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How do I calculate the extra space need that is taken up by a driver or port in a box?





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 itís 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 itís 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 18Ē 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 Ė 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. Itís 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 donít 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 18Ē driver will give you 3 liters of volume because of the indentation of itís 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 donít worry about it taking up space in the cambers, like wise donít worry about any fiberglass of wadding that you place inside the chambers. There inclusion takes up very little space and you wonít 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.


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