Crane Song AVOCET II - Manual (Page 43)

contribution. Reverse the polarity of the sub. The polarity which produces the loudest bass is the
correct polarity . Mark it on the plugs, and don’t forget it!
Next comes an iterative process (“lather, rinse, repeat until clean”). Here’s a summary of the four-
steps: (1, 2, & 3) Using filtered pink noise, we’ll determine the precise phase, amplitude and
crossover dial position for
any one crossover frequency . (4) Then we’ll put Rebecca back on and see
if all the bass notes now sound equally loud. If not equally loud, then we’ll go back to the filtered pink
noise and try a different crossover frequency. We keep repeating this test sequence until the bottom
note(s) has been made “even” without affecting the others. With practice you can do this in less than
half an hour. Adjust each subwoofer individually, playing one channel at a time.
And now in detail:
1) Crossover frequency (lowpass)
Play filtered pink noise (or the Mix CD’s multifrequencies) at your best guess of crossover frequency,
say 63 or 80 Hz. Notice that the signal has a
pitch center, or dominant pitch quality. If the subwoofer is
misadjusted, adding the sub to the satellites will slide the pitch center of the satellite’s signal. Reverse
the sub’s polarity (set it to
incorrect polarity). With the sub gain at a medium level, start at the lowest
frequency, and raise the frequency until you hear the dominant pitch begin to rise (literally, the center
“note” of the pink noise appears to go sharp, to use musical terms). Back it off slightly (to a point just
below where the pitch is affected), and you have correctly set the crossover to this frequency.
Recheck your setting. That’s it.
2) Phase
The sub should always be on a line with or slightly in front of the satellite. With the woofer a moderate
amount in front of the satellites, the phase will generally need to be set something greater than 0
degrees. Return the sub(s) to
the correct polarity. Play the same frequency of filtered noise and
increase the amount of “phase” until you hear the dominant pitch rise. Back it off slightly, recheck your
setting, and that’s it
3) Amplitude
The subwoofer’s settings are exactly correct when its amplitude is identical to the satellite’s at the
crossover frequency. The subwoofer gain is the easiest to get right because there will be a clear
center point, just like focusing a camera. Play the filtered noise, and discover that the pitch is only
correct at a certain gain, above which the pitch goes up (sharp), and slightly below which it goes
down (flat). “Focus” the gain for the center pitch, which will match the pitch of the satellites without the
sub. Recheck your work by disconnecting and reconnecting the sub. The pitch should not change
when you reconnect the sub, otherwise the gain is wrong. To be extremely precise, increase the gain
in tiny increments until you find the point where the pitch rises when the sub is connected, then back
the gain off by the last increment. This process is extremely sensitive.
4) Rebecca
Play
Spanish Harlem again. If all the levels of the bass notes are even, you’re finished with steps 1-4.
If you hear a rise in level below some low note, then the crossover frequency is too high and vice
versa. Do not attempt to fix the problem with the subwoofer gain, because that has been calibrated by
this procedure, which leaves nothing in doubt except the choice of crossover frequency. Go back to
step one and try again. Once all the notes are even, your crossover is perfectly adjusted. Write that
frequency down. Then, for complete confidence, check the nearest frequency above and below (go
back through steps 1-4), proving you made the right choice. This piece of test music is sufficiently
useful that there will be a clear difference between each 1/3 octave frequency choice and it will be
comparatively easy to determine the winner. The trick is not to rely on our faulty acoustic memory, but
on the ear’s ability to make relative comparisons.
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