Dissecting the Bandpass box formulas

[AllenB]

To choose the right driver for a bandpass box, I wanted to know the effect of driver parameters on the box. This way I could adjust the size and cutoff frequencies by finding the right driver.

Four parameters are needed for a design. Three driver parameters: f_s, Q_ts and V_as. You choose F_low to suit your needs.

This table shows the effect of increasing these four parameters by showing what happens to the box when you do.

             | Increasing| Increasing| Increasing| Increasing
             | F low     | Qts       | Fs        | Vas
_____________|___________|___________|___________|________
F high cutoff| Increases | Decreases | Increases | No effect
Rear volume  | Decreases | Increases | Increases | Increases
Rear Qtc     | Increases | Increases | Decreases | No effect
Front volume | No effect | Increases | No effect | Increases
Front Ftuning| Increases | Decreases | Increases | No effect
Gain         | Increases | Increases | Decreases | No effect

So if you want to change the box size, frequencies or gain, try these things when looking for a driver or choosing your f_low.

1) To decrease the front box volume, choose a lower Q_ts or a lower V_as. (no other parameters make a difference)

2) To decrease the rear box volume, choose a lower Q_ts, a lower V_as, a lower F_s, or a higher F_low.

3) To lower the F_high cutoff, choose a higher Q_ts, a lower F_s, or a lower F_low

4) To increase the gain, choose a higher Q_ts, a lower F_s, or a higher F_low.

Changing some of these may have a side effect, for example: If you want to reduce the F_high cutoff but you don't want the box size to increase, the only way to do this is with a lower driver f_s.

A tip when comparing two drivers to see which will give you a smaller box, two parameters: V_as and Q_ts, have an effect on both the front and rear box sizes. V_as has a direct effect, i.e: halving V_as halves both parts of the box. Halving Q_ts will halve the rear box, but will quarter the size of the front box, therefore, Q_ts is slightly more useful in reducing box size, and Q_ts will also reduce gain.