|22nd February 2009, 09:21 AM||#21|
Join Date: May 2008
The biggest boost from the T-Bass circuit will be, where the impedance reaches the minimum. According to your graph that seems to be between 35 and 40 Hz.
From there on downward the attenutation will become smaller. At 0,1 dB you can assume the attenuation as 0. A 12 dB/octave Butterworth filter will attenuate the signal down to ~8 Hz, an 18 dB/octave Butterworth filter down to ~22 Hz, a 24 dB/octave Butterworth filter down to ~25 Hz.
You would probably be better off, if you let the subwoofers run in parallel with the main speaker woofers, and equalize them then for flat response. That way you have the same phase progression for all of them, and you don't get cancellations from crossover-induced phase turns that are different for main speakers and subwoofers.
If you've always done it like that, then it's probably wrong. (Henry Ford)
|23rd February 2009, 04:37 AM||#22|
Join Date: Jun 2005
Thanks for reminder. You're right on those filter things. Points taken. However there's something different in my points of view and the "problems" I have to deal with...
Previously I also thought of making the center woofers in parallel with main baffles, however I think the current xover point is a little bit too high to do so -- it's at about 160~180Hz acoustically. If I would take this route, the 'mid' has to play lower to have a good integration. Say, 100Hz sounds good, 80Hz is better, I guess. Now in this system, the midrange can't do this well enough, so I don't and won't force it to. If I get some other "better" midbass drivers, that'll be another story.
On the woofer side, such pro sound 18"ers have inherent strong midbass. Way too much for home use actually. OB loss on the low end emphasizes this character furthur. I always have to cross them at least 1 octave lower (plus some minus adjustments on EQ) to get a reasonable flat response. Otherwise the range of 50Hz~200Hz is inevitably overflowed, way too much, no matter what. So now I made the 2nd pair of woofers creeping in at lower frequency, thus more or less compensate this.
As to the losses caused by those filters, I don't care very much, since the voltage gain along the whole system is always much more than enough.
On the amps, the extreme low impedance loads (now it's 1 Ohm) on both channels of my poor little Hafler is already not a 'healthy' setup. Making it even lower is indeed too cruel and the losses along interconnections might be too high in proportion, thus degrade the overall performance. A "better" amp is under planning, but somewhat low down in my priority. Now I just spread the loads to different amps, and this might be easier for them. (Still tough, though.)
I ran some more RTA scans yesterday, and saw something more clearly (or, more problems were found). The dip mentioned previously is actually at a point between 63 and 70Hz, which is slightly different from my previous memory.
And I found something insteresting: when the xover of center sub is set at 40Hz, the acoustic output actually extends to about 100Hz !! (pretty flat in the range of 30~100Hz) While there's no dip under such setup.
Raising the xover to the highest (160Hz on the dial), the 70Hz dip appears. And the frequency response on the higher end is only slightly over 100Hz, with a 3~4dB lift on 50~100Hz range. How funny!
Obsevations (measurements) above were on the center sub only, (main baffles were shut down). I guess many factors involved -- baffle size and shape, position, the room modes, characters of woofer itself, T-bass setting.... etc. This also shows my lackings in the measuring skills and equipment.
Fortunately, when the main baffles are on, all of them blend nicely. The sub-bass filling is very good, which is obviously the help from the center sub. Their SPL is added up and averaged out in some of their individual irregularities. Now it's quite flat from high 20's upto the midbass, and also very even in the room. Without the center sub, the extending can't be that low, and there'd be some holes in the room here & there. Overall speaking, I'm quite satisfied with the result.
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