Boxnotes worked example
Modelling in WinISD, it takes two 6inch ports to get the airspeed down to an acceptable level when driven with 800w
The ports will be entered as 152mm diameter, 4mm wall thickness and length of 1392mm
Enter Panel, driver and port details
Check that Panel thickness is set to 19mm ....and.... that
Double thickness Front is ticked
Use the Dropdown list to select a driver Size of 18 inch
Set Port length field to 1392 ....and.... Set
Internal diameter field to 152
Set Port wall thickness field to 4 ....and.... Set
Number of Ports field to 2
From the menu choose Port Flanges and select One End Flanged
Enter volume
Enter 200 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 134 litres, which is about 66 litres short of what we want
Go back to the Internal volume field and add this amount - change it to 267 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 200 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 1st graphic is the one which shows 1 driver.
This tells us that to fit our driver, the minimum box width and minimum height is 470mm.
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 1387mm
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 three problem areas ...
The red lines show that the front-to-back resonances are reinforcing the port resonance
The blue lines show that the top-to-bottom resonance is reinforcing the
side-to-side resonance.
Not surprising since the dimensions are the same
The green lines show that the driver-to-side wall resonances are reinforcing each other.
This can be addressed by moving the driver sideways.
Lets get that one out of the way first....
To make some room, increase the box width from 470 to 500mm.
Double-click the Box Depth field to recalculate the dimensions
Change the Offset from vertical centreline to 10mm.
Our spectral display now look like...
Lets see how a box height of 1000mm looks.
Enter 1000 into the Box height field and double-click the
Box Depth field to recalculate the dimensions
The new Box Depth comes in at 657mm, and the display looks like....
Now a standard size of MDF sheet is 600mm, so reduce the box depth to 600 + 19mm (thickness of front panel)
Enter 619 into the Box depth field and double-click the
Box Height field to recalculate the dimensions
This pushes the height out to 1066mm
The box top-to-bottom resonance is 167hz.
It would be nice to get this a bit higher, so lets make the box a bit wider
Change the Box width field to 531 and double-click the
Box Height field to recalculate the dimensions
The height is now 1000 and the top-to-bottom resonance has lifted to 178hz.
The driver is still sitting at the default 200mm down from the top.
This is giving a driver-to-bottom-wall resonance of 220hz.
Lets move it to the centre of the box and see what happens.
Change the Box top to driver centre field to 500mm...
We now have a cluster of resonances around 350hz. with the driver exactly half way up the box, the
driver-to-top-wall and
driver-to-bottom-wall resonances reinforce.
To make matters worse, they coincide with the box
side-to-side resonance.
Lets move the driver up a bit further..
Change the Box top to driver centre field to 400mm...
Now its crowded around 300hz...
Change the Box top to driver centre field to 300mm...
Much better with the resonances are all spaced out.
The exception is the pair around 300hz. This is the driver-to-back-wall and the
box-front-to-back combination,
which you cant do anything about.
It would be nice to have the
Box-top-to-bottom resonance a bit higher than 178hz, but dropping the height of the box would
mean making it deeper or wider. This would move the resonances around in the 300hz area,
which we have just managed to space out nicely, so this is probably about as good as can be done
Note that the ports are too long to fit into this box without several bends. That's a whole other issue though....
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