Anyone familiar with the properties of sound waves care to c

___crisis___1 · Post by **___crisis___1** » Tue Aug 09, 2005 9:54 am

ok, so i have read about the properties of sound quite a few times... and always missed one key factor that i just discovered last night.

i know about rarefaction, and compression, and how these are created... and i was always under the impression that the airparticles moved toward the listener, that those waves were created by compressed and rarefacted airparticles moving toward the listener. however... i read last night that these particles of air do not move toward the listner... they are simply moved back and forth, compressed and rarefacted. so, can someone maybe clear up how sound travels through the air?

what is it that moves through the air? and what are the particles in the air that get moved created from? this just raises so many more questions...

darkrezin · Post by **darkrezin** » Tue Aug 09, 2005 10:22 am

As far as I know (and I'm not an expert) it's vibrations that travel through the air.

In other words, the sound energy makes the air vibrate - there is no tangible substance in the air that's causing the sound.

Hope this makes sense (and I hope it's actually accurate too).

Post by **garyb** » Tue Aug 09, 2005 11:10 am

it's the WAVE that moves, not the particles.
this doesn't matter much to the listener, however.

decimator · Post by **decimator** » Tue Aug 09, 2005 11:58 am

Yep, it just moves back and forth.
Our earth "air" is a various mix of molecules at a given pressure with a ... ( ? ) ratio of vacuum between.
Sound is the " rug up and down " energy that propagate through an "inert" medium, then our humain brain computes ...
On water, more dense medium, your fart can be heard by a sensitive whale on an another ocean ( scary thought )

astroman · Post by **astroman** » Tue Aug 09, 2005 1:08 pm

on calm water you can easily observe the spreading of waves if you throw something in it - or fart if you prefer...

the particles of the medium (water or air in this example) pass the mechanical pulse to their neighbours and start to oscillate.

cheers, Tom

Post by **hubird** » Tue Aug 09, 2005 4:48 pm

--->

___crisis___1 · Post by **___crisis___1** » Tue Aug 09, 2005 5:06 pm

ok, so the fact that sound cannot travel in space is due to the fact that... there is no air? i don't get what keeps sound from moving through space... is it lack of oxygen? so when you say "moving through the air..." are you really speaking of the waves moving through the amount of oxygen in that space?

alfonso · Post by **alfonso** » Tue Aug 09, 2005 5:14 pm

It's just there aren't molecules of any kind to vibrate.

Post by **Guest** » Tue Aug 09, 2005 5:20 pm

alfonso got it.

Sound travels in waves & by making molecules vibrate

In space, there are no molecules to vibrate
in order for sound to travel.

Post by **garyb** » Tue Aug 09, 2005 5:21 pm

not oxygen, but air.
space is mostly vacuum. it takes matter for a wave to propagate(like the ocean). the wave is the function of the wave itself, not the matter that transmits it. the classic example that demonstrates this is the cork on the ocean example. you can experience this for yourself in the water. as the wave passes the cork, the cork rises and falls vertically. the cork really doesn't move much on the horizontal axis however, the wave passes and leaves the cork more or less where it started. the wave is not the same as the water's current(which DOES transport things) which would be more like wind.....

for more info on waves and particles, check quantum physics and the study of light...

Post by **hubird** » Tue Aug 09, 2005 5:25 pm

it's the energy (of the wave) that moves, as Gary said.
You know that little design thing, with three bals hanging in a row in touch?
If you pul one ball aside and let it fall back, the middle ball will stay exactly on it's place, the third bal however absorbs the energy and flies to the opposit side, etc.

exactly this way the air molecules transport the energy from the sound source (the frequency-dependent pushed conus of the speaker) to the next molecules, but each molecule stays about on it's own place in the room, like Astroman's water that transports the wave, not the water.

Only close to the source, i.c. close to a woofer sound box (or an earthquake in case of a tsunami), you get the totally different proces of 'moved' air (cq. water), due to the massive move of the speaker conus front and back.
Even in that area the frequency waves are perfectly perceptible, both processes hardly influence eachother, as the percieved 'wind' doesn't function as a 'wave'.
cheers.

[ This Message was edited by: hubird on 2005-08-09 18:38 ]

___crisis___1 · Post by **___crisis___1** » Tue Aug 09, 2005 7:00 pm

ahhh... that three balls analogy hit the spot... it is suddenly a little clearer to me. a few things i must clear up though:

so... the amount of particles moved per cycle is dependant on wavelength correct? if the wavelength is smaller, the frequency is higher, which cause it to vibrate, or oscillate, faster, which causes a less amount of particles per rarefaction/compression cycle... correct?

Post by **hubird** » Tue Aug 09, 2005 8:13 pm

it's not an analogy, it's exactly about transporting the energy not the movement

rarefaction/compression I don't know, but I guess it expresses the frequency

Me$$iah · Post by **Me$$iah** » Tue Aug 09, 2005 8:44 pm

Ball thing is called an 'archimedes cradle' I believe and often has five or more balls in.

Also sound travelinig in waves by vibrating the particles in the air is bang on.

And to finish an analogy about why no sound in space:-
drop a stone into a bath full of water and see the waves created....thats earth(here,now)
Now empty the bath of water and drop a stone in. Now see the waves, no thes no water to make waves...thats space (no atmosphere or particles close enough to vibrate[carry waves])

love the cork analogy that explains it in a great visual way, tho I think it works better when you actually see the cork not moving but just bobbing up'n'down

great thread

Cheers
-Me$$iah

firubbi · Post by **firubbi** » Tue Aug 09, 2005 10:47 pm

so is it,
60hz = 60 times vibrations/sec ? or
10Khz= 10,000 times vibrations/sec?
thanks. good topic to learn

___crisis___1 · Post by **___crisis___1** » Tue Aug 09, 2005 11:14 pm

yes, actually. pretty simple huh? it's odd to think of how much this has in common with a cpu. it's rating in speed is measured by how many calculation are done per cycle, which also runs on an oscillator. this oscillation is also measured in frquency... or how many complete up and down tick cycles per second. 1 up + 1 down tick = complete cycle. 1 cycle is 1 hz, so 1 megahz is 1,000,000 hz, and one gigahz is 1,000,000,000 hz. there for a... let's say 2.4 ghz cpu, is capable of performing 2.4 billion calculations per second!

pretty damn fast! but this is of course seperate from the FSB, which runs on a multiple of this.

wayne · Post by **wayne** » Wed Aug 10, 2005 1:29 am

This is cool

paulrmartin · Post by **paulrmartin** » Wed Aug 10, 2005 4:17 am

rarefacted? Do you mean refracted?

spacef · Post by **spacef** » Wed Aug 10, 2005 4:23 am

On 2005-08-09 18:21, garyb wrote:
for more info on waves and particles, check quantum physics and the study of light...

isn't it just a part of fluid mecanics ? (don't know in english, but in french : mecanique des fluides)... not sure though...

at least fluid mecanics allows to understand easily how waves propagate in a fluid (air, water...) and why it doesn't propagate in space (not enough fluid). cavitation also allows to understand easily phase cancellation etc... in another analogy...
http://www.e-scio.net/ondes/son.php3 (in french)
simplified compression is the "strength" of the wave (the "peaks" of waves in water for ex), the resistance it opposes to the environment, the concentration of the energy (peak) (move your finger in the water for ex, also, sound bang, aerodynamism, tsunami, winds.. etc...etc)...
rarefaction: i suppose, the loss of energy after a certain time...

more complicated : http://www.silcom.com/~aludwig/Physics/ ... sound.html
easier http://en.wikipedia.org/wiki/Fluid_dynamics

specifically : http://www.glenbrook.k12.il.us/gbssci/p ... ndtoc.html

lots of other stuff on the web...

well, back to cofee...

[ This Message was edited by: spacef on 2005-08-10 05:49 ]

Post by **garyb** » Wed Aug 10, 2005 4:49 am

spacef-yes, of course...
REFRACTED, is likely the word as Paul suggested. refracted simply means scattered. refracted waves are waves that have been redirected, bounced off a surface.
RAREFACTED would be something made more rare or less dense, so maybe some description of the action of the wave's propagation is what was alluded to....the compresssion and then release as the wave passes through the medium...fluid mechanics as spacef suggested.

[ This Message was edited by: garyb on 2005-08-10 05:54 ]