"Room Correction" claims to compensate for the anomalies resulting from the
recombination of direct and reverberant sound field information. It does
this by comparing the test signal to the recorded signal, and applying DSP
to complement the end result, mainly by applying equalization to boost
response dips, and attenuate response peaks. In a simple world, this would
be a good idea. However, in our world, serious problems arise. These are:
● Lack of precision in EQ, measuring, or application.
Without very precise processing and very high resolution, the EQ adjustment
is often way too broad. For instance, it may attenuate a band with a peak
present to average a flat response. However, that band, say 1/3 octave, may
not just have a slightly lower peak, but a couple of new dips above and
below that frequency. It may measure flat with a low resolution measurement
like a 1/3 octave spectrum analyzer, but in effect you've just shifted your
problem into multiple problems. In the cases where EQ can be of substantial
benefit (i.e., subwoofer notch filter attenuation), the measurement must be
capable of resolving 1/12th an octave or smaller, and the EQ itself should
be of the fully adjustable variety, where not only amplitude can be
adjusted, but also the specific frequency, and the width of the boost/cut,
or 'Q' of the filter.
● You can't undo reverberation. The reverberation of
a room is similar to that of a bell. If you hit a bell and ring it, you
can't undo the ring by hitting it differently. If you hit it differently, it
may sound different, but it's still ringing.
● With the possible and contentious exception of notching
frequencies low enough to be relegated to subwoofer reproduction
exclusively, our auditory system will pick up the changes in the direct
sound. Instead of sounding like the room went away, it will sound like we're
in the same room, but something screwy happened to the original event. In
attempting to fix one problem, we've done little to fix the problem, and
have created another.
● Even if the above problems could be addressed, and the
room 'corrected,' there is the fact that the acoustic character of the room
changes with listener position. That is, even if we could provide a correct
compensation for one listening area, the compensation would be a total
crapshoot when it came to the listening experience just two feet (one seat) away
from that position.
Bottom line: Simply applying EQ to a speaker does not "correct" the room.
Although you can counter frequency response aberrations at the source (such
as may be inherent in the speaker), you can do nothing for issues beyond the
To correct a room, you must physically change what is in the
room and where it is in the room. This
means a minimum of sound absorption material to attenuate the strongest and
most detrimental reflections, as well as spending serious time positioning
and aligning the subwoofer, or in some cases using multiple subwoofers to
smooth out the response. Then and only then does EQ become the "final
Even after applying room treatments, many automatic room correction systems fall short in that
they only provide correction when taking into account a single listener, the
one who sits with their head in a vise at the exact same position the
microphone was in during set-up. We can illustrate this by going through the
auto-set-up routine with the microphone in different positions and then
checking the settings: It will choose very different settings depending
on which seat the microphone was placed at when you ran the auto routine. In
some cases, the
people seated elsewhere end up with WORSE sound!
In fact, you could use a device
equipped with two microphones and a custom-built collecting surface, often
referred to as our head, complete with outer ears, which is a bit more
sophisticated than a single microphone, but you might find the adjustments tailored to
a single location artificial or distracting. In other words, the sound could
even be worse at the optimal listening position.
Is EQ useless? Certainly not! It simply requires more
intelligent and sparing implementation.
An outstanding Auto EQ scheme is the Audyssey system we've tested
in a couple of consumer products that have licensed it. As we understand it
the Audyssey originated as an attempt to automate the complex and time consuming
task of aligning the sound systems of commercial theaters. One of its
fundamental features which sets it apart is that it takes several
measurements throughout the listening area and applies a "best overall" EQ
carefully and sparingly. Coincidentally, when we spoke to the Audyssey
creators, they agreed that there is only so much you can do, and when
it comes to electronic 'correction,' some things are better left undone.
Even then, the most dramatic benefits achieved by the Audyssey 'correction'
technology lie in loudspeaker correction. If you start with good
loudspeakers, the benefits of electronic processing are far more subtle.
Assuming you have a good subwoofer, start by setting all
your speakers to Small, using 80 Hz as the high-pass frequency (we have an essay
which makes a pretty good case for this), set your distances according to
what they actually are, and set your levels with either the built-in
noise generator (if you have a THX unit) or suitable external equivalent using a $35 SPL meter from Radio Shack. After getting used to the sound of a standard
calibration, fiddle a little bit here and there (experiment with different
crossover frequencies within the operating range of both the main and the
subwoofer, and massage your channel levels maybe one dB, or in rare cases
If you are really daring, get your hands on a CD or DVD with low frequency
sweeps to get a sense for what frequencies are a problem for your subwoofer
and use a fully parametric (meaning that it includes "Q" as an adjustment), extremely specific, manual EQ to cut (never
boost) the problem frequencies with surgical precision, and then let your
ear be the final judge.
- Brian Florian and Colin Miller -