Axel Ridtahler presented full range drivers with exponential horn (front
loaded) made of acrylic glass. Both, full range as well as bass, work like
an open rear, dipole system. The horn ensures a vivacious, lively sound and
provides - due to the virtual baffle extension - a perfect link to the subwoofer
system at around 150 Hz. And besides, the dipolar radiation over the entire
frequency range assures an extremely well balanced sound projection, forming
a uniform and integrated whole. The full range sounded extremely convincing,
vocals were delightful, a homogenous sound as one would expect from good drivers
of this type.
The bass is a special treat. It's a variety of the dipole transducer. This
arrangement, developed and patented by Axel Ridtahler shows a considerably
lower resonance and is extremely compact. The optimised radiation pattern
doesn't correspond to the typical "8" any longer, which is why this type of
speaker is not a genuine dipole. To distinguish between a classical dipole
and Ridtahler's variation the latter is affectionately called RiPol(e). There
are two types of RiPoles: the BMS version - like the one we discuss here -
with two bass drivers facing one another and the DRS version with only one
bass driver, like described in Hobby HiFi.
We would like to point out that it does matter which bass driver is to be
employed, which parameters these drivers need to have and how the cabinet
is designed. In our example one active BMC RiPole was linked up to one horn.
The picture on the left shows the RiPole schematically from the top with
the cover removed.
The picture to the right shows the protruding magnets (next to Axel Ridtahler's
hand) and the opening in the centre. Beneath the subwoofer a resonance decoupler
is visible, beneath that a granite plate.
The listeners were rewarded with:
Axel Ridtahler explained the theory behind his sort-of-dipole. The most interesting
aspect is that the resonance frequency drops through impedance transformation
compared to the same driver not being mounted! This is astonishing since mounting
a driver in a conventional cabinet increases the resonance frequency considerably.
In this context quite a fashionable loudspeakers system was discussed where
the bass driver is operating below its resonance
frequency (URPS). However, the alleged advantage of this design could
not be confirmed by the audience.
Instead, all listeners were enthusiastic about the Ripole's performance.
We would like to thank Axel Ridtahler for this extraordinary experience. We'll
certainly keep in contact with him to gain more information and obtain more
ideas for our DIY community. There is already a heated discussion going on
and first results will be available soon.
The company Audioelevation, licensee of Axel Ridtahler's patents, is in charge
of marketing the RiPole; some RiPole kits are available
Publications about dipole (sub)woofers and RiPoles:
- STEREO 10/2002, pages 136 -138
- stereoplay 9/03, pages 30 - 38
- hifi&records 1/2004, page 58
- Hobby Hifi (DSR) 4+5/2002
- image HiFi 3/2003
Axel Ridtahler's comparison of different subwoofer systems you'll find here
© Iris Strassacker 2005. All rights reserved.