Definition of sound Waves

[Rhea]

This might be a question with a totally obvious answer. I was wondering, when does a sound wave become a sound wave? What I mean is if I just push some air with something (a flat piece ot paper for example), we don't call that a soundwave.....but how is it any different? Is it that it is not making the air particles oscillate?
In a sound wave the air (or whatever medium) oscillates about it equilibrium position with a certain frequency.....does that not happen when you just push some air with something? And that's the distinction.

[TeleMad]

One difference is the frequency itself. The human ear cannot detect sound vibrations that aren't of a high enough frequency (there are actually two factors involved: frequency and intensity, but I'm neglecting the second one here). So you might have to make the paper vibrate at a steady rate of 100 times per second, or more, in order to be able to hear it produce a sound. In fact, I and other kids used to do this. We'd get a box of Good N Plenty candy and when we were done, fold the flap up, place our mouth on it, and blow. The flap would vibrate up and down fast enough to produce a "note".

As a matter of fact, that's what a trumpet does. The lips vibrate at a certain frequency and produce a buzz: the mouthpiece and tubing of the trumpet act as a "mechanical" amplifier. Pitches are changed either by increasing the length of the tubing the air must pass through (holding down one or more valves reroutes the air into and through additional tubing, lowering the pitch) or the person tightens up his lips to produce more vibrations per second (which produces a higher pitched buzz, and therefore, a higher pitched note).

I should point out that my above comments dealt only with human hearing. Other animals have different frequency ranges that they can here: some much lower, some much higher.

[Tormod]

Hi Rhea! Welcome to Hypography!

Acoustics is the only "real" physics class I've ever taken, since I have a music degree from college.

A pretty straigh forward definition of "sound waves" is:

a wave that transmits sound

as defined in Princeton's WordNet.
http://www.cogsci.princeton.edu/cgi-bin/webwn?stage=1&word=sound+wave


You ask "when does a sound wave become a sound wave". The answer is of course that a sound wave is always a sound wave. The difference between a sound wave and any other wave is that a "sound wave" transmits sound - as defined above. When you push air with something you do actually create a sound wave, but it needs to have a certain amout of energy to be audible.

Technically, sound waves do not consist of oscillating air particles, but of waves travelling through a medium, like air, water, glass, metal, and similar mediums. Sound travels faster through solids and gases (like helium) than through air. You don't need air to transmit sound.

It is easy, however, to prove that it is enough to push air to create a sound wave. Just blow with your mouth! It is easier to hear it if you blow into a microphone or a glass bottle, but you can hear the wave that is produced quite easily.

A more technical description is given by NASA:

A mechanical disturbance advancing with infinite velocity through an elastic medium and consisting of longitudinal displacements of the medium, i.e., consisting of compressional and rarefactional displacements parallel to the direction of advance of the disturbance; a longitudinal wave. Sound waves are small-amplitude adiabatic oscillations.

The also state that:

Gases, liquids, and solids transmit sound waves, and the propagation velocity is characteristic of the nature and physical state of each of these media. In those cases where a steadily vibrating sound generator acts as a source of waves, one may speak of a uniform wave train; but in other cases (explosions, lightning discharges) a violent initial disturbance sends out a principal wave, followed by waves of more or less rapidly diminishing amplitude.

http://roland.lerc.nasa.gov/~dglover/dictionary//s.html


For a more in depth discussion of sound and hearing, here is an interesting page:

Physics Tutorial 2: The Physics of Hearing
http://www.audioholics.com/techtips/...ofHearing.html

[Rhea]

Thanks Telemad. I am new to the forum so I've been a bit timid about posting.
I understand what you are saying. I am still a bit confused. There are frequencies that we humans can't here but we still call them sound waves right? So if I'm vibrating the paper slow and can't hear anything, could we say that a sound wave is still being produced at a frequency too low for us to here? Or not even a piece of paper...what if I just push some air with my hand 3 times per second. Of course I wouldn't here anything.....but when can we start calling it a sound wave.....does it have to be a certain frequency for it to be classified as a sound wave and not just some air being pushed?

[Rhea]

Thanks Tormod. You responded while I was writing my previous response....and I didn't notice it. I understand what you are saying. So my question then becomes. what is the definition of sound?
If I stick my hand in a pool of water and move it with a constant frequency.....could we say that a sound wave is being produced?

[Tormod]

Rhea, the definition of sound is pretty much explained in the last link I provided.

A frequency as such does not produce a sound wave. A sound wave is a mechanical wave, not electromagnetic (so while we can translate a EM wave, say, a light pulse, into sound, what actually happens is that the electromagnetical wave is translated into a mehcanical wave).

I guess we can say that a wave becomes a sound wave when it is strong enough to be picked up by a mechanical sound device without needing translation or amplification of any kind. So it would depend on how hard you move your hand in the water...but if you stick a waterproof mic in the pool it would start to pick up the movements you make. However, you would need to move your hand very fast (say back and forth at least 50 times a second) to make an audible wave. So what the mic picks up is the wave produced by the motion, not the particular frequency with which you move your hand.

[TeleMad]

Rhead: There are frequencies that we humans can't here but we still call them sound waves right?

"If a tree falls in the woods and there's no one around to hear it, does the tree make a sound?" Or something like that. The question kind of assumes that it is humans that determine whether or not a sound has been made (and then asks whether or not that is correct).

For us to hear a sound, compressional waves of air must enter our ear and strike our tympanic membrane - our eardrum. This causes the membrane to move in and out according to the pressure applied to it by the compressions of air. The movement of the membrane moves three tiny bones - called the ossicles - in the middle ear. These bones in turn transmit force to a small oval window. The oval window is a small hole that leads into the inner ear: the hole is covered by a membrane. So the bones pushing against the oval window cause it to move in and out, and the movement of the oval window sets up pulsed motion in fluid within our inner ears. The moving fluid pushes minute "hairs" out of position and in so doing, opens up channels that allow ions to flow into cells. This movement of electric charge triggers a nerve impulse to the brain which it then deciphers as sound.

What does all of that mean? Well, if the air compressions don't have enough energy, then we won't hear a sound. They won't be strong enough to push the tympanic membrane and so on. In addition, our system does not register waves that are too far apart in time (I don't know if our brain ignores them, or if some other mechanical reason is involved). Only sounds that exceed what's known as the threshold of hearing are audible, and to do that they must have a high enough frequency/intensity combination. That is, a low frequency but high intensity sound might be able to be heard, and a high frequency low intensity sound might too, but a low frequency/low intensity sound cannot.

But if we stop focussing on humans, we see that other animals can hear sounds that we don't. Bats navigate by echolocation - we can't hear their "chirps", but they sure can. Is that a sound wave? Sure is. But in everyday usage, when someone speaks of a sound or a sound wave, they usually mean a sound that we can hear. Technically though, many sound waves can't be heard by humans.

[Freethinker]

I feel sorry for Rhea. First though, welcome to our group Rhea. Glad you decided to jump in. But the reason I feel sorry for you is I assume you were hoping for a simple direct answer. What we find once again however is that defintions are seldom simple. And most of that is because of human language being so poor at communicating information accurately and completely.

One answer, which can be shown to be correct, is that human's establish what is "sound" based on our physiological abilities. We can "hear" certain frequencies at certain energy levels. As Telemad says.

compressional waves of air must enter our ear and strike our tympanic membrane - our eardrum.

yet even this is incorrect as at lower frequencies the sound is transmitted thru cranial bone transmission.

Then there is the application of the term "sound" to extremely low frequencies. Far below what any species we are aware of could ever hear. It has been determined that black holes make a "sound". According to research they, like a sax, play in the key of B flat. Problem is it is 57 octaives below Middle C.

Black Hole Sound Waves

http://science.nasa.gov/headlines/y2003/09sep_blackholesounds.htm


Such a simple question. So many complicated answers.

Hope one of them helps along the way.

[Tormod]

Originally posted by: Freethinker
One answer, which can be shown to be correct, is that human's establish what is "sound" based on our physiological abilities. We can "hear" certain frequencies at certain energy levels.

I think both Telemad and I explained that the definition of sound is not limited to human hearing, and thus is not related to our physiological abilities except in everyday language.

Then there is the application of the term "sound" to extremely low frequencies. Far below what any species we are aware of could ever hear. It has been determined that black holes make a "sound". According to research they, like a sax, play in the key of B flat. Problem is it is 57 octaives below Middle C.

The article in question fails to mention that soundwaves are mechanical in nature and require a medium to carry them. The "empty" space of interplanetary space is not a medium through which sound can travel (thus the famous movie slogan from Alien, "In space, noone can hear you scream"). So the astronomers have converted the electromagnetic frequencies into mechanical waves for the purpose of "sampling" the sound of a black hole.

[Tormod]

Actually, upon re-reading the article it actually says the sound waves travel through the stellar gas cloud.

But the 64,000 dollar question is...how did the sound travel from there to here?