Sunday, 14 July 2013

Beyma 15XA38Nd Box - Further design

Following on from previous blog entries: here and here, I have recently dug out the Martin King's MLTL spreadsheets and applied a ported MLTL design to the Beyma 15XA38Nd. From all the various modelling studies and trials in MathCAD and in Basta, the bass section of the Beyma really does need a larger box to get the most out of the bass response. In the previous scenario, by using a 300 litre box, the modelled response in Basta was almost flat down to 30 Hz with no bass lift. What that really means, is that the design will be an exercise in compromise - bass verses box size. 

But the question in the choice between bass response and box size must include room gain. As I read in a few sites and books, modelling flat down to 20 Hz does not guarantee a good clean bass. The room modes will inevitable add to the bass lift, and if it is flat, it may need to be equalised to reduce the bass response, so that the overall bass response in the room is better. The below shows the difference between a 140 and 250 litre cabinet in Basta. The differences according to the MJK's MathCAD models is less apparent. The difference is basically a cabinet depth of 400 mm verse 650 mm - quite a change. Obviously if I played with the width as well, the depth increase would be marginal. As modelled, the width is 438 mm and the height is 1,100 mm, quite big already. 

Basta with 140 l cabinet.

Basta with 250 l cabinet

Based on the Basta model, here is the various points at which the predicted response hits 90 dB (-6 dB) and 95 dB (-1 dB). Generally the other frequencies sit between 96 and 96 dB.

140 litres = 34.5 Hz and 52.6 Hz (∆ 18.1 Hz)
160 litres = 32.8 Hz and 48.8 Hz (∆ 16.0 Hz)
180 litres = 31.7 Hz and 45.2 Hz (∆ 13.5 Hz)
200 litres = 30.9 Hz and 41.8 Hz (∆ 10.9 Hz)
220 litres = 30.4 Hz and 39.1 Hz (∆ 8.7 Hz)
250 litres = 29.8  Hz and 36.3 Hz (∆ 6.5 Hz)
300 litres = 29.1 Hz and 34.0 Hz (∆ 4.9 Hz)

I have also added the difference in Hz between the -6 and -1 dB points. It interesting to note that there is quite a difference going from 180 to 200 litres, as compared with the other steps in volume. What the delta represents is the sharpest of the corner, obviously as the volume gets larger, the corner point - per se - becomes more and more sharp, and eventually becomes a hump. At 1000 litres the hump is about 101 dB, but the 90 dB point is at 28 Hz.

Looking at the various graphs, I would have to say that as a pure modelled frequency response 240 litres does look to be very appealing. But as mentioned before, I must consider room gain, and then determine the volume based on that parameter. So now, the next step is to estimate the room gain in my two potential listening rooms....

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