Tuesday, November 22, 2011

The keyboard oscillator

For testing speakers, particularly subwoofers,  I find that nothing beats the conveninence and flexibility of a keyboard synthesizer.  Most Kurzweil "programs" (what others might call soft instruments) are very complex.  But it's not hard to cook up a sine wave oscillator useful for testing subs, building on the simple "Default Program" number 200.  Here is what I generally set up:

Tone: sine wave
Pressure sensitivity: 0
Control #8: volume
ADSR: sustain 100%, else 0%

The area between C0 and C1 (16 and 32 Hz) is interesting.  I think my EQ boost in this area helps restore strong response in this difficult region.  But each different note causes a different feature of the room to start rattling.  Only below 18Hz or so does it seem like keys do nothing.  And there I wonder if I haven't programmed my SVS PB13 to cut out too high.


I can't reiterate enough how useful a keyboard oscillator is for system adjustment.  It just sits there calling me to plunk a few more notes, checking out some other aspect of the sound.  Changes I've made so far:

+5dB at 27Hz changed down to +3.5dB

Room mode cut: 44Hz -9dB Q: 2.2 (sounds all to the better)  This is the most important room mode to tame within the subwoofers operating region where it is most important to eliminate high Q resonances (the sub tends to stimulate room modes far more than the panel speakers...room resonances can almost be ignored on the Acoustats).  Back before I started using Tact correction in the Living Room (January 2010, I remember it well if not memorialized here) I had been using two tuned resonance cancellors at something like 38 and 45Hz.  The keyboard makes it easy to check these things out and be sure you haven't gone too far (I know 9dB is a lot but it is vaporized in the wind of the resonance).

PB13 subwoofer tuning: I've put in two plugs into left sub ports (leaving 1 of 3 ports open) which permits me to dial back the sub low frequency cutoff (lfc) two notches (I had previously dialed back the lfc from 20Hz to 18Hz anyway.)  So now I'm choosing the 16Hz cutoff and I'm doing so within recommended usage.  Putting the lfc in in a lower-than-recommended position now buys very little difference (1dB) at 16Hz: measuring (with GR 1933 meter) I get 69 or 70dB at 16 Hz, 69 with recommended filter setting, while 70dB is about the average response level 20-80Hz, though response clibs to 72.5dB at 18Hz.  As it now stands, 70dB at 16Hz is barely barely audible (the lower setting changes this to barely audible); the strong 72.5dB at 18Hz is nicely audible.

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