question on speaker efficiency (SPL)

Discussion in 'Amps and Cabs [BG]' started by CaptainWally, Mar 29, 2006.

1. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
so i think i have this right:

SPL = x dB using 1W 1M away from speaker...

dB actually = dBm?

EDIT: Oh wait, dBm wouldn't make sense would it? It's a comparison of the actual amout of physical pressure in the room vs. the additional pressure from the cab, right?

Anyway, what I don't understand is how this relates to different sized cabs. So you've got a 2x10 @ 100 dB and you've got a 4x10 @ 100 dB....

Does this mean the 410 is less efficient since it requires all 4 speakers to create the same SPL?

2. ukbassboy

Mar 19, 2006
UK
Good question... I have no idea what the answer is though.
I wait in anticipation.

3. mikezimmermanSupporting Member

Apr 29, 2001
Hmm, I guess it depends on what you mean by "less efficient". Normally, though, I'd say if you've got a 2x10 and a 4x10 with the same output level (100dB at 1m) from the same input current (1w), they are equally "efficient" in a simple cabinet vs. cabinet comparison. In a general sense, efficiency is just a measure of the amount of output that's generated by a specific amount of input--it doesn't account for the number of drivers, size, weight, or anything else that might well matter to you in using the cab!

Of course, if you're talking about the individual efficiency of each of the 10" drivers, the comparison goes a bit differently...

Mike

4. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
So...the fact that one cab requires 4 speakers to create 100dB and the other only requires 2 speakers isn't a factor?

That's counterintuitive.

5. illidian

Jul 2, 2004
If a 4x10 has an effeciency of 102dB, you're looking at 96dB/speaker, in theory.

A 2x10 with an effeciency of 102db, the speakers have a sensitivty of 99dB/each.

Sensitivity of cabs is measured of the entire cabinet. Sensitivity of speakers is measured with a single speaker.

6. billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
You're talking about sensitivity, not efficiency, and it's related not only to the number of drivers in the cab but also the system impedance. If a 2x and a 4x are using the same drivers inside they won't have the same impedance, and that explains your scenario. If the 2x is 8 ohms then the 4x would be 4 ohms. If both had the the same 1 meter/1 watt sensitivity, it takes 2.83 volts to develop a watt into 8 ohms, and only 2 volts to develop a watt into 4 ohms, so the 4x would be louder with the same voltage input. But if the two cabs did in fact have identical drivers they would not have equal sensitivity anyway, the 4x would be 3dB higher due to mutual coupling.

7. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
Cool - thanks!

does the term "efficiency" really have any place in cabinet specs? or is that a legit term that mean something else?

So I'm still a bit confused though. I went to the SWR site and the both the 4 ohm and 8 ohm 4x10 Goliath cab have an "SPL" of 105. as i understand it, that's not consistent with your explanation is it?

and what does the dB mean anyway? I know dB is, but what 2 thing are we comparing here - pressure in silent room vs. pressure in front of cab @ 1M?

8. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
so when i see a quoted cab spec, does the spl refer to the whole cab or just a single speaker in the cab?

9. PassinwindI know nothing.Commercial User

Dec 3, 2003
Columbia River Gorge, WA.
Owner/Designer &Toaster Tech Passinwind Electronics
The "quiet" reference is the average threshold of human hearing.
http://en.wikipedia.org/wiki/Sound_pressure_level

10. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
ok, so it IS pressure of cab sound to a standard reference pressure.... 20 micropascals.

i guess variance in pressure in test rooms isn't enough to make a difference. e.g. denver vs. sea level?

11. billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
No.
They both would have the same wattage sensitivity, but the 4ohm would have 3dB higher sensitivity for equal voltage input.
Decibel, and the measurement should be taken either in an anechoic chamber or outdoors.

12. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
Ok, I think I'm getting it...the voltage is manipulated for each cab so that is gets 1W based on each cab's resistance.

I still don't understand why the SWR 210 (both 4 and 8 ohm ver) is 105 and so is the 410 (4 and 8 ohm ver).

13. Bassin' 'RoundBanned

Apr 30, 2005
So if you want loud, look to a 4 ohm, 4x cab. Or if you want to haul it around, a 6x or 8 x.

14. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
Uh, ok, thanks ...

15. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
so here's a related question...

is doubling the power to cab A (assuming its within its rating) going to yield the same SPL as keeping the same amount of power and adding an identical cabinet and adjusting voltage to account for impedence difference?

16. billfitzmauriceCommercial User

Sep 15, 2004
New Hampshire
Owner, Bill Fitzmaurice Loudspeaker Design
Either the drivers are different or the specs are bogus. I'd lean towards the latter.
No, because you get an additional 3dB of sensitivity by adding the second cab, so with equal power input the 4x will be 3dB louder. Since the 4x also will handle twice the power it would be 6dB louder than the 2x when both are driven to full power.

17. Bob Lee (QSC)In case you missed it, I work for QSC Audio!Commercial User

Jul 3, 2001
Costa Mesa, Calif.
Technical Communications Developer, QSC Audio
Having multiple drivers will usually boost sensitivity on-axis (assuming the drivers are arranged on the same plane), but will decrease sensitivity in some off-axis directions. This is called lobing.

Properly horn loading a driver can increase its efficiency by increasing the acoustical impedance. Because of the very large wavelengths at bass frequencies, though, it's often not practical.

18. CaptainWally

Oct 21, 2000
Sandy Eggo, CA

19. CaptainWally

Oct 21, 2000
Sandy Eggo, CA
MMM, did you note what I said about adjusting the voltage so the the 210 has x2 watts?

20. Petebass

Dec 22, 2002
QLD Australia
Bob, them's fightin' words to Bill.