Worked example
A large box is deliberately chosen for this example to make sure we have some problem resonances to deal with
Let us start with some typical parameters derived from a speaker design program such as WinISD
Two 10 inch drivers
Working volume of 130 litres
Three 102mm ports 645mm long with a port wall thickness of 2mm
The box is to be made from 18mm MDF with a double thickness front panel.
Enter Panel, driver and port details
Check that Panel thickness is set to 18mm ....and.... Check that Double thickness Front is ticked
Use the Dropdown list to select a driver Size of 10 inch ....and.... Set No. of drivers field to 2
Set Port length field to 645 ....and.... Set Internal diameter field to 102
Set Port wall thickness field to 2 ....and.... Set Number of Ports field to 3
Enter volume
Enter 130 into the Internal Volume field. The field background turns light red to indicate that it has been changed and that the box dimensions have not yet been recalculated
Double click on the Box depth field to calculate initial dimensions
Looking at the Tuning Summary, we see that the Working volume is about 108 litres, which is about 22 litres short of what we want
Go back to the Internal volume field and add this amount - change it to 152 litres and double click on Box depth field to recalculate dimensions
We now see that the Working volume is close enough to our target of 130 litres
Set minimum dimensions
Click the Min button to bring up the Minimum Dimensions screen. This screen inherits the driver diameter and box panel thickness from the main screen. Click the graphic that corresponds with the driver arrangement. In this case the 2nd graphic is the one which shows 2 drivers.
This tells us that to fit our two drivers, the minimum box width is 276mm and minimum height is 534mm.
Click the Transfer button to copy the minimum width and height values onto the main screen.
Click the OK button to return to the main screen.
Double click the Box depth field to calculate its value, producing a value of 1325mm
Adjust box dimensions and examine resonances
We now have all the parameters entered and have a working box, albeit badly dimensioned.
Looking at the Spectral Display, we see two red resonances at the left of the display.
Checking the Resonances Display, we see a Driver-to-rear-wall resonance of 67hz and a Box-front-to-back resonance of 135hz. Obviously for a subwoofer these are unacceptable.
Change the Box depth field to a value that would be more practical. The easiest size of MDF sheet to handle is 600mm * 1800, so adding 18mm for the front panel and subtracting 5mm for cleaning up the rear edge with a router gives us a box depth of 613mm.
Set the Box depth field to 613 and double click the Box height field
We see that the height is now 1168mm, with a Box-top-to-bottom resonance of 151hz. The Driver-to-rear-wall resonance is 155hz, which is close enough to 151hz for the two to occur simultaneously, which would produce a large peak in response. There is also a Driver-to-bottom-wall res of 90hz
Reduce the Box height field to 900mm and double click the Box width field
The Box-top-to-bottom resonance is now 199hz which is OK for a sub.
The Driver-to-bottom resonance is now 126hz, which is slill too low, so move the driver down to the centre of the box.
Set the Box top to driver centre field to 450. This changes the resonance to 199hz, but this is the same as the Box-top-to-bottom resonance and will reinforce it. To avoid this, move the driver back up by 100mm.
Set the Box top to driver centre field to 350, which changes the resonance to 161hz
The Driver-to-rear-wall resonance of 151hz is fairly close to 161hz, so lets trade off box depth against box width to move the Driver-to-rear-wall resonance to half way between 161 and 199 - lets aim for 180hz.
Set the Box width field to 400 and double click the Box depth field, causing it to recalculate. Luckily we found our desired resonance of 180hz on the first go, with box depth being 537mm.
