Undergrad Understanding Sound Propagation in Liquids: Longitudinal and Transverse Waves

[IWantToLearn]

Why sound waves propagate through liquids as longitudinal waves, and it is propagate on the surface of the liquid as transverse waves?

 

[Drakkith]

Are you referring to the normal gravity water waves like you see in the ocean? I ask because I've never heard of sound waves traveling on the surface of a liquid.

 

[IWantToLearn]

I've never heard of sound waves traveling on the surface of a liquid.

me too!
but i heard someone (supposed to be an expert)talking about it
and i am searching the internet to get any information about this phenomena right now

 

[IWantToLearn]

but i think that mechanical waves through liquids propagate as transverse and longitudinal,
and i think that, on the surface of the liquid the propagation is both longitudinal and transverse, mainly transverse, and inside the liquid i think also its both transverse and longitudinal , mainly longitudinal

i think the cause of that is the attraction forces between the molecules

but sound is a mechanical wave , can we say that it is propagating on the surface of a liquid as a transverse wave!?

 

[BruceW]

yes, mechanical waves on the surface of the fluid can be partly transverse and partly longitudinal. But mechanical waves inside the fluid are longitudinal only. Unless the fluid is viscous enough I guess. Anyway, here's some links:

http://en.wikipedia.org/wiki/Rayleigh_wave
http://en.wikipedia.org/wiki/Surface_acoustic_wave

 

[Bobbywhy]

When a frog transmits its call while partially submerged in water it generates three kinds of signals: the acoustic signal in the water, the acoustic signal in the air, and the circularly spreading surface waves emitted by the vibrating throat sac. Another frog that is partially submerged in this same body of water receives these three different signals. First, the acoustic signal in water arrives, and then the airborne acoustic waves, and lastly, the surface water waves arrive.

In a two-frog pond with an undisturbed water surface, a semi-submerged frog receiving the surface waves could deduce the relative bearing of the origin of the circular surface waves by comparing wave arrival times according to their azimuth angle. A pond with many frogs croaking creates a complex pattern with all the surface waves interacting both constructively and destructively. I have photographed this surface wave pattern at night using the reflection of a nearby streetlight.

 

[Bobbywhy]

Examples of transverse waves include seismic S (secondary) waves, and the motion of the electric (E) and magnetic (M) fields in an electromagnetic plane wave, which both oscillate perpendicularly to each other as well as to the direction of energy transfer. Therefore an electromagnetic wave consists of two transverse waves, visible light being an example of an electromagnetic wave. See electromagnetic spectrum for information on different types of electromagnetic waves. An oscillating string is another example of a transverse wave; a more everyday example would be an audience wave.
http://en.wikipedia.org/wiki/Transverse_wave

Longitudinal waves, also known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also referred to as compressional waves or compression waves.
Longitudinal waves include sound waves (alternation in pressure, particle displacement, or particle velocity propagated in an elastic material) and seismic P-waves (created by earthquakes and explosions).
http://en.wikipedia.org/wiki/Longitudinal_wave

 

[davenn]

Examples of transverse waves include seismic S (secondary) waves, and the motion of the electric (E) and magnetic (M) fields in an electromagnetic plane wave, which both oscillate perpendicularly to each other as well as to the direction of energy transfer. Therefore an electromagnetic wave consists of two transverse waves, visible light being an example of an electromagnetic wave. See electromagnetic spectrum for information on different types of electromagnetic waves. An oscillating string is another example of a transverse wave; a more everyday example would be an audience wave.
http://en.wikipedia.org/wiki/Transverse_wave

good link Bobbywhy saved me the effort :)

As any good seismologist knows, Traverse waves WONT travel through a liquid. This is because liquids don't have any shear strength.
This is also how they discovered that the outer core of the Earth is liquid as only the compressional seismic P waves will pass through.
It is also why if you are on a boat during a local earthquake, You will feel the thump of the P wave striking ... may witnesses attest to it feeling like running aground... but you don't feel the traverse ( side to side) shake of the S waves.

cheers
Dave

 

[IWantToLearn]

Thanks for all, but i didn't got a clear answer, can sound waves propagate on the surface of a liquid in the form of transverse waves?
and if they do, then why?

could the reason be the surface tension?

 

[Drakkith]

I would think so. When the longitudal wave hits the boundary at the surface the molecules would seem to be pushed *out* before falling back down. This seems to generate a transverse wave in the surface. Well, it does in my head, I could be wrong.

 

[Bobbywhy]

All electromagnetic waves are transverse. Example: electrostatic and electromagnetic fields oscillate at right angles to the direction of propagation. They need no medium; EM waves travel in a vacuum.

Mechanical waves can be either transverse or longitudinal. They require a physical medium to propagate in.

Longitudinal waves are waves of compression. Examples are sound in air or in water.

Transverse waves are disturbances that are perpendicular to the direction of travel. One example is surface water waves.

Yes, acoustic waves may travel on the surface of water. Here are two youtube videos showing just that:


http://www.youtube.com/watch?annota...&feature=iv&src_vid=tI6S5CS-6JI&v=8LEeENVSG-k

And yes, surface tension has much to do with their propagation!

 

[IWantToLearn]

Yes, acoustic waves may travel on the surface of water. Here are two youtube videos showing just that:


http://www.youtube.com/watch?annota...&feature=iv&src_vid=tI6S5CS-6JI&v=8LEeENVSG-k

And yes, surface tension has much to do with their propagation!

Thanks Bobbywhy :)

 

[DennisN]

Thanks, Bobbywhy! I really enjoyed clip 2, it was beautiful!