SPL formula?

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dedlyjedly

Copper - Posts: 94

Joined: November 17, 2004

Location: United States

Posted: December 27, 2004 at 3:02 PM / IP Logged

doesnt the reference efficiency in effect sum up most of the other variables of the sub such as xmax, cone surface area, size of magnet. etc?!

then there are other variables such as enclosure, vehicle, speaker placement, etc that will play into db drag scores.

MECP certified, 5yrs experience, you probably otta listen bitch!!

haemphyst

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Joined: January 19, 2003

Location: Michigan, Bouvet Island

Posted: December 27, 2004 at 3:32 PM / IP Logged

NO!!! The efficiency of a driver is a stand-alone specification. All of those items listed CAN have an effect on the Re of a driver, but the driver's Re will be a spec of it's own. If I plug in all of the other T/S parameters for a driver, my JBL Speakershop cannot give me the Re of a driver (Actually, I never tried it... I'll give it a shot... It does, but it is based on the Qts, a combination parameter). If those were the items that "made up" efficiency of a driver, then how do you explain a one inch tweeter with a 100dB efficiency, and an excursion of .01mm and 10 watts power handling, but an 18 inch driver with 3 inches of excursion, and 2000 watts power handling only gets 87-90dB. Shouldn't the 18 inch driver with 3 inches of throw be WAY louder than the tweeter? (with one watt input - lets compare apples to apples) Here is the physics coming into play again... SPL is based on RATE OF CHANGE! A tweeter can move back and forth FAR faster than a woofer - and while the wavelengths are MUCH smaller from the tweeter, the speed with which a driver can change direction DIRECTLY affects the rate of change (between rarifaction and pressurization), therefore it's output capability at a given power level.

I know, somebody is gonna say "But we're talking about woofers, not tweeters." A driver is a driver is a driver... if I had a tweeter with 6mm of Xmax, it will be FAR LESS EFFICIENT than the above mentioned tweeter. The laws of physics do not change, just because the frequency band being reproduced does. Consider uthink's beloved Nakamichi component set - OK fine, it has GREAT output, at the expense of extension, and he himself admits that the bass response could be better. Me, I am running Morel MW-162s in my doors (which personally I feel knock the Naks d**k in the dirt), but I have to drive my 6 inch drivers with 370 watts per channel. Oh well, I am happy to do so, because I can go all the way to 50Hz (and with SPECTACULAR authority, I might add) with them. I am willing to trade efficiency for extension.

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

kfr01

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Joined: April 30, 2003

Posted: December 27, 2004 at 3:55 PM / IP Logged

dedlyjedly, incorrect. Reference efficiency is a very valuable parameter. Alone, however, it really tells us very little about the driver's potential performance.

xmax is not used to calculate reference efficiency.

SPL is derived from the physical parameters Mms and BL.
Mms is the driver's moving mass.
BL is the motor force.

Neither of these parameters carry any direct relationship to xmax.

Reference efficiency can be calculated other ways - none of which involve xmax.

Reference efficiency does not tell us anything about total potential SPL in any way. (thermal power handling, xmax, etc.)

Edit: Damn, haemphyst beat me to the punch, well, our posts didn't completely overlap, so I'll leave this as is. :-)

New Project: 2003 Pathfinder

Poormanq45

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Joined: October 27, 2004

Location: United States

Posted: December 27, 2004 at 10:57 PM / IP Logged

Well, the reason I am thinking that Excursion has alot to do with SPL is that the Driver is basically like a piston. Think about the engine in your car. As the piston moves up in the cylinder, more pressure is created.

Try this. Put a subwoofer in an enclosure specifically designed for it. Wire everything up. And then create an enclosure to seal off the front of the driver from getting an "unlimited" supply of air to move. Put a Digital Water Column tester in both the front and back enclosure. Find some way to measure the excursion of the driver while it's playing. Feed the driver a signal. Now, only measure the Excursion and the pressure reading of the Water Column. If you exclude the measurement of power that you are feeding the driver, you should be able to find a correlation between SPL and Excursion.

I understand that high excursions cause inefficiency. But there must be some way to calculate the SPL of a driver. And the basic 3Db increase = Doubling power as you guys know simply does not work.

haemphyst

Platinum - Posts: 5,054

Joined: January 19, 2003

Location: Michigan, Bouvet Island

Posted: December 28, 2004 at 12:16 AM / IP Logged

Dood, as I have stated several times, SPL is RATE OF CHANGE! It matters not one WHIT about how much pressure a piston can produce in either the positive or negative waveform, (which is what your water column is measuring) but HOW FAST CAN IT GO FROM ABSOLUTE MAXIMUM TO ABSOLUTE MINIMUM... Otherwise seemingly identical drivers can have different efficiencies, based on MMS, Le, Bl. If any one of these changes, it WILL affect the overall output from a driver, but Xmax WILL NOT AFFECT IT!

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

kfr01

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Posted: December 28, 2004 at 12:45 AM / IP Logged

Poormanq45 wrote:

If you exclude the measurement of power that you are feeding the driver, you should be able to find a correlation between SPL and Excursion. I understand that high excursions cause inefficiency. But there must be some way to calculate the SPL of a driver. And the basic 3Db increase = Doubling power as you guys know simply does not work.

Haemphyst, I totally agree with what you're saying, but I did some thinking and our friend here must be talking about how xmax LIMITS spl at a given frequency. Here xmax is useful, I think.

displacement (vd) = xmax * effective piston area (sd)

The more air you can displace the louder you can go - the piston hypo. Since piston area is fixed, a limiting factor of displacement, and thus SPL, is xmax. So, given an unlimited amount of power, effectively throwing out any considerations of efficiency, and forgetting about thermal limitations, xmax would be useful in determining xmax limited SPL. Once I realized what our friend poormanq might be getting at I searched a bit....

I found this spreadsheet you might find helpful. The famous Mr. Linkwitz hosts this: http://www.linkwitzlab.com/spl_max1.xls Part A is titled, "Excursion limited rms sound pressure level (SPL) for a driver in a closed box."

In other words, this calculates MAX SPL for a GIVEN FREQUENCY.

You can view the formulas on the spreadsheet, or just use it to do the work for you.

Anyway, thanks for pressing this issue. I found Mr. Linkwitz's sheet fascinating. Poormanq, have you downloaded WinISDpro yet? It does some of this for you too. I think you may enjoy playing around with it.

New Project: 2003 Pathfinder

kfr01

Gold - Posts: 2,121

Joined: April 30, 2003

Posted: December 28, 2004 at 10:37 AM / IP Logged

I'm curious to hear what Steven has to say about this, if anything.

(ppssst... when are my Extremis drivers going to arrive? ;-)

New Project: 2003 Pathfinder

Poormanq45

Silver - Posts: 597

Joined: October 27, 2004

Location: United States

Posted: December 28, 2004 at 9:43 PM / IP Logged

Thank you kfr01. I realize now that I was very unclear in what I was asking. It's amazing that you were actually able to figure out what I was asking.

Thanks for the link.

[quote]Dood, as I have stated several times, SPL is RATE OF CHANGE![/quote]

Isn't the rate of change called the Transient Response?

SPL = Sound Pressure Level.

kfr01

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Joined: April 30, 2003

Posted: December 29, 2004 at 11:14 AM / IP Logged

Poormanq45 wrote:

Thank you kfr01. I realize now that I was very unclear in what I was asking. It's amazing that you were actually able to figure out what I was asking.

Thanks for the link.

[quote]Dood, as I have stated several times, SPL is RATE OF CHANGE!

Isn't the rate of change called the Transient Response?

SPL = Sound Pressure Level.[/QUOTE]

No problem. However, Haemphyst was still right, "Sound Pressure Level" still is all about the rate of change. At full excursion, if moving too slow, pressure drops off. It just becomes a giant motor moving in and out. Excursion of the woofer just sets the LIMIT on how FAR the driver can move while it is moving.

This is very important to understand when comparing speakers!!! As long as BOTH speakers are being driven within their excursion limits, excursion becomes a NON ISSUE. i.e. If I have two subwoofers, one that reaches xmax at 1600w at 40hz with a 30mm xmax. The other has 20mm of xmax, still a great deal, and reaches that with 600w at 40hz.

If you "only" have 500 watts of power to those speakers, EXCURSION WON'T EFFECT SPL ONE TINY BIT. RATE OF CHANGE WILL.

This is where the EFFICIENCY of the driver is so important. With 500w, the lower excursion, probably more efficient, driver will have MORE SPL, even though the potential xmax is less.

Anyway, have you downloaded WinISD yet?

New Project: 2003 Pathfinder

Poormanq45

Silver - Posts: 597

Joined: October 27, 2004

Location: United States

Posted: December 29, 2004 at 5:31 PM / IP Logged

Yeah I downloaded WinISD Pro.

So, why do companies like Adire Audio have such "large" excursions on their subs? 22mm each way IIRC.

So what's the point of designing the sub, surround, and motor to be able to have that large of an excursion if it is known that the greater the excursion, the lower the efficiency? Why don't all companies strive to increase the efficieny of their drivers instead of designing them to have large excursions?

Is it just a marketing gimmick to appeal to the uninformed american public? I mean, it's the American mentality that more is Always better then less SPL formula? - Page 2 -- posted image.

. I was talking to a guy in the local Car audio shop today about the Adire Shive and Bramha that they had. He kept mentioning the excursion. I guess he sells more speakers if he says a "high" excursion number SPL formula? - Page 2 -- posted image.

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