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In Reply to: RE: thank you for your valuable insights posted by josh358 on February 16, 2010 at 16:31:27
why is this so difficult for speakers like planars/ribbons?
H.F.N.
Follow Ups:
That's a good question. Loud, complex works are difficult for any reproducer, but planars seem to have a particular hard time (while handling with aplomb instruments that give other technologies fits). I suspect that it's because choral music provides poor psychoacoustical masking for the distortion characteristics of planar magnetic drivers. The smoothness, purity, and varying pitch of vocal timbres seem to reveal every rattle and buzz, and the lateral resonances -- the "mylar sound." Basically, the fact that the driver is a cross between a snare drum and a kazoo. You can also hear the intermodulation products as the pitches glide with respect to one another and the rough high order harmonic distortion, particularly in loud passages on single-ended drivers like the less expensive Maggies, and ribbon subharmonic distortion. Also, planar magnetic speakers are second only to electrostatics in their ability to reveal distortion in the microphone and electronics. Sometimes, I find it hard to tell whether the speaker itself is producing a distortion or simply reproducing it accurately. You play the recording on dynamics and you don't hear the distortion, or you hear less of it, but you don't hear much of anything else, either -- just a soupy mush.
I think the problem with choral music is that the bulk of it is louder than an orchestral piece, with mezzo forte being the lowest rung during most of the piece. The second problem is that it has no bass so all of the energy is concentrated in the midrange. Hence the problem with single ended planar magnetics. Beyond a certain output volume, only the positive half of the output is reproduced fully. The other half is limited by the proximity of the magnet board and gets heavily compressed. While we are not particularly sensitive to this in bass frequencies, we are going to notice it on the midrange. And since nearly the entire output is there in choral music, the loud passages end up having an asymmetrically distorted waveform.
Push pull designs (MG20.1) fare better but may run out of space between the magnet boards and sound compressed at high output (Eminent Technology).
The solution is to simply have allot of headroom (MG20.1) and/or many drivers (The original Infinity IRS) Neo8 and other line arrays in VMPS, LS9. etc. or its equivalent in surface area in the Apogee bass.. The headroom means a larger distance between the magnet boards, which means either more and stronger magnets (Apogee) or lower sensitivity. It should be considered also that there is a limit to the amount of magnets due to saturation effects.
This brings up something I've long wondered, whether it wouldn't be feasible to reduce IM by predistorting the signal to compensate for position-dependent variations in the magnetic field strength.
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