posted
Okay this is a theory which I have had for quite some time. I think we are all thinking about what causes Sound Pressure the wrong way. The pressure the mic is most concerned about is not the overall change in air pressure in the cabin. It is the pressure or force applied to the mic diaphram by the excitied particles (in Wave form)created by the movement of the cones. Think on this one. If the mic was concerned with air pressure why would SPL readings be higher at the dash or say in the kick pannel than in the open middle of the car. I can go in to much greater length on this matter but time is short. Hope you guys understand what I am talking about.
------------------ Knowledge=SPL As does and open mind Team Hertz Canada Pres
Posts: 221 | From: Calgary, AB | Registered: Nov 1999
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posted
Mr Hertz- I think that SPL is higher in the places that you mention because you are in effect boundary loading the mic.
ShadowStar- Ports aren't linear in their operation. At or above the tuned frequency(to a certain degree before antinodes cause destructive intereference at the high end) frequency of the box, the port is creating constructive interference- like having another speaker. But below the tuned frequency, the back wave is ever so slightly out of phase with the speaker- creating destructive interference.
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Posts: 7843 | From: Charlotte,NC USA | Registered: May 1999
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posted
Just some thoughts to add to the conversation.
When you play your speakers, and create sound, you send a "pocket" of high pressure followed immediately by a pocket of low pressure through your car. The pressures vary a little above and a little below the static pressure of the vehicle. The SPL mic has a lightweight membrane that is moved by those pressures, in for higher and out for lower. There is a magnet attached to that membrane that when moving in another magnetic field creates a current. That current then travels to an electronic device where you can measure voltage. It is essentially an "AC" circuit. The amplitude of the wave (electric) is the SPL.
Increasing the cabin pressure, or lowering the cabin pressure simply shifts the wave, but does not alter the amplitude. The only effect in changing the cabin pressure is the loading or unloading of the speaker by the air spring in the box. Ported boxes should not change with air pressure. Sealed boxes can have a difference if the static pressure inside the box is different than the static pressure of the vehicle, and then the SPL will only be higher one way, not both. The air density (theoretically) should not change the SPL as it is in "both" sides of the equation and cancels out.
Also, one other thing to note, cone area does not really play a role in SPL, other than exciting more air that could impact the mic. If the mic was at one end of a tube with a speaker at the other, with the tube being the same diameter of the speaker, then the speaker with the larger excursion, not area will produce more SPL. In the vehicle environment, the only effect of more cone area is to excite more air molecules that could bounce around and impact the mic. One highly concentrated (and laminar) wave off of a small speaker, aimed directly at the mic can produce surprising SPL numbers. You all will know how well it works when I finally get the bugs out of it.
Just my thoughts. I learned this information from a Noise Control Engineering class that was required by my Aeronautical/Astronomical Engineering degree at Purdue University, so effectively, that's what the guys at NASA would say.
------------------ Overcoming by out-thinking
"Forget about subsonic, I'm trying for supersonic, and I don't mean a high note."
posted
I don't know how to describe this more clearly to you all. Hmmmmm. Air density does have an effect on SPL I have witnessed it time and time again. Elevation, temperature, humidity etc. all play a part in effecting SPL readings due to changing air densities. This is because in denser air the particles are more tightly packed, therefore the transfer of energy (motion) from one particle to the next (like a chain reaction)is more efficient. It took me a long time to realize what actually causes SPL, and it is not an overall increase and decrease of the air pressure in the cabin. And the reason it is easier to make less woofers with more power go louder than more woofers with less power is simply due to cancelation. When the energy is emmiting from only 1 or 2 woofers which are very close together the output is in effect more focused.
------------------ Knowledge=SPL As does and open mind Team Hertz Canada Pres
Posts: 221 | From: Calgary, AB | Registered: Nov 1999
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Well, lets see- Speed of excited particles hitting mic diaphragm going up, actual pressure exerted on the mic goes up, correct.. The amount/speed of collisions between air molecules IS pressure.
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
Ok...so if the displacement of air molecules in a smaller area create higher levels of spl then why make flat subwoofers? In my opinion the greater amount of Volume the speakers cone can hold will yeild higher amounts of SPL since it displaces more air. EI: Take any standard 12 subwoofer, Fill it with sand and measure the volume of air the cone holds. This volume is relative to the amount of air per stroke the woofer moves correct? So if you were to make the louder subwoofer; you would intern make the volume of the cone larger? But most companies make the speaker larger (15,18,20) to increase the volume of air the cone will hold. Why not make the cone larger by increasing the length from the Surrond to the coil?
IE: Take an Egg shell and cut it in half (very carfully of course)
Most subwoofers that offer High excursion give thier subwoofers 1.5-2 inches of rubber surround so it can move more air per stroke. Yet if the same suround was applied to lets say my egg shell (with a diameter of 12") it could possibly move twice as much air as a standard 12 inch woofer.
Just for fun Ill try and plug some unrealistic numbers in this math problem.
1 12inch woofer with 1.5inch rubber surround and a cone volume of lets say 40. Multiply the excursion by the volume and you get 60.
Now a 12inch woofer with 1.5 inch rubber surrond and a cone volume of lets say 60. Multiply the excursion by the volume and you get 90.
Thats 1.5 times the amount of air per driver. Im sure my figures are not that far off. But none the less you get my point. Wouldnt a speaker with more cone volume give better spl? In this case the usual dustcap would not exist. No flat parts on the subwoofer at all. Could this hurt its performance?
If any of you talk to JL, Orio, or any other speaker company about this....Can I please get a free pair of these woofers..LOL Seriously please...PRETTY PLEASE ;oP
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Posts: 1 | From: WI | Registered: May 2000
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As stated earlier, a true inverted dome shap would be sweet.. However, hard to realize. Cabasse makes a 23" inverted dome subwoofer but it isn't really setup for high excursion..
Now, if you made that subwoofer with a high excursion, it would have a little more displacement because of more surface area.. So are we gonna start having "ribbed" or "terraced" subwoofers???
ShadowStar
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HEY! I'm looking for a CHEAP used Clarion Thunderdome.. Hook me up!
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
First of all. The displacement is only in the in-out direction (no comments). Therefore the slant or amount of cone should not matter, only the diameter. The cone shaped ones send a cone shaped wave through the air. This does not reflect off of surfaces perfectly. A flat woofer sends a flat wave, which makes the reflections nicer.
Also I stated earlier that air density doesn't matter. It does, but the effect is so small that those of us in the aerospace industry can ignore it. It will effect your tuning frequency more so than anything else. If you can find the right freqency for the conditions, then it shouldn't matter.
------------------ Overcoming by out-thinking
"Forget about subsonic, I'm trying for supersonic, and I don't mean a high note."
posted
back to the shuttle thing. when you watch a launch and you see all the water spraying under the shuttle it is not for cooling purposes. water mutes sound. just like talking underwater. i think i read that one of the mercury rockets a long time ago blew apart the launch platform because of the spl which is why they now use water. matt
posted
When I took a tour of Kennedy Space center, they said the water was for cooling. They had many locomaotive engines pumping water for cooling purposes.
About the fat surround issue- Isn't that just a band aid for poor engineering? Companies use a large surround to control the cone after the voice coil has been launched out of the gap. Right?
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Posts: 7843 | From: Charlotte,NC USA | Registered: May 1999
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posted
vibrations and waves animations.... this link http://www.gmi.edu/~drussell/Demos.html will show and hopefully help some of you to better understand acoustical physics... it covers...vibrations of simple as well as complex oscillators, and continuous systems modes. mechanical waves (ACOUSTICS)....basic wave phenomena.....sound waves(longitudal and transverse, doppler effect, simple source radiators, cylindrical radiators..... MORE complicated wave phenomena...like...phase chang upon reflections and phase speed versus group speed..
even human hearing and how the ear works it even covers electricmagnetic waves and matter waves.....ENJOY take from it what you can.....
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Posts: 122 | From: St.Louis, MO | Registered: Feb 2000
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http://www.gmi.edu/~drussell/Demos.html i tried to post this link in a interesting fashion, just a few minutes. i dont know where the post went but i do ot think that it went where it was supposed to go...so. i will make this re-try alittle shorter.... basically the site gives descriptions of different types of waves(oscillation,mechanical,electromagnetical, and matter waves) in brief but well rounded descriptions from simple waves to complex waves, as well as being accompanied by animation....in the way that these topics are illustrated here, it may not be very easy for everyone to understand it all(even i dont), but i do think that most everyone will be able to get a least a basic and better understanding of acoustical waves and how they operate. ..... enjoy...and take from this what you can.
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Posts: 122 | From: St.Louis, MO | Registered: Feb 2000
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posted
Just a thought; wouldn't the pressure the motor structure can place on the cone have a equal if not greater effect (than excursion) on the amount of spl a driver can produce. Even if a driver has 3" of excursion capability couldn't a driver thats only got 1" of excursion capability be able to out perform it as long as the motor structure was more powerful within that 1" distance? Since the pressure of the sound wave can only be equal to the amount of pressure placed on the air in the first place. Just thought I would throw this out there since everyone seems to be bent on the excursion factor.
Posts: 736 | From: Albertville MN USA | Registered: Mar 2000
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Only in conditions where the pressure in the cab (compliance of cabin air) causes the woofer movement to slow can there be an effect due to a weaker or stronger motor.. yes, at SUPER high pressure, only a stronger motor will be able to make the subwoofer throw to full excursion, thus pressurizing the enviroment more. But, if you have not reached that critical pressure where the cabin pressure and motor force are equal does the motor force's power on the cone (past that making full excursion) matter.
ShadowStar
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HEY! I'm looking for a CHEAP used Clarion Thunderdome.. Hook me up!
Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
That is a very good point. Would be interesting to see a plot where you could show the point where different woofers reach there limit as to how much pressure they can possibly produce (no matter how many of them there are). Separate the men from the boys per say. Would get to find out what kind of limits are really placed by the components rather than the setups and how close we are to reaching those limits.
Posts: 736 | From: Albertville MN USA | Registered: Mar 2000
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http://www.stemnet.nf.ca/~yliu/acoustics.html#effects this is a link to a page which is titled "AN UNOFFICIAL GUIDE TO ACOUSTICS" just wanted to drop the link here for those of you who may be interested....
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Posts: 122 | From: St.Louis, MO | Registered: Feb 2000
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posted
Nasty- I'm not Dr. Hertz, but I can answer that. Sound waves travel travel faster through liquids than gasses because there are fewer molecules present in a gas for the energy of the wave to be applied to. In other words liquids are far more dense with much less space between them. When a wave passes one molecule of a liquid, it only has to travel a small distance before it slams into the next particle, thus transfering the energy. In a gas on the other had the molecule must travel a longer distance before impacting the next. Also I believe that intermolecular collisions are almost perfectly elastic, so that there would be no (or very little) time delay for the energy to be transferred to the next molecule. Gas molecules have to travel farther and will have a "flight time" before they impact the next molecule. This flight time, I think, is what slows down a wave in a gas. THis should als hold for solids as well. I'm shutting my face now 'cause my brain hurts. Hope this helped, Steve
ps- Thanks for posting how you made your kickpanels! That info. was invaluable! Cool kicks in your Prix! ------------------ Coming to rock Gainesville eventually (ie-before the next ice age) 2 - Eclipse 8815 subs 2 - D300HC A/D/S components 1 - RF800a2 (or PPI)
(Look! In the sky! Is a bird? Is it a plane? No, it's Bumpin' Yota -enforcer of massive bass in sound systems and antagonist of distortion and wimpy bass everywhere!)
[This message has been edited by Bumpin' Yota (edited 05-25-2000).]
Posts: 4518 | From: Land Lord of Iggy's Head Apart Complex Tower #2 | Registered: Jan 2000
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