Friday, February 4, 2011

Fun Fun Fun with Tact Measurements

All the last times I've done Tact Room Correction System (RCS) 2.0 corrections on my living room system, I've been in too much of a hurry to really do it right.  That's not saying much, I'm sure that will be true this time too.

I'm torn between wanting to get it done badly, and wanting to really dig deep into understanding how I can make my system sound better, using the wealth of potential diagnostic information the Tact can show it it's measurements.

Look only at the steeply rising curve on the right; ignore the room noise from dishwasher etc on the left (low frequencies).

That's my supertweeter, with the full +15dB level and a 15.5kHz crossover.  But it looks nothing like the idealized picture that I had in mind (though it's not too far from my Genrad measurement) of a supertweeter with flat response 10-35kHz crossed over at 15.5kHz.  Instead, it looks like a highpass filter around 18kHz or so, starts out pretty steep, then gets shallow after awhile, then steep again.  You probably can't read the scale, but the range is 50dB and the tweeter is showing 45dB attenuation from the quasi peak at 18kHz and 3kHz.  Unfortunately, the Tact isn't showing frequency response above 20kHz.  My measurements show a flat range from 16Khz (just above crossover) to 24kHz; none of my microphones are calibrated now and none were calibrated above 20kHz, so I really can't be sure of anything above 20kHz and I have no reason to doubt Elac's 35kHz spec, though perhaps that's a 3dB downpoint.

Anyway, even if we assume more-or-less flatness above 18kHz, why is there so much attenuation between there an the crossover point of 15.5kHz?  Funny you should ask, because a Linkwitz-Riley crossover attenuates each driver 6dB at the crossover point.  What is the attenuation shown above at 15.5kHz?    Let's look at the raw numbers from the Tact (which are not easy to understand at first glance).

18.290kHz     -57.34
15.493            -59.97

That's only about 2.7dB attenuation, not the 6dB ideal.   So the suspicious looking drop from 18kHz isn't even sufficient?  Without checking the theoretic curve, I can't be sure, because even 20kHz isn't sufficiently above the 6dB attenuation point at 15.5kHz to be "flat", the crossover is already attenuating a bit at 20Khz, so I would need to know what the theory would say the response should be at 20kHz (or 18.29kHz).

Anyway, I think it's about right, actually, though it might still be a tad too much attenuation at 15.493 (the closest the Tact lets me measure to 15.5kHz).

Now where do I measure -6dB?  The closes point I can find to -6dB from the maximum level of -57.34dB is at 13.811 kHz.  This seems to fit with my intent to cross over at that point, but I'm not sure is correct.

Also, the microphone may not be very accurate at these frequencies, and perhaps I erred by aiming it directly at the speaker, and perhaps I am not using the correct Tact, and perhaps (many people
believe this) the Tact microphone calibrations are not very accurate, one might even be better off using a generic calibration with fewer tiny irregularities (some do this).

So there's lots of wiggle room here.  Perhaps this is indeed showing the intended LR crossover applied to a roughly flat supertweeter response.  Or perhaps, for some reason, this setting is providing the "acoustic" crossover that is more like 14khz, which is after all what I intended to do.

OK, that's one aspect of this, how accurate is the top above the crossover.  The answer is, it appears to be not too far off, actual acoustic crossover somewhere between 14kHz and 15.5kHz.

But the other aspect is below the crossover.  Is it really crossing over at 48dB per octave.  We would expect to see about 48dB attuation around one octave lower.  (I don't think you actually achieve 48dB plus 6dB initial attenuation, because during the first octave the attenuation is just ramping up, or down, so to speak.)

At 10.6kHz, about half an octave below the nominal crossover point of 15.5kHz, the response is attenuated by 18.3dB.  That doesn't sound adequate (though it is clear I really need a nominal Linkwitz-Riley crossver calculator) because I would think the attenuation should be more like 24dB.  And clearly the supertweeter is not much getting out of the way of the Acoustat peak around 14khz, though it is attenuated roughly 6dB there.

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