[yancrazy]

Tuning a room is quite often a massive mathematical problem to deal with and as you have already run some tests on your control room i think you can handle some more info that i dug up from an future music mag i have at home.


Why is it that you always seem to hear people talking about problems with bass in their studio, and rarely with
high frequencies? Why does it seem to be that those low frequencies are so problematic? Or is it just myth? Well,
it is true that low frequencies are more problematic, and the reason has to do with the radiation space in a room.
The radiation space is the geometrical surroundings of a speaker, and is frequency dependent. If the speaker is in
the middle of a field (for example), there are no limitations to its radiation space and it's known as 'free standing'.
As soon as you place it in a room, however, the radiation space starts to be limited. If your speakers are soffit
(wall) mounted, or right next to the wall, they can 'see' only half the space now, and if placed by two walls, the
radiation space is divided by four. A typical corner, with two walls and a floor would cut this to an eighth of the
radiation space of our monitor in the middle of a field. But what does this mean in practise?
A decrease in the radiation space means an increase in energy density in that immediate area (i.e. higher SPL
levels), and the theoretical value is 6dB for every time the space is halved. So soffit-mounting your speakers will
give 6dB of boost, and in a corner (with two walls plus floor) this will give 18dB of boost. But remember that we
said earlier that this is frequency dependent, and the way it works is that the radiation space is limited by
'reflecting surfaces that are large compared to the wavelengths'. So for low frequencies this means the walls, the floor and the ceiling, for mid frequencies it's mainly the speaker baffle and objects near the speaker, while for high
frequencies it's entirely the speaker baffle and driver itself. Now you understand why it's the bass frequencies that
cause all the trouble?
1. +6dB bass boost at boundaries: Now we've understood the effects of limiting the radiation space of a
speaker (i.e. placing it in a room), and that it's the low frequency effects that are most apparent, let's look at some
practical examples. Many big studios choose to soffit-mount their monitors (Wall mount), and by
doing this they will experience a 6dB boost in the low frequencies, since the radiation space is halved. In this
case, the bass response of the speaker should be cut by at least 6dB to compensate. Placing a speaker very
close to the back wall will also have a similar result, and the same action should be taken.
If your speakers are in the corners of a room, you'll experience a bass boost of between 12dB and 18dB,
depending on how close they are to the floor (above about 1.3m the floor won't have a significant effect). Again,
this needs to be rectified by cutting the bass response of the speaker. Great - your bass frequencies are under
control? Or are they? There's another factor that has to be taken into account when deciding how far away from
the walls you place them...
2. Frequency cancellation caused by wall reflections: Another reason why many studios wallmount their
speakers is to avoid cancellation of frequencies caused by wall reflections. This effect is easy to explain: if a
speaker is a quarter wave-length away from a reflective wall, the reflected wave returns to the speaker with a halfcycle
phase difference (2 x 1/4 cycle, kapiche?) - i.e it's in antiphase.
If this is a perfect reflection then the cancell-ation will be complete, though in reality this is unlikely to be the case
(due to absorption by the surface). Nevertheless, the effect can be very audible indeed, and a problem that needs
to be solved. Naturally, this will occur in more than one direction too, so several frequencies may experience
cancellation.
The solution? Apart from flush mounting (which eliminates reflections) you can either place the speaker very close
to the wall, or considerably away from the wall. In the first case, the cancellation frequency is so high (work it out
yourself using the following equation: wavelength = velocity of sound (343m/s) / frequency) that the effect is
overlapped by higher directivity and higher density of room resonance modes.
If you take this option, remember that placing your speaker close to the wall will result in 6dB of bass boost
(covered in point 4) and you'll need to take corrective action.
The other solution is to move your speakers well away from the walls, where the cancellation frequency is so low
that it'll have little effect on the music anyway. But just how far away? Well, using the above equation, a distance
of three metres will give a cancellation frequency of 29Hz, which isn't going to cause anyone problems. Even at
two metres, the cancellation frequency is an acceptable 43Hz (2 x 4 = 8, 343/8 = 42.875), but of course this all
depends on how much space you have to spare.
3. Positioning of subwoofers: Subwoofers have been used increasingly in recent years in conjunction with
nearfields to give an extended frequency response in the absence of (a) a large room (b) budget for a pair of fullrange
speakers and (c) both of the former. But does it really matter where you place the subwoofer?
Frequencies in this bandwidth are omni-directional anyway, so surely it doesn't matter where they're placed? Well,
not quite... (you knew I was going to say that, didn't you?). Firstly we should look at the +6dB effect at boundaries
(covered in point 1). If you have a large room, you can place the subwoofer 3m away from the closest walls, and
you'll avoid the 1/4 wavelength frequency cancellations covered in point 2.
The subwoofer can be either facing the floor, or into the room, but you will have to adjust the unit's response to
take into account the effect of the boundary (the floor) by taking off 6dB of gain. In a smaller room, the unit should
be placed close to the front wall(s) (between 20 and 80 cm) to avoid any frequency cancellation due to the 1/4
wavelength phenomenon. If this is in the centre of the room, with two boundaries (floor and wall) then the
response of the unit should be adjusted by 12dB, and in a corner (three boundaries) it should be adjusted by
18dB.
There is another factor to take into account, and that's the general room behaviour, its standing waves and axial
modes. We will be covering these issues in next month's instalment, so we won't go into too much detail here.
However, a quick explanation would be that by placing the subwoofer in a pressure minima (which, coincidentally,
would generally be the case if you placed the unit on the floor behind the mixing desk and between the nearfields
in a small room) it causes the subwoofer problems in generating high enough SPLs. One solution is to move the
sub slightly off the centre line, which puts it in a more balanced sound pressure zone.

Sorry for the wall of text.