PS AUDIO: The bipolar mirage and why it is important

PSA

Paul McGowan writes:  The easiest way to picture how a bipole loudspeaker is configured is to imagine two full range Monopole speaker boxes back-to-back.  Sound comes out of both the front and the back of the combined boxes in this configuration and, if you connect them in-phase (red to red and black to black) you get a bipole.

The first bipole I remember was the Mirage series of loudspeakers back in the day when API of Canada owned Mirage and they were considered high end loudspeakers.  Mirage is today owned by Klipsch and I haven’t paid too much attention to where their product focuses are.

The advantages of a bipole are somewhat the same as a dipole (which we’ll discuss next) and their problems are as well.  Radiating sound out of the rear of the loudspeaker in equal proportions to the front might sound goofy – after all why do you need to fill the back of the room with sound?  Surely the sound pointing at your ears is most important?

The answer lies in using the room to your advantage.  If you take your loudspeaker pair out into the backyard and play them you’ll be pretty much underwhelmed with the sound and certainly disappointed in the imaging – there pretty much won’t be any.  This is because we need the room reflections to help us build a believable soundstage in our rooms.

Exciting the rear of the room at the same time we launch audio directly at us helps create an illusion of soundstage and depth we need to support the recordings we listen to.  Monopoles can create a lifelike soundstage in a room without launching this rear facing wave because at lower frequencies loudspeakers project all around them, the rear wall included.  But monopoles can’t project their tweeter’s energy at the rear wall and so their imaging can sometimes be trapped between the speakers.  To solve this, some monopole designers add a rear facing tweeter.  But back to Bipoles.

The problem with a bipole is something called comb filtering.  This is a fancy term that basically means some frequencies are cancelled, while others are reinforced or ignored.  Picture a comb as a frequency response graph where each of the teeth on the comb are frequencies at a proper level in volume – what you want – and the space between the teeth are cancellations or lower volume at the next higher or lower frequencies.

Comb effects can be lowered or worked with by clever design from the loudspeaker manufacturer and so the results are truly dependent on the skill of the designer.

From a user standpoint the Bipole presents some challenges not encountered with a standard Monopole loudspeaker.  Distance from the rear wall (it needs space to work) and treatment of that rear wall are critical elements in successfully integrating a Bipole into your room to take advantage of its added spatial properties.  The same can be said of a Dipole.

A Dipole is basically a Bipole but instead of the rear wave being in phase, as it is with the Bipole, the Dipole is 180 degrees different and that makes a huge difference in the way the speaker needs to be designed as well as how you place it in the room and what you do to the room to accommodate it.

EDITORIAL NOTE: The opinions expressed in the above post do not necessarily reflect those of our editorial team – just in case you wondered. Neil McCauley