# total efficiency of 2 cabs?

Discussion in 'Amps and Cabs [BG]' started by Matthias, Aug 19, 2001.

1. ### Matthias

May 30, 2000
Vienna, Austria
Here is a question to our tech experts:

How do I calculate total efficiency of two cabs with different efficiency ratings, for example a 115 with 98dB and a 210 with 100dB?

I just know that you have to add 3dB if you have 2 equally loud sources and that you gain another 3dB if they are close together (stacked cabs) due to coupling effects or something like that.

Thanks,
Matthias

2. ### Luis Fabara

Aug 13, 2000
I dont know any formula,
but my thinking got me to 102dB

103 dB = Is not achievable.
101 dB is Achiavable with 98 +98..

So.. (98 + 100) /2 = 99
99 + 3 = 102 dB

Mayble Im completely wrong, but that´s my empirical aproach.

3. ### MikeyD

Sep 9, 2000
Hi, Matthias,
First, I must point out that sensitivity and efficiency are not the same. Sensitivity *can* indicate efficiency if all else is equal (e.g., directivity). The reason is that efficiency is the total acoustic output power of the cabinet in 3-space (i.e., a complete envelope around the cabinet) divided by the total electrical wattage input. Sensitivity is the sound pressure level measured at *one* point in space.

Second, the mutual coupling only works at the lowest frequencies, and diminishes to nothing eventually (typically in the upper bass region). Others have indicated that sensitivity is usually measured anechoicly at 1 kHz, so it is doubtful that mutual coupling contributes anything to the sensitivity at that frequency. However, at lower frequencies, you can get some benefit from the coupling.

On your question, if you can put equal wattage into both cabinets, the 115 will give you 95 dB at 1/2-watt, and the 210 will give 97 dB at the same wattage. If you combine these, you get 99.1 dB (you have to add their individual acoustic power intensities, which involves using logarithms). This is the likely number at 1 kHz. If the drivers are placed within 2 feet of each other, mutual coupling can assist starting at below ~140 Hz. But putting your cabinets on a floor, or in a corner, etc. boosts the lows as well.

- Mike

4. ### Matthias

May 30, 2000
Vienna, Austria
Luis: Yes, that's what I thought about, too. But as I have learned from this forum that acoustics is not so simple, I thought I'd better ask...

Mike: Thanks a lot for this detailed answer (again). I appreciate your contributions to this forum very much - I've learned a lot in the past year, and a big share of it came from your posts.
(not to forget Joris, Throbbinnut, Bruce L and Bruce G, Dave B and many others of cause)

Matthias

5. ### Matthias

May 30, 2000
Vienna, Austria