Dipole loudspeakers – an uncomplicated explanation

Paul McGowan writes 

Let’s cover dipoles today, what they are, why I like them and then we’ll move back to the rebuilding of the new PS Sound Room as we ready ourselves for the most famous dipole/monopole hybrid of all time, the legendary Infinity IRS that we’ll be installing soon. BTW, I posted the first video of this journey on YouTube for any of you that would like to watch the setup process.  You can access it here http://youtu.be/LJDv3-5ArIs this is the intro and then we’ll have another update this weekend to watch.

The last Monopole loudspeaker I owned was a pair of Revel Salons of which I was very happy with for quite some time.  In fact, it was the only Monopole I have ever owned, starting out my audio career with Magneplanars, Acoustats, Martin Logans, Infinity RS-1′s – all dipoles or dipole hybrids until the advent of the Revels.

Every speaker I have owned, with the exception of the original Magneplanar MGII’s, had subwoofers coupled to them for extended bass.  This bass has always been a pair of self-powered sealed box Monopoles and I would not consider owning a pair of loudspeakers that tried to do everything in one box.  Certainly there can be successful systems without subwoofers in separate cabinets, but rooms are so picky when it comes to supporting proper bass at the listening position that a separate box subwoofer is almost always the way to go.

A dipole is simply a loudspeaker that radiates sound both from its front and the rear at the same time.  Unlike a Monopole which radiates only from the front, or a Bipole that radiates front and back – but in phase – the dipole front and rear radiators are always out of phase.  Out of phase simply means that as one surface is pushing the air forward, the other surface is pulling the air back towards it – or put another way – one surface is creating greater pressure in the room while the other surface is doing the opposite, creating less pressure.

If you point two drivers at each other – one creating greater pressure while the other is lowering pressure by the same amount – the net result is zero pressure change in the room.  Imagine a balloon with two mouth pieces, one at each end of the balloon.  When the one side is blowing air into the balloon, the other is removing an equal quantity from the balloon – and then that process is reversed – thus the net volume of air in the balloon never changes.  This is called phase cancellation and typically it isn’t what we want.

Because all good sound system work in a room and depend on the room for the sound quality, dipoles can be a double edge sword for setup.  The ideal setting for a dipole is away from the back wall far enough so the rear wave of sound pressure doesn’t cancel out the front wave of direct sound.  One might think that you would be better off damping and absorbing the rear wall reflections to a point where there is no possibility of sound cancellation from the back to the front; but then one would be wrong.

Dead overly damped rooms are not good for sound creation (as I mentioned yesterday).  The best rooms use room reflections to their advantage and dipole loudspeaker setups are dependent on a rear, reflective surface that has been properly diffused to work correctly.

Remember in our earlier discussion I spoke of comb filtering?  Diffusion and distance from the rear wall can lower and even eliminate comb filtering in the listening room.  This is because a diffused back wave isn’t specific enough to cancel out the front wave at any frequency, thus lowering and limiting any comb filtering effects.

Monopole loudspeakers have no back wave at higher frequencies – then a growing back wave as the frequency goes down – thus creating an odd mix of monopolar response all the way down to omnipolar in the bass, and so their ability to create a lifelike soundstage at all frequencies becomes very difficult – in fact to my ear – Monopoles almost never create a true wall to wall soundstage that is behind the loudspeaker pair properly.

Sure they can create very lifelike imaging – Wilson’s are a great example of superb Monopole imaging – but the soundstage is somewhat trapped between and behind the speaker pair, and rarely wall to wall and beyond as you can get with dipoles.