Sunday, 5 May 2013

Beyma Ported Cab

To design a simple box for my Beyma 15XA38Nd, I have employed Basta. Using a 200l ported box with two 75mm diameter 147mm length ports (tuned to 25 Hz). The box is also fairly standard at around 1m in height, 0.5m in width and 0.4m in depth. Something easy to construct.

My speaker box model also includes a Linkwitz Transform at 20 Hz along with a 4th order low pass at 1,800 Hz in tandem with a 2nd order high pass at 15 Hz produces a near flat frequency as model between 30 Hz to 700 Hz. With -3dB points around 20 Hz and 1 kHz. See below.

Crossover at 1.8 kHz and maximised bass response

There is quite a lot of filtering in there to achieve this. The 1.8 kHz crossover was set from the standard Beyma passive crossover (2nd order) tuned to that frequency. I think that 1.8 kHz seems a bit high for a 15" driver, so if I set the model to 1 kHz instead, it gives a much more 'flat' predicted response. Note also that the Xmax of this driver is 4.0mm and according to the model we hit 3mm at around 20 Hz with 1W of input. I may have to tune the high pass from 15 Hz to something higher to limit the response. The below shows using a 4th order HP at 20 Hz, which limits the Xmax to under 2mm at 20 Hz. 
Crossover at 1 kHz and slightly reduce bass response to limit Xmax.

Given my listening room is only 5x4m in size, I think that a flat response down to 30 Hz may well be plenty enough. My question now is how to experiment with the various cross-over points? The easiest solution would be to employ a PA style crossover, something cheap from Behringer or similar. Which would allow me to test and tweak to my heart's content. 

Another option would be to make the box closed, and with the same settings as the second scenario above. The response is still pretty decent. 

Closed box with same parameters as above.
To be honest, if the speaker performs anywhere near what the model shows, it'd be great. Even if I were to use a closed box or vented with the standard passive 2nd order filter, the response is still pretty decent. See the two simulations below.

Passive crossover ported box, no EQ.

Passive crossover closed box, no EQ.

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