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Push-pull woofers

Posted by Wayne Parham (M) on May 18, 2003 at 13:04:37

In Reply to: Re: PPSL posted by djk on May 18, 2003 at 04:20:25:

Hello again Dennis!

This configuration looks interesting, and I'm starting to look at it closely. The π cornerhorn can easily be built with dual woofers and it has a plenium of sorts. Since the woofers aren't exposed, it would not be as unattractive as a baffle-mounted setup. Then again, baffle-mounted speakers can be built that have one woofer only run to 100Hz or so, to reduce interference anomalies in the lower midrange. Those are just a couple of my thoughts for implementations of this configuration in addition to the plenium you've described.

You wrote:

> > EV no longer has the datsheets for the MTL4 on line, and the
> > KEF 104 has been discontinued for awhile too.

I assume these two manufacturers used this configuration, yes? I wonder why these models were discontinued. It might have been aesthetics - Speakers made this way might be perceived as being "peculiar looking."

A quick search on the internet shows that EAW is using the push/pull woofer configuration , and I also notice that they use some other design techniques that I personally find attractive.

I appreciate your discussions here and I'll check into this some more.

I wrote:

"But the thing I'm wondering about is how do you get the anomalous harmonics to be 180 ^o out of phase while having the fundamental in phase."

And you replied:

> > The wavelength of 200hz is ~68", my first model had the magnet
> > structure of the inverted driver only 1" away from the front of
> > the other cone. The cones are so close together and the
> > wavelengths so long that it still subtracts.

I wasn't worried about distance problems shifting the phase because the frequencies of interest are second harmonics of the woofer's resonant frequency region. You've rightly shown just how long these wavelengths are.

More what I was concerned with was a confirmation that the second harmonics would be generated exactly in phase with the fundamental as opposed to 180 ^o out of phase. I wouldn't expect any other phase relationship, but it would not be unreasonable to expect either or both variants, and I'm not sure at what proportions.

For example, if the suspension were more resistive in the forward direction than the reverse on one cone and just the opposite on the other, then I would expect second harmonics to be of opposite phase in these two speakers. In that case, it would actually perform better to have the woofers connected in the "normal" configuration, as this would cancel the harmonics due to their opposite phase relationship.

Then again, this would be an example of poor quality control and I'm not sure that's the sort of issue we're dealing with here. I suspect it has more to do with non-linearities near excursion limits, and where the amplitude of the voice coil's magnetic polarity becomes significant.

I wrote:

"Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders."

And you replied:

> > The 3rd harmonic reduction is a function of the low-pass behavior
> > of the plenum. IE: if the plenum acts like a 12dB low pass at
> > 200hz it will roll off the 3rd harmonic of 100hz by roughly 6dB.
> > Obviously no useable reduction can occur much below 80hz or so.

Yes, the configuration can do nothing about odd-order harmonics. This is very similar to a complementary-symmetry amplifier configuration, which have benefits in the reduction of even-orders. I understand what you're saying about the acoustic chamber, but in regards to the summing of the two speakers, we're talking about even-order improvements here.

Wayne

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