waves move a lot like a wave when sounds waves are in fact waves... why do you think they call them sound WAVES haha... of course this coming from a yankee fan... haha just bustin your balls