Math on speaker air movement

[alexclaber]

Originally posted by notanaggie
Otherwise, you could say a speaker system with 8 speakers was 9dB more efficient than a system with 1. That is nonsense.

Nonsense? I thought that's exactly how it was - double the number of drivers and you gain 3dB efficiency as well as doubling the power handling (another 3dB of output).

Alex

 

[Geoff St. Germaine]

Originally posted by notanaggie
No, you are IGNORING the point.

Absolute impedance IS the issue.

If you connect two 8 ohm speakers producing 103 DB each, you get 106. BUT you "made" a 4 ohm speaker which draws twice the power, and that is where the "extra" 3 dB came from. You are cheating just the way you said was wrong.

A speaker which has 1 driver at 103 db/1watt will produce the exact same SPL as a speaker with 2 drivers which each produce 103 dB/1watt, as long as the impedances are the same, meaning both systems are 8 ohms.

Otherwise, you could say a speaker system with 8 speakers was 9dB more efficient than a system with 1. That is nonsense.

I understood that the 3dB loudness gain was registered at the same power setting to the sytem as a whole. I.E. two identical 210s powered with 350W would be 3dB louder than one 210 powered with 350W. Could be wrong though.

Geoff

 

[Jerrold Tiers]

Originally posted by Geoff St. Germaine
I understood that the 3dB loudness gain was registered at the same power setting to the sytem as a whole. I.E. two identical 210s powered with 350W would be 3dB louder than one 210 powered with 350W. Could be wrong though.

Geoff

No, and here is why.

If the total power input is the same, in one case it goes to a single speaker, and in the second case it goes to two speakers.

When it is going to two speakers, if they are identical, half the power must go to each.

Then, if each speaker can produce 102dB with one watt, but they only get half a watt each, each produces 99dB. The two speakers, each producing 99dB SPL, total up to the original 102 dB.

That is the problem. With multiple speakers, the same power is divided up among them, reducing the power to each one. Yes they total up adding, but it is an addition of several smaller power increments. Four speakers get 1/4 the power each, and produce 1/4 the acoustic output power each, compared to a single speaker.

But suppose the multiple speakers total to a lower impedance and draw more power. Then because the power input is higher, the SPL output is higher. But the efficiency in dB per watt is not increased.

As was mentioned, if there is beaming, acoustic coupling, etc, there can be a difference in *apparent* output.

But when for instance the sound is beamed more straight ahead, it is simply the same total power re-directed. The sound that would have gone off to the side is sent straight out front instead.

A multi-speaker system can sound better than a particular single-speaker system. It can even sound louder, but the basic electro-mechanical efficiency doesn't change.

 

[bgavin]

Voltage remains constant in a parallel circuit. Voltage drops across each component in a series circuit. Efficiency never enters into this.

Let's do the math: Power = (Voltage^2) / Resistance

Given:
4 volts output from the amp
8 ohm single speaker load (DC resistance)

(4^2) / 8 ohms = 2 watts

Replace the single 8-ohm load with a parallel conected pair of 8 ohms for a 4-ohm load.

(4^2) / 4 ohms = 4 watts

Each driver consumes 1/2 the power (2 watts) because power is always additive in a circuit. Ptotal = P1 + P2, with 2+2=4 for proof.

Doubling the cone area at the same input power produces 4x the acoustic power, or +3dB.

I'm done. You can have the last word.

 

[bassmantele]

Originally posted by bgavin


Doubling the cone area at the same input power produces 4x the acoustic power, or +3dB.

According to your explanation, doubling the cone area had nothing to do with it. It was halving the speaker impedence that made the change, no? Doubling the cone area and keeping the impedence the same would result in the same power.

When I asked Ted Weber about this, he said that it was coupling that produces the increased volume.

 

[Scutterflux]

If this argument isn't over now!

 

[Geoff St. Germaine]

Originally posted by Ampeg Insider
No, and here is why.

If the total power input is the same, in one case it goes to a single speaker, and in the second case it goes to two speakers.

When it is going to two speakers, if they are identical, half the power must go to each.

Then, if each speaker can produce 102dB with one watt, but they only get half a watt each, each produces 99dB. The two speakers, each producing 99dB SPL, total up to the original 102 dB.

That is the problem. With multiple speakers, the same power is divided up among them, reducing the power to each one. Yes they total up adding, but it is an addition of several smaller power increments. Four speakers get 1/4 the power each, and produce 1/4 the acoustic output power each, compared to a single speaker.

But suppose the multiple speakers total to a lower impedance and draw more power. Then because the power input is higher, the SPL output is higher. But the efficiency in dB per watt is not increased.

As was mentioned, if there is beaming, acoustic coupling, etc, there can be a difference in *apparent* output.

But when for instance the sound is beamed more straight ahead, it is simply the same total power re-directed. The sound that would have gone off to the side is sent straight out front instead.

A multi-speaker system can sound better than a particular single-speaker system. It can even sound louder, but the basic electro-mechanical efficiency doesn't change.

Cool, that makes sense. Thanks.

Geoff

 

[Femto]

If you put a signal into a single speaker with an impedance of 4 ohms, you would get x amount of sound pressure.

If you then put a signal from exactly the same source into two speakers with an impedance of 8 ohms each, wired in parallel you may not get exactly 2x amount of sound pressure but it's going to be more than with the single speaker, right?

So we have two different speaker systems (One 4 ohm speaker and two 8 ohm speakers wired in parallel) which both present the same impedance to the amplifer. The only difference is the area of the cone.

I know there are most probably various equations and functions I have overlooked but I think we're getting far too technical over something which started out not requiring all the deep deepness.

Am I on the right track with this one?

Hey, I didn't notice that last post from you, Geoff until I hit submit. Doh! Still....am I on the right lines though?

 

[joegeezer]

I absolutely love reading this stuff. You guys are very knowledgable, and beyond my math skills. I can only say that I notice oodles of guys raving about the 6-10 cabs these days, and I think its only because hauling an 8-10 is more of a b*cth, than a 6-10. From my ears and years, no combination of cabinets in any form, that I have ever tried, beats out an 8-10 cab for bass. Multiple speakers, moving lots of air, does it for my ears. I see lots of new stuff in multiples with 8-8s and the Phil Jones multiple 5s, but I still say, even though, I'm 52, I walk with a cane, and have no knee on my left leg, I still grunt it up, and haul around my 8-10. They just can't be beat by anything, in my opinion.

 

[billfitzmaurice]

A speaker which has 1 driver at 103 db/1watt will produce the exact same SPL as a speaker with 2 drivers which each produce 103 dB/1watt, as long as the impedances are the same, meaning both systems are 8 ohms.

Wrong, because you're not taking into consideration mutual coupling , which increases sensitivity by 3dB.

Otherwise, you could say a speaker system with 8 speakers was 9dB more efficient than a system with 1. That is nonsense.

It is nonsense. First, efficiency is not measured in decibels, sensitivity is. A parallel wired eight driver system will have 18dB higher voltage sensitivity than a single driver, although if wired in a typical series/parallel arrangement the wattage sensitivity would go up by only 12dB, while the system impedance will be halved.

basic electro-mechanical efficiency doesn't change.

Yes, it does. Typically direct radiators have an efficiency of around 5%, while efficiencies of 40-50% are not uncommon in horn loaded systems.

Much of what has been said here is both [reasonably] accurate and [mostly] irrelevent, for if you don't compare sensitivity at specific frequencies you're comparing apples and walnuts anyway. The displacement figure isn't totally meaningless, but almost.

They just can't be beat by anything, in my opinion.

He hasn't heard a T24, has he, Bruce?

 

[joegeezer]

No , I haven't Bill, but would like to. I'm still working on the Omni12. If I like that, I'll have to try the T24.

 

[bgavin]

Or stand in front of a T36 if you really want your guts blown out.

My T24 keeps up with my son-in-law's Ampeg 610HLF without breaking a sweat. It provides a bottom the Ampeg can only wish for.

I figure a T36 is at least +10dB hotter at 40 Hz than the T24. That equates to sounding twice as loud at the same power level.

[ edit ]

If one wants a big rig that moves a lot of air, the Tuba 36 in 28" width plus a DR280 on top does the trick. The T36 is 945 square inches of radiating plane, and 500+ square inches of mid-bass plane for the DR280. This is the ticket for outdoor gigs, but is massive overkill in a venue with full PA support, same as the 8x10 rigs. Like the 8x10 rigs, it is also a pain to cart around.

 

[Le Petard]

Just in case anyone is still monitoring this thread I have a question only about the movement of air. I would like a clear and concise explanation of how an oscillating speaker can create actual "flow", that is airflow in 1 direction. I have seen videos of young ladies sitting in these cars with the amp'd up stereos and suddenly their hair "frizzes" out in all directions. This I can understand - a very rapid oscillation of air in and out through a orifice (in most cases the rolled down car window). And that makes perfect sense to me as a speaker cone moves back and forth. But how does the back and forth movement cause the air to flow in 1 direction in videos where the hair is not just hanging there but actually being blown out of the window as if there is a large fan behind her head ?

 

[Rick James]

In that scenario the car itself is a bandpass enclosure, with the open window acting as a port. The airflow is the same as what you might feel holding your hand in front of the port on your bass cab. It may look like the airflow is only in the one direction, but it's not, it just changes direction so rapidly that it's not easily seen. Couple that with the high pressure inside the cabinet, or in this case the car, and the low pressure outside and what you see on the video is the result.

 

[Le Petard]

I have seen a video (source unknown) of a foam head with toilet paper taped on to simulate hair, mounted on a stand maybe 3 feet or so outside of the vehicle window. Once the audio is "applied" the pseudo hair flies straight out and away from the vehicle as if propelled by a fan. This is not what I would expect from a "back & forth" movement of air resulting from the movement of a speaker cone. The flow was undeniably 1-way. If one uses a floor bass drum as a simulator, the air port in the drum head would be the window. The floor kick would be the source to move the skin "cone" back and forth. At the air port you will feel the air as it move first outward then back in again. I fully understand this result and it is expected. What is not expected would be for the air to move in a 1-way direction out of the floor bass through the air port and that is what is being demonstrated by this video. Is the video fake? I would provide a link or even the video, but the claimant won't provide me with either link or file.

 

[agedhorse]

I have seen a video (source unknown) of a foam head with toilet paper taped on to simulate hair, mounted on a stand maybe 3 feet or so outside of the vehicle window. Once the audio is "applied" the pseudo hair flies straight out and away from the vehicle as if propelled by a fan. This is not what I would expect from a "back & forth" movement of air resulting from the movement of a speaker cone. The flow was undeniably 1-way. If one uses a floor bass drum as a simulator, the air port in the drum head would be the window. The floor kick would be the source to move the skin "cone" back and forth. At the air port you will feel the air as it move first outward then back in again. I fully understand this result and it is expected. What is not expected would be for the air to move in a 1-way direction out of the floor bass through the air port and that is what is being demonstrated by this video. Is the video fake? I would provide a link or even the video, but the claimant won't provide me with either link or file.

Don't believe everything you see.

It might be possible to generate enough electrostatic field from the movement of air around the foam to make it appear this way, but I suspect that this is not the case.

 

[edro]

If a single endpoint anchored string object moves, it must be acted up by some force.
If it deflects continuously in one direction, then the force is not audio, nor vibrational, as both are waves.

That leaves a static condition force. The possibilities would be constant air flow, electrical charge, or 'hollywood'.

Air in an enclosure is gonna be wave....as in suck and blow... Ain't it...

I could consider the possibility of the air interaction with the polystyrene surface as in the wave motion across surface to build an electrical charge. That being said, I am curious though about the 'out ' part...."out and away from the vehicle"... Hmmmm... I would think a charge would result in the reference point being the surface of the polystyrene head....

Then there is always the hollywood method...

 

[Le Petard]

The fact that I have not been allowed to closely examine the video on his phone causes some suspicion. I have had no luck finding the video on-line. The 2 photos I have attached show what I expect to see as a result of a properly built low bass audio system and a rendition of what the "toilet paper/foam head" version looks like. 1)Head in widow area - hair kinda floats, 2)hair 3 feet outside window area is blown violently away from audio source. I highly suspect it is a faked video.