Woofers working together, +3dB or +6dB?

[freos]

I'm a bit confused right now.

I've always thought that if you add two woofers with the same specifications you get a +3dB gain. That's consistent with the results I get in WinISD and sources as this one:
http://www.epd.gov.hk/epd/noise_educ...1/intro_5.html

However, right now I'm putting together a crossover. I'm using Passive Crossover Designer ( http://www.pvconsultants.com/audio/crossover/pcd.htm ) and I have the proper files for the drivers.

When using one woofer and one tweeter, and looking at the numbers, I see that at the crossover point I get +6dB (e.g. the woofer is at 86 dB, the tweeter at 86 dB and it adds up to 92 db). This is consistent with information about audio crossovers from Wikipedia, see this image:
http://en.wikipedia.org/wiki/File:Li...utterworth.png

So, how come adding woofers in WinISD and "common knowledge" about how sound sources works together says +3dB and crossover facts says +6dB?

Things get even more confusing when I directly in Passive Crossover Designer choose to use 2x woofers and get +6dB compared to one woofer all the way from 10 Hz up to the crossover point, where in WinISD the same combination of two woofers gives a +3dB compared to one woofer.

Can anyone straighten this out for me?

[pjpoes]

I believe the reason for this confusion is that you gain 3db's for the increased woofer and 3db's for the halving of impedance. If you wired the woofers in series instead of parallel you should get no gain. Let's say you have an 8 ohm driver, if you wire it in parallel, you have the two woofers and 4 ohm impedance, so as compared with the other drivers, that driver is drawing more current, and thus, has a higher voltage sensitivity. Two 8ohm woofers in series will be 16ohm, the voltage sensitivity will be massively reduced, but you still get the 3db increase from the two woofers, giving a net gain of zero.

I'm sure there are others who can speaker more specifically about why this is, but as far as designing crossovers are concerned, that's really all you need to be concerned with. Oh keep in mind that if using the woofers for midrange as well, series will double the inductance, parallel halves it. The response will change with these two wiring configurations. For bass only it won't likely matter much (you should be using actual measurements anyway), but for midrange it will.

[Saturnus]

Well, it's a common problem that many get confused over so don't worry.

Connecting 2 drivers on the same output naturally results in a doubling of the sensitivity, meaning a +3dB gain. It doesn't matter whether you connect the speakers in series or parallel, the impact on sensitivity is the same.

However, when you parallel connect 2 driver you halve the impedance which means that for the same voltage output of the amplifier the power will also double, meaning another +3dB gain.

And naturally the opposite is true if you series connect 2 drivers and thereby double the impedance and halve the power on the same voltage output, meaning a -3dB loss.

The difference between series and parallel connections only come into play if your measuring equipment measures in volts and not in watts.

What it really means is that the same amplifier can produce 6dB higher SPL, or 4 times as loud, on 2 drivers connected in parallel as on a single driver or 2 drivers connected in series.

The differences when you calculate crossovers should be obvious. If you have a working system with a tweeter and woofer which is balanced for sensitivity, and you suddenly decide to add another woofer then you will firstly add +3dB for having 2 drivers on the woofers output. Now depending on whether you series or parallel connect you will either get a -3dB loss or a +3dB gain (or top of those +3dB from before, remember). This is because the woofer section will now draw half or twice as much power in relation to the tweeter section which is unchanged.

[pjpoes]

wait now, that's not fully accurate, it does matter in crossover design. If you wire them in one way over their other, the woofers will be seen by the amplifier differently as compared to the other drivers. If you don't account for this, your crossover will be wrong, the bass being either too high or two low.

[bumpyride]

A pretty good exlanatin:

http://www.linkwitzlab.com/faq.htm#Q21

[freos]

Aha, I get it now, thanks for the replies!

The numbers I get from Passive Crossover Designer are based on constant voltage, while the numbers from WinISD are based on constant "wattage" (is that even a word?). I.e. 2.83V into 8 ohms is 1W, but into 4 ohms it is 2W, hence the other 3 dB gain.

I had to sleep on it, but now it all makes sense

I'm just used to thinking everything in "@1W/1m", but I'm well aware of the fact that lower impedance = more power if the voltage is the same. I just didn't link those two together yesterday and it was driving me crazy. Great to get help with this kind of stuff when you get stuck!

[navin]

Putting this theory into practice.....

Focal uses 3 woofers in their Electra 1038 Be 2 / 836V floor standing speakers.
Focal Electra 1038 Be: High end floorstanding hifi loudspeakers
Focal Chorus 836 V floorstanding hifi speakers for high end Home Audio system

I am hoping to build something similar using drivers from Wavecor.
Woofer: WF182BD03_04
Mid-woofer: WF120BD03_04
Tweeter: TW030WA05 to 08

If we took 3 4 ohm woofers in series we'd get 12 ohms. 2.83V at 12 ohms = 0.67W.

Each woofer is 91db@2W (2.83V @ 4 ohms) hence at 0.67W each woofer would be see 0.9V (0.22W) and hence would only be puttting out 81.5db (91-9.5db). The combination of 3 wofoers would hence be delivering around about 86.5db (81.5+4.77db) right? Hence this would match the 8 ohm WF120 midwoofer. Am I correct in assuming this?

Would a better way of looking at this would be that at 8.49V (3x 2.83V) each woofer would see 2.83V (aka 2W) and be putting out 91db each and about 95.8db (91+4.77) as a group. At 8.49V the 8 ohm mid would see 9W and hence be putting out 95.5db (86+9.5) and so would be well matched to a group of 3 4ohm woofers. Does this make sense?

What am I doing wrong?

[navin]

Originally Posted by navin

I am hoping to build something similar using drivers from Wavecor.
Woofer: WF182BD03_04
Mid-woofer: WF120BD03_04
Tweeter: TW030WA05 to 08

I could not edit my older post but I had a related question.

Given that I will be using the 7" only till about 300Hz should I use the Wavecoor Woofer or Subwoofer?
SW182BD01_02
WF182BD03_04

The woofer's inductance is less than 0.1mH while the subwoofer's Le is 1.5mH. Also the BL/mm ratio of the subwoofer is 1/3 that of the woofer.

[wolf_teeth]

Originally Posted by pjpoes

wait now, that's not fully accurate, it does matter in crossover design.

What Saturnus said is all true. The level or mode of wiring does matter in the xover design to have the tweeter (and now multiple woofers) remain balanced with the bass. Therefore changing the woofer section will possibly require an alteration of the tweeter section. Saturnus just did not mention that.

Later,
Wolf

[whgeiger]

See Message #8:
Mutual coupling question
Regards,
WHG