Dipole Loudspeakers

A dipole radiates equally forwards and backwards and has a null at the sides, and it keeps that pattern at every frequency. A box loudspeaker does not: at low frequencies it radiates in all directions, and it only becomes directional higher up, once the baffle and the driver are large compared with the wavelength. Its polar pattern — and with it the amount and the spectral balance of the energy it puts into the room — changes as frequency rises.

That matters because in a room you hear the direct sound plus the reflections that follow it. Floyd Toole, Sean Olive and other researchers found that listener preference ratings are consistently higher for loudspeakers with a uniform polar pattern: the reflected sound then carries the same spectral balance as the direct sound, instead of being a coloured copy of it.

What is a dipole speaker?

A dipole speaker radiates from both sides of the driver. The front and the rear send out the same sound in opposite phase: when the cone pushes air forwards, it pulls it from behind. To the sides the two waves cancel, so the pattern seen from above is a figure-of-eight, loud to the front and back and almost silent at ±90°. The simplest dipole is an open baffle: a driver on a flat board, with no box behind it.

A box speaker works the other way. The box swallows the rear wave, and the front radiates in all directions at low frequencies. That box also has to be damped, and whatever leaks out of it colours the sound. A dipole has no box to colour, and it puts less sound into the side walls, the floor and the ceiling, so the room adds less of its own.

Every driver a point source

The Baffless system is a 4-way design in which each driver works only around its first dipole peak — the range where its baffle is still small compared with the wavelength being reproduced. Each driver therefore behaves as a true acoustic point source and keeps a proper figure-of-eight pattern. An open baffle wider than the wavelength stops being a dipole: the pattern breaks up, off-axis response becomes frequency-dependent, and the power radiated into the room no longer tracks the direct sound. That is why there is only the bare minimum of material around each driver.

Measured directivity

Measured directivity in free field, 20 Hz–20 kHz, with frequency-dependent windowing at 1/6.9-octave resolution. One trace per horizontal angle, −180° to +180°; the valleys are the ±90° directions, where the system radiates virtually nothing.

From 20 Hz to 5 kHz the measured directivity index stays between 4.6 and 6.3 dB — a textbook dipole is 4.8 dB. Higher up it stops being a plain figure-of-eight: around 7–9 kHz the pattern widens and the index falls to 3.0–3.3 dB, and from 12 kHz upwards the main lobe narrows and side lobes appear, taking the index to 7.6 dB at 12 kHz and 10.2 dB at 20 kHz — the range where the driver itself is large compared with the wavelength. What does not change is the front-to-back symmetry: the rear lobe stays comparable to the front one all the way to 20 kHz.

Measured horizontal polar response at eleven frequencies from 20 Hz to 20 kHz, free field, each plot labelled with its directivity index. The figure-of-eight and its ± 90° nulls hold from 20 Hz through the midrange; above 7 kHz the lobe structure becomes finer, but the system is still radiating as a dipole at 20 kHz.

Dipole all the way, including the bass

The Baffless system is dipole across the entire frequency range — from 20 Hz to 20 kHz. This is unusual. Most open baffle designs use a conventional box for bass and go dipole only for mid and high frequencies.

Dipole bass is inefficient, and maximum SPL will not compete with a monopole — but in a normal-sized room that is rarely the limiting factor, and it is exactly in small rooms that dipole bass is the cleaner of the two. It is more articulated, because a source with nulls at its sides couples far more weakly to the room's modes. Elias Pekonen's modulation-transfer analysis of dipole versus monopole bass shows the mechanism behind why it sounds so natural.

The same nulls change what the room contributes overall. With virtually no output at ±90°, side-wall reflections and the modes those walls support are simply not driven. The wall behind the speakers becomes the surface that matters, and its delayed reflection adds a sense of space rather than smearing the stereo image.

Questions about dipole speakers

How do dipole speakers work?

Front and rear radiate the same signal in opposite phase. The waves cancel at the sides, which gives the figure-of-eight pattern. Because the two waves also cancel each other at low frequencies, the bass falls off and has to be equalised; in an active system with DSP that is a fixed filter.

Dipole vs open baffle: what is the difference?

Open baffle is the construction: a driver on a flat board without a box. Dipole is the radiation pattern. An open baffle behaves as a dipole only while the board is small compared with the wavelength; a baffle wider than that stops being a dipole and its off-axis response becomes uneven. That is why each Baffless driver works only in its own dipole range, with the minimum of material around it.

Dipole vs bipole vs monopole?

A monopole is an ordinary box speaker: it radiates forwards, and in all directions at low frequencies. A bipole radiates front and back in phase, so it has no side nulls and sprays sound everywhere. A dipole radiates front and back in opposite phase and has deep nulls at the sides.

Are dipole speakers good?

For music in a normal room, yes. The pattern stays the same across the range, so the reflected sound has the same balance as the direct sound, and research by Floyd Toole and Sean Olive found listeners prefer such uniform behaviour. Dipole bass couples weakly to the room’s modes and sounds more articulate. The cost is efficiency: maximum SPL will not compete with a box of the same size.

How low can a dipole speaker go, and how loud?

Every speaker can produce some output at 20 Hz. Even a tweeter can, only so quietly that nobody would hear it. So the real question is not whether a speaker reaches 20 Hz, but at what sound pressure level it plays the whole range from 20 Hz to 20 kHz. Loud deep bass needs a large volume of moving air, that is, cone area multiplied by excursion. A dipole needs even more, because its front and rear waves partly cancel at low frequencies, so dipole bass also needs equalisation and more amplifier power. The Baffless system is sized for this: it stays a dipole from 20 Hz upwards, and the measured figure-of-eight holds at 20 Hz. See the Dipole System.

Where should I place dipole speakers?

Away from the wall behind them. The rear wave reflects from that wall, and the further away it is, the later the reflection arrives and the more it adds space instead of blurring the image. As a starting point, about 1 m or more from the front wall. The side walls matter much less, because the sides are where a dipole is silent.