What's a bipole with front+back units?

[andyjevans]

Hi guys,

Could you in your infinite wisdom explain what we call a speaker that has two units moving together, one firing forwards and one firing backwards. The speaker units can also be braced by a solid pipe under pressure joining the two magnets to each other at the rear ends. I once heard such an arrangement and thought it sounded great.

I posted elsewhere about using PVC pipe, and one possibility might be a T section of pipe with the bottom of the T pointing into the down pipe and the ends of the T firing front and back. Hope I'm clear here! Could be done in an ordinary box of course - doesn't need pipe.

What are the pros and cons of this? In particular what effect does it have on bass response using two full-range speakers? Also, can I take it that front and rear units need to be identical, or can they differ? For example the rear firing unit having a lower fs value.

thanks

andy

[andyjevans]

Correction to the above - I think I mean dipole, i.e. when one speaker goes out the other goes in, so the cones are both going frontwards together and going backwards together.

Does this require an infinite baffle or can the box be ported/transmission line or whatever?

Again, can someone explain bipole/dipole in terms of bass response?

thanks

andy

[dshortt9]

Bipole is the correct term. Planet10 made a couple of speakers like this and I have as well. It works well for me and has given me the best bass of all I have built. Here is a brief explanation by Planet10
Transmission Line Speakers -- Technical Articles
If you search for posts by me you will find a picture of my speakers, 2 - 10" Klipsch drivers from an RF-7 per speaker, a simple 1st order series crossover to a Heil AMT tweeter. Best sound I have heard including high end commercial speakers. Of couse I'm biased. Can be driven by a single 300B but the 4 ohm impedance begs for just a bit more power if you are a metalhead.

[Scottmoose]

If you have a box with forward & rearward firing speakers wired in phase with each other then it will produce a bipolar radiation pattern. If the drivers are wired out of phase with each other, then it will give a dipolar radiation pattern (like an OB).

The latter is pointless as a box speaker if the drivers share the same volume because as one cone moves out, the other moves in, so there is essentially no air displacement / resonance / spring / whatever you like to call it in the box, which will essentially do nothing.

[dshortt9]

Both speakers should be identical. The box design can be whatever, TL, IB, or ported. Mine are ported.

[andyjevans]

I suspect Scottmoose is right here - I'm talking about a dipole with two speakers. But is it really true that the box does nothing? Does two speakers effectively imply Isobarik?

andy

[dshortt9]

Scottmoose is right in his definition of bipolar and dipolar behavior. You need to define which way each speaker is facing and how they are wired - in or out of phase. Try not to be ambiguous.
The T shaped design would be considered either ported if the bottom of the T iis open, Infinite if the bottom is closed. Two speakers require twice the Vb for correct operation. The T shape should only be used with the speakers in phase so the front one pushes towards you and the rear one pushes away from you at the same time. This is bipolar operation.

Isobarik is a design different than the above. The two drivers are placed either "cone to magnet" and wired in phase with one another or "cone to cone" or "magnet to magnet" and wired out of phase with one another so that their cones move together when driven with an audio signal, keeping the pressure of air in the chamber substantially constant (the "isobarik" condition). There is a sealed chamber between the two. The hidden speaker has either an infinite, ported, or T-line baffle.

[Scottmoose]

Indeed.

Think about it for a second Andy. To use a BR as an example, such enclosures use the air like a 'spring' coupling the rear of the cone to another 'cone' of air entrapped in the port area (to use Badmaieff / Davis's very simple description). That's all fine & dandy, but if you wire up two drivers out of phase in the same box volume, as one cone moves out by x amount, so the other cone moves in by the same amount. Result: no pressure change in the enclosure, so vent output will be nil, or as near as makes no difference. The same applies to any other enclosure type where the two drivers share the same volume if they are wired out of phase; you need a pressure change for a box to do anything: remove that, and it won't.

Re isobaric loading, as noted, is rather different (as an aside, you can add cone-to-magnet to that mix as well in some designs). Probably most famously used in the infamous Linn Isobarik.

[ultrakaz]

Hello, does anything happen to the impedence of the two drivers, sealed with appropriate volume and wired in phase?

[dshortt9]

Impedance is one half that of one driver in the selected Vb.
Assuming the two drivers are placed in a sealed box of twice the selected Vb.

If ported, use twice the Vb of one driver and double the ports, to make calculations easy. I'm not sure how one would calculate a single port unless your box design software has that option.

For a TL, you are on your own, but Planet10 has information on that.