Wednesday, February 9, 2011

my latest invention: Boom Control

By now, it's pretty clear that using digital signal processing (DSP) both to analyze my system and correct it are very useful.  I'm really enjoying the new cleanness that comes from having used notch filters to negate some very serious buzzes in my Acoustat panels.  I imagine now I was hearing those buzzes all the time, and I thought they were coming from something else, like the subwoofers.  I'm worried that the 116Hz resonance may not actually be easily fixed by giving the panel membranes a hair dryer heat treatment.  It may be something related to the frames or individual panels, and a repair may require added bracing, etc.  Lots of people find they have to brace their Acoustat 1+1's for better sound.

But moving on, DSP also has limits.  Flattening the frequency response at the listening position may not remove serious resonances (room boom) elsewhere in the room.   Now, you could take the position that these resonances don't matter.  But they often add a "strained" quality to the sound, as you hear walls shaking, etc.

This is an old argument.  Those who sell acoustic treatments often say DSP is worthless, and vice versa.  The truth is that both DSP and acoustic treatments can be useful under some circumstances, and with some limitations.

DSP is the more magic technology.  I find it amazing the way I can simply dial in a few notch filters to fix serious problems with my speakers that could take weeks of frustrating work to fix.  And within the range of tasks DSP can accomplish, it has amazing dept, it is amazing to be able to make things like notch filters with a Q of 10 (which seems to be about right for one semi-tone, 8.9 works for two semi-tones).

Now I have come up with a DSP answer to the old DSP limitation of not being able to flatten the frequency response everywhere in a room at the same time.  It's not a perfect answer, but it's better than nothing.  And it can't be worse, because you can simply turn it off (in my case, with a remote control).  I call it Boom Control.

Some recordings need this, others are better without it.  Typically, you don't mind the room boom outside the listening position while listening to an acoustic recording.  But you may find it intolerable while listening to a recording with electric bass.

It seems that 6dB make a nice filter depth for this sort of thing.  I have room boom outside the listening position principally in the region 40-55 Hz.  For some recordings with electric bass, it seems helpful to engage a 6dB filter with Q of 1.6 at 42 Hz.

Unfortunately, although intended to reduce the boom outside the listening position, it does also reduce the bass frequency response AT the listening position.  It may make some rock recordings sound slightly anemic.  On the other hand, it may make some otherwise intolerable recordings very listenable.  So, it's a compromise, but it works.

The effect of the reduction around the room periphery seems more than I would have expected from a 6dB reduction, while the bass at the listening position seems reduced a little but not 6dB.  Basically, you switch on the Boom Control, and all the bass rattling (or most of it, anyway) goes away.  Suddenly you have a real image with bass instruments, etc, right in front of you where it should be.  That's how it works sometimes.  Other times, you feel "where did the bass go" and then you switch off the Boom Control and you get all you want back again (because perhaps a lot of it was in the 40-55Hz range being attenuated).

Boom Control is essential for the record Bass Ecstasy (which, btw, I find quite fun to listen to in a guilty pleasure sort of way).  On Dark Side of the Moon, I prefer to leave the Boom Control off because the engineering of the record has already eliminated the boom and it sounds a bit too dry with Boom Control.

I have dialed in this filter into EQ #1 on my Tact RCS preamp.  I can turn it on or off with remote control.

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