mbl 101D Radialstrahler
|mbl 101D Radialstrahler
|Sound In The Round
|14 April 2000
If there were such a thing as the seven wonders of the audiophile world the mbl 101D would surely make everyone’s list. To just call it a loudspeaker doesn’t do its inventor justice. This sound delivery system, handcrafted to order in Germany, accomplishes its purpose to perfection. But what is its mission exactly? What is its function in a stereo or surround sound system and is its perfection relevant to such reproduction? Certainly it is a loudspeaker but just as certainly it is the most unconventional one you are likely to hear.
First Some Background In Loudspeaker Physics
There are two extreme philosophies possible in loudspeaker design. First one can imagine a speaker that operates like a laser beam. That is, it sends sound out in a straight line from a single point directly to the listening position. It is so sharply focused that even if the room were untreated, little sound would be reflected until the beam hits the wall behind the listener and if this wall were actually a door one could have a home listening room that was virtually inaudible. Such a speaker pair would produce a very sharp stereo stage and have an exceptionally flat frequency response since higher frequency room modes, boosts, and suck outs would not be excited. This type of speaker would correspond to the description often made that stereo is a window into the performing space. So one would listen through this window and hear the acoustics of the space where the recording was made, unaffected by the room supporting the window. The sweet spot would of course be limited in this approach. The only speaker I know of that has attempted to do this commercially is the top-of-the-line French Cabasse speaker.The other diametrically opposed approach is to make a sound reproducer that is a full range pulsating sphere. That is, the sphere radiates sound equally at all frequencies and in all directions, horizontally and vertically. Such a speaker only makes sense in a home listening room stereo system if that room is able to reflect all this indirect sound back toward the listeners. Such a speaker is probably more ideal for use as the surround speakers in a 5.1 arrangement than the dipoles usually recommended. There is no psychoacoustic theory or mathematical formula that indicates why a pulsating sphere should be useful in a standard stereo system or a home theater for that matter so one operates on prejudice here. But ever since Bose introduced the Series 901, many audiophiles have been impressed with the spatial qualities of speakers that output most of their sound in every direction but toward the listener. One can opine that such reproduction is quite suitable for the “They Are Here” type of sound reproduction. If you posit a recording, made in a dry studio of a single instrument like a piano or a small jazz combo, played back in a live home environment then the acoustic signature of the home listening environment will be impressed on the reproduced sound and the performers will appear to be in the room, if the size of the ensemble is small enough to make this logical. This is why the Chesky type of small combo recording is so favored by audiophiles and by manufacturers demonstrating at shows in small hotel rooms. However, this type of reproduction is less effective for symphony orchestras, operas, or recordings that include hall ambience that conflicts with the playback room ambience. Of course, the room reflections stimulated by an omnidirectional speaker can and almost always blur the localization of the performers, but one can argue that localization in live concert halls is not that great either and this is a matter of taste. Even if pulsating spheres don’t image very well, they have the advantage of being able to project this fuzzy image over a very large area, so they make sense if a lot of guests are expected.
The 101D is a four way omni-directional speaker. Instead of a single pulsating sphere, it uses one pulsating football woofer, one pulsating oblate spheroid hardball midranger, one pulsating oblate spheroid handball tweeter, and one non-pulsating 12-inch cone subwoofer. The square tapered subwoofer enclosure rises to support a football shaped radial woofer speaker standing on end. A vertical rod from the top passes down the center of the football and attaches to a voice coil below the football. The voice coil causes the many petals that form the football to elongate or shorten. This motion expands or contracts the ball to produce sound symmetrically 360 degrees about the axis. The two higher frequency radiators are shaped more like the theoretical spheres discussed above, but work on the same squeeze and stretch principle. The crossover frequencies are 105, 600, and 3500 Hz. This design ensures that the radiation is really uniform in the 360-degree horizontal plane. Likewise, this minimal motion design ensures that there can be no cone breakup or Doppler shift wherein motion of a cone at one frequency is able to modulate sound produced at other frequencies if the cone is not perfectly rigid.The appearance of this speaker is really cool. It could stand on its own as a piece of sculpture even if it never produced a sound. The workmanship is impeccable as it should be at this price level, a cool $35,800. It weighs 176 pounds and stands 67 inches tall. The manufacturer claims a response of 20 to 40,000 Hz. but does not indicate if this is the 3-dB point. The impedance is 4 ohms and the peak power rating is 2,200 Watts. Heat dissipation at this power level is facilitated by immersing the voice coils in ferrofluid. While not a high efficiency speaker, I had no difficulty driving it to earsplitting levels with a cheap hundred-watt amplifier during testing. It seems to be comparable to electrostatic speakers in this regard.
The Usual Placement Claptrap
Remember that there is no mathematical science or physics that can be used to determine the best position in a room for any speaker, mbl or not. All such rules of placement are empirical in nature and subjective. But for what it’s worth, mbl suggests that you site the speakers in the part of the room that is most lively. They also urge you to put the speakers along the longer wall if the room is not square. They also would like each speaker to be one and one half times further from its near-side wall than the speaker is from the wall behind it. Finally, the listening position should be further from the wall behind it than the speaker is from its rear wall. Presumably these rules will tame some lower frequency peaks and nulls but I would not worry about these rules too much. The manual also tells you what to do if you don’t follow orders and put the speakers along the short wall.
Measurements And Listening Tests
I originally obtained these speakers for research purposes. The idea was that if a software stereo dipole could be formed using an omnidirectional speaker then the sweet spot could be made large enough to accommodate three people without any deterioration in localization. The experiment worked and I hope that someday there will be a more affordable version of this type of speaker to facilitate the development of more realistic, crosstalk free, sound reproduction. I then moved these speakers into my exercise room, which is quite large, and exceedingly live with hard sound reflecting surfaces everywhere. I have never really liked to listen to music in this room while exercising because the brightness of the gym and moving from machine to machine made it difficult to hear anything remotely resembling a stereo effect. With the mbl’s installed, (on the long wall as it turns out) the result was amazing. Almost anywhere on the 22 piece weight machine circuit one could hear the sound loud and clear with a sense of spaciousness that was most gratifying. Of course specific localization was a bit haphazard but left was largely left and right stayed to the right.
I do not normally measure speakers since I seldom review them, but on this occasion I did a considerable amount of testing. Using a sound pressure meter I walked around the speaker and sure enough the output remained constant within a few dB all the way around at any frequency. By ear, I could hear no change in level at any angle or at any frequency above the 105 Hz crossover on up. I don’t know about the 40 kHz response claim, but the bass response is flat down to 30 Hz and then rolls off rapidly. While its not going to outperform a great subwoofer, it certainly produces some of the cleanest bass I have ever heard using an oscillator. There was no audible second or third harmonic distortion at any of the difficult low frequencies between 25 and 75 Hz and no rattling or cabinet vibration to be felt. I had to put my hand through the subwoofer portholes to feel anything at all and the low bass was very hard to localize as low base should be when there is no distortion to give its position away.
Getting close to the speaker to avoid room effects and doing a slow frequency scan, no significant departures from flat response were heard or otherwise observed. Of course out in the room, with sound bouncing around in all directions, the peaks and dips were clearly evident at any frequency. The same would be true of most speakers in a live room and is why room treatment is usually a good idea for normal speakers.
I usually listen to the harpsichord while working out. Except for the Soundlab M-2 electrostatic loudspeakers, I have never heard the transient glory of this instrument so clearly as via these speakers. Again omni speakers in a live room will always do a good job of putting a solo instrument in the room with you. But the exceptionally low distortion and extraordinary transient response that this mbl technology is capable of, makes this a singular sonic experience.
Because of their truly high-fidelity performance, it might make sense to ignore their circular directionality, and use them in a room that is well sound treated. The advantage would be that the imaging would be excellent without the room reflections while the sweet spot would be much larger than that obtainable with most cone, or ribbon speakers. There would be a significant loss in efficiency since most of the sound being emitted would be absorbed by the room treatment. But high-powered amplifiers are readily available and the power handling capacity of this speaker seems to be infinite.
Finally a curved electrostatic panel has similar properties and advantages but the rear wave is of the opposite polarity and, if not absorbed, will induce unpredictable anomalies. Thus, if a live room is your cup of tea and room treatment is anathema to you mbl is a great way to go for music and home theater mains or surrounds if money were no object. I would expect that in time, more affordable omnidirectional designs will appear. As the pioneer, mbl should be very proud of their accomplishment.
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