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Good Vibrations<br />
So, you’ve got a monitor... what do you do with it? Acoustic oracle, White Mark’s David Bell,<br />
talks to <strong>Audio</strong> <strong>Media</strong> magazine about the basics of speaker placement, rooms, and stands.<br />
A<br />
udio <strong>Media</strong>: It’s common knowledge that<br />
you can’t consider speaker per<strong>for</strong>mance<br />
without considering the room. Could<br />
you start us off with some basics?<br />
David Bell: A room and a set of speakers<br />
should always be considered as a unit.<br />
Both influence what you hear, and the<br />
interaction between both influences what you<br />
hear almost as much as the choice of either.<br />
So, if you to have a production space<br />
with free-standing speakers and you put<br />
the speakers largely where they need to be,<br />
and symmetrically in the room, the room<br />
will interact with the speakers in a number<br />
of ways. It will itself have modes which will<br />
cause bass colouration, and the positioning of<br />
the speakers will alter the interaction of the<br />
speaker with the boundaries. By that I mean<br />
it will reflect off the walls and you’ll have<br />
‘coherent interference’. This will cause notch<br />
filtering, and is principally noted in the lower<br />
mid and the upper bass, which is really where<br />
the character of voices is stored.<br />
Even free-standing speakers ought to be<br />
set up by someone who knows what they’re<br />
doing. You will get interaction between the<br />
walls, the ceiling, the floor, the speaker, and<br />
you. You need to optimise that. You can do this<br />
by listening to stuff that you know well, and<br />
adjusting the speaker position – moving it six<br />
inches will make a huge difference.<br />
As a fundamental thing, it’s a good idea not<br />
to have it the same distance from the floor, the<br />
ceiling, and both walls so that the interactive<br />
effects of those boundaries are not lying on top<br />
of each other, and there<strong>for</strong>e smeared a bit.<br />
A second thing is that the speaker is<br />
trying to give you a full bandwidth of output<br />
out of its speaker cones. It should be borne in<br />
mind that as frequency goes down, as much<br />
will come out of the back of the speaker<br />
as comes out the front. The tweeter fires<br />
higher frequencies <strong>for</strong>wards in a cone – it’s<br />
directional. It becomes less directional as<br />
the frequency its carrying goes down to a<br />
wavelength comparable to its diameter<br />
and larger.<br />
The same is true of the midrange and the<br />
bass, but the bass tends to be much more<br />
360-degrees radiating, because basically<br />
the box vibrates.<br />
On A Sure Footing<br />
AM: What advice do you have on stands?<br />
DB: You need to mount it on something<br />
solid, so that the mounting you’ve got<br />
doesn’t absorb frequencies specific to the<br />
mounting... This is particularly true where<br />
you’ve got a nice bit of furniture, and you’ve<br />
got a computer keyboard and a little mixer,<br />
and you stand the speakers on a little<br />
bookshelf shelf at the top of it. At least put<br />
a paving slab on top – something heavy that<br />
the speaker can run against.<br />
There’s an awful lot of rubbish talked about<br />
high end, really expensive, rubbed-againstthe-thighs-of-virgins<br />
magnesium-alloy...<br />
What you want is a sodding great heavy thing<br />
to stand your speakers on – and it’s no good<br />
putting it on a wonderful speaker stand,<br />
wonderfully linked up to a diaphragmatic<br />
wooden floor, because that’s just the same.<br />
Although if your speaker stand is full of<br />
lead, it’s then going to be quite heavy, so the<br />
resonant frequency of the floor membrane it’s<br />
standing on is going to be much lower.<br />
AM: Is there any sense in de-coupling<br />
speakers – by using foam products,<br />
<strong>for</strong> example?<br />
DB: You have to be careful. If you get a<br />
piece of foam and stand a speaker on it,<br />
the frequency at which it is de-coupled –<br />
the resonant frequency of that compressed,<br />
sprung system – is proportional to some<br />
interesting constants, times one over the<br />
deflection. The further you deflect it,<br />
the lower the frequency it works at.<br />
We try to get our floors to deflect about<br />
4 GOOD VIBRATIONS<br />
6-8mm, and that gives you between 8-14Hz<br />
resonant frequency. So anything from double<br />
that up – from 20Hz up – is then de-coupled…<br />
But if you’ve got, say, 2mm of deflection, your<br />
resonant frequency would be higher, and<br />
would only clear once you’re well into the<br />
audible range. Thus, you’ve got a problem at<br />
the bottom end – it’s getting absorbed by stuff<br />
that’s not very well de-coupled.<br />
This is why proper mounts are better than<br />
bits of plastic. They manufacture them so that<br />
if you put a certain weight on it, you will get<br />
8mm of deflection, and it’s known to be within<br />
the elastic limit of the squashy stuff, so that it<br />
will work properly at that frequency.<br />
Up The Ante<br />
AM: What about moving on to bigger rooms,<br />
bigger speakers, and bigger budgets?<br />
DB: You can start designing the room to try<br />
and increase the effectiveness of the roomspeaker<br />
combination. One of the things<br />
you can do is to minimise the effect of bass<br />
reflection by soffit mounting the speakers –<br />
building them into the wall. This is done not<br />
just by sticking a piece of wall fabric under<br />
the speakers, but by building them into a<br />
substantial wall in a big, heavy box. We then<br />
pack the front wall – we use mineral wool,<br />
though other people do others things.<br />
The bass end becomes more efficient<br />
because it’s all going <strong>for</strong>wards. If you correctly<br />
soffit mount a speaker, you’ve got the early<br />
bass reflections under control. And then if<br />
you carefully design the distance between you<br />
and the walls, you can further improve those<br />
early bass reflections.<br />
Architectural Considerations<br />
We now need to look at the room itself. Cubic<br />
rooms are a bad plan. Everyone’s heard of the<br />
various imperfect ratios of height to length<br />
to width. These all work in different ways,<br />
and to different standards; and different<br />
ratios have different devotees. But effectively