posted
Mouser, How are you going to deal with the requirements for the size of your ports? They are going to have to be huge!!!!!!!! At least if you don’t want them to restrict your SPL. Ive been reading up on ducts and there ability to radiate into free space or half space and they are just like RF/ microwave transmission lines. If you try to make a lot of fast moving air move the air in your cabin, there will be a reflection coefficient and most of the power will not be transferred where you want it. A Horn will help a lot from what the math says though.
Keep your ports big, and shaped like a horn !
Also, you said playing a resonant frequency in extreme cars doesn’t work well.
An extreme car shouldn’t have a resonant frequency. They are usually sealed pretty well. Resonant frequencies come from leakage. If your car is perfectly sealed, You should see a constant gain as the frequency gets lower and lower.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
dank: everything except a vaccum(which can't really be classified as a thing) has a resonant frequency. The air in your car is not exempt. (sealed boxes resonate too)
the acoustical analogous circuit for air is an Acoustical Compliance (CA) in parallel with an Acoustical Mass(MA) The combination of the two form a parallel resonant "circuit" and is easy to calculate. I dont have my acoustics notes in front of me so I dont want to guess at what the equations actually are. They are related to Volume, speed of sound, and air density though.
I think the reason square waves don't work is that the meter isn't going to be able to pick up the high frequency component where the increased pressure is there. In unfiltered mics, it should do very well.
Phase alignment at low frequencies is pretty easy provided you have a half a wavelength of room to work with and enough damping to reach a steady state in a smallish amount of time.
jt
Posts: 56 | From: atlanta | Registered: Nov 2000
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posted
What I am saying is that a sealed enclosure will not have a resonant frequency by itself. You have to add a mass that can oscillate. In a car, there is an air volume, which has a compliance (cmH20\ml) and leakage. The leakage is mass that can oscillate back and forth in the leak. This is where the resonance comes from. So when people determine the transfer function of their car, they will find a slope (usually +12dB/octave) heading downward. In addition to that depending on the amount of leakage they have, they will find a sweet spot or big bump along the curve that is not due to the volume and dimensions alone, but do to the Compliance and leakage.
In extreme vehicles, there is very little leakage, so it is more important to tune to a spot where the combo of box peak and transfer function add up to the highest spl. So for each speaker, it would be tuned to a different spot for max spl. I think this is what Mouser was getting at.
[ 09-26-2001: Message edited by: mr.dank ]
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
I didnt say air didnt have mass, Im saying it needs something like leakage ( a duct) to act as a moving mass so that the compliance and it (the moveing mass) can act togather as a mass and spring system. A solid box does not have a resonant frequency if the walls are rigid and there is no moving mass.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
are you trying to tell me that a circuit with an inductor and capacitor will not resonate? If they are Ideal(read zero resistance), they will resonate forever.
The exact same thing applies to air. And a perfectly sealed box.
jt
Posts: 56 | From: atlanta | Registered: Nov 2000
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posted
ummm, Ive been saying that a box perfectly sealed, does not have a moving mass and therefor does not have an equivalent inductor (inertance) modeled into the equation.
If you add a source to drive it, then it has the mass and a resonance frequency, but not by itself. And by controlling the mass, you control the resonance frequency of the SYSTEM.This applies to an extreme vehicle.
And I’m not saying that everything doesn’t have a resonance frequency, I’m just saying that for these applications, and for what we do, there is no resonant frequency in a perfectly sealed box. a box by itself will not resonate!!!!!!!
[ 09-26-2001: Message edited by: mr.dank ]
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Well, you certainly have a good point, but I have to side with Jteef on this one. The resonance frequency of a sealed box is an inherant property of that box, and is present regardless of whether or not the box is actually resonating or not. For instance, suppose that you have a perfectly sealed box(with no driver) suspended in midair out in the middle of nowhere(by some fishing line perhaps). Now imagine that you place a sub(in a box) in close proximity to the perfectly sealed, suspended box. Now, turn on the sub and drive it with a frequency generator, varying the frequency from say 10Hz to 1KHz. Also, imagine that you have some sort of sensor mounted on the suspended, perfectly sealed box. Now, sound waves from the sub are propagating through the air and some of them are hitting the perfectly sealed box. If you were to monitor the amplitude of the vibrations on the perfectly sealed box(using the sensors) you would notice that when the sub is playing different frequencies, the amplitude of the vibrations on the perfectly sealed box would change. At one(or possible a few) frequencies, the amplitude of the vibrations in the perfectly sealed box would peak out. That's the resonant frequency of that box. That resonant frequency is determined by the nature of the materials used to construct the box as well as the volume and density of air inside the box. Now, instead of using the perfectly sealed box in this mental experiment, you could just as easily substitute an extreme vehicle. Thats my arguement. Anyway, this is a pretty interesting discussion, let keep it going... Ash Oh yeah, and as far as the moving mass goes, the actual air inside the vehicle would act as the moving mass...and to a very small degree the materials used to construct the vehicle would act in the same way...
posted
Ash, Everything you said is correct. The whole key to this suspended box which resonates is like you said, the material used. That is what resonates. The air inside the box adds compliance and has an effect on the resonant frequency. If the walls of this box where perfectly rigid, It would not resonate. Now lets go back to an extreme vehicle. This extreme vehicle will be sealed airtight and be perfectly rigid. Now once you place a sub with a box in there and tune it, there will be a resonant frequency. It is dependant on the speaker and how it is set up. Now lets go to my car, it is not perfectly sealed and it doesn’t matter what I tune my box to, there is always a resonance frequency at 62-63 Hz. It doesn’t matter what speaker or what box. It is always there. This is because of the leakage, which acts like a suspended mass and creates a SYSTEM, which has a resonance frequency. The extreme vehicle does not have this suspended mass and therefore no system which can resonate until you add the speaker. The bottom line of what im saying is that, Air in a box has compliance, not a resonant frequency. The compliance is part of a system that resonates, and it is the system that has the resonance frequency not the air!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
if you need load and low and have the room a transmission line line system is the best .with this system you change the speed of the sound in the incloser two cause a phase shift of sound
Posts: 7 | From: texas | Registered: Sep 2001
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posted
Hey Mr. Dank I think that I can agree with you now. I didn't realize that you were taking the walls of the box(and the extreme vehicle) to be perfectly rigid. If the walls of the box are completely rigid it cannot resonate. This goes all of the way back to the fact that information cannot travel faster than the speed of light. However, I think we all know that there is NOTHING on this earth that qualifies as perfectly rigid. Even an extreme vehicle is not perfectly rigid, and therefore will have a resonant frequency on its own. Now, granted, the difference in response in dB's between when the resonant frequency and other frequencies are played will be minute, and will most likely not matter much when compared to the gains caused by tuning the sub/box system to its resonant frequency. Good discussion... Ash(smoking another bowl as I write this!)
quote:with this system you change the speed of the sound
where are we underwater now?
In Reply to ash:
You can not tell the resonant frequency of this perfectly sealed, suspended box because the driver you are using, "sub" in this case WILL have peaks and valleys in its frequency response. As well, the atmosphere and objects around the box will effect what your microphone picks up on. You have to count all these things because in the real world everything is either a reflector or a propogator of sound.
Weather Sound is resonating through the box or not, it does have an inherit resonant frequency, as does all things. And since we dont use just boxes by themselves it is kind of pointless to find the resonant frequency of the box itself without first having a driver (as all drivers are different).
To get the kind of gain from box+driver matching, you first have to have the res freq of the driver, then using some formulas (that i have somewhere written down) you build your box to the appropriate volume(really hard to do). Once you have done that, whatever the matched frequency is you should have a perfect box for that driver.
You also have to think about 1/4 wave and standing waves, so those are important as well.
Obviously this will only work for a sealed box, so all you big port guy's out there dont jump all over it.
Semi-offtopic: One more thing, it seems to me that people think that the only way to tune a box is to port it. While this might look cooler and yield a smaller box it is not the only way. And as you all know, ported boxes have some major disadvantages. Bigger box = Lower res freq, Smaller = Higher etc....
Do the math!
-------------------- Andrew Kemendo Team Pro Creations FAA Pilot #2718765 Oderint dum metuant
posted
By limit I meant the vacuum limit, and by 193.x I meant on the second half-wavelength. As you can see in the graph I showed that the SPL can get higher than 14.x, but not lower than -14.x PSI (in relation to earths average atmospheric pressure).
Do you know how they measured that? Did they take the first half wavelength, and treat it as if the air pressure level was high enough for the second half-wavelength to be fully present, or did they take an average of the two half-wavelengths? I ask because in "normal" situations it doesn't matter.
posted
Maybe I am wrong, I dont know. It was my understanding that an enclosure is made up of 3 things. Acoustic compliance, mass, and resistance. The compliance would be represented by an ideal capacitor, the mass by an ideal inductor, and resistance as the losses in the system.
The flexing walls of a non rigid box would contribute their own sound based on the size of the panels, their density, etc. But this does not have anything to do with our situation except that the energy required to flex these walls would be represented as a loss and included in RA.
This leads me to think that if you have a perfectly rigid box with no air leaks, and excite the air on the inside at the resonant frequency of the series combination of the Acoustic mass and compliance, it would sing for ever. You wouldn't be able to hear it outside the box, but you would inside. Why would this be different in an extreme vehicle? Except that you'll never actually have a lossless car. Seats, non-continuous lines(corners), etc might have a greater effect than how rigid the roof is. (it is possible that this frequency is outside of the audio band...i haven't done any calculations to verify)
Ash: have you taken Dr. Patronis' acoustics class yet ?
jt
Posts: 56 | From: atlanta | Registered: Nov 2000
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posted
jteef, when you model the equivalent electrical system like that. You have Inertiance (kinda like mass)=> inductor, Compliance => volume of air, and resistnace => losses. The inertiance doesnt come from the volume of air in the box or its mass, It comes from a mass that is movable such as a port or speaker.
Ash, Im with you.
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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hey, how did g kitzmann change his post without getting a edited by message?
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
ok, then in an extreme vehicle with a rigid sealed body, you still have the mass of air inside, and the cone of the speakers to move it. Wont there be resonance? Or will it just be out of band since the air leaks have been sealed up?
Posts: 56 | From: atlanta | Registered: Nov 2000
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posted
Ya you will have a resonance, but it depends on what speaker you put in there and what you tune it too. In normal cars there is a resonant frequency, and it doesn’t matter what speaker you put in, It’s always the same frequency. So you tune the speaker and box to that frequency, turn it up, and then say DAAAAAMMMMMNNNNNN!!!!!!!!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Looks like you guys didn't figure it out anyway, perhaps you should pay some attention to dB Don's forum after finals and then you can see what's going on with phase correction and patented ports...LOL.
Or perhaps just ask Nate...
For the compliance of the air to work against the mass it needs to load against it....in a sealed box it won't other than by having parallel walls and having it bounce around that way.
-------------------- *hmm need to think of a new sig*
posted
Heh Mouser I am not even sure what the original question was...I just jumped in talking about the perfect sealed box...lol oh well whatever, man! I need to review my notes on electrical modeling of acoustic systems anyhow... Ash
-------------------- ****************************** ****************************** Posts: 1130 | From: Los Angeles, CA | Registered: Aug 2001
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posted
A horn is basically a "displacement scalar" which will change displacement X of the driver which corresponds to bore x stroke into displacement Y of the air which corresponds to bore x stroke x some constant effected by the box.
The loading of the driver allows it to impart more energy to the air by forcing the driver to squeeze air into a higher velocity, more directional movement. Then, the horn body modifies this energy to frequential SPL gain, which is an effect of the shape of the horn body.
Both sides of the driver may produce this "throat velocity" assuming that there is a horn type loading on both sides of the driver.
If the driver is in a situation where it can excurt most effectively (IE unloaded by the box, only loaded by the horn) then you get the ideal efficiency of the system because there is the least effects working AGAINST the cone motion.
The phase between the outputs of both sides of the box would have an ideal relation at 180^ (that is, the rarefaction portion of one side physically couples with the rarefaction of the other side due to an enforced time delay between the outputs- IE, by making the back wave travel a longer path, for a given frequency, you can effectively use both sides of the woofer stroke after 1 cycle to double your effective displacement per movement of cone)
If you align the phase thusly of two high energy horn loaded sub sides, you get two times the actual air displacement with very little cancellation effect.
The unloading of the box by making the box large, if done correctly, can also align the phase of the two outputs. Then, adding in your well calculated horn effect and tuning it correctly to match the car, you get a maximum system efficiency that is QUITE ridiculous.
So, Colin.. Did I miss my mark?
ShadowStar
-------------------- You can't build a reputation for what you're GOING to do.. But you can build one for TALKING about it!
It's all about knowledge, love and respect.
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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Now I understand the mechanics of my '99 built box... Cheers Shadowstar! My first rear loaded folded horn box was built for an old 15" Clarion (sq sub). With a lowly 150rms pushing it and "placed at the BACK of my van" had the ability to throw a full six pack off the "FRONT" section of the roof. My last pair of folded horns containing cheap 15" managed to crack my glass patio doors. While playing the THX cinema sweep. LOL The only downside is the size of the boxes.
-------------------- ----------------------------------- Team Troublestarter (Tea~boy, One lump or two?) Posts: 171 | From: England | Registered: Dec 2000
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