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Another interesting note, the Tannoy true point-source Dual Concentric™ monitors are free from the physical offsets previously
described, which means that the behavior of the Dual Concentric™ monitors will not change in the horizontal, vertical or any
other plane you can pick. The other neat thing about Dual Concentric™ monitors is that no matter how close you get to them
you can't get the image to fragment into separate high and low frequency sources.
Now, if you were to follow the all too common practice of lying your two way monitors on their side to give you better sight lines
over your meter bridge, you can see (and hear) what will happen. With the monitor on its side, moving your head horizontally
means you are now moving through all those rays, or lobes, where the wavefronts from the woofers and tweeters interfere with
each other. The midrange frequency response will be different for each head position. All two way component monitors, no
matter who manufactures them, need to be used with the multi-driver axis vertical (that's just the way it has to be when you're
in the near-field). And if you're wondering how three-way near-fields work with a whole bunch of speakers stuck all over the
baffle, well... you'll quickly realise why we stopped at two way speaker systems.
3.2: POSITIONING
This is the monitor equivalent of a wheel alignment. Where do you aim the speakers to give you the smoothest and most consistent
sound, and how far apart do you place them to give you a good stereo image? The basic rule is to follow the layout of an
equilateral triangle. The distance between the two monitors should be roughly the same as the distance between one monitor
and your nose in the listening position where you are leaning forward on the console armrest. See the following diagram.
The speaker axis (shown on the diagram) should be aimed at the halfway point between your furthest forward and the furthest
back listening positions (as indicated by the two heads on the diagram). This is typically a range of about 24" (600mm). If you
can, you should line your ears up with the vertical speaker axis (half way between the woofer and the tweeter). Remember the
earlier drawings showing your ears and the speaker, these were to get your normal listening position lined up in the best spot
possible. If this would have you resting your chin on the console, you could tilt the monitor back slightly. This keeps your head
in the sweet spot whether you're leaning forward adjusting level or EQ, or leaning back and listening to the mix. Don't go crazy
trying to get this exact to three decimal places, within a few inches will suffice. Your Tannoy monitors have a wide sweet spot
both horizontally and vertically to reduce the variations in sound quality as you move around doing your recording engineer
stuff. Turning the monitors in like this has an added benefit of keeping the high frequencies from reflecting off the walls and
outboard gear.
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, maximising 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 equaliser, 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
equalisation 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 minimise 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 to the speaker's response, because the
ear/brain reads reflections arriving within 10 milliseconds of the direct sound as being part of the speaker's response, these reflections
can pull the stereo image around in different directions at different frequencies. Enough theory, now to the lab.
Here's where we have fun with science. Take the microphone stand and place it at the mix position. Attach one end of the string to
the top of the mic stand, and stretch the string out to the front of one of the monitors. This is the direct sound path from the speaker
to your mix position. To locate all the surfaces that will contribute reflections within that magic 2 millisecond window, add 600mm(24")
to the string you have stretched out. Take a small piece of gaffer tape and attach the string to the baffle (NOT the drivers!). Now,
every surface you can touch with any part of that string can contribute a reflection to your mix position.