If I wire two speakers in series, how much gain can I expect to get? - diyAudio
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Old 4th May 2006, 12:35 AM   #1
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Default If I wire two speakers in series, how much gain can I expect to get?

Sorry if this is a stupid question (I sort of think it is).

I'm now designing a simple WTW speaker. After your valuable help, I've been able to use nearfield + groundplane measurements to get the exact value for BSC, I've minimised reflections, and I'm feeling very confident in my new design.

That is... until I considered the following.

I'm using a pair of Focal 5N421 mid-woofers and a Neo3 PDR tweeter.

The way I measured the drivers was the following:

1) Mic in front of the tweeter.
2) Connect amp to tweeter; measure freq. response.
3) Without changing volume, measure freq. response of upper woofer.
4) Without changing volume, measure freq. response of lower woofer.
5) Without changing volume, measure freq. response of the whole speaker.

Now, I've properly calibrated the Z distance in LSPCad, and everything matches.

But, my question is:

Since I measured each speaker individually, wouldn't that mean that, when connected in series I shouldn't get any gain whatsoever?

As it is, LSPCad is telling me I'm gaining 6 dB from connecting both speakers in series. I assume that, since I measured individually each driver, there's no way for LSPCad to know about this.

Then again, I have compared my measurement of both upper and lower woofers with the simulation of LSPCad. And they are both the same in magnitude!

Is this OK? Can I expect +6dB when I connect woofers in series? I thought this was the case with parallel drivers.

Here's the LSPCad screen I'm getting... thanks for your help!
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Old 4th May 2006, 12:54 AM   #2
infinia is offline infinia  United States
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Hi
Looking at your graphs I see closer to 5 dB increase.
Theory predicts 3 dB increase due to adding double the radiating surfaces only. Not too sure where the 2dB error is from but since it is uniform in frequency, perhaps a distance calibration error?
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Old 4th May 2006, 01:11 AM   #3
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I break the problem into the acoustical (efficiency) gain and the impedance sensitivity difference.

Consider that for two drivers connected in series you have doubled the cone area so the acoustical improvement is +3 dB. But you have doubled the impedance (say 8 ohms to 16 ohms for 8 ohms drivers) so that the sensitivity difference is -3 dB. The net improvement is the summation of the acoustical and sensitivity which is +3 -3 = 0 dB change.

For parallel drivers the acosutical improvement is +3 dB (doubled cone area) while the impedance is halved so that the impedance sensitivity change is +3 dB. The total is +3 +3 = 6 dB.

The usual caveat is that their sound fields must overlap. This usually means that their center to center spacing of the drivers must be less than one wavelength at their highest freqeuncy of operation.
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Old 4th May 2006, 01:20 AM   #4
infinia is offline infinia  United States
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Jim you are correct in everything you say, but look closer at the test. The circuit never changes only the mic position.
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Old 4th May 2006, 03:05 AM   #5
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Quote:
Originally posted by infinia
Jim you are correct in everything you say, but look closer at the test. The circuit never changes only the mic position.

I should have made that clearer - I never moved the mic from its position in front of the tweeter. Neither did I change the distance in JustMLS. All I did was switch cables from every driver.

Jim, I'd assume your post to be correct: +3 dB from double the area, -3 dB from doubling the impedance, 0 dB net gain. But the measurements I took with both woofers in series are the same LSPCad predicted for the circuit.

It makes no sense to me!

Should I scale the driver's SPL data to -3 dB each? or -6 dB? Maybe I could scale both drivers' output to equal one driver output...
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Old 4th May 2006, 03:32 AM   #6
infinia is offline infinia  United States
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Hi
I would assume in this measurement case the woofers would be physically isolated from each other then. Can you try gated measurements at further distances. That is, try other measurements to gather clues.
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Old 4th May 2006, 05:12 AM   #7
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I dont use lspcad, so im curious what wooferSup and wooferInf are.
Did you label the upper and lower woofer that? If not is the program displaying something else?
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Old 4th May 2006, 02:09 PM   #8
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Quote:
Originally posted by nunayafb
I dont use lspcad, so im curious what wooferSup and wooferInf are.
Did you label the upper and lower woofer that? If not is the program displaying something else?
Oh, that's spanish for "Upper Woofer" and "Lower Woofer". I just labeled them as such.

I've given some thought about this situation, and I remembered I took a measurement with both woofers in series and the tweeter in parallel. I'd assume this is the "real" reference for the SPL levels between tweeter and woofers, so, as long as my simulation reflects these levels, I'll be OK... I mean, the real issue is the relative level between woofer and tweeter... so my "all drivers wired" measurement should be my reference level... right?
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Old 4th May 2006, 03:40 PM   #9
infinia is offline infinia  United States
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Quote:
Originally posted by fjhuerta


I remembered I took a measurement with both woofers in series ...............I mean, the real issue is the relative level between woofer and tweeter... so my "all drivers wired" measurement should be my reference level... right?

As long as you the "designer" feel comfort that your measurements are really reflecting theory. Then all is good. I would try to find the error by repeating tests with some better accuracy. Maybe try near and far microphone tests with out changing the woofer circuit. The difference should be +3dB with calibrating for distance losses. In addition, as Jim pointed out the tweeter referenced at the amplifier terminals will be 0 dB from the two series connected woofers far field. Clear as mud now?
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Old 4th May 2006, 04:20 PM   #10
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Quote:
Originally posted by infinia

The difference should be +3dB with calibrating for distance losses. In addition, as Jim pointed out the tweeter referenced at the amplifier terminals will be 0 dB from the two series connected woofers far field. Clear as mud now?
I think it's clear

I tried another new thing. I loaded the parameters for one woofer and saw the frequency response graph. Then I told LSPCad there were two identical woofers in series (in the driver configuration section). I got something like +3dB out of it. I then configured the drivers in parallel. The difference was +6dB this time. It still doesn't make any sense. By using the driver configuration section, LSPCad should know the difference in amplitude should be 0 dB with two woofers in series, but it simulates +3 dB. And, the weirdest thing of all is that this simulation equals my measured response.

I'm so confused, I'll take those measurements again!
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