# Is sound wave a transverse wave?

#### jerry0696

i had always belived that sound wave was a longitudinal wave till i come across something saying that its a transverse wave in solid.can anyone explain is it so??

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#### phyzguy

i had always belived that sound wave was a longitudinal wave till i come across something saying that its a transverse wave in solid.can anyone explain is it so??
In a fluid like air, sound waves are only longitudinal, since fluids don't support shear forces. Solids support both longitudinal and transverse sound waves, and they typically travel at different speeds.

#### jerry0696

In a fluid like air, sound waves are only longitudinal, since fluids don't support shear forces. Solids support both longitudinal and transverse sound waves, and they typically travel at different speeds.
if solid support both does that mean that sound is a transverse wave in solids??

#### phyzguy

if solid support both does that mean that sound is a transverse wave in solids??
There are longitudinal sound waves in solids.

There are also transverse sound waves in solids.

#### jtbell

Mentor
does that mean that sound is a transverse wave in solids??
Sound can be a transverse wave in a solid, but it doesn't have to be transverse. It can be longitudinal instead, like in a gas or liquid.

#### jerry0696

Sound can be a transverse wave in a solid, but it doesn't have to be transverse. It can be longitudinal instead, like in a gas or liquid.
but is sound transverse ,longitudinal or both in solid??
everyone is telling me that sound can only be longitudinal (my teachers)

#### voko

Inside a solid, sounds is longitudinal. However, at the surface of a solid, sound is transverse. The surface is a like a membrane, its oscillations can only be transverse.

#### davenn

Gold Member
However, at the surface of a solid, sound is transverse. The surface is a like a membrane, its oscillations can only be transverse
Am desperately trying to picture that

thanks
Dave

#### AlephZero

Homework Helper
Am desperately trying to picture that

It is almost true by definition, for a common sense definition of "sound". If the surface of the solid doesn't move transversely to the surface, it doesn't transmit any pressure vibrations to the air. (OK, let's ignore the tiny amount of energy that would get into the boundary layer and be dissipated by the air viscosity before it got anywhere else.)

If course a solid can vibrate with no transverse motion at the surface, for example torsional vibrations of a disc on a shaft, but that type of vibration doesn't generate "sound".

#### jerry0696

Inside a solid, sounds is longitudinal. However, at the surface of a solid, sound is transverse. The surface is a like a membrane, its oscillations can only be transverse.
you are telling me that it is a longitudinal wave in the solid but transverse wave at the surface.
do you know any book that can give me further explanation on this topic??

I think the answer to the OPs question is no. Sound waves are longitudinal and not transverse. By definition sound waves are related to those types of waves that can be sensed by the ear and the ear drum reacts to longitudinal vibrations.
A solid may be able to vibrate in different ways but only the longitudinal components of those vibrations can be transferred to any surrounding air as sound waves. It reminds me of the longitudinal and transverse vibrations that can be set up on a stretched string. Both set up longitudinal vibrations (sound waves) in the surrounding air.

#### voko

There are different modes of vibration in solids but only those that can set up longitudinal waves in the surroundings are related to sound.

#### voko

There are different modes of vibration in solids but only those that can set up longitudinal waves in the surroundings are related to sound.
If you attach a solid body directly to an acoustic sensor, a transverse wave will excite it just the same as a longitudinal wave would.

If there is an air gap, transverse wave will still excite it. It is quite obvious if you think for a second: most loudspeakers in the world are membranes, and an oscillating membrane is indistinguishable from a surface acoustic wave.

The eardrum can be considered as a membrane and this responds most strongly to longitudinal vibrations. It is sound that's being discussed,not vibrations in general.

#### voko

The eardrum can be considered as a membrane and this responds most strongly to longitudinal vibrations. It is sound that's being discussed,not vibrations in general.
What's your definition of "sound"? And why is your definition relevant for me or anyone else?

#### jerry0696

I think the answer to the OPs question is no. Sound waves are longitudinal and not transverse. By definition sound waves are related to those types of waves that can be sensed by the ear and the ear drum reacts to longitudinal vibrations.
A solid may be able to vibrate in different ways but only the longitudinal components of those vibrations can be transferred to any surrounding air as sound waves. It reminds me of the longitudinal and transverse vibrations that can be set up on a stretched string. Both set up longitudinal vibrations (sound waves) in the surrounding air.
apparently sound can be longitudinal and transverse wave but air and fluid cannot support transverse wave and sound can be transverse wave in solid
i need a book to comfirm the hypothesis

What's your definition of "sound"? And why is your definition relevant for me or anyone else?
Vibrations which travel through the air or another medium and are sensed by the ear.
(Concise Oxford English Dictionary)

The relevant part of this discussion is that the vibrations "are sensed by the ear" and the ear responds to longitudinal vibrations. Any transverse components eg along the plane of the ear drum will not be effectively passed through.
I'm not sure at what level of education the op is at but I have a reasonable idea. In the UK system any reference to sound being carried by transverse waves will lose marks.

#### voko

Vibrations which travel through the air or another medium and are sensed by the ear.
So ultrasound is not sound then?

The relevant part of this discussion is that the vibrations "are sensed by the ear" and the ear responds to longitudinal vibrations.
I am pretty sure that if a solid bar is made to touch the eardrums, a surface acoustic wave on it will be registered.

The only reason you can talk about the ear responding to longitudinal vibrations is because ordinarily it senses vibrations in air, which does not support any other. So you are effectively reducing sound to a phenomenon occurring only in air (or perhaps other gases and liquids). Which is at odds with the widely known and recognized concept of sound in solids.

#### sophiecentaur

Gold Member
This is yet another thread dealing with definition and classification. A lot of it is wasted effort because people are not talking from the same standpoint..
You can 'define' sound as the sort of vibrations that the ear can detect by longitudinal vibrations of the air in the ear or you can define it in terms of the frequency range. This is not really relevant to the modes of transmission of vibrations through media yet contributors are running round in circles trying to convince each other one way or another.
There is very little to be gained in the way of understanding, merely by putting things in columns of what they are and what they aren't. The UK has recently found itself lower in the education league tables. Not surprising, as kids are often taught their Sciences by teachers who stopped doing Physics, Chemistry or Biology after Double Award Science. Classification is favourite under those circumstances. How ye sow, so shall ye reap.

#### dauto

That's a questions about semantics not physics. The answer to the question depends on what the definition of a sound wave is. Usually the longitudinal waves in a solid are called p-waves while the transverse waves in a solid are called s-waves. both of them can excite longitudinal waves in the air which will be perceived as sound.

#### sophiecentaur

Gold Member
The effect at the surface will be due to a surface wave, unless the sound wave is normal incidence. Surface waves tend to be neglected in many situations particularly when Seismic waves are taught in school. But they are very relevant to buildings, which are on the surface and when sound energy is coupled into and out of a solid surface.
More over simplification due to classification, I think.

So ultrasound is not sound then?
I think it's well understood by all here that ultrasound is sound and we could go for more detailed definitions such as defining "sounds which are audible to humans" or to bats or to any other animal species. But I don't want to get involved in semantics but want to see OPs question answered at a suitable level.
I'm assuming that the OP is currently at a level which is equivelent to GCSE or perhaps AS or A level (UK qualifications).If so the specifications require that students should understand that.................."Sound waves are longitudinal" (P1.5.1b AQA physics)

#### sophiecentaur

Gold Member
So Ultraviolet is 'Light' and Infra Red is 'Light' too - but microwaves and X Rays aren't? Classification really gets in the way of things.
That quote from the AQA Specification just makes my point about classification taking over from understanding. It would have been just as easy to cover themselves and say that sound waves in air are longitudinal. Let's face it, the 'sound' that is transmitted along the ossicles, is in the form of lateral vibrations on levers. But I would be surprised if the person who approved the spec gave it as much thought as has been expended on this thread.

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