Tannoy ProtoJ, Reference Manual

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3.3 Console Reflections
The number one killer of smooth nearfield monitor response is the mixing console. Most people park
their nearfield monitors on top of the meter bridge, which makes the console top the most influential
reflecting surface for the sound you hear when you’re mixing. How important is that reflection? If it
were such a big deal, wouldn’t people have done something different by now? Let’s face it, this
industry is not exactly quick to acknowledge a situation that makes it apparent we’ve ignored a prob-
lem for the ten years or so since nearfields became popular. So you still have reservations as to the
significance of this reflection? OK, dig out your pink noise generator, it’s time for an experiment.
You’ll need an assistant from the audience for this. Have your assistant hold your monitor up in the
air about 24” (600 mm) from the top of the console while feeding pink noise through it. Make sure
you’re listening right on the speaker axis (that line between the woofer and tweeter), and have your
assistant slowly lower the monitor onto its perch on the meter bridge. Hear that change in midrange
character as it gets within the last foot of the console? How different did it sound when it was up in
the air. There’s two significant things happening here. The first problem is comb filtering (interfer-
ence) caused by the reflection from the console top taking a fraction of a second (and a tiny frac-
tion!) longer to get to your ear than the sound directly from the speaker. And you thought you could-
n’t afford a Flanger for your studio. You may have noticed that in the last foot prior to touchdown on
the console the main sonic change was in the midband area. There is a real cruelty associated with
laws of physics, the speed of sound in air gives us wavelengths in the midrange area that just seem
to coincide with the dimensions of everything we mount speakers on, maximizing the problems cre-
ated by these reflections. The previous diagrams (horizontal and vertical) that show the interaction
between a separate woofer and tweeter also demonstrate the problem generated by the reflection.
Substitute the reflection for one of the drivers in the diagram and you’ve got a pretty good picture of
what happens when the original wavefront and the reflected wavefront meet. The big problem with
this comb filtering is that you can’t fix it with any terrestrial equalizer, once the waves cancel there is
nothing left to boost. You may also notice that as you move your head, the filtering changes, so any
equalization you attempted to apply for one position would be detrimental to all the other listening
positions.
We mentioned that there are two things happening here. The second thing is a change in directivity
caused by the addition of a boundary (the console top). When you want to shout at someone out-
doors you cup your hands around your mouth to increase the directivity of your voice. By placing the
monitor on the meter bridge, you’ve done the equivalent of putting one hand to your mouth. In the
lower midrange (200 - 800 Hz), where the wave length is long enough at 16” - 55” (400 mm - 1400
mm) that you don’t get cancellation, you get the wave fronts adding together. This causes an incre-
ment in the level over the lower midrange area at the same time that holes appear in the midrange
from comb-filtering effects. Now that you’ve begun to believe it is impossible to use nearfields on a
console top, we’ll talk about what you can do to help alleviate these problems. The first thing you
need to do is be able to identify the surfaces that are close enough to do serious harm. You can do
this at great expense by using a real time analyser, and spend a few months learning to use it, OR,
you can grab a length of string, some gaffer tape, and a mic stand, and get set for another experi-
ment.
First the theory. For unblemished stereo imaging and frequency response you would want to listen in
a completely reflection free environment, like an anechoic chamber, where all you would hear is the
image and the direct sound produced by the speakers, no nasty reflections anywhere. For most peo-
ple this is impractical. Next best thing, if you can establish a listening position free of reflections
arriving within 2 milliseconds after the direct sound (that’s the time it takes sound to travel about 24"
or 600 mm or less which represents all frequencies from 500 Hz and up), and minimize reflections
arriving within 10 milliseconds of the direct sound, you can maintain a remarkably stable stereo
image, and uniform response throughout the mix area. The "direct sound" is just that, it is the short-
est straight-line path that sound can take from the speaker to your ear, no bounces, no reflections.