I'd like to showcase my latest loudspeaker design, a dipole loudspeaker that has practically no baffle at mid and high frequencies. Drivers are two Visaton W300S woofers in a small U-frame, a Visaton AL170 for the midrange and a Bohlender&Graebener Neo3W for treble. Crossing and EQ are done with digital equipment from Behringer - the Ultra-Curve equalizer and the DCX crossover.
The crossover between bass and mid is between 300 and 350 hz. The mid to high crossover is around 1.7 khz. I've tried a lower crossoverpoint between bass en mid, but I ran out of thermal dynamic range on the AL170; not so strange, considering the approximately 11 dB of boost needed at 200 hz. A lower crossoverpoint between mid and tweeter is not practical, as the natural rolloff of the Neo3W without baffle is very steep below 1700hz.
The speaker was finished only yesterday and the crossoverdesign is at this point based on measurements done inroom. This means anechoic to not much lower than 500 hz. Below 500 hz the response is optimized for steady-state response at the main listening position. When it gets a little warmer outside (It's freezing cold now), I'll do some anechoic measurements in the garden.
My previous dipole speaker was a passive design with a large baffle, used to not let efficiency become all to low. In direct comparison to the Linkwitz Orion it became clear there was room for improvement. My dipole had constant directivity up to approx. 800 hz, whereas the Orion has constant directivity up to almost 2 khz. The Orion delivered a much more credible and specious soundstage and the sound was less tiring. (side-note: Although the Orion was one of the best speakers I had heard at that time, in my opinion the Pluto - at mid and high frequencies - sounded even better than the Orion)
The reason to use no baffle for the midrange and treble is to keep dipole-radiation to as high a frequency as possible. By maintaining dipole radiation, dispersion remains well-controlled. Simulations in Tolvan Edge showed that using a very small baffle would be the optimum with respect to directivity-control. The midwoofer has its first dipole-bump (where sound radiated at the front and the rear add up in phase) at 1.5 khz. Below this frequency the response starts to roll off with 6 dB/oct. This means this speaker is very inefficient. That is the price to pay for a pure dipole radiation-pattern. The peak is very symmetrical and can thus be easily equalized.
[IMGHTTPDEAD]http://img708.imageshack.us/img708/13/edgebafflelesdipole0tot.jpg[/IMGHTTPDEAD]
[IMGHTTPDEAD]http://img709.imageshack.us/img709/2351/dipoolal170baffleles0to.jpg[/IMGHTTPDEAD]
Initially I wanted to use the B&G Neo3PDR, which has wider dispersion at higher frequencies than the Neo3W. However, this wide dispersion leads to much more severe diffraction effects in the high treble. The beaming of the Neo3W helps to keep dispersion well-controlled. The dipole-bump is centered around 3.5 khz. Below 1.7 khz it rolls of very steeply, due to both the dipole loss and the drivers' own frequency response. All in all it needs a lot less boost than the midrange.
There is not very much to say about the low end. The speaker uses two 12" woofers in a U-frame with a small depth. The quarter-wave resonance -centered around 400 hz - is not very pronounced and is easily equalized. I don't think it is audible. The response is pulled flat to 35 hz. Below this frequency a high-pass keeps the drivers from over-excursion.
[IMGHTTPDEAD]http://img191.imageshack.us/img191/2365/dscf6008.jpg[/IMGHTTPDEAD]
Well, how does it sound? Very good (a lot better than the prototype looks, anyway)! I'm not too good at describing sound. I think the measurements speak for themselves. I've not yet compared these directly to the Orion, but I think these dipoles sound even better! At least the measurements look quite a lot better than those of the Orion. As an experiment - to keep the sound-power-average as smooth as possible - the on-axis response above 10 khz is lifted. This is not really audible to me. Note that the green curve is the averaged response.
[IMGHTTPDEAD]http://img709.imageshack.us/img709/3811/nultotnegentiggraden.jpg[/IMGHTTPDEAD]
Response from on-axis to 90 degrees off-axis + average curve.
The on-axis dip around 6.5 khz is there to compensate for diffraction-effects that are visible off-axis. I'll probably experiment a bit with dampening material at the edges of the tweeter to reduce diffraction.
With more time on tweaking the crossover between mid and tweeter I think I should be able to reduce the dip at 1.5 khz.
The crossover between bass and mid is between 300 and 350 hz. The mid to high crossover is around 1.7 khz. I've tried a lower crossoverpoint between bass en mid, but I ran out of thermal dynamic range on the AL170; not so strange, considering the approximately 11 dB of boost needed at 200 hz. A lower crossoverpoint between mid and tweeter is not practical, as the natural rolloff of the Neo3W without baffle is very steep below 1700hz.
The speaker was finished only yesterday and the crossoverdesign is at this point based on measurements done inroom. This means anechoic to not much lower than 500 hz. Below 500 hz the response is optimized for steady-state response at the main listening position. When it gets a little warmer outside (It's freezing cold now), I'll do some anechoic measurements in the garden.
My previous dipole speaker was a passive design with a large baffle, used to not let efficiency become all to low. In direct comparison to the Linkwitz Orion it became clear there was room for improvement. My dipole had constant directivity up to approx. 800 hz, whereas the Orion has constant directivity up to almost 2 khz. The Orion delivered a much more credible and specious soundstage and the sound was less tiring. (side-note: Although the Orion was one of the best speakers I had heard at that time, in my opinion the Pluto - at mid and high frequencies - sounded even better than the Orion)
The reason to use no baffle for the midrange and treble is to keep dipole-radiation to as high a frequency as possible. By maintaining dipole radiation, dispersion remains well-controlled. Simulations in Tolvan Edge showed that using a very small baffle would be the optimum with respect to directivity-control. The midwoofer has its first dipole-bump (where sound radiated at the front and the rear add up in phase) at 1.5 khz. Below this frequency the response starts to roll off with 6 dB/oct. This means this speaker is very inefficient. That is the price to pay for a pure dipole radiation-pattern. The peak is very symmetrical and can thus be easily equalized.
[IMGHTTPDEAD]http://img708.imageshack.us/img708/13/edgebafflelesdipole0tot.jpg[/IMGHTTPDEAD]
[IMGHTTPDEAD]http://img709.imageshack.us/img709/2351/dipoolal170baffleles0to.jpg[/IMGHTTPDEAD]
Initially I wanted to use the B&G Neo3PDR, which has wider dispersion at higher frequencies than the Neo3W. However, this wide dispersion leads to much more severe diffraction effects in the high treble. The beaming of the Neo3W helps to keep dispersion well-controlled. The dipole-bump is centered around 3.5 khz. Below 1.7 khz it rolls of very steeply, due to both the dipole loss and the drivers' own frequency response. All in all it needs a lot less boost than the midrange.
There is not very much to say about the low end. The speaker uses two 12" woofers in a U-frame with a small depth. The quarter-wave resonance -centered around 400 hz - is not very pronounced and is easily equalized. I don't think it is audible. The response is pulled flat to 35 hz. Below this frequency a high-pass keeps the drivers from over-excursion.
[IMGHTTPDEAD]http://img191.imageshack.us/img191/2365/dscf6008.jpg[/IMGHTTPDEAD]
Well, how does it sound? Very good (a lot better than the prototype looks, anyway)! I'm not too good at describing sound. I think the measurements speak for themselves. I've not yet compared these directly to the Orion, but I think these dipoles sound even better! At least the measurements look quite a lot better than those of the Orion. As an experiment - to keep the sound-power-average as smooth as possible - the on-axis response above 10 khz is lifted. This is not really audible to me. Note that the green curve is the averaged response.
[IMGHTTPDEAD]http://img709.imageshack.us/img709/3811/nultotnegentiggraden.jpg[/IMGHTTPDEAD]
Response from on-axis to 90 degrees off-axis + average curve.
The on-axis dip around 6.5 khz is there to compensate for diffraction-effects that are visible off-axis. I'll probably experiment a bit with dampening material at the edges of the tweeter to reduce diffraction.
With more time on tweaking the crossover between mid and tweeter I think I should be able to reduce the dip at 1.5 khz.
