Woofer distortion

[Hyldal]

What gives the lowest woofer distortion, if you have two woofers, playing at the same sound pressure level?

1. woofers close copled face to face (isobarik I guess its called)
2. one woofer facing normal out of the box, and another next to it mounted out of the box, with the magnet side out.

[ch83575]

In #2 are you wiring the driver in reverse polarity? Or, are you using it as like a "backward" woofer? Actually, I don't think it matters, seems to me that #2 doesn't make much sense as there is no air spring if you wire it in reverse polarity like an isobaric (the box would be constant pressure). If it is positive polarity then it would make much less distortion and noise if you just turned it around the right way and called it another woofer.

Maybe I am missing something here, but it seems like #1 would have isobaric distortion cancelation and #2 really doesn't have any benefits at all. In both ways you are using the woofers back as the radiating source, and while I have never read conclusive testing to support that it causes any problems at low frequency, it just seems like a bad idea to me. What about mounting them in an isobaric cavity magnet to magnet? Remember... you still need a box, separate from the isobaric constant pressure space, or else you are doing some kind of iso-OB... maybe that would be cool...?

[sreten]

Hi,

2 can play 6dB louder than 1 so at the same level the answers always 2.

/sreten.

[Hyldal]

What I mean by number 2 is that bothe woofers are on the front baffel but one of them have be put with the magnet facing out.

I guess this will reduce even order distortion? and at the same time you have two driver to move air so at the same suündpressure level, distortin wil be lower?

[Svante]

Originally posted by Hyldal
What gives the lowest woofer distortion, if you have two woofers, playing at the same sound pressure level?

1. woofers close copled face to face (isobarik I guess its called)
2. one woofer facing normal out of the box, and another next to it mounted out of the box, with the magnet side out.

This is not an easy question.

Mounting the drivers in opposite polarity reduces even order harmonics, both in the isobaric case and side-by-side.

Allowing both woofers to push air out of the box (as in side-by-side mounting) reduces the cone amplitude and thereby the distortion.

On the other hand, keeping the same box volume and mounting the drivers side-by-side results in a different frequency response which makes comparison difficult. If bass is lowered, should distortion be measured at that lower level?

Anyway, in Basta! one can simulat a maximum output level (MOL) based on a number of citeria that must not be exceeded, such ac cone amplitude, vent velocity etc. Ultimately, exceeding these critera leads to distortion, so studying the MOL curve says something about the distortion in the systems.

Here are three systems using a box volume optimized for a single driver. The thin line is the MOL, the dashed line is the cone amplitude.

It can be seen that the side-by-side mounting never has a lower MOL than the other systems, indicating that distortion would be lower for that mounting. Then again, the MOL in the Basta! simulation disregards any positive effects of push-pull mounting.

On the other hand, the side-by-side mounting really requires a larger box volume; the response is rather poor. Then again comparing differently sized systems with regard to distortion is IMO sort of unfair. It is almost always easier to make a large system with less distortion.

[AndrewT]

Hi,
does using a larger cone area inherently have less distortion due to lower amplitude for the same SPL?

[FullThrottleRic]

It depends a lot on the motor design of the driver as to how much, but as a general rule yes

[Svante]

Originally posted by AndrewT
Hi,
does using a larger cone area inherently have less distortion due to lower amplitude for the same SPL?

I'll rephrase that: Getting a larger cone area by adding a second driver results in less distortion at the same sound level due to less cone movement.

Extending the cone area of one driver by increasing the cone diameter modifies so many other things in the driver so it is hard to tell for sure.

The above is for low frequencies. At high frequencies, the cone amplitude is not the problem.

[sreten]

Originally posted by Hyldal

What gives the lowest woofer distortion, if you have two woofers,
playing at the same sound pressure level?

1. woofers close copled face to face (isobarik I guess its called)
2. one woofer facing normal out of the box, and another next
to it mounted out of the box, with the magnet side out.

Hi,

the other answer to the question is any cancellation of 2nd harmonic
distortion is ~ the same with the caveat that some drivers linearity
is designed to react favourably with box air compression non-linearity
and reverse mounting one / isobaric may kibosh this optimisation.

/sreten.

[ch83575]

Originally posted by sreten


Hi,

the other answer to the question is any cancellation of 2nd harmonic
distortion is ~ the same with the caveat that some drivers linearity
is designed to react favourably with box air compression non-linearity
and reverse mounting one / isobaric may kibosh this optimisation.

/sreten.

I agree, but we are all still talking about problems in compliance dictated frequencies. What if the woofer is in a 2-way? I would think it would be rear masking, diffraction and excursion noise that I would be most worried about. I would think that these problems would kick in at fairly low midrange frequencies so it might even be a problem is some 3-way designs depending on crossover frequency. I mean, lets face it, the back wave of a driver is not what the designers meant us to be listening to.