Hi all! Since it's all gone slightly quiet, I thought that I would ask the forum about an issue to which I do not know the answer.
I'm thinking here of the contrast between my current 2-way B&W 805S speakers which are a ported design with a carefully shaped cabinet (which in fact has no parallel sides), a single mid-bass driver only 6.5" in diameter (although having a Kevlar composite cone to minimise cone break-up), a completely separate aluminium dome tweeter located on top of the main cabinet, and is claimed to have a very simple passive crossover circuit which has a high nominal crossover frequency of 4kHz and a pair of Spendor BC3 loudspeakers I owned back in the 70's, these were massive rectangular boxes of a 4-way design, comprising a 12" woofer, an 8" mid-range driver, a tweeter AND a super-tweeter above that. Despite the size of the these speakers the 12" driver was loaded in a reflex arrangement with 2 forward facing rectangular ports; the 8" driver had its own sealed enclosure concealed within the main cabinet; but most significantly the whole arrangement was controlled by a literally massive full 4-way, half-section passive crossover which had inductors the size of small transformers. As a consequence they represented a very difficult load with volatile & often reactive element dominated impedance. There were too many individual sound-sources to integrate easily within one loudspeaker and some problems with phase aspects around the 3 separate crossover points which if I recall correctly were 800Hz, 3000Hz & 12000Hz. Ok, there's the trade-off, agile & well-imaging stand-mounts but with limited LF vs. more complex (usually, not always) floor-standers with more extended LF & probably higher power capacity but often with larger, less agile woofers and inferior soundstage presentation. Off-axis response is also usually easier to control with a simpler design. However the most obvious reason to go for stand-mounts remains that they tend to work better than larger boxes in small rooms & are usually much less fussy about speaker positioning. This much I think I understand, but please feel free to comment or correct me if you disagree with anything I have suggested above. I would obviously accept that a poor implementation will remove the potential benefits of any given loudspeaker's basic layout. What interests me however is what happens from the power amplifier's perspective when a stand-mount receives signals at frequencies below its LF roll off point, where air resistance within the cabinet is mechanically reducing or preventing the motion of the LF driver. I wondered at first whether this effect might in some way cause the speaker impedance to rise for frequencies significantly below its LF roll-off point, and spent some time scouring the internet for a published impedance vs. frequency chart for my 805S's. I did finally find one, although it had not been produced by B&W themselves, which seemed to indicate that impedance did initially rise in the LF region, especially in the reflex reinforcement range, but then seemed to fall again so that for frequencies which would be audible to the ear but which my stand-mounts would struggle to emit even faintly, it was back around the nominal 8 Ohm mark or lower. This seems to me to imply that the speaker must still be drawing significant power from the amplifier for such very low frequencies, although it is producing little or no audible output. I can only assume that this power must be dissipated by heating up the voice-coil of the driver which cannot respond mechanically to such signals. This seems an unsatisfactory situation which might in extreme cases cause damage to the driver even though the signal was clean & not clipped. The extended LF response is certainly there at the power amplifier output posts - that is precisely what provides the signal for my active subwoofers. These have an input impedance of 10KOhm however, so draw no power to speak of at all, which is why their addition has no effect whatsoever on the performance of the stand-mount speakers. If this is true, why do stand-mount loudspeakers not include a steep high-pass filter at 30Hz or so, to divert this power from the drivers unable to turn it into sound, and presumably simultaneously reduce the load on the power amplifier by increasing the impedance it is loaded with below that frequency point? Have I missed something here? Dave (puzzled) ------------------------------------------------------------------------ Golden Earring's Profile: http://forums.slimdevices.com/member.php?userid=66646 View this thread: http://forums.slimdevices.com/showthread.php?t=106914 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/audiophiles
