'isobaric' tweeter mounting for decreased distortion

[sardonx]

Sorry i know this would not techincally be called isobaric but apparently it got your attention.

When you use isobaric configuration with a woofer if you mount them one behind another you significantly decrease 2nd order harmonic distortion by eliminating magnetic irregularities. I would think the same could be done with a tweeter. If you had 4 identical tweeters you could make a stereo pair by mounting them back to back and reversing polarity of course like in isobaric config. Has anyone done this?
I think i will try this when i eventually get another pair of the tweeters I have.

Although i do realize that many people dont want to decrease 2nd order harmonic distortion.

This gives me another idea though. If this was done with a conventional speaker box the rear tweeter would be firing into the box.. which would be fine, it woudn't be that hard to damp it, it's only high frequencies. But say you were making a dipole speaker and both the front and rear firing tweeters were in free air - wouldn't this be a more true dipole design than most designs which use just a front firing tweeter? I have seen box designs which have rear firing tweeters but never a dipole.

Someone please correct me if i'm wrong in any of these assumptions.

[geolemon]

I had always understood isobaric mounting to iron out suspension nonlinearities...
And only (as you say) when mounted face-to-face (or back-to-back)...

The suspension nonlinearities make sense to me...
In a clamshell-style isobaric (or back-to-back), there's always one cone moving out of the basket, and one moving in, regardless of the phase of the signal.

But this is simply a benefit of a push-pull arrangement.. two woofers, non-isobaric can do the same thing...

How does the magnetism come into play?

[GRollins]

Regarding woofers: Isobaric loading means that the pressure generated by one woofer loads into a small chamber which then loads into the second woofer.
Since most tweeters are sealed, you're going to find it difficult to get the pressure of the two drivers "together," if you follow what I mean. You'll need something like an open frame ribbon tweeter, or perhaps a tweeter with a vented pole piece (though those are usually tuned, and I'd advise against trying to play games with the tuning).
Isobaric loading can be used to reduce distortion, but it can also be used to increase the apparent volume of an enclosure.
I'm not quite sure where you're going with the dipolar tweeter question; if the tweeter is front-firing, then by definition, it's not dipolar. That said, and assuming that you have on hand some open-back tweeters, you could have an Isobaric dipolar tweeter.
The problem that you'll have to watch out for is that high frequency wavelengths are much, much shorter than their low frequency counterparts. Well, duh, right? But...with Isobaric woofers, the chamber between the two drivers is very small--only a small fraction of a wavelength. Ah, but at higher frequencies, you could run into resonances within the connecting chamber, due to the very short wavelength. I'm not saying that it wouldn't work out, but you have to be very careful, indeed, with the cabinet design.

Grey

[Nappylady]

If you try this, you will find the effect to be completely unnoticeable and unmeasurable.

This kind of cancellation happens with the actual sound waves, not the magnetic fields. I discovered this when I built dipolar full-range speakers that had a very nearly sealed back. The cancellation of even-order harmonics didn't work, and there was this terrible comb filtering, moving from the top of the speaker to the bottom.

Since tweeters are generally strictly cardioid, the effect you mention simply will not happen with them.

If you want to eliminate the second-order distortion in a tweeter, you could always make dual voice coils, like in the newer Adire woofers. See http://www.hometheaterforum.com/htfo...hreadid=136939 . (Someone correct me if I'm wrong!)

Good luck, in any case.

[sardonx]

Grollins and Geolemon

You two have seriously misunderstood what i'm talking about. Lets take this one topic at a time. My bad for cramming too much into one post.

1. I know what isobaric is and what it does in a woofer application. By isobaric tweeter configuration i meant that just two tweeters are mounted magnet to magnet. Normal closed back, dome tweeters - that's all. This is not an isobaric configuration since it has nothing to do with changing compliance and changing the volume of an enclosure. That's why the first thing i said whas "Sorry i know this would not technically be called Isobaric BUT..."

2. My proposal. Most dipole speakers have one tweeter and one woofer mounted in free air, or in an open baffle if you will. The woofer has an open back while the tweeter doesn't. This is a fault in itself, wouldn't you agree? To fix this, just MOUNT ANOTHER IDENTICAL TWEETER BEHIND THE EXISTING ONE AND REVERSE THE POLARITY. VOILA! NOW YOU HAVE A TWEETER SYSTEM THAT FUNCTIONS IN AIR JUST LIKE THE WOOFER THAT IS RIGHT BELOW IT.

Now do you see how this will make a more true dipole than one with just a single tweeter?

[sardonx]

Nappylady
You made your post while i was still replying to the first two!

Thanks for replying though... although I think i'm more confused now than i was before.

[sreten]

Originally posted by sardonx
Grollins and Geolemon

You two have seriously misunderstood what i'm talking about. Lets take this one topic at a time. My bad for cramming too much into one post.

1. I know what isobaric is and what it does in a woofer application. By isobaric tweeter configuration i meant that just two tweeters are mounted magnet to magnet. Normal closed back, dome tweeters - that's all. This is not an isobaric configuration since it has nothing to do with changing compliance and changing the volume of an enclosure. That's why the first thing i said whas "Sorry i know this would not technically be called Isobaric BUT..."

2. My proposal. Most dipole speakers have one tweeter and one woofer mounted in free air, or in an open baffle if you will. The woofer has an open back while the tweeter doesn't. This is a fault in itself, wouldn't you agree? To fix this, just MOUNT ANOTHER IDENTICAL TWEETER BEHIND THE EXISTING ONE AND REVERSE THE POLARITY. VOILA! NOW YOU HAVE A TWEETER SYSTEM THAT FUNCTIONS IN AIR JUST LIKE THE WOOFER THAT IS RIGHT BELOW IT.

Now do you see how this will make a more true dipole than one with just a single tweeter?

1) the reduction of second harmonic distortion is due to the
moving parts not the fixed magnets. It works with drivers
mounted face to face (clamshell style) and woofer in parrallel
but one physically mounted in the opposite direction.

2) The TLB design has this configuration, its not popular with
commercial manafacturers for obvious commercial reasons.
Its also possible to wire them with bipolar connections, in
this case the vibrations from the two units would cancel.
Some dipole midrange speakers have tweeters with a vented magnet.


sreten.

[GRollins]

Having one tweeter facing forward and another backwards is old as the hills. You can wire them in phase to simulate an omni-directional source, or out of phase to simulate a dipole, depending on your philosophy.
I think you'll have a better chance at discussing the idea if you drop the term Isobaric; that only contributes confusion.

Grey

EDIT: Note that simply having two tweeters back-to-back will do nothing to decrease distortion, except possibly in the sense that each may have its cone excursion reduced for equivalent SPL.

[geolemon]

Originally posted by GRollins
Having one tweeter facing forward and another backwards is old as the hills. You can wire them in phase to simulate an omni-directional source, or out of phase to simulate a dipole, depending on your philosophy.
I think you'll have a better chance at discussing the idea if you drop the term Isobaric; that only contributes confusion.

Grey

EDIT: Note that simply having two tweeters back-to-back will do nothing to decrease distortion, except possibly in the sense that each may have its cone excursion reduced for equivalent SPL.

I agree, the use of the term Isobaric (which means constant pressure, in reference to the trapped ambient-pressure air used to 'couple' the two drivers) is confusing... because it's not isobaric.

And the two strategies mentioned - pseudo-dipole or pseudo-omnidirectional - both seem very feasible.

I've also seen a couple tiny drivers... 2" cone units that reach up to nearly 20,000Khz (I'd bet there are some out there that do - my experience with these teeny drivers is nil)...
...you might be able to use some of these type of drivers to do a truedipole, which would even eliminate your original concern.

[BTW]

Originally posted by sardonx
Sorry i know this would not techincally be called isobaric but apparently it got your attention.

I think i will try this when i eventually get another pair of the tweeters I have.

Although i do realize that many people dont want to decrease 2nd order harmonic distortion.


Someone please correct me if i'm wrong in any of these assumptions.

I think Mirage or Definitive have speakers which have a tweeter firing backwards at the same time. They call it a Bipole, which is not to be confused with Dipole ie single driver in OB configuration.