Issue #3 -
FILTER DESIGN AND DSP
Of all the challenges that we face every day like
product design and fabrication, marketing and just
running our businesses one stands out as most
demanding of our skills, most fun and most
gratifying: filter design.
Now, if you didn’t already know this about him, Dad
likes to take chances, likes to push his luck, likes to
find out where the limits are. Of course that has to
backfire once in a while, right? Get over it, that’s the
cost of exploration.
But when he started drawing a twin woofer two way
my eyes crossed! Come on, Dad, you taught me this
stuff when I was a kid. Remember the wave kinetics
demonstations in the bathtub? Twin woofers
decouple at the wavelength of the distance between
their centers. It’s nature. The idea is fine for a three
way where the woofer cut off is pretty low. But for a
two way? When has that ever worked? Haven’t
you’ve heard it when the other guys try it? Plus you’ll
have to run the tweeters down so low - POP! How
much did you pay for those tweeters?
Anyway, what would you gain in the bargain? Twice
the output. Or much more bass extension. Or some
combination. So there is an allure, I’ll give him that.
Dad figured we could use our DSP smarts and the
right tweeter to limit the woofers’ band all the while
protecting the tweeters. This strategy would not
have stood a chance with old fashioned passive
filters. I have to remind myself we’re not in Kansas
And we’re using a great new woofer that, with its
huge coil, can really dish it out. It’s just begging for
some serious DSP equalization at the bottom and we
are certainly using enough juice to bang out the kind
of bass energy that I demand.
All right, what the heck. We can always back off to a
single woofer design. So, OK. Go for it.
Right away we realized that DSP was opening yet
another door to the next level up. How far could we
take it? Off and on between our other work we keep
returning to the Kestrels and each time they get
better. You know, filter work is kinda like an eye
exam: you keep making changes until you can’t get
any more improvement. So, in a funny sort of way,
when it goes on and on for weeks that’s a really good
So, where are we now? Not only did the decoupling
issue yield to my attack and the tweets are happy,
but the big coils and big, smartly EQed power really
delivered the kind of power-band guts and resolve
that puts a smile on my face. AC DC from a two
way? No way, right? Hells Bells Yes!
Today, Dad the insomniac snuck in for an early
session before I got in. As I climbed the stairs to the
lab he was playing Duke Ellington’s Indigos album. It
was beautiful. Gobs of energy, wide open dynamics.
Timbrel detail top to bottom. Real real horns.
Midrange-orama. The stuff that most people just
don’t get. This little speaker punches way, way, way
above it’s weight. I can’t believe I’m saying this: but
they really do use the whole Kilowatt!
Are we done with the filter? Probably, I’m already
pinching myself; we could be at the limit. We’ll see.
But we’ve sure pushed that limit back a step or two.
And the beauty of DSP is that, should we take
another step, say, a year from now, we just email the
new file to our Kestrel owners, keeping them up to
Try doing that the ‘old way’. Ha!
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