Golden Earring wrote: > > > 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? >
Passive speakers generally do not have any electrical high pass filters, but most active speakers do. The reason why is as given above - below their low frequency limit most speakers are quite willing to absorb power, but generally produce very little acoustical output and/or mostly distortion. ------------------------------------------------------------------------ arnyk's Profile: http://forums.slimdevices.com/member.php?userid=64365 View this thread: http://forums.slimdevices.com/showthread.php?t=106914 _______________________________________________ audiophiles mailing list audiophiles@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/audiophiles
