Tuesday, February 8, 2011

Buzzes fixed with notch filters

Yes I know now my Acoustat speakers should eventually be taken apart, as much as can be done, and have their membranes heat treated to restore original tension.  Hopefully that will get rid of the buzzing resonances at 116Hz and around 172 Hz.

And in recent posts, I've described how I moved the cutoff frequency for the Acoustats from 104Hz up to 121Hz to get rid of the buzzing (that was good for a Tact level of 83dB out of 99.9 on "A Story Within A Story" by Pat Metheny).

That's a neat trick, but added to the midbass depression around 100Hz.

Last night I took a slightly more direct approach and used the Parametric Equalizer option on my Behringer DCX 2496 digital crossover to notch out two notable buzzy resonances in the Acoustats.  I added a -15dB notch with Q of 8.9 at 116 Hz, and a -6dB notch with Q of 7.9 at 172 Hz.

With these notches in place, I could restore the crossover point to 104Hz (restoring some of the lost midbass) and still play A Story Within A Story as a Tact level of 93dB without getting the buzz.  That's more than 10dB of added dynamic range.

With the "Sine Wave" program I created on my Kurzweil K2661 synthesizer, I can easily go up and down the keyboard to identify the problem buzzes and other resonances, and then test how well I've notched them out afterwards.  Before the notch, I did not want to press the A2 or A#2 keys for fear of getting painful sound.  Afterwards, I can play all the keys in that region, and they are all about at the same level, with A2 and A#2 only sounding slightly depressed.  I identified the need for the second notch at 172 Hz purely by using the keyboard.

The sound with all the new DSP programming I've done recently is incredibly clean, spacious, enjoyable.  The dynamic range is far greater, and I can listen to fairly heavy bass without cringing.  In addition to the Pat Metheny, I listened to Dark Side of the Moon and Bass Ecstasy.  DSOM was like hearing it for the first time, there was so much more inner spaciousness from recent changes, including more forward listening position.  Bass Ecstasy could finally be turned up loud enough to be really enjoyable.

Bottom line: if you do not fix buzzes the "old fashioned way" by eliminating them at the source, it is critical that you do something about them anyway using notch filters.  It's worth sacrificing a tiny bit of frequency response flatness to get rid of painful and potentially destructive resonances.  Little is more annoying than having your loudspeakers buzz.

(BTW, the legendary BBC-designed LS 3/5A speaker actually has an very terrible resonance in it's Bextrene woofer around 1kHz.  The KEF B110 woofer was heavily treated, but still has the resonance, so it gets notched out in the crossover, one of the reasons the LS3/5A crossover is so complex.  So many of the most highly appreciated audio products depend on the same kind of tricks that I use.)


Playing on the Kurzweil, which is in the corner of my living room, I was struck by how loud the resonances around 42 Hz are, even after full Tact room correction.  It turns out that these resonances (which span the range 35Hz to 50Hz) seem to be most offensive in the corners and elsewhere around the periphery of the room.  In the central portion of the room, these resonances are not directly audible.  The Tact system is only correcting the resonances detected from the position(s) of the microphones.  It can't correct resonances that only increase the SPL somewhere else in the room.

Actually, the Tact does have a multiple-measurement feature which I believe is intended to deal with problems like this.  But I'm not clear on how to use it or how it works yet.

A problem like this is something that requires a compromise solution of some kind.  To reduce the BOOM around the room it may be necessary to take a slight hit on the frequency response at the listening position.  But how much?  Well, that's up to me, a computer can't make the decision.

One obvious solution might be split-the-difference.  Assuming flat response at the listening position, but 20dB peak in the corner, chose a 10dB adjustment.

I tried a 6dB solution like this.  I had figured out long ago that a 42Hz notch with Q of 1.6 worked nicely on these resonances (it had already been dialed in on my DCX, but was turned off).  So I pulled that region down by 6dB.

Unfortunately, it still sounds boomy in the corners, and 6dB reduction around 42Hz in the listening position makes for a wimpy bass sound on some recordings, while still sounding stressed on certain others (like Bass Ecstasy).

A better solution here would be bass traps to reduce the resonances acoustically.  But those are expensive.

For now, I'm going to have 3 EQ levels dialed into my Tact as remote control selectable options.  A 3dB reduction (barely noticable loss of bass, but helpful reduction in boom at periphery), a 6dB reduction, and a 10dB reduction.

For the real kick-back bass, I'll turn the EQ off.  For entertaining guests, I might use the 10dB reduction.


Both of the above are uses of digital signal processing (DSP) to deal with specific problems.  Now when one is also using correction, that raises an additional issue.  Should one do the correction measurements with the notch filters in place, or just add them in afterwards?

It occurs to me that in cases like these it is desirable to add the notch filters afterwards.  Thus these are "post correction filters".

If the notch filters are added in prior to correction, the correction system may attempt to un-notch them to some degree.

However, it may be that the correction system, by design, doesn't do much for notches.  It is longstanding conventional wisdom that audio response notches should not be boosted.

Given lack of understanding about how Tact actually works, it might be desireable to try this both ways.

Likewise for the peripheral room correction.  This is a nice kind of thing to have as remote control selectable option as a post-correction filter.

In principle, the need for post correction filters is reduced by the Tact having "room curve" facilities.  You could draw needed notches right into the room curve.  But once again, not knowing exactly how to draw a Q of 8.9 notch makes this a bit tricky.  Also, it's far easier to make and test small adjustments by turning knobs on the Behringer than running the complicated RCS 2.0 software.

Nothing beats an automated system like RCS for flattening (or curving in desireable ways) the overall frequency response.  And it works OK on most resonances.  But some things require a bit more focus to do correctly, and in tricky situations setting up notches and other post-correction filters manually after full correction might be the way to go.

I would like Tact better if it were a more open system, say for example if you could chose which correction algorithms you wanted to use.  But it is still far better than "fully automated" systems in that Tact lets you can draw target room curve, and see what measurements are being made so you can fix things up prior to correction.

There has been a trend toward fully automated DSP systems recently.  I don't trust those at all.

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