Crane Song AVOCET II - Manual (Page 45)

What To Do When the Results are Less Than Perfect
When interpreting
Spanish Harlem , don’t get too hung up on little “dips” in level. Dips are less
objectionable to the ear than peaks. First, attack problems with resonant notes and then look at the
dips. Everything may not be rosy the first time around. Supposing that the subwoofer helped the
bottom note(s), which means the crossover is at the right frequency, but some upper note in the
progression has been affected. This means the subwoofer position is not optimized, or the subwoofer
has some frequency response anomaly. As the sub is moved towards the room corners, the low bass
response goes up, previous dips become peaks. There’s cancellation/reinforcement between the subs
and the satellites, which changes complexly as the sub is moved. Thus, adjusting the subwoofer
position is a powerful method to even out the bass, but this type of trial and error is too complicated
without test equipment. You could slide the woofer slightly, adjust the crossover as above, listen, move
it again, readjust, and listen, but our acoustic memory is too short to tell when we’ve hit the perfect
spot.
Advanced Techniques
Integrating the Instruments with the Ears
Here’s where it gets complicated. If you are having problems with uneven bass, we can no longer rely
strictly on our ears. If you’re comfortable with measurement instruments, then let’s proceed. First,
listen to Rebecca and mark down the problem frequency or frequencies, either peaks or dips. You’ll
use that knowledge when you bring in the big guns, the 1/3 octave analyser. The good thing is that
Rebecca has already told you where the problems are, so you’ll know how to separate the forest from
the trees in the 1/3 octave display. I used
Spectrafoo (an excellent analysis program for the Mac) in
transfer function mode with wide band pink noise into both satellites and subs (one channel at a
time).
Spectrafoo time aligns the stimulus and response, which helps to separate direct from reflected
sound, more accurately representing what the ear hears. Spectrafoo revealed a rising response in my
room below 40 Hz, and more important, a little dip in the combined response circa 63 Hz which
corresponded with my perception that note was perhaps a little weak. By moving the sub around very
slightly and watching the display, I was able to exchange the weakness against the surplus without
aggravating any other peaks.
The strength of this method is we’re continuously integrating our powerful (almost objective) listening
judgments with the “over-powerful” analysis tool. We’re using the analyser for general trends, not
absolute amplitudes; that’s what I mean by separating the forest from the trees. The position of the
test microphone should be in the exact listening position. Wear earplugs to keep your ears fresh when
you’re not required to listen. After moving the woofer, don’t forget to readjust the crossover gain and
phase with our listening technique.
If all goes well, Spanish Harlem will be even better adjusted and we can rest assured that our system
is really really tweaked. Now sit back and enjoy. Oops, your work is never done. Now that you’ve
adjusted your system, I’ll let you in on one more secret: Servo amplifiers have internal adjustments
that affect woofer damping, make the bass “tighter” or “looser.” but that’s another story.
45