Fig 1. This combination is driven by a Quad 405. One channel for the ESL and the other for the sub.
The Contrapunt, a dipole sub woofer
The ESL-63, thoug an excellent loudspeaker, cannot produce the full contra octave and cannot produce sound levels higher than 96 dB. The contrapunt is designed to cover these aspects, but when these extremes are not met the use of the sub woofer should be inaudible when it is compared to the ESL63 full range.
It was quite a discovery that it is possible to build a sub without an enclosure. Compared to normal sub woofers there are several specific advantages with dipoles. Normal subs with an enclosure excite many more room modes, due to their omnidirectional character. Some bass notes are louder and some are softer. This leads to an uneven frequency response in a normal living room. In contrast due to the directional nature of a dipole there are no standing waves to the ceiling and side walls. The bass of the dipole is deep because the resonance frequency of the woofer is not shifted upwards by an enclosure. Energy is transmitted without the delay and the back-lash that is caused by a box. Bass energy build up is immediately.
I have given this sub a name, the Contrapunt. It can be used with speakers for the upper range that can be crossed at 110Hz. For every channel a sub woofer should be used. Combined with a Quad ESL-63, the ESL can play up to 12dB louder and the lower bass range is extended. An extra advantage of the Contrapunt is that when it is used with the Quads, the dipole frequency characteristic of the combination of the ESL and the sub woofer will be the same on both sides.
The design of the sub woofer is simple, but at the expense of an extra amplifier and an electronic crossover/correction filter. Measured frequency range is 24 to 110Hz within 3dB. They should be placed at least one meter before the rear wall. In my living room they have 4 meters free space behind.
The quality of a dipole sub is different from what most listeners anticipated as the sound of an open bass system. Also for me. The sub is precise, no matter if it has to generate pressure and attack for house music or to reveal the delicacy of a slap bass. Bass doesn't need time to build up, it is very well damped, better than when a enclosure is used. Low-frequency tones are reproduced life-like. The combination of the Quad ESL-63 and the sub woofer shows in symphonic works precise positioning and discrimination of instruments, and a great dynamic range while maintaining an impressive reproduction of sound stage and space. Don't be fooled, the sound pressure with 2 subs in a 50m2 livingroom is 100dB at 31,2 Hz. The dipole sub doesn't add coloration. When the extremes this dipole subwoofer is designed for are not existent, i.e. SPL below 96dB and no content below 40Hz, switching off the contrapunt and switching on the ESL-63 to full range is (nearly) inaudible. When it is not needed it is not there.
A specific characteristic of a dipole is the cancellation frequency that is determined by the size of the baffle. A compensation is needed of 6dB/octave below the cancellation frequency of the baffle, which starts when the shortest path between the positive front and the negative back wave is 0.5 wave length. This quality is also the cause of the directional radiation pattern that looks like the figure 8. Standing waves in the listening room are reduced, but a lot of acoustic power is principly lost with a dipole. Two factors are of prime importance to get a adequate sound pressure level at the lowest frequency's. First the cancellation frequency should be as low as possible, which becomes difficult when a small baffle is used, and secondly the woofers should be able to move a lot of air to compensate for the acoustic loss. To extend the path between back and front the baffle is folded. It is possible to make the folded baffle deeper, like Sigfried Linkwitz has done, but it will make the cavity react like an open organ pipe at 1/4 wavelength, which needs a notch filter in the crossover, not a real problem though.
The resonance frequency of the drivers should be below the lowest note that is to be produced. Two Peerless 831857's (CCX315) are used. Although I thought at first there should be several woofers suited for this application, several builders reported noise problems with vented types through the pole plate. The combination of Qts (measured 0.5-0.55), resonance frequency (24Hz), stroke (18mm pp), and air tightness make this model one of the few that is suited. Maximum used power for full stroke in this design is 40 Watts/ woofer at 30Hz. See the FAQ for notes about using a speaker with an other Q factor.
Most 12" woofers produce at 30Hz at maximum linear stroke a total harmonic distortion of more than 10%. The push-pull configuration reduces even harmonic distortion with approximately 50 %. Regretfully the nasty uneven distortion is not reduced :-( . (There may be some misunderstanding, even harmonic distortion is the same as uneven order harmonic distortion.)
Motional Feedback Woofer
When one is used to the undistorted clean bass response of the Quad's, sometimes -the human ear is not very sensitive to harmonic distortion in the bass region- the need is felt for a further reduction of harmonic distortion of the sub woofer. This can be accomplished by using a servo controlled woofer. Distortion can become below 3% at high levels. On the picture is in the middle the acceleration element of a Philips 12" MFB woofer. This gives a stunning roaring real life performance of low organ notes. The sound of bass drums (in real life dipole's) and the stiff Fender precision bass are very accurate. But the trade-off is that in many rock recordings the harmonic distortion of the playback system is anticipated during recording, so bass may be a bit too lean.
Geert Meddens, this page started in sept. 1999.
Fig. 2 The electronic crossover and correction unit.
Filter Scheme and Graphs
Although the slope of the high pass is electrically 12dB/oct, the accoustic slope is determined by the combination ESL and filter. The same applies to the accoustic combination of the woofer+filter. The slopes are carefully tailored with the characteristics of the speakers to give a flat frequency response. In the crossover region the phase response of the low pass (subwoofer+filter) and high pass (ESL+filter) are nearly the same (20 degree difference). Two other features are incorporated in the filter. The characteristic of the subsonic high pass filter at 20Hz also compensates for the fall off due to the low Q factor of the woofer. The high pass section in front of the ESL gives the region between 150-200Hz a lift of 1.6dB to compensate for a nearly unaudible flaw in the frequency characteristic of the ESL. Although the filter seems quite normal, it is carefully designed and tested to function for the combination of an ESL63 with this dipole sub. This filter should work with an other speaker than the ESL. This speakers shoud be able to reproduce one octave below the crossover frequency.Archief Bennebroek
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