how to determine sensitivity of two drivers wired in paralell?

[audiophile36]

Say I have a 93db 8ohm driver and an 85db 8ohm driver wired in paralell,what is the overall sensitivity of the combined drivers...I know the load will be 4ohms instead of 8ohms.

[OzMikeH]

Since the drivers are not identical the frequency response will be different for each so it depends on the frequency. Also at higher frequencies comb filtering comes into effect so it also depends on position of measurement mic.

Two 93dB drivers can give you 96dB, now that second driver was 8dB less efficient than the first one. in percentages that is less than 25% of the volume of the first driver for a given input.
So at a guess I would say about 125% of the volume of the 93dB driver. I'm pretty sure the combined efficiency would be less than 94dB.

I can't think of any situation where doing such a thing would do more good than harm.

[kmaier]

I would agree that combining two dissimilar drivers would probably cause more problems than benefit, but here's a link which would probably help in the calculations:

http://physics.mtsu.edu/~wmr/log_4.htm

Regards, KM

[goudey]

Assuming perfect acoustic coherent sum (e.g., lower frequencies at some distance):

SPL = 20*log10( 10^(SPL1/20) + 10^(SPL2/20) )

In this case, 93 + 85 = 95.9 dB

The formula produces the obvious 6 dB net gain for matched drivers.

Thats just the math. Such a combination of drivers is usually a bad idea (as previously noted).

[WithTarragon]

There are several problems

1. the 8 ohm number is nominal and you should not assume it is exact. The impedance will vary with frequency and in some portions of the spectrum the impedance may not be purely resistive. This variation will not be the same in both transducers. I have no idea what drivers are being used.

2. Yes, the impedance in parallel will be less than the nominal 8 Ohms - bit we don't know what it is (see above)

3. No, this statement is incorrect
"In this case, 93 + 85 = 95.9 dB"
At best the summation will be 93 plus a fraction.

4. Using two different transducers in parallel is not a good idea.

-Tom

[audiophile36]

Thank's for the responses. The reason I ask is because I am running open baffle speakers with a single full range driver and a woofer to augment the bass,and wanted to know the net sensitivity of the speaker.I not sure of the woofer sensitivity so I used 85db as a worst case scenerio.If that is such a bad idea wouldn't every 2-way speaker suffer in the same manner ? Almost every commercial speaker uses two or more different drivers with different sensitivities so I am having a hard time understanding what is so bad about combining two different drivers.

[audiophile36]

Originally posted by WithTarragon
There are several problems

1. the 8 ohm number is nominal and you should not assume it is exact. The impedance will vary with frequency and in some portions of the spectrum the impedance may not be purely resistive. This variation will not be the same in both transducers. I have no idea what drivers are being used.

2. Yes, the impedance in parallel will be less than the nominal 8 Ohms - bit we don't know what it is (see above)

3. No, this statement is incorrect
"In this case, 93 + 85 = 95.9 dB"
At best the summation will be 93 plus a fraction.

4. Using two different transducers in parallel is not a good idea.

-Tom

Thank's for your detailed response.The drivers in question are the Fostex Fe126e 93db 8ohms and a woofer of unknown sensitivity.The specs on the woofer state 8ohms,FR 45hz-3khz,15watts nominal 30 watts max.I must say that the speakers do sound good.
[IMG][/IMG]

[ßart West-VL.]

Here they use two different drivers, on purpose.
I think it can sound good as ones faults can be smoothed out by the others quality.
It really can give a nice sound, just try it, you might be surprised.

Two drivers of 90dB have a combined sensitivity of 93dB, not 96dB.
You get the 96dB by doubling the power, because of the halved resistance.

Bart

[goudey]

regarding:

"3. No, this statement is incorrect
"In this case, 93 + 85 = 95.9 dB"
At best the summation will be 93 plus a fraction."

My original statement is correct, and the above statement is quite incorrect for the simple case of two closely spaced drivers. But when the two drivers are significantly separated, and the sound propagation paths very different, the sound pressures will not add "coherently". And if a crossover divides the signal to the two drivers, there is no sensitivity enhancement.

From the pictures, it looks like the woofer will augment the low frequencies significantly, but not the higher frequencies (different path lengths, falling high frequency response of the woofer).

My formula is based on simple linear acoustic wave propagation models, available in acoustics texts. But the formula is too simple for the speaker arrangement shown.

Regarding using two different drivers, if it sounds good, don't worry! The first post did not hint about the actual configuration. The picture helps. Good photo, and it looks like it would sound good. I also like the open baffle sound.

Is there a crossover involved?

A frequency response measurement would help guide improvements.

[audiophile36]

Originally posted by goudey
regarding:

"3. No, this statement is incorrect
"In this case, 93 + 85 = 95.9 dB"
At best the summation will be 93 plus a fraction."

My original statement is correct, and the above statement is quite incorrect for the simple case of two closely spaced drivers. But when the two drivers are significantly separated, and the sound propagation paths very different, the sound pressures will not add "coherently". And if a crossover divides the signal to the two drivers, there is no sensitivity enhancement.

From the pictures, it looks like the woofer will augment the low frequencies significantly, but not the higher frequencies (different path lengths, falling high frequency response of the woofer).

My formula is based on simple linear acoustic wave propagation models, available in acoustics texts. But the formula is too simple for the speaker arrangement shown.

Regarding using two different drivers, if it sounds good, don't worry! The first post did not hint about the actual configuration. The picture helps. Good photo, and it looks like it would sound good. I also like the open baffle sound.

Is there a crossover involved?

A frequency response measurement would help guide improvements.

Thank's for the response.There is no crossover,I let the woofer run full range up to the Fostex.