Dipoles have been a matter of discussion for some time; in the following a comprehensive interview covering many angles.
Gerd Lommersum interviewed by
Peter Strassacker (9/2005)
Peter: Gerd, what is the difference between a dipolar and a conventional loudspeaker?
Gerd: A dipolar speaker also radiates rearward. Therefore, the room plays a more important role. The rear radiation is reflected by the walls and is then received by the listener. The other extreme is a close range monitor with almost point-source characteristics. Is the listening position close to the speaker, then the room influence is relatively low.
Peter: How does a dipolar speaker perform dependent on the placement in the room?
Gerd: As I said earlier, the dipolar speaker also radiates rearward; the reflection by the wall is then received by the listener. The reflected portion of the sound is dependent on the distance between speaker and the wall and on the absorbing properties of the reflecting surface.
Peter: What are the important aspects?
The reflections shouldn't reach the listener's ears to soon after the direct
radiation has been received. Therefore, placing the speakers too close to
the wall doesn't make sense. A distance to the wall of at least 1 metre is
Peter: Should, therefore, a dipolar speaker be positioned freestanding?
Gerd: Yes, considering a typical dipole this makes a lot of sense. But one should also employ the reflecting walls, especially the rear one.
Peter: What design characteristics should be considered?
The classic dipole (electrostatic planar transducer) radiates the same signal
forward as backward, but with inverted phase. It's totally different when
using cone drivers as dipoles. The magnet obstructs a large part of the diaphragm,
the basket hinders rear radiation, causing compression. Here is an example
using the Vifa 10 BG 120 in a 200 mm baffle
Picture above: it's clearly noticeable that the rear frequency
response is totally different. There are peaks at 1.5 and 2.5 kHz and there
is a level drop with regard to high frequencies with peaks at 7 and 20 kHz.
If money is no object, the dipole should have a fully adequate rear radiating driver as well, i.e. there should be two drivers in push-pull configuration.
Peter: How strong should the rear radiation of the dipole be?
When the distance between speaker and walls is considerable, then the rear
radiation level should be similar to the front radiation. When the distance
is smaller, then the rear radiating level should be reduced. Ideally, the
rear radiation should be adjustable by the listener. But that's only possible
when a separate rear radiating driver is employed.
Peter: What frequency dependencies are important?
Gerd: At low frequencies dipoles have a considerably lower efficiency (see details). ). The advantage is a lower room resonance.
At high frequencies a rear radiating full range driver won't be able to deliver
an acceptable sound (picture above).
Peter: And how do I get hold of dipole that takes all this into consideration: - Subwoofer for lowest frequencies - Separate driver for rear radiation - Adjustable level for rear radiation
Just take the Dipo 1 (2016: no longer available) that I designed recently.
Despite conventional cone drivers this loudspeaker gets as close as possible to a
classic planar transducer.
Many thanks, Gerd.