Hi guys
I can’t help but think that if the mechanism were clear, that you could get back on track.
Picture air molecules, these have mass and are “springy” AND they are under a static pressure. Sound, a tiny disturbance above and below the static pressure has a finite propagation speed, why?. Electrically, molecule to molecule an instantaneous pressure transmission can be thought of as a series L and parallel C elements except there are millions and millions of them unlike a simple series electrical transmission line they in a 3d pattern. Each transmission takes a finite time because of the tiny “delay” that element produces.
To be clear here I do not mean acoustic transmission line but the electrical.
Like an electrical transmission line, one finds the velocity is directly related to the L and C of that circuit. In cable, relative to the speed of light, cable transmission speed can be as slow as 60% the speed of light for instance by having larger L’s and C’s than a pair of open bare wires.
In air, one can speed up the speed of sound by heating it (increasing the stiffness of the spring portion) and some gasses have somewhat different sound velocities as there L’s and C are different.
On can think of acoustic impedance related to the ratio between the force per area involved and the motion involved.
A given “power” radiated in air produces much more motion per pressure than if one radiated the same power into water for example. In water, tiny radiator motions produce huge pressures.
The speed of sound in water is governed by the exact same L and C equivalent circuit however.
The coupling between water molecules is much stiffer than with air, so much stiffer that even though water has much greater density, the speed is still much higher.
It is not “infinity stiff, if it were the speed of sound would also be infinite.
A metal like titanium or a ceramic like quartz also has a speed of sound set by the same thing as air.
To our senses things like Quartz and as we have been taught in school water, seem incompressible but the transmission of sound, its speed and acoustic impedance all depend on this same compressibility to mass relationships. It is why a Piezo electric crystal is a much better acoustic match in to water than into air.
Anyway hope that helps maybe adding some mental images.
Best,
Tom Danley
"Zorg you are a monster" (Vito Cornelius)
I can’t help but think that if the mechanism were clear, that you could get back on track.
Picture air molecules, these have mass and are “springy” AND they are under a static pressure. Sound, a tiny disturbance above and below the static pressure has a finite propagation speed, why?. Electrically, molecule to molecule an instantaneous pressure transmission can be thought of as a series L and parallel C elements except there are millions and millions of them unlike a simple series electrical transmission line they in a 3d pattern. Each transmission takes a finite time because of the tiny “delay” that element produces.
To be clear here I do not mean acoustic transmission line but the electrical.
Like an electrical transmission line, one finds the velocity is directly related to the L and C of that circuit. In cable, relative to the speed of light, cable transmission speed can be as slow as 60% the speed of light for instance by having larger L’s and C’s than a pair of open bare wires.
In air, one can speed up the speed of sound by heating it (increasing the stiffness of the spring portion) and some gasses have somewhat different sound velocities as there L’s and C are different.
On can think of acoustic impedance related to the ratio between the force per area involved and the motion involved.
A given “power” radiated in air produces much more motion per pressure than if one radiated the same power into water for example. In water, tiny radiator motions produce huge pressures.
The speed of sound in water is governed by the exact same L and C equivalent circuit however.
The coupling between water molecules is much stiffer than with air, so much stiffer that even though water has much greater density, the speed is still much higher.
It is not “infinity stiff, if it were the speed of sound would also be infinite.
A metal like titanium or a ceramic like quartz also has a speed of sound set by the same thing as air.
To our senses things like Quartz and as we have been taught in school water, seem incompressible but the transmission of sound, its speed and acoustic impedance all depend on this same compressibility to mass relationships. It is why a Piezo electric crystal is a much better acoustic match in to water than into air.
Anyway hope that helps maybe adding some mental images.
Best,
Tom Danley
"Zorg you are a monster" (Vito Cornelius)
Hi Tom,
That was as clear as mud, but that's exactly what folks want to see here - an explanation that's hard to read and ambiguous.
BTW, I'm not actually Zorg. He is in attendance in this thread though...
That was as clear as mud, but that's exactly what folks want to see here - an explanation that's hard to read and ambiguous.
BTW, I'm not actually Zorg. He is in attendance in this thread though...
Learning project, not costly.
I have a tiny back room and some small, "full range" low efficiency (80 dB) speakers which I could use for a system there:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=297-428
http://www.partsexpress.com/pdf/297-428g.pdf
High SPLs aren't needed and if I made non-diffracting cabinets they could sound decent. If I gave them tweeters and subs they'd sound better, if they had CD wave guided tweeters they'd sound even better.
This would be a good learning project for me because I could really use it to teach myself how to build good crossovers, try out some ideas I have for making compound curved cabinets, and because all the ingredients are either very small or very cheap, it would be low cost.
Since I already have cheap, low efficiency "woofers" it doesn't make sense to use a compression driver I'd have to pad down by a zillion percent. So it makes sense to use a conventional tweeter.
Cone or dome? I'm thinking the previous discussion here, before things went to a bad place, suggests using a dome presents difficulties in matching things to the waveguide throat, and also perhaps having to modify the the driver's face plate.
But what about using a cone tweeter? I could put it in a tiny enclosure and have it duct directly to the waveguide throat which could be whatever radius is advantageous.
Has anybody done this?
PE has these:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=270-032
Which are closed in back, I think, and are the right price. I prowled through Value Village today but they didn't have anything that struck me as worth salvaging.
I have a tiny back room and some small, "full range" low efficiency (80 dB) speakers which I could use for a system there:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=297-428
http://www.partsexpress.com/pdf/297-428g.pdf
High SPLs aren't needed and if I made non-diffracting cabinets they could sound decent. If I gave them tweeters and subs they'd sound better, if they had CD wave guided tweeters they'd sound even better.
This would be a good learning project for me because I could really use it to teach myself how to build good crossovers, try out some ideas I have for making compound curved cabinets, and because all the ingredients are either very small or very cheap, it would be low cost.
Since I already have cheap, low efficiency "woofers" it doesn't make sense to use a compression driver I'd have to pad down by a zillion percent. So it makes sense to use a conventional tweeter.
Cone or dome? I'm thinking the previous discussion here, before things went to a bad place, suggests using a dome presents difficulties in matching things to the waveguide throat, and also perhaps having to modify the the driver's face plate.
But what about using a cone tweeter? I could put it in a tiny enclosure and have it duct directly to the waveguide throat which could be whatever radius is advantageous.
Has anybody done this?
PE has these:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=270-032
Which are closed in back, I think, and are the right price. I prowled through Value Village today but they didn't have anything that struck me as worth salvaging.
MJL21193 said:
Quite the opposite. The energy used to compress water is efficiently passed through the medium, think of Tom's LC analogy. "Pushing water" would dissipate energy much faster due to the friction of moving molecules past one another. Waves are propagated through a medium such that the final position of the medium is the same as the starting position. Surface waves propagate through the ocean, but water is not moved horizontally, except in a returning cycle, until something causes the wave to become unstable and break. Only when this happens is water permanently displaced horizontally. At this point, the energy is dissipated.
Sheldon
http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
"The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right."
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/wavepropagation.htm
"Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. "
"The wave is seen as the motion of the compressed region (ie, it is a pressure wave), which moves from left to right."
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/wavepropagation.htm
"Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. "
Fact: Water is denser than air. The molecules are closer together.
Fact: Sound travels faster through water than through air.
Fact: Sound travels further through water than through air.
Fact: It requires enormous amounts of energy to compress water even slightly.
Fact: It takes very little energy (gravity on a falling pebble for example) to set waves in motion in water.
Conclusion: There isn't enough energy in a sound wave to compress water. The sound wave is merely setting the water in motion, like the falling pebble would.
Put a driver under water, see what happens when it plays. The cone pushes the water forward (high pressure) then it pulls it back (low pressure). At no point is the water being compressed.
One complete cycle of cone movement in and out will set in motion a single wave that will expand and eventually die out.
Fact: Sound travels faster through water than through air.
Fact: Sound travels further through water than through air.
Fact: It requires enormous amounts of energy to compress water even slightly.
Fact: It takes very little energy (gravity on a falling pebble for example) to set waves in motion in water.
Conclusion: There isn't enough energy in a sound wave to compress water. The sound wave is merely setting the water in motion, like the falling pebble would.
Put a driver under water, see what happens when it plays. The cone pushes the water forward (high pressure) then it pulls it back (low pressure). At no point is the water being compressed.
One complete cycle of cone movement in and out will set in motion a single wave that will expand and eventually die out.
Bratislav said:
Thanks for the links/quotes. I was searching myself and came up with this in google books:
Google books on velocity of sound waves (oceanography)
Note the form of expression (2) in which the velocity V is a function of the specific heat, density and compressibility . Since K (compressibility) is in the denominator, if the liquid were not compressible, K=0, the speed of sound V would be infinite. Of course K is not zero.
Also the line right after:
No ambiguity whatsoever.
Dave
Bratislav said:
Air is easy to compress, that has already been established. That is the root of the lower efficiency of a cone to air interface - air has low density.
Water is much denser, and is a much more efficient transmitter of sound, for the very reason that it will not be compressed by the sound wave. Vibration is easily carried through it.
Your conclusion is wrong. Sound can only propagate through fluids as a compression (longitudinal) wave. Shear waves can only propagate through medium that keeps molecules in place (solids). Compression wavefront requires particle density change to propagate, which is by definition compression.
Seriously, arguing this does not make any sense. Semantics aside, compression is like pregnancy - there is no 'little' involved in either.
Seriously, arguing this does not make any sense. Semantics aside, compression is like pregnancy - there is no 'little' involved in either.
Here is a good explanation of how sound works. Most here should already be well versed on the topic, but apparently not.
I have tracked it down for you guys, it was even written by people with a few letters after their name..PhD.
I'll quote a highlight:
"There are three requirements for sound to "occur" in an environment: (1) a vibrating source to initiate sound, (2) a medium to transmit sound vibrations throughout the environment and (3) a receiver to hear or record sound vibrations.
Sound is initiated in an environment by a vibrating source. Vibrating sources are many and varied in the World -- vocal cords, a membrane of animal hide or synthetic material, a stretched string that is plucked or bowed, objects such as wood, stone, clay, metal and glass that are struck, rattling of beads in a small enclosure, clapping of hands, singing of birds, grunts and groans of animals, buzzing of lips in a small resonating tube, splitting of an air stream, small pieces of reed attached to a tube and set in motion by the action of human breath, and many, many other natural vibrating sources. "
And:
"A vibrating source transmits its vibrations through a medium by causing the medium to move, or vibrate, at exactly the same speed of vibration as the source itself. The movement of the gas or liquid medium is identical to surface waves found on any large body of water. Surface waves on water move up and down, and they transmit energy from one point to another &emdash; from a source (tidal action, wind, a passing ship, an earthquake) to receiver (the shoreline). Sound transmission through the air is accomplished by a similar physical process. "
I have tracked it down for you guys, it was even written by people with a few letters after their name..PhD.
I'll quote a highlight:
"There are three requirements for sound to "occur" in an environment: (1) a vibrating source to initiate sound, (2) a medium to transmit sound vibrations throughout the environment and (3) a receiver to hear or record sound vibrations.
Sound is initiated in an environment by a vibrating source. Vibrating sources are many and varied in the World -- vocal cords, a membrane of animal hide or synthetic material, a stretched string that is plucked or bowed, objects such as wood, stone, clay, metal and glass that are struck, rattling of beads in a small enclosure, clapping of hands, singing of birds, grunts and groans of animals, buzzing of lips in a small resonating tube, splitting of an air stream, small pieces of reed attached to a tube and set in motion by the action of human breath, and many, many other natural vibrating sources. "
And:
"A vibrating source transmits its vibrations through a medium by causing the medium to move, or vibrate, at exactly the same speed of vibration as the source itself. The movement of the gas or liquid medium is identical to surface waves found on any large body of water. Surface waves on water move up and down, and they transmit energy from one point to another &emdash; from a source (tidal action, wind, a passing ship, an earthquake) to receiver (the shoreline). Sound transmission through the air is accomplished by a similar physical process. "
MJL21193 said:
I can only guess his PhD was not in physics or related field. Maybe arts or something similar. This paper would not pass a peer review of a first year physics student, I can tell you that.
I think we're mixing up volume velocity and pressure. These are the V's and I's of the acoustic domain(depending on whether you're a mobility or an impedance kinda a guy).
But a V drives an I and an I needs a V to exist. I think we're all in vehement agreement here.
Can we get back to the great stuff in the beginning of this thread.
But a V drives an I and an I needs a V to exist. I think we're all in vehement agreement here.
Can we get back to the great stuff in the beginning of this thread.
Bratislav said:
PhD means beans to me, but some around here put much stock in it. I put it out there because it can be a source you fellows can believe, as you obviously don't like what I'm saying. I'm getting tired of hearing "you are wrong".
You all seem to be missing a critical point: You are saying that in order for sound to move through water, it needs to compress the water. You think this because it's called a COMPRESSION wave. In reality, that has more to do with pressure, pushing. After all, a speaker makes the air vibrate by pushing it, right?
It's like saying that in order to push an object, you need to crush part of it, This is not the case.
Just because Earl says it's so, doesn't mean it's correct. You just need to think about it for a bit.
Look up and bear in mind how much force is required to compress water. This is the key.
Dr. Geddes, you might want to find that mold for the car waveguides. A post was made, and about 10 people have expressed interest so far. Whether all of them come up with the cash, who knows...but I know you have at least 2 sets to consider sold.
I have more than a couple years working with composites and could lay them up for you...if you didn't have a local fiberglass shop.
I have more than a couple years working with composites and could lay them up for you...if you didn't have a local fiberglass shop.
MJL21193 said:
MJL21193 said:
Dont want to be rude but is it done for good this time? I enjoy the talk about "Geddes on waveguides," hence the title. Dont mind this discussion but it needs a separate thread.
And yes Dr. Geddes I think we will be able to make the waveguides work if you are willing and can find the mold. Oh and someone asked what the minimum recommended crossover would be? Thanks
Trying to get back on the wave guides thing.
Depending on the size and shape of the car wave guide, I would be interested in trying to mate it with a small driver to that it can be placed on top of the dash.
Depending on the size and shape of the car wave guide, I would be interested in trying to mate it with a small driver to that it can be placed on top of the dash.
I kind of lost track of where the wave guide equation is, but will look it up. The main thinking about the wave guide is that if the flare is such that the wave departs from the guide wall due to flare rate, then there is less constant directivity. In order for the wave to stick to the guide wall, the high frequencies must diffract quite a bit at the thoat to let the walls guide the wave. This is not an easy task because we now need to consider throat size, wave length, and throat expansion curve.
I always understood that as far as waves in water are concerned that although the wave moves, molecules move up and down making a circular pattern and do not themselves move forward with the wave. This is mentioned in several boat design publications. I additionally though that sound was made by successive slight compression and rarefaction of air. I have no clue from reading the argumentative discussions going on here as to whether or not what I understood is correct.
jamikl
jamikl
Hi jamikl
Sound propagates in gaseous media (e.g. air) only through longitudinal - compressional- waves ("successive slight compression and rarefaction") .
In theory* shear ("up and down") and surface (elliptical path)waves are not supported in gaseous media.
*There are reports that such waves can be generated at the interface of gaseous media with different rheological qualities, and at the interface of gaseous-solid materials under specific and may be extreme cases.
Regards
George
Sound propagates in gaseous media (e.g. air) only through longitudinal - compressional- waves ("successive slight compression and rarefaction") .
In theory* shear ("up and down") and surface (elliptical path)waves are not supported in gaseous media.
*There are reports that such waves can be generated at the interface of gaseous media with different rheological qualities, and at the interface of gaseous-solid materials under specific and may be extreme cases.
Regards
George