posted
Not too sure if anyone has heard of this topic before but the common SPL folklore goes something along the lines of: "once you compress air over 5% it no longer acts as a gas for modelling purposes, it behaves as a semi-liquid"
Now I really have no idea if this is true and really it doesn't matter too much as real world testing has always proven more reliable for SPL then 'theories'. None the less though it got me thinking, what is the formula to figure this out? I've read/heard that 194dB = 14PSI = 1 Atmosphere of pressure. So how can we convert these figures into 5% compressibility of air? Just curious to see if mathematically this percentage compression can even be achieved in a common SPL vehicle and if so what is the dB reading at which we should see this phenomenon occuring?
As always, just curious???
-------------------- I'm so happy, Cuz today I found my friends, They're in my head. Posts: 130 | From: Melbourne, Australia | Registered: Jun 2002
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posted
WOW I have no idea what you are talking about ...a physics professor may know that. But anyway, what will nitrogen or helium do to car audio as sound move faster(or slower) in these mediums? Just curious also...
-------------------- 98 Cadillac Deville
Premier TS-C504s Cadence SeAqua amps 12" Kicker L7
posted
I don't know who said that but it is completely wrong. If you compress air 5%, that corresponds to a pressure of only 0.77 psi above atmospheric pressure, which is hardly anything. Or a sound wave with a max amplitude of .77 psi above atmospheric pressure would be about 168 dB.
You are correct in that about 194 dB is equal to atmospheric pressure, because of this it is also the loudest sound possible in the Earth's atmosphere. For 194 dB the compression of the air would be 50%. Even for these pressures, which are still relatively low (only about 29 psi), air can still be modeled as a gas with very good accuracy.
Posts: 327 | From: Pleasanton & Sacramento Ca | Registered: Dec 2001
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posted
Thanks for the info, out of curiousity how did you derive that 5% compression equals 0.77 psi above atmospheric pressure and therefore 168dB?
It's more the formula than the theory that interests me, becuase as I said above at the end of the day real world testing of SPL is far more beneficial in the long run than relying solely on 'theories'
Thanks again...
-------------------- I'm so happy, Cuz today I found my friends, They're in my head. Posts: 130 | From: Melbourne, Australia | Registered: Jun 2002
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posted
Well, there is a pretty broad and distinct difference between gas and liquid phases and it certainly doesn't occur at 5% pressure differential at room temperature and standard atmospheric pressure.
However, air is a medium for the propagation of waves, and as such it must have some sort of hysterisis (thats where the medium can't keep up with the transducer: Like when you wave a hose in the air, the water coming out lags behind the hose tip, thats hysterisis) and so whenever you emit a sharp pressure peak, you have to accept the medium's ability to convey it immediately.. I'm not sure at what pressure differential that occurs at, however, if you think about a, say, 2000hz wave, there is a pressure peak emitted in 1/1000th of a second, and as we can hear sounds distincly at and above 2khz, I would say that air is pretty freakin quick as far as pressure hysterisis goes.. A 5% compression of air of air is a pretty big SPL however.
5% compression of air = .05 * 14.7psi = .77 psi
~194db = a variation of 14psi from atmospheric pressure, therefore, 184db = 1.47psi variation, and 181 = ~.74, so 5% pressure variation is at or around 181db thats pretty freakin loud however I still don't think Mike Hughes is gonna be mixing Oxygen Soup in his van any time soon
ShadowStar, the funny man
-------------------- 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: 2579 | From: Somewhere In the Northeast | Registered: May 1999
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posted
I remember talking to some guys at Glenn research a few years ago about the maximum sound pressure level in atomosphere. the maximum compresibility has to do with the physics of a speaker system more then anything else. When you reach 14.7 psi in an enclosed car using a speaker you have compressed the air to half its original volume. On the back stroke you rest at 0 psi and create a vacuum on the back stroke. in the ported enclosure models I saw this causes the sound wave to be supersonic(travleing faster then 1130 fps at sea level)which is against physics of sound.
If you model an spl system as an air compressor in a fluid dynamic model instead of a sound model it shows that you can generate higher pressures and if the force is there hypersonic wavers are possible
-------------------- Just some thoughts from a nobody. Posts: 3921 | From: Lakewood,Ohio | Registered: Aug 2003
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quote:Originally posted by ShadowStar: 5% compression of air = .05 * 14.7psi = .77 psi
~194db = a variation of 14psi from atmospheric pressure, therefore, 184db = 1.47psi variation, and 181 = ~.74, so 5% pressure variation is at or around 181db thats pretty freakin loud
I think it's 6 db for a doubling/halving of pressure and 20 db for a 10x change in pressure. So 194-20=174 for 10% of atmospheric, 174-6=168 db for 5% of atmosphere.
posted
6 dB is what you use to express the gain when you double the voltage into a fixed load.
Here, 3 dB is double.
BTW, 194 dB is not the limit, you can still clip the wave and go up to infinite psi on the forward stroke and 0 psi on the back stroke. Notice the infinite on one side, there is no 194 dB limit.
194 dB is the theoretical limit of faithful sound reproduction. Beyond that, distortion is guaranteed. But we're talking about SPL, so who cares about distortion?
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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quote:Originally posted by Nomad84: I think it's 6 db for a doubling/halving of pressure and 20 db for a 10x change in pressure. So 194-20=174 for 10% of atmospheric, 174-6=168 db for 5% of atmosphere.
Yes you are right. This can be easily proven with the equations used for finding SPL. The equation is
SPL = 20*log(pressure/reference pressure)
The reference pressure is 2.9*10^-9 psig
For 14.7 psig the SPL is found by: 20*log(14.7/2.9*10^-9) ~ 194 dB.
For .77 psig the SPL would be: 20*log(0.77/2.9*10^-9) ~ 168 dB
There can be pressure waves in the Earth's atmosphere that are above 14.7 psig but they are not classified as a sound wave; they would be a shock wave. For any pressure wave with this much pressure, it would have to travel faster than the speed of sound.
If you have ever seen any of the old military videos where they were testing out nuclear weapons you will see a shock wave. They have so much power that they will destroy anything in their path and will kill humans instantly.
Posts: 327 | From: Pleasanton & Sacramento Ca | Registered: Dec 2001
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quote:Originally posted by emu: 6 dB is what you use to express the gain when you double the voltage into a fixed load.
Here, 3 dB is double.
What you are saying about here is sound power level, not sound pressure level.
Sound power level has a different equation which is: dB = 10*log(power/reference power) The reference power is 10^-12 watts
So if you double the power level of a sound wave the increase in dB is 3dB. If you double pressure the increase would be 6dB.
Posts: 327 | From: Pleasanton & Sacramento Ca | Registered: Dec 2001
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posted
The important thing to realize here, is that a standing (static) pressure of 14.7psi is 0dB. At a maximum sound wave of 194dB, you're standard pressure is still 14.7psi, yet your peaks are now 0psi and 29.4psi.
It is possible to obtain a higher SPL, however once you go beyond a 14.7psi +/- pressure change, your centerline changes from 0psi to one half the maximum + pressure. I'm not exactly sure how the SPL scale changes (I don't think it's linear, maybe logarithmic/exponential?) so it's kind of hard to place where the 5% compressability occurs.
Some food for thought though is that once you get above 140, you can begin to 'feel' the pressure change in your ears; so the question is, how big is the pressure deviation at let say, 140/150dB?
Justin
-------------------- Hooked on sonics worked for me! Posts: 766 | From: Tucked65's Trunk | Registered: Mar 2003
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quote:Originally posted by Thedrumming2ba: The important thing to realize here, is that a standing (static) pressure of 14.7psi is 0dB. At a maximum sound wave of 194dB, you're standard pressure is still 14.7psi, yet your peaks are now 0psi and 29.4psi.
You are correct what you say here.
quote:It is possible to obtain a higher SPL, however once you go beyond a 14.7psi +/- pressure change, your centerline changes from 0psi to one half the maximum + pressure. I'm not exactly sure how the SPL scale changes (I don't think it's linear, maybe logarithmic/exponential?) so it's kind of hard to place where the 5% compressability occurs.
Here you are wrong. The centerline cannot change, it is the pressure that will return once the sound is gone, which is atmospheric pressure(~14.7 psia). The only way to have a higher sound level would be to raise the static pressure.
Pressure waves above 29.4 psia can occur in the Earth's atmosphere, but they are no longer a sound wave, they are a shock wave. The only way for this high of a pressure wave to occur is if the wave travels faster than the speed of sound. Since this wave is supersonic, it is classified as a shock wave.
Posts: 327 | From: Pleasanton & Sacramento Ca | Registered: Dec 2001
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posted
On the topic of pressure, if a SUB is placed in a BOX, lets make it a small box of around 2.0 cubic feet. How many pounds per square inch of weight will be forced against the REAR of the cone from the Backwave?
I ask this question because I have a theory on why so Many people with IDMAX get Mis-aligned voice coils after 2-3 months of use.
I think because the SUPPORT for the rear of the cone isn't stable enough to support the extreme pressure of the backwave when 1000 watts of power is applied to the sub in a small enclosure.
But this is just a theory, although this subject has sparked some outrage at soundomain, I just feel that any HIGH POWER SPL sub should be Built to withstand ALL KINDS OF FORCES...even forces that we don't see but are there during EXTREME SPL APPLICATIONSand not just have a BIG MAGNET.
In my opinion JL's W7 models offer more support to the SUB during EXTREME PRESSURE applications with their patented voice coil alignment technology. And cone support technology.
Which is why is decided to purchase one because they are built with common sense in mind.
I do not know much about other SPL subs and how they are built...although some can create LOTS of SPL for $cheap$ and little power...But I find that most SPL subs on the MARKET today only depend on a stiff FRAME and a CONE Material that can stay linear during peak excursions. And omit voice coil alignment problems that may occur when even more power is applied to the driver.
(This thread is just opinion only and is not meant to spark any arguments.)
posted
Correct me if im wrong but sound waves don't travel faster than the speed of sound, only the object that makes the sound can travel faster than the speed of sound. Shock waves are soundwaves bunched together. When I think about the possibility of creating pressures of 29.4/0 I say that it must be directly at and behind the speaker, whats the chances of a complete vacume wave(0psi) reaching the mic before it's diminished by the air rushing in. I know what your thinking, you get higher spl at the dash than when you put the mic next to the speaker in many cases. True but it seams that the losses would be greater at higher levels. To get a 194dB spl @ the mic you would need to displace at least half the volume of the enclosure/cabin by speaker movement, not ports because they would be the weakest link. Just a thought.
Posts: 35 | From: Al | Registered: Feb 2002
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posted
did anyone see that subwoofer,that looked like a rotary vane transducer?? i think it was produced by a big car audio company,and quickly dissapeared..
quote:Originally posted by toxicbass_1: did anyone see that subwoofer,that looked like a rotary vane transducer?? i think it was produced by a big car audio company,and quickly dissapeared..
Good discussions
Pheonix Gold Cyclone?
Where's my cookie?
-------------------- 2005 Florida Street C Champion 2004 SBN Street C Champion 2004 SBN IDBL No Wall Champion 2003 NOPI Nats Street B Champion
quote:Originally posted by toxicbass_1: Pheonix Gold Cyclone?
Where's my cookie?
*gives a pretend cookie*
lol thats it,i wonder if it was any good..the guy with his site thoht it was
*crunch crunch crunch*
From what I heard they gave monsterous output below 40hz, but everyone said not to try anything above that cutoff so strong midbasses were required. That's what I was told anyhow, I'd much pefer to play with a few cyclones...
-------------------- 2005 Florida Street C Champion 2004 SBN Street C Champion 2004 SBN IDBL No Wall Champion 2003 NOPI Nats Street B Champion
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