Installation manual

Community S-Series - Operation and Installation Manual - Page 38
So here we have a loudspeaker installed in a room. We already know that this loudspeaker
exhibits a flat response in a free field environment, such as outdoors or in an anechoic
chamber. But what happens when it’s installed in a room?
Logic dictates that whatever changes occur to the response of the loudspeaker in the room,
are dependant entirely on the effect of the room (unless, of course we wired the
loudspeaker wrong, or broke it in transit…which we didn’t).
Now as we listen to our loudspeaker, we hear things we didn’t hear in the free field
environment. It sounds bass heavy. It sounds like there’s a buildup of energy somewhere;
say around 300 Hz. We also hear something happening at about 600 Hz. What do we do?
Let’s measure it. Let’s assume we have a narrow band, a high resolution FFT-based
6
measurement instrument and a perfectly flat microphone (these do actually exist). Should
we measure it nearfield, say about 1 meter away? Why not? Somewhere we heard that’s a
good thing to do.
We place the microphone about 1 meter from the loudspeaker and we look at the response.
It’s quite flat. Not like it looked when we measured it outdoors, but not all that different.
Overall, the lower frequencies exhibit a gradual rise in amplitude as they drop in frequency,
but there’s also some ‘rolling hills’ up to about 800 Hz.
We grab our graphic equalizer and try to smooth out these rolling hills and the rise in the
bass response. A cut at 63 Hz merely puts a hole in the response at 63 Hz; it doesn’t fix
the rolling hills. But the loudspeaker does sound less bass heavy when we run the music
track. More cuts at 125 and 250 again help it to sound less bass heavy, but we can clearly
see we’re ‘chopping up’ the response curve. Maybe these minimalist guys are right….too
much EQ really chops things up! Too bad there’s not a filter on this thing that produces the
inverse of the whole response shape.
Let’s try moving the mic to the mix position. That’s seems to be a good idea. Put the mic
where the sound operator is.
Wow. Now there’s a whole new picture. The holes from the graphic can barely be seen
anymore. Instead, there’s a big bump at 362 Hz and again at 725 Hz, and the whole low
end is even more accentuated.
We try using the graphic to flatten the response. We try for a long time, but no
combination of filters will flatten the low end. Pulling down 315 takes part of the 362 Hz
bump out, but not all of it. Pulling down 400 just puts a hole above the bump at 362 and
makes the bump look even bigger than before. Same problem at 725 Hz. “This isn’t
working! It must be true… you can’t really EQ a room.”
7
Someone says, “Let’s try this parametric equalizer instead.” You’re ready to do anything.
After setting it up, you’ve found it has a shelving filter with an adjustable turnover
frequency. You try cutting it 8 dB and the whole low end quickly flattens, except for the
362 Hz bump. But the slope’s not quite right. There’s still a quick rise around 900. You
move the turnover frequency up to 900. Like magic, the whole low end is now flat except
for the bumps at 362 and 725. Engaging a bandpass filter, you dial up a peak of 4 dB
making the bandwidth quite narrow. In a few seconds, you’ve easily centered the peak
squarely on the bump at 362 Hz. Now you cut it and fiddle with the Q. In a few more
seconds, the bump is gone. No trace.
You repeat the process at 725. Again it’s gone without a trace. But this has to play havoc
with the phase, doesn’t it? Something has to be wrong. It’s too easy.
The guy who owns the FFT tells you that because you’re looking at the transfer function of
the loudspeaker in the room, you can also see the phase response if you want to. He
pushes a few buttons and there on the screen is a phase response trace, along with the
frequency response trace. It looks remarkably flat from about 200 Hz up to 1 kHz or so.
You bypass the equalizer and the bumps are back, along with the big rise in low end.
Remarkably, the phase trace now shows two wiggles, dead centered on the 362 and 725
6
FFT is an acronym standing for Fast Fourier Transform and is based on the Discrete Fourier Transform, a mathematical algorithm
defined by French mathematician Jean Fourier. FFT measurement instruments are vitally important to the study of sound and
vibration.
7
No matter what technique you use you can’t, of course, EQ a room; you can only EQ the sound system in the room. But much of
the world refers to the process of equalizing a system as ‘room-tuning.’