posted
I just thought I would share a couple things with the group and I would like any and all comments.
First, there is a fundamental point that EVERYONE needs to know.
THE MEDIUM IS NOT THE WAVE
Air allows sound waves to travel across it. But, sounds waves only transmit energy, not matter!
The transmission media- lets take air as the medium. When the wave passes through, the energy imparted causes air molecules to move slightly in the wave direction.. This causes collsion with other air molecules, transmitting energy.
BUT, the total displacement of each individual air molecule is MUCH less than the displacement of the total wave energy in the same time period (and thus their velocity is MUCH smaller!)
Therefore, if you look at it this way, you could keep the frequency and wave speed of a wave the same, but increase the substituent molecules' velocity to a theoretical maximum of the wave speed, thereby increasing wave energy, right? (Wave energy could be quantified as kinetic energy of all substituent molecules moving with general wave motion, considering kinetic energy as 1/2 * the mass of air * velocity squared, and therefore any point equidistant from the radiating source (at a given time) would have some portion of this energy, right?
PRESSURE (Force / Area) on/at the microphone is measured by what portion of molecular kinetic energy (the energy of the molecules that reach the microphone) is imparted on the microphone.
Since the molecules lose velocity but retain the same mass (when striking the diaphragm), and the microphone mass doesn't change but its velocity increases (with direction in agreement with molecular motion) ( but not necessarily wave direction, but lets leave that till later), the imparted energy must therefore be the momentum of the moving air molecules for the pressure phase of sound.
Since momentum is M = mass * velocity, the higher the molecular velocity, the higher the energy, right? Well, note that Force = Mass * Acceleration, which is directly applicable (the microphone diaphragm applies a Newton's 3rd law pair force to absorb the momentum of the air molecules in the form of an impulse over the duration of diphragm/molecule proximity) and since this force corresponds to the Force of pressure on the microphone...
then the velocity of air molecules that reach the diphragm is one half of the SPL equation, the other is the mass of air that reach the measurement diphragm. ( Side note! ::::Since the area of the diaphragm is limited, wouldn't this limit the mass of air that can strike the diaphragm at one time, and commutate much of their energy directly into mass coupling at the mic of multiple molecules?) SPECULATION?!)
I would guess this is why a port which generally has higher velocity air molecules coming out will generate a higher SPL when measured at 1 foot than a subwoofer will.
This is just musing, but I thought it might prove insightful.. Although sound waves are inherently dissipational, their energy can be directed as a function of the velocity of their substituent molecules!
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
If the velocity is half of the equation for SPL, wouldn't higher frequencies with the same displacement yield higher SPLs? Since higher frequencies would have the cone accelerating faster, and therefor the air having higher velocity, the over all SPL would be higher. Am I on track? Or way off?
------------------ rybaudio@freeze.com
The goal: Accurate musical reproduction in the mobile environment
Posts: 3957 | From: State College, PA | Registered: Sep 2000
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posted
Ryb- The effect is even more prounounced than that, actually.. For, if you think about it, it takes 4 times the excursion to equal the energy in a wave one octave lower than the reference.
Remember the run on high frequency bass that Alma "started", yielding high(er) SPL numbers? Guess why? :-D
ShadowStar
Jc- Everybody's going surfing, surfing USA... :-D
Dank- Icky saki, dank-san.
------------------ Still looking for that CHEAP Thunderdome :D
Got Ears? Get Oz!
You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
-Eclipse-Rockford Fosgate-Oz Audio-Precision Power Inc.-AudioControl-Stinger-Tsunami-Clifford
It's all about knowledge, love and respect.
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
SS- Yeah, I see what you mean. I see how tweeters can displace a small amount of air, yet yield the same SPL as a sub moving much more air. Interesting, reminds me how much I love physics.
------------------ rybaudio@freeze.com
The goal: Accurate musical reproduction in the mobile environment
Posts: 3957 | From: State College, PA | Registered: Sep 2000
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posted
Hi guys. I am sort of new to this topic and am not by any means a physics major. I am actualy a GED guy that studied it as a hobby. (I do now have college as well, but never studied physics).
Shadow, correct me if I am wrong here, but to hit the SPL with as high a frequency as Alma hits, you would have to have twice the cone volume as someone who put the same thing half the size in a vehicle with half the cabin space and running a lower octave. Is this correct? (this is a debate among me and some of my employees that are spl freaks)
Also, if mass*volume is such the factor, why not place one 8" sub in the glove box and port through the defrost vent on the passenger side? wouldn't this create the highest level of pressure being so close, therefore much less room for loss?
Forgive me, I am half asleep and may not be making sense. (considering I don't when awake that isn't a far cry)
------------------ '00 F150 Red Quad Cab 2 JL Audio 10W3 RF BD1000a1 RF Punch 200a4 Clarion drx8675z Head Unit Clarion DPH910 Processor
Posts: 112 | From: Carrollton, TX USA | Registered: Mar 2001
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quote:Originally posted by OURich: Hi guys. I am sort of new to this topic and am not by any means a physics major. I am actualy a GED guy that studied it as a hobby. (I do now have college as well, but never studied physics).
Shadow, correct me if I am wrong here, but to hit the SPL with as high a frequency as Alma hits, you would have to have twice the cone volume as someone who put the same thing half the size in a vehicle with half the cabin space and running a lower octave. Is this correct?
-----Hmm. You're kinda comparing apples to oranges-
If you have a given displacement at a given frequency, ideally the smaller the volume in front of the drivers, the higher the pressure variation and thus the higher the SPL.
To get the same amount of energy in the wave at 1/2 the frequency (down an octave) you must increase the displacement roughly by four times.
I'm not sure the effects of lowering the volume in front of the drivers vs. changing the frequency, but they must be inversly proportional, so one must be able to assign constants to their relationship, but that would be experimentally verified and probably would have other factors.
So, to lower the frequency and octave and holding everything else similar you would need to increase the displacement x4.. If you cut the volume in front of the subs in half, I would assume that this would double your displacement percentage, and so you could just double the displacement, but this is a rough guess. ------
quote:Originally posted by OURich: (this is a debate among me and some of my employees that are spl freaks)
Also, if mass*volume is such the factor, why not place one 8" sub in the glove box and port through the defrost vent on the passenger side? wouldn't this create the highest level of pressure being so close, therefore much less room for loss?
Yes.. Why not? I mean... Ever put headphones on? Thats a terribly small amount of power and a very small diaphragm and it makes very loud noise, right?
Well.. Putting that driver in the glove box would be akin to putting the mic in the port, etc.. Questionable competition legality would probably be a reason not to do it, but it would definately give you some big numbers!
quote:Originally posted by OURich:
Forgive me, I am half asleep and may not be making sense. (considering I don't when awake that isn't a far cry)
Ehh.. I don't make sense 3/4 of the time, don't worry about it
ShadowStar
------------------ Still looking for that CHEAP Thunderdome :D
Got Ears? Get Oz!
You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
-Eclipse-Rockford Fosgate-Oz Audio-Precision Power Inc.-AudioControl-Stinger-Tsunami-Clifford
It's all about knowledge, love and respect.
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Okay, that's all easy enough, but what do you think the effect would be if Alma reinstalled the stock air vent in the Bronco that would be just next to the mic. It would in effect create a large bandpass box, with the mic at one end of the port. Knowing how much output bandpass boxes can make from evena small port, and figuring that the port would be working with the sub at the right frequency it would surely yield a large increase in the SPL capabilities. I ran this past Don first and he was in agreement.
Or should I be keeping this a secret???
Colin
------------------ Spy Shots Warning SPL in my car exceeds government health guidelines, civilians are advised to proceed with caution. Failiure to comply may result in serious injury or death.
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