Tannoy Reveal user manual Console Reflections

3.3 Console Reflections

The number one killer of smooth near-field monitor response is the mixing console. Most people park their near-field 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 problem for the ten years or so since near-fields 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 600mm (24”) 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 (interference) caused by the reflection from the console top taking a fraction of a second (and a tiny fraction!) longer to get to your ear than the sound directly from the speaker. And you thought you couldn’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 created 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 outdoors 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 (200-800Hz), where the wavelength is long enough at 400mm-1400mm(16”-55”) that you don’t get cancellation, you get the wavefronts adding together. This causes an increment 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 near-fields 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 experiment.

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 people 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 600mm 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 shortest straight-line path that sound can take from the speaker to your ear, no bounces, no reflections. The 2 millisecond reflection window really affects the character of the sound at the mix position, drastically altering the response of the speaker in the critical audio bands of 500Hz and above. The 10 millisecond reflection window does some more subtle things