2 Woofers in Parallel Config - 4 Options - diyAudio
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Old 20th November 2006, 09:38 PM   #1
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Default 2 Woofers in Parallel Config - 4 Options

I am building a woofer box. I will use 2 x ScanSpeak 26W8861 10" Revelators in a 100 ltr sealed box. They will be wired in parallel to give a 3dB higher SPL and reduce the cone excursion to half to help reducing linear distortions at high SPL.


Click the image to open in full size.


There are many options:

1. Normal configuration: Both woofers are in the front and they look the best.

2. Push-Pull Top-Bottom: According to the LDCB, this cancells odd order harmonic distortions. But the box is more difficult to design to make it look good in the front panel.

3. Push-Pull Front-Back: According to the LDCB, this cancells odd order harmonic distortions. But one woofer is mounted to the back so I am not sure if this is any good if crossed over between 100Hz to 300Hz. I guess this will also have some benefits as in 4.

4. Linn Olson's Parallel: This won't cancell any odd order harmonic distortions but will cancel the cabinet vibrations because the 2 drivers are applying the opposite force. This would have major benefits for the MTM sitting on top of the woofer box.

Now these are all in theory. Have you done any of those? which one do you prefer? Pros and Cons?

My preference would be 3. for XO below 100Hz and 2. for XO above 100Hz.

What do you think?

Best regards,
Bill
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Old 21st November 2006, 05:33 AM   #2
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I do push pull as 2 drivers facing each other in the same hole, similar to isobaric. design them on bass box pro. out of phase wiring.
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Old 21st November 2006, 08:43 AM   #3
lndm is offline lndm  Australia
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Wouldn't the third option cancel the driver's even order distortions, not the odds? (that is to say where a driver moves out differently to in)

ScanSpeak motors tend to have a symmetrical drive and so don't really produce much even order distortion and the only simple way to reduce odd order distortion is to keep the driver within limits.

Therefore, I might guess that your best options are either number one, or isobaric as Cameron Glendin suggested.


BTW, hey Cameron
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Old 21st November 2006, 09:47 AM   #4
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2 and 3 cancel even harmonics not odd. Otherwise all your thinking is sound.

I would not bother with isobaric unless you are short on space for the box/want a small box, as it wastes the SPL capability of one of the drivers.
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Old 21st November 2006, 10:05 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

No.4 is I'd say your best bet. Compliantly mounting the drivers
and adding a force cancelling linkage / rod is a possibility.

/sreten.
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Old 21st November 2006, 12:07 PM   #6
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(JPK) Hi Bill. Unfortunately configurations 2 and 3 do not cancel even order distortion as advertised. Even order distortion arises from asymmetries in the driver motor and suspension, and also from the air suspension. The compression/expansion process of the air in a sealed box is nonlinear and asymmetric and thus a source of even order distortion. If the drivers are mounted in free air, like a dipole or true infinite baffle then the push/pull configuration will cancel even order distortion. But when the drivers are in a sealed box both drivers are compressing or expanding the air in the box in the same manor regardless of the mounting configuration. Below is a set of 3 simulations of two drivers mounted in various way. At the far left two identical drivers with suspension asymmetry are mounted in a pull/pull (or push/push) configuration. In this case, only the driver nonlinearities are considered. The dark blue data is the result for one driver and the pink is the result for the sum of both drivers. The nonlinearity of the air suspension of the box is omitted from the simulation. The air spring is considered to be perfectly linear. The center figure shows the result for the same configuration, but with the drivers mounted in push/pull. As you can see, in this case where the air compliance nonlinearity is neglected, the even order distortion components are significantly reduced or eliminated as indicated by the difference between the blue and pink data points. This is the result that commonly leads to the conclusion that push/pull always reduces/eliminated even order THD. However, in the figure to the far right the configuration is identical to that of the center figure but the nonlinearity of the air in the box is considered. Note that the even order distortion, particularly 2nd order, returns and is almost as high as it was in the extreme right figure.



Click the image to open in full size.

Interestingly, if the drivers are mounted in push/push or pull/pull configuration, one configuration can result is lower even order distortion. This depends on how the asymmetries in the drivers interact with the nonlinearity of the box air compliance. Mounted in one configuration they will augment each other, as in the figure to the left below. In the other configuration the asymmetries in the driver tend to be canceled by those of the air compliance and the net even order distortion is lower.

Click the image to open in full size.

Obviously, these results depend on the degrees of nonlinearity and the driver excursion, but the idea that mounting driver in a push/pull configuration reduces or eliminates even order THD for a sealed box system just isn't correct.

Finally, your last figure, 4, also won't necessarily reduce cabinet vibration. It will result on no net force on the cabinet meaning the cabinet will not want to "walk", but panel vibration will still result from the driver motion and changing air pressure in the box. A rigid bar connecting the motor of both drivers would help cancel the forces applied directly to the baffles, but there would still be pressure variations in the box to excite the panels.
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Old 21st November 2006, 10:17 PM   #7
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Thank you all for your ideas. Much appreciated.

According to John K, and I am convinced, there is not a great advantage of using push-pull. Since I won't know if push-push or pull-pull will give lower distortions (driver dependent) until they are mounted in a cabinet and get measured, at this stage I guess I won't be using push-pull because of their odd look and difficulty in mounting one driver due to the depth of the cabinet. The biggest trouble is that in a sealed box, once one of the drivers is mounted, it can not be taken out again unless the back panel is removable.

So that comes down to only Option 1 or 4.

Although 4 does not necessarily eliminate / reduce cabinet resonance / vibration, it does prevent cabinet rocking to some extent so I guess the MTM above would still see less movements. But this may be a small gain and I have to consider other factors.

With Option 4, let's presume the crossover is at 200Hz for my MTMWW speakers. The wave length of 200Hz is 1.7m. Let's assume the cabinet depth is 600mm, considering a monopole with a 4pi radiation pattern, the sound of the back woofer would come in 127 degree delay. At 100Hz 64 degree delay. Would this cause significant sonic problem?

I can see for 80Hz below, there may not be much of a problem with the delay. As a matter of fact, I guess the back firing woofer, due to its close distance to the front wall and to the floor, would give a substantial dB gain. But at higher frequencies, this may not work.

Please correct me if I am wrong as I am in a learning process.

Best regards,
Bill
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Old 22nd November 2006, 06:14 AM   #8
Shaun is offline Shaun  South Africa
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HiFiNutNut

From what is said above, it seems that with good quality woofers there is not much advantage to deviating from the conventional driver configuration. Might I suggest that you turn your attention to the cabinet, then? There's lots of info on the forum and www about panel damping and/or bracing. Methods that I intend exploring myself (and have not yet tried) are matrix bracing and mechanical bracing of driver motors.
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Old 22nd November 2006, 07:23 AM   #9
lndm is offline lndm  Australia
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Old 22nd November 2006, 07:33 PM   #10
Svante is offline Svante  Sweden
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Quote:
Originally posted by john k... Obviously, these results depend on the degrees of nonlinearity and the driver excursion, but the idea that mounting driver in a push/pull configuration reduces or eliminates even order THD for a sealed box system just isn't correct.
Interesting simulations...!

So, the reason for the effect is that in some cases, assymetric nonlinearities in the cone suspension can cancel the nonlinearities of the air spring.

I wonder... How common is this in real life? You show simulations, but have you tried the corresponding thing with real systems? It seems to me that one would have to know quite a bit about the nonlinearities of the suspension in order to cancel it with an appropriate box volume.

Furthermore, often the main source of second harmonic is the voice coil inductance, and its nonlinearity. To my understanding, this cannot be linearized by the cavity non-linearity, since the nonlinearity of the inductance of only indirectly correlates with the forces that the current will generate.

Yet another point, the cancelling effect only works with closed boxes. For bass-reflex boxes the ratio of air compression and cone amplitude varies with frequency, and so the nonlinearity of the air spring and cone suspension cannot cancel, for all frequencies.

The reason I question the validity of the simulations* is that I have seen more than one measurement on real systems where the distortion was actually reduced when turning one driver inside out.

But, point taken, the nonlinearity of the air spring will remain. This only tells me that the box should not be made too small. Or put differently, the sound level inside the box should be kept reasonably low. I did some simulations ( ) on this myself, and as a rule of thumb, and if the cone motion would be completely controlled by the air spring, the 2nd harmonic distortion would be 1% if the SPL inside the box is 150 dB.

The bottom line here is, I think, that there are so many effects occurring at the same time that the net effect of them will have to be tested on real cases, and it is probably so that the best solution depends on the configuration and driver properties.

*I mean, the simulations are probably correct, but I would imagine that they represent a special case where the nonlinearities of the suspension and air happens to cancel. I don't think that happens that often in real life.
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