Originally Posted by dr_hsu
Unless I am missing something about your passive summing (I don't quite understand why you had 4 arrows going from the 'Bass EQ' to the passive summing box, for example), you can just feed the output of the Marchand crossover to the left inputs of the ULSs and the sub out through the EQ into the right channel inputs of the ULSs? You don't need any passive summing.
That would require running two audio cables to each sub instead of one. Everything was already wired and running by the time I discovered the rolloff in the BassQ.
There are 4 arrows because the BassQ has 4 separately tuned outputs, one for each subwoofer. It's a simplified form of Harman's SFM process.
The Marchand is ambiguously labeled as a crossover (that's what the unit does) but in my system it is only a 20 Hz low-pass filter. This is all in order to compensate for inherent rolloff in the BassQ unit. I didn't buy 4 ULS-15s to have the bottom end limited to 20 Hz!
The two outputs of the SSP carry the same signal, but the Aux out is time delayed 22 ms to compensate for the latency of the BassQ, so the <20Hz part of the spectrum splices perfectly back together with the EQ'd part >20Hz. And therein is another advantage of the passive summing: I can see the splice on a scope and with REW before the signal enters the sub. I guess I could have simulated it to prove out using the sub as the summing.
How high do you have to set the volume on the subs right now? Note that since the subs are not all co-located in one spot, the SPL at your listening chair is not easily determined without any actual measurements.
They are not co-located because that would not have helped my room's modal cancellation problems.
Best to just perform a measurement yourself. Put the SPL meter at ear level on your sweet spot (meter set to 'flat' position or if not to 'C'). Set the volume on all the subs to max. Play a 40 Hz tone and turn up the master volume on your processor till you get 90 dB. Then measure how many mV the summing box is putting out to the subs. This measurement will take into account all the acoustic interactions of your actual room (at some frequencies the output of one sub can be cancelling the output of another sub while summing at other frequencies).
I know how to measure it, being an EE and all. I just don't want to mess with the settings on the sub (set it to max, then try to return it to the original position) as they are calibrated by the BassQ so they interact a certain way to cancel room modes, and I don't want to haul out the mics to run it again. The system works and sounds great as it stands.
I just figured as the designer of the sub you might have the sensitivity information readily at hand. If not, or you prefer not to share it, that's perfectly OK. I was just asking.