Monday, February 7, 2011

Room Correction Weekend ends with A Story

I did some other fun things also, but the bulk of the first weekend in February 2011 was spent making microphone measurements of my audio system using my Tact Room Correction System (RCS) 2.0 Preamp, making correction curves, and listening to them (Saturday night didn't end until 7am because I couldn't quit listening because it sounded so good).  A gazillion measurements were made, and I made many new important discoveries.   Photos were taken of many graphs, but it will take a week just to sort through them.  It ended on a mixed note, however, so-to-speak.  Playing through Sonos the Pat Metheny track "A Story Within A Story" revealed a buzzy bass note in the intro.  Damn.  I need, someday which will probably not be soon, to take the Acoustats apart and give their membranes the hair dryer shrink treatment.  However, I can fix the buzz by running the Acoustats as I did last year, Crossed over above 120hz.  (Actually, I think I previously crossed at 116, but with current correction curves I need to cross at 121, and higher might be even better wrt reducing potential for buzz.)

This time, since January I had tried to cross the Acoustats in at 104Hz to avoid a disturbing room resonance.  The subs cross out at 85Hz also to avoid that resonance, the resonance pick up the tab in the middle giving reasonably smooth response (and nicely boomless) without correction.

Great idea I thought, which could be applied to most speaker systems: stagger the sub crossover around the room ceiling resonance around 100Hz.  And the Acoustats are a "full range" speaker (as some people define it, anyway) that has audible bass (with room gain) down to 40Hz or so.  So there not only shouldn't have been a problem changing crossover from 116 down to 104Hz, that should give the system cleaner "panel bass" (actually, many people think panel bass is fake sounding, but flat speaker lovers usually think cone bass is fake sounding).

But there is a problem, because of my 20 year old panels and my desire to listen at pretty substantial (not ear damaging) levels, and because of room correction itself.  Because the Acoustats have a deep midbass depression in their response between 110 and 400 Hz, the room correction is fixing that with midbass boost.  That midbass boost is pusing the speaker into noisy distortion around 110Hz.  Without the boost, I wouldn't get the distortion without playing considerably louder (though I have not tried this, I simply switched to correction Bypass and the buzz went away at the same level).

But the panels clearly have a problem, and even if boost is required to make them sound bad on A Story Within A Story, they could be distorting less noticeably on other music, and in some cases, without boost.

You could pin the problem on the crossover point, the room correction, the loudness level, and ultimately the speakers.  In the sense that the speakers shouldn't have this problem at this frequency and level, it is a speaker problem that ultimately needs to be addressed (and yes I have even thought of buying a totally different kind of speaker, like Magnepan 1.7's or Linkwitz Orion).  But meanwhile, I can work around it by judicious choice of crossover, and as long as I don't play too loud.  Using current room correction curve (measured for 104Hz panel crossover) #2, but with post-correction change to 121Hz crossover, I can play A Story Within A Story to 81 gain level on Tact preamp without distortion.  At 82 the distortion is barely audible, at 83 he distortion sounds like "just a normal part of slap bass" (but it's being exaggerated by speaker distortion to sound qualitatively different than the actual recording).

I don't want to give up the correction.  The perfectly calibrated midbass boost brings life back to music.  The music sounds so much more real with good midbass, even if I have to compromise that midbass slightly by leaving a hole in the midbass between 104Hz and 121Hz by moving the panel crossover up to 121Hz.  That is peanuts compared with having the whole range depressed.  By the way, I think the whole range depression may result partly from the infamous dipolar cancellation.  Linkwitz deals with this in his design by calculating the effect and deliberately equalizing it.  I'm canceling the effect by measuring the system and room correcting it.

So I'm listening with the small hole in the midbass instead of the big midbass depression that I had previously.  Eventually I'll have to go back and do a whole new series of corrections based on the new crossover.  I was going to tell you how much work that was this time (I did two sets of 6 corrections over the weekend...I had to run a second set because I made a couple of mistakes in the first set; each set has two measurements (for helpful redundancy) for regular, no supertweet, and no sub or supertweet conditions).  But I before I do that, I also need to see if I can push the subwoofer to cross over slightly higher.  That work will require more measurements and tests too.

I can just press the Bypass button on the Tact remote to bring uncorrected response.  It's no longer the uncorrected response from January, as prelude to Tact correction I changed tweeter highpass to 20kHz and changed from LR48 to BU24, level reduced by 2dB net, and fine-tuned crossover delay.  That made for something like perfect impulse reproduction, at least with the supertweeter adding to the Acoustat nicely and even making impulse sharper.  And now, of course, bypass now has both the hole in the 104-121Hz midbass AND the 110-400Hz depression, whereas before it just had the depression, so now bypass is slightly worse in the bass, arguably better in the treble than it was before.

Anyway, though the bypass is slighly different than before, I don't believe it has gotten much worse.  But compared to the corrected response, you just don't want to listen to it anymore.  Sure it's very open.  But it's very thin and bright sounding, with too much highs above 1kHz (just consistently bright) and no midbass.

That was why I do room correction!

While great progress was made, now I know that even more work will be required that I was expecting to be sufficient, with no end in sight (such as working on the speaker itself).

But that's the way life is, isn't it?  If there were nothing more to be done, how fun would that be?

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