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The RiPole, a special kind of dipolar subwoofer

Axel Ridtahler Axel Ridtahler,
developer of the RiPols,
designed for corporate
sound systems

About this special subwoofer
Axel Ridtahler, the father of the RiPole,
interviewed by Dr. Peter Strassacker (3/2005).

Peter Strassacker:
What inspired you to come up with the RiPole?

Axel Ridtahler:
About 30 years ago I became quite involved in electrostatic loudspeakers and did some experiments and investigations in this regard. You know, electrostats are dipoles by nature. During these years I met Harold Beveridge, Peter Walker and Rolf Rennwald. An inherent problem with electrostats has always been the lack of bass or the correct linking of subwoofers to this type of radiator. I have been directing my attention to this problem for quite some time now. It simply seemed logical to me to use the same type of radiator for bass as well. In the course of research and optimising work I came across the effect of lowering the resonance frequency. For the first time in my audio life the so important resonance frequency was lowered by placing one or two bass drivers into a cabinet.

My first research I did for a friend in 1990. Our first dipoles were still enormous. The results were, however, so impressive that I continued my investigations. In 1998 we applied to have this design patented. The patent was approved on November 9th, 2000 (Pat. No. 198 30 947).

Peter Strassacker:
It is often said that dipoles lack low bass. How did you trick the laws of physics?

Axel Ridtahler:
A dipole is clearly defined physically, showing a figure of eight radiation. The RiPole doesn't display - due to its special cabinet - the exact symmetrical radiation pattern like a dipole (sic: it's therefore not a real dipole). The difference in front and rear radiation intensity causes a subdued omnidirectional behaviour and, therefore, a different, more favourable frequency response (=> therefore RiPole!). I did some research research regarding the omnidirectional properties of different systems (like closed cabinet, dipole and RiPole) and compared the results. It became clear that the 'cabinet' prevents the air, moved by the diaphragm, to escape too quickly, therefore, increasing the radiation resistance. Exactly this effect is the reason that the resonance frequency drops compared to a 'free air' driver, ensuring the reproduction of lower bass compared with a classic dipole. But, that's all according to the law of physics, I didn't trick it.

Peter Strassacker:
What's the radiation pattern of a RiPole, what's the difference to traditional subwoofers?

Axel Ridtahler:
A dipolar bass radiates directional: to the front and with different intensity and 180 degrees phase inversion to the rear. This reduces standing waves. Standing waves give the impression of slow, low bass that doesn't decay. Powerful subwoofers just pump the room full of bass - what remains is the impression of loose and sluggish bass. On the other hand, a RiPole generates, so to speak, low frequency 'antimatter', preventing this negative effect altogether. Bass is radiated and the opposing radiation just prevents the unwanted standing waves - the signal is not washed out by its own multiple overlays. Nevertheless, I take care not to speak of the 'nonsense' of fast bass.

Everyone who is sitting in the radiation area of a RiPole will notice the floating low bass. The acoustic short circuit only occurs in the area 90 degrees of the radiation axis, by the way, a perfect spot for gears sensitive to feedback like turntables and valve amps. These devices, including RiPoles themselves, are not affected by low frequencies!

Peter Strassacker:
Thanks very much Axel, for an impressive presentation.

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