Friday, January 21, 2011

More Crossover Tests and Thoughts

Thursday night I tested my new high supertweeter output level using a General Radio 1933 Sound Analyzer, and it suggests I am doing OK.

The Genrad has built-in switchable octave band filters.  I played pink noise and tried the different bands, pointing the random incidence microphone toward the ceiling mostly.  The highest band, 16Khz, was pretty much in line with the other bands.  If I turn off the supertweeter, the 16Khz band goes way down.

In the first test i did, the 16Khz band was 2dB higher than the 8kHz band.  I could easily make it level with the 8kHz band by turning down the supertweeter level a few dB.  But then in a later measurement, with the microphone pointed slightly differently, the 16kHz band was 1dB lower than the 8Khz band.  The highest bands were the 32Hz and 64Hz bands, they were at least 5dB higher than the others.  (Those bands are still in need of room correction with my Tact preamp, but having them boosted slightly makes for better sound anyway.)

More listening to pop recordings such as "Bass Erotica" suggests the new crossover alignment is really on track if not perfect yet.  As currently configured, the system is more fun to listen to than it has ever been.  It's good all around (if still a bit lumpy in the bass) but the rhythm, pace, and intertransient silence have never been equaled in any of my previous systems.

I particularly think the idea of spacing the low frequency crossover around the 90-100Hz vertical room resonance is fabulous.  You are going to have that resonance anyway, so you might as well take advantage of it to take some load off your speakers.  Separating the low pass and high pass crossover points reduces the effect of interference that can occur when both speakers are playing simultaneously.

During Crossover Redesign Days 2-3 I spent much time sweeping the low end with my B&K oscillator, and polished the crossover points slightly, moving the subwoofer lowpass from 72 to 84 (to reduce a depression in that range) and moving the panel highpass from 100Hz to 104Hz (because there was still a peak at 100, but when I raised highpass to 107Hz I start getting a depression).

I might try fiddling with this some more...I could possibly eliminate any peaking around 90Hz entirely, but since I am getting to the point where depressions open up if I lower the lowpass or raise the highpass, it would seem I may have already gotten about as much benefit from this trick as I can.  And it does nothing for the main room resonance at 45Hz, that will have to be fixed by the Room Correction System.

I've tried different crossover points for the supertweeter also, currently 14.8khz seems to work best (at least when I tested it at the old +10db level).

One concern has also come up in my mind.  It seems pretty clear the Acoustats are rolling off pretty steeply above 14kHz.  I had thought they were supposed to have high frequency response to 18 or 22kHz.  Perhaps mine need some capacitor replacement.

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