Secrets Q & A
- Written by Brian Florian and Colin Miller
- Published on 30 November 2007
"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 loudspeaker.
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 tweak".
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 two dB). 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.