Can the speed of SOUND actually slow down?

[japan rocks/andromeda]

This is sort of related to the "can the speed of light slow down"thread.Will it really slow down? I think so because the sound gets softer the farther away it is.Why not slow down and get softer?

[iNow]

The speed of sound is not a constant, but instead depends entirely upon the medium (or substance) through which it travels.

 

The speed of light, however, IS a constant, and is independent of the medium (or substance) through which it travels.

 

 

What you mention about sounds getting softer as they get farther away relates to its intensity, not its speed. The sound will dissipate (or break apart more and more) the farther away it is, which is why it gets softer.

[Mr Skeptic]

The speed of sound is not dependent on its loudness, it is dependent on the elasticity of the medium it travels through and the density. Suck the helium out of a helium balloon, and you increase the speed of sound withing your lungs and voicebox, which will make your voicebox "sound" like it is smaller, and you will talk at a very high pitch (like a chipmunk on cartoons).

[Caleb]

I think so because the sound gets softer the farther away it is.Why not slow down and get softer?

 

Sound can also be absorbed into objects and bounce off of objects.

[iNow]

Sound can also be absorbed into objects and bounce off of objects.

 

I think that light can do so, as well.

[John Cuthber]

 

The speed of light, however, IS a constant, and is independent of the medium (or substance) through which it travels.

 

 

Sure about that?

[Sisyphus]

Sure about that?

 

It is true, technically. Light appears to slow down in different mediums because photons are successively absorbed and re-emitted, but individual photons are still always traveling at C.

 

Or more precisely, unlike sound, light isn't a wave through a medium, so it's not really correct to talk about air or glass or whatever as the "medium" through which it travels at all.

[John Cuthber]

From that point of view the only medium in which they ever travel is a vacuum in which case the point is moot.

Incidentally, how far from, say a hydrogen atom, does the photon have to be in order not to be slowed down by it? The reason I ask is that as far as I understand it, the electron probability density falls rapidly but it never quite reaches zero. In that case there's no such thing as "between two particles".

the idea that I am here so I slow down every photon in the universe (slightly) seems odd but I can't see why I don't.

[Sisyphus]

From that point of view the only medium in which they ever travel is a vacuum in which case the point is moot.

 

Agreed, though it's probably best not even to call vacuum the "medium."

 

Incidentally, how far from, say a hydrogen atom, does the photon have to be in order not to be slowed down by it? The reason I ask is that as far as I understand it, the electron probability density falls rapidly but it never quite reaches zero. In that case there's no such thing as "between two particles".

the idea that I am here so I slow down every photon in the universe (slightly) seems odd but I can't see why I don't.

 

Being near it doesn't slow it down. It only exists moving at C. Photons travels at C -> photon is absorbed by atom (thus ceasing to exist) -> atom emits a new photon.

[iNow]

Sure about that?

 

Yes sir, I sure am, and I can thank swansont for being able to explain why (which Sisyphus has already done admirably on my behalf above).

[Mr Skeptic]

A classical electromagnetic wave, on the other hand, would travel slower through a medium with a higher index of refraction.

[japan rocks/andromeda]

maybe since light refracts when it hits a clear glass cup...oh but sound cant do that can it

[iNow]

maybe since light refracts when it hits a clear glass cup...oh but sound cant do that can it

 

Absolutely, sound can reflect. That's why so much attention is paid when building concert halls, theaters, and auditoriums with such a specific shape... They want to reflect the sound in very specific ways to achieve very specific goals (like great sound quality regardless of where you sit).

 

However, the ability of sound to reflect is also unrelated to its speed.

 

 

To clarify the above posts... Some people think that light does slow down. They use observations of it bending when hitting glass, or refracting when hitting water to justify that idea. Their assumption is that the speed of light has changed, and that when they see light bending through the glass or through the water it is because light has slowed down. However, that is untrue. The only thing that changes is the apparent speed of light... It looks like it has slowed down... it appears to be going at a slower speed... but it really has not. The speed of light is constant... the speed of light is invariant.

 

What happens is the light hits the target, and it gets absorbed and re-emitted... it bounces around like a pinball in a pinball machine before continuing on its path. It hits stuff... it interacts with stuff... it gets absorbed by stuff... it re-emits out of stuff... However, regardless of all of that activity, the photon (the light) itself was always moving constantly at c... at the speed of light. It just looks like it slowed down when being viewed by our limited human eyes... It just looks like it slowed down because it bounced around in the glass, or in the water... so it took longer than it normally would to get from point A to point B... but that is not because it changed its speed... it just followed a longer path.

[John Cuthber]

A block of glass doesn't have holes in it to let the light through. The electron density is never zero in the solid. How can the light get through "travelling" at c through the spaces between the atoms when there are no spaces?

 

The reflection of sound depends directly on the acoustic impedance which is, IIRC, the product of the density and the speed of sound in a material.

[timo]

I'm glad to see that I am not the only person on earth that seems to consider this "photons get absorbed and re-emitted by the atoms but travel at c between them" an internet myth. John already mentioned problems with this assumption. Let's put a number on a related problem: Let the average distance between two atoms be 0.2 nm. Let the wavelength of your photon be 500 nm => you "vacuum" within one wavelength contains roughly 2500 atoms. The fact that electron density is not tied to the location of the nuclei just makes the assumption of a vacuum even worse.

 

Actually, Mr. Sceptic already mentioned my main problem with the statement:

A classical electromagnetic wave [...'] would travel slower through a medium with a higher index of refraction.

To add to this statement and formulate my problem: How is quantization of the field supposed to change that?

Wouldn't a natural quantization of the EM field in a medium be to ... well, quantize the given field in a medium? Photons in particle physics are defined as quanta of non-interacting vacuum solutions of the EM field. I find quantizing the classical solutions in material much more natural than assuming a vacuum, quantizing the classical vacuum solutions, and then trying to correct for the fact that you're not in a vacuum (however you'd actually do this correction).

That said I am not an optics guy or even QM or solid state physics expert so perhaps in practice the vacuum approach actually is the way to go - but I do not see any good reasons for that, particularly not in the simple form it is typically presented.

 

Sorry for being off-topic considering the original question, but perhaps some split of the thread is in order, anyways.

Edited April 22, 2010 by timo
removed statement that might distract from my point

[Sisyphus]

A block of glass doesn't have holes in it to let the light through. The electron density is never zero in the solid. How can the light get through "travelling" at c through the spaces between the atoms when there are no spaces?

 

I didn't say it was traveling in spaces between atoms. The photon can and indeed must go through atoms. It is, however, absorbed at a single point. Before then, it is traveling at C. Electron density means the probability at a given location that that is where it will interact, for example being absorbed. So a photon is quite likely to be absorbed close to the nucleus, and very unlikely (but never impossibly) to be absorbed further away. But until it is, it travels at C.

 

To put it another way, in quantum mechanics interactions either occur completely, or they don't at all. A photon traveling through an "electron field" and being slowed by it in some continuous function is, as Mr Skeptic says, thinking classically.

Edited April 22, 2010 by Sisyphus

[timo]

So a photon is quite likely to be absorbed close to the nucleus, and very unlikely (but never impossibly) to be absorbed further away.

Do you know that or do you assume that? A solid is not simply a collection of ordered effectively-free atoms but a higher-level structure with new features (example keywords: conductance band, valence band, Mössbauer effect, phonons, your signature ).

 

To put it another way, in quantum mechanics interactions either occur completely, or they don't at all.

If you shoot a photon on an electron at rest the final state is unique. If you believe that neither the initial state of a reaction nor the Hamiltonian contain (hidden) random variables then it is quite obvious from the Schrödinger equation that the evolution of an in-state through an interaction region into an out-state must be strictly deterministic. There is no probability for an interaction to occur or not on this level. Probabilities come into play when you want to measure properties of the out-state. Since you are not measuring "between the interactions" (assuming that notion made sense which I doubt) your picture of a random absorption every that-many atoms and free vacuum propagation between these interactions breaks down - and it is not obvious to me that you can pull it out of the equations again if you do the math properly. Edited April 22, 2010 by timo
couldn't resist mentioning the sig (no offense meant).

[michel123456]

Absolutely, sound can reflect. That's why so much attention is paid when building concert halls, theaters, and auditoriums with such a specific shape... They want to reflect the sound in very specific ways to achieve very specific goals (like great sound quality regardless of where you sit).

 

Actually, they don't want to reflect sound at all, they want to absorb it in order to avoid perturbations of the original sound coming from the source. They use hollowed panels so that sound penetrates the holes and reflect inside a small cavity inside without coming out. The panels are placed in asymetric positions in order to disperse the remaining reflected sound waves.

 

It seems there is more interest about speed of light than about speed of sound, but SOS is interesting too. Sound waves travel much better in solid objects rather than fluids. This particularity has been used in a wide range of interests, such as the invention of stethoscope and seismology. Thats why you must collate your ear to a door if you want to hear a conversation on the other side, or collate it to the ground to hear the coming buffalos (if you are an american native). And a lot of other interesting phenomenas.

[iNow]

Actually, they don't want to reflect sound at all, they want to absorb it in order to avoid perturbations of the original sound coming from the source. They use hollowed panels so that sound penetrates the holes and reflect inside a small cavity inside without coming out. The panels are placed in asymetric positions in order to disperse the remaining reflected sound waves.

 

That's good to know, and I appreciate the correction. My primary point stands, however, that sound can and does reflect, even though the example I chose to help visualize it was such a poor one.

 

 

How about this... Yes, sound reflects. Ever yelled into a canyon and heard an echo?

[D H]

Actually, they don't want to reflect sound at all, they want to absorb it in order to avoid perturbations of the original sound coming from the source.

Actually, they do want sound to reflect. A concert hall is anything but a soundproofed chamber. A properly designed concert hall will have lots and lots of desired reflections; these are the essence of what makes the sound in a concert hall sound so rich. Reflections are not desired for a rock-and-roll concert, but that is because the sound technicians electronically control the sound amplification and sound effects.

 

Concert halls are un-amplified. Orchestra members, opera singers, and stage performers disdain electronic amplification. To have their music or voices come across to the audience the concert hall has to have the right acoustics. The best acoustics for chamber music is not the same as the best acoustics for an opera or a play. The acoustics engineers modify the acoustics of the hall/theater to suit the application.

[Mr Skeptic]

Actually, they don't want to reflect sound at all, they want to absorb it in order to avoid perturbations of the original sound coming from the source.

 

They want some reflections, but not in such a way that it would interfere at the locations where the crowd sits. Without any reflections, the area sounds "dead".

[michel123456]

They want some reflections, but not in such a way that it would interfere at the locations where the crowd sits. Without any reflections, the area sounds "dead".

 

Sort of. For those interested I found this http://www.anstendig.org/Acoustics.html. Acoustic is a very specific work in the attributes of specialized consultants. As an architect I know only the basic principles, as those for acoustic insulation, which are paradoxally not the same as those applied in a concert hall, although based on the same physical characteristics of sound.