
The red area is the rear chamber and the driver or blue and yellow bits
have intruded into this red space. So the red space or rear chamber will
have to be made bigger by the same amount that the driver occupies to
take into account for it’s intrusion.
So anytime you have a box and you put something into it you have made
the space inside it smaller, you will then have to make the box bigger
to get it back to it’s same volume. Lets take a rear chamber for a
bandpass design. You have calculated that it needs to be 100 liters, the
rear of the driver takes up on average 7 liters for an 18” unit, so if
we mounted the driver in a chamber of 100 liters the internal volume
would now be 93 liters. But we need 100 liters, so we would need to make
the rear chamber 107 liters. But what about the port. You also have to
work out how much space the ports have taken up inside of the chamber.
Say we have a shelve port (a shelve port is like the front chamber port
on the C Sub design) and the port has a 120 mm opening, the cabinet is
500 mm wide and the port extends 380 mm into the cabinet. We work out
its volume by multiplying the height by the width by the depth. So that
would be 120 x 500 x 380. Wrong. The opening to the outside world of the
port is 120 mm but because it is made from 18 mm material we have to
take this into account. So the calculation would be 138 x 500 x 362. You
notice that I put 362 not 380 for the depth, this is because we are
working with an internal volume not an external one. The port is 380 mm
deep from the front of the cabinet, but because of the 18 mm cabinet
front the internal depth taken up by the port is 380 – the 18 mm front
panel. So, 138 x 500 x 362 = 24.97 liters. The answer you get will have
loads of numbers in it, if you take off the last 6 numbers and put the
decimal point there that will leave you with the answer in liters. So
the figure I got by multiplying all the numbers together was 24978000.
If I put the decimal point 6 figures in from the right I get 24.978000
or 24.97 liters. It’s close enough to call it 25 liters, so our 100
liter rear chamber now has a 7 liter driver and a 25 liter port
occupying space inside it. If we made the chamber 132 liters then after
the driver and port have taken out there volumes we are left with 100
liters, which is the right volume for this design to work correctly.
You do the same for the front chamber with working out the extra size
you need to make it because of the front port or ports taking up space,
but this time we don’t have the driver chassis and magnet taking up
space. In fact we have the opposite, the cone of the driver gives us a
bit more space in the front chamber, so we can make the front chamber
slightly smaller because of the volume the front of the driver has given
us. An average 18” driver will give you 3 liters of volume because of
the indentation of it’s cone, so make the front chamber 3 liters smaller
to allow for it. An example would be that we need the front chamber to
be 60 liters, we have already worked out that the front chamber port has
a volume of 28 liters and that the driver is giving us 3 liters. So we
add the 28 liters of the port to the 60 liters we need and get 88 liters,
we then take off 3 liters because of the space the driver is giving us
and get 85 liters. So you would need to build a front chamber with an
internal volume of 85 liters to get the 60 liters the chamber needs to
be.
Unless there is a lot of internal bracing don’t worry about it taking up
space in the cambers, like wise don’t worry about any fiberglass of
wadding that you place inside the chambers. There inclusion takes up
very little space and you won’t notice a difference if you do make the
internal volumes bigger to accommodate them. If you had a lot of
internal bracing then you might want to work out how much space it is
going to take up. You do this by multiplying all the bits that go into
making the braces. So, if you had 3 braces that were 400 mm x 80 mm x 18
mm they would take up 1.7 liters, a small volume not really worth making
the chamber bigger for. 
