# Sound refraction?

## Main Question or Discussion Point

Does sound also refract like light?
I know it obeys laws of reflection like that of light ( angle of incidence = angle of reflection)
How we are able to hear sound when there are doors closed, windows closed etc. Light is eventually stopped by opaque high sized objects. So doors should stop longitudinal wave vibration of particles of sound from reaching us?
Also are there any sound proof doors?
I thought earlier that sound doesn't refract but find it's way by some crevices somewhere. Smarty

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So can we make all analogies of light to sound except one is transverse wave and other is longitudinal wave.
Can you explain this bit of questions also?
How we are able to hear sound when there are doors closed, windows closed etc. Light is eventually stopped by opaque high sized objects. So doors should stop longitudinal wave vibration of particles of sound from reaching us?

DrDu
Note that seismic waves are also a kind of sound waves:
http://en.wikipedia.org/wiki/Seismic_wave
The refraction of these waves gives valuable insight into the structure of the earth.
Also the refraction of ultrasound is used extensively in medical imaging.

Drakkith
Staff Emeritus
How we are able to hear sound when there are doors closed, windows closed etc. Light is eventually stopped by opaque high sized objects. So doors should stop longitudinal wave vibration of particles of sound from reaching us?
You could say that most objects are 'semi-tranparent' to sound waves, so you need special materials to fully block the sound. Hard, solid objects don't absorb sound very well, while soft materials like foam are very good at absorbing sound.

So can we make all analogies of light to sound except one is transverse wave and other is longitudinal wave.
Can you explain this bit of questions also?
Light and sound are both types of waves and will obey all the properties that waves exhibit. Most of what happens to light also happens to sound if you're talking about the wave properties.

Note that seismic waves are also a kind of sound waves:
http://en.wikipedia.org/wiki/Seismic_wave
The refraction of these waves gives valuable insight into the structure of the earth.
Also the refraction of ultrasound is used extensively in medical imaging.
So how does this explain my bit of questions?
I am not asking an insight of earth.
You could say that most objects are 'semi-tranparent' to sound waves, so you need special materials to fully block the sound. Hard, solid objects don't absorb sound very well, while soft materials like foam are very good at absorbing sound.

Light and sound are both types of waves and will obey all the properties that waves exhibit. Most of what happens to light also happens to sound if you're talking about the wave properties.
This was valuable info. Thanks.

DrDu
One of the first effects that was explained using the refraction of sound waves was the observed audibility of canon sound over large distances by Chladni and others at the beginning of the 19th century. Canon sound from a military operation could be heard up to 50 km away and then again in a distance of 120 km up to 250 km. The second zone of audibility is due to sound getting refracted in higher zones of the atmosphere in inversion layers.

Yeah this makes sense to me
But
How speed of sound increases in water and of light decreases , if they both have wave properties @Drakkith and @DrDu ?

Drakkith
Staff Emeritus
Please don't tag me if I'm already in the discussion. I don't have time to answer right now. Someone else might be able to help you in the meantime.

DrDu
The speed of light increases both for sound waves and for light waves. However, you have to be careful here which speed you are talking of. The relevant speed to explain refraction is phase velocity, i.e. the speed with which the maxima in a sinusoidal sound wave travel. This speed is different from the group velocity which describes the speed of a wavepacket and is relevant to information transport. For a sound wave, the two velocities differ little, but for a light wave, phase velocity is higher in a medium than in vacuum while group velocity is always smaller.

Doesn't know the basics of group velocity or phase velocity.
Was finding Wikipedia a bit jargon.
The speed here that I am asking is
that sound speed in air is around 340 m/s and in water it increases and in solids further more.
But for light
Light when refracts from a rarer to denser medium it speed decreases like in water from air.
Light speed in air is c but it is decreasing when refracting?

DrDu
Sorry, I got somewhat confused. You are right: in most media, the phase speed of light is in deed less than the speed of light in vacuo. So if you want, light behaves here different than sound waves, at least when passing from air to some solid material.

Now Drakkith was saying that if considering waves they have same properties.
So how in this case the two behaving differently?

DrDu
They behave the same, but parameters are different. Even for light, the refractive index n is different for different wavelengths. So it shouldn't be too astonishing that n may have different values (<1 vs >1 ) for different kind of waves.

Yeah thanks but wanting a bit more explanation.

Drakkith
Staff Emeritus
Now Drakkith was saying that if considering waves they have same properties.
So how in this case the two behaving differently?
That is close, but not exactly what I said. I said that light and sound both obey all the properties that waves exhibit, meaning that both light and sound undergo refraction, diffraction, have a wavelength/frequency, etc. The reason light and sound behave differently when entering a medium is that they are both very different waves.

See the following article for an explanation of the speed of sound in a medium: http://en.wikipedia.org/wiki/Speed_of_sound#Basic_concept

Light is composed of alternating electric and magnetic fields, which have very different rules in a medium. In free space, the EM wave (light) has nothing to interact with. But in a medium, the wave interacts with large numbers of charged particles.