Friday, February 11, 2011

Sometimes it's best to do things the right way (LR48)

In earlier post I described various strategies for crossing over (or not) my electrostatic panel speakers and my subwoofers.  Last night, I found that one of the strategies I have passed over for awhile does indeed work very well.  That is the strategy of having a symmetrical crossover, with high pass and low pass cutoffs set for the same frequency and using the same crossover function (such as Linkwitz-Riley or Butterworth) on both sides with the same ultimate slope (such as 24dB/octave).

Listening to some FM radio for the first time in a couple months, I was first impressed by the clarity of the sound and incredible spacious imaging.  (This mainly results from moving the listening position forward about 8 inches, also from recent changes to the sub/panel crossover.)  But I was also appalled by an apparent dryness, bordering on harshness.

Now I've been fiddling a lot with the crossover controls recently, but when I made the above observation I had the panel highpass at 104Hz with BU24 (butterworth).

First I tried changing that to 84 Hz, same as the lowpass on the subs.  Big improvement, now the sound is sweet and less dry (funny how improving midbass makes the midrange sound nicer, but not surprising really).

But then it actually sounded a bit boomy.  So I changed the BU24 to LR48 (Linkwitz-Riley) and that cleaned up the bass.  Now it was sounding very nice and I just kept on listening for quite awhile.

Also funny that when I started and was thinking the sound was harsh, I turned off the humidifier like yesterday.  But that didn't seem to have as much affect as changing the crossover.  After changing the crossover, I was enjoying FM radio with the even noisier dishwasher running.

Now the idea that both highpass and lowpass sides of the crossover should have cutoffs at 84Hz, and that they should both use LR48, that sounds pretty obvious, right?  So why wasn't I doing this already?

Well, with somewhat less careful recordkeeping, I did try this last year.  But one of my goals way back then was to take advantage of my dual subs and improve my ability to play more loudly by offloading bass from the panels (which can't play back much bass without bottoming) and onto the sub (very high output capability).

And I had a test, one version of Polyvtsian Dances, where a tympani made by Acoustats rattle.  I found that I could get rid of the rattle by moving highpass point to 121 Hz.  So that was where I set the crossover and where it stayed for about 12 months.  (Actually, for other reasons, I set the sub lowpass even higher, and then let the Room Correction fix up the response, a very bad approach I now know.)

Well I haven't repeated the rattle test, but now I have a notch filter at 116Hz specifically to handle the rattle.  So now I may be able to have good sounding bass without the rattling.

The other point was that there are terrible room modes in the range of 90Hz-110Hz.  I could in effect EQ these down by spreading the sides of the crossover apart (I call this underlapping).

It turns out to be a very bad idea to have sub playing in the 90Hz and up because it excites room modes horribly.

But it works out fine to play the electrostats down to 84 Hz, because their dipolar character basically doesn't excite those full-wave-and-up resonances.

Hearing how much better it works to have full crossover at 84 Hz, I don't think I'll be trading that away for loudness capability any more.  Except I may have a secondary setting for those days when I need more loudness.

I have also been thinking about the possibility of making my notch filters amplitude dependent.  The Behringer does provide that option, but it might do more harm to the sound than good.  But along those lines...why not have amplitude dependent crossover point?  When you play really loud, the crossover automatically ratchets up to a higher frequency to protect midrange panels.

Fully crossing over both sides with LR48 how does the bass sound?  Almost like pure electrostatic.  Interestingly, LR48 like all LR crossovers actually reduces energy around the crossover point (helping to avoid feeding those resonances) but maintain amplitude in the axial response, so that straight ahead the drivers add up arithmetically.

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