# how to determine sensitivity of two drivers wired in paralell?

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#### 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

Several problems

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

Re: Several problems

WithTarragon said:
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.
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#### 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

goudey said:
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.

#### Cloth Ears

goudey said:
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).

goudey,

Is that useful for other acoustic summing? For example, a drivers' output and the (theoretical) aid of an open baffle... or are the edges of a bafle so far away as to make this summing incorrect?

#### planet10

Paid Member
goudey said:
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.

Just to clear up some confusion, this formula is for sensitivity (ie fixed voltage into the load/not fixed power (efficiency)) and is only valid if your amplifier is a perfect voltage source.

almost all of the gain is from working the amplifier harder.

(ignoring the response anomalies from the OB)

2 identical drivers in parallel with a perfect voltage source will output 6 dB more level (3 dB from the doubling of efficiency and 3 dB from the amp outputting 2x as much power)

2 identical drivers in parallel with a perfect current source will output the same level (3 dB from the doubling of efficiency and -3 dB from the amp outputting half as much power)

The only way to make an arrangement like this work with a passive network is to have a woofer considerably more efficient than the mid-tweeter, and in your case you really need to bi-amp, as you have it, you probably loose more than you gain. To get the best out of the 126 you do need to filter the bottom end*ie a big cap.

dave

#### WithTarragon

planet10 said:

Just to clear up some confusion, this formula is for sensitivity (ie fixed voltage into the load/not fixed power (efficiency)) and is only valid if your amplifier is a perfect voltage source.

almost all of the gain is from working the amplifier harder.

(ignoring the response anomalies from the OB)

2 identical drivers in parallel with a perfect voltage source will output 6 dB more level (3 dB from the doubling of efficiency and 3 dB from the amp outputting 2x as much power)

2 identical drivers in parallel with a perfect current source will output the same level (3 dB from the doubling of efficiency and -3 dB from the amp outputting half as much power)

The only way to make an arrangement like this work with a passive network is to have a woofer considerably more efficient than the mid-tweeter, and in your case you really need to bi-amp, as you have it, you probably loose more than you gain. To get the best out of the 126 you do need to filter the bottom end*ie a big cap.

dave

Okay, I guess I am missing something. I take two drivers (of the same nominal impedance of 8 ohms). Driver A gets 1 volt and produces 85 dB SPL, while Driver B gets 1 volt and produces 93 dB SPL.

Well, if I have one sound source that is 93 dB SPL and I add a second source of 85 dB SPL, I will only measure 93 dB plus a fraction when both are played simultaneously. With this difference it does not matter whether the two signals are coherent or not.

Yes, I agree, if the the two sources were equal in SPL, then if the two signals were also correlated (equal SPL, phase and frequency) then I would measure a 6dB increase when palyed simultaneously. If they were different frequencies (uncorrelated) then there would only be a 3 dB increase.

Why am I (or someone else) confused on this?
-Tom

#### audiophile36

Well I went ahead and biamped the OB's. The 8 inchers are now subwoofers...I am using plate amps to power the woofers and have them crossed over at 120hz...I still have the Fostex 126e runing full range. I thought the sound was good running both drivers fullrange,but Biamped and crossed-over is on another level.

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