Sunday, 25 August 2013

Beyma Coax 15XA38Nd Option - Ported Box

The enclosure speaker design requires the use of a Linkwitz Transform circuit to get a predicted flat response down to the low 30 Hz range. As an alternative, I modelled a ported version using the same enclosure size. The ported has an area of 250 cm3, and is back mounted about 150mm from the base of the cabinet. See below on the comparison between the two on Basta. I have chosen to predict the output at 8W power amp.
Comparison Between Closed an Ported.
The ported response is quite similar until 50 Hz, where it starts to diverge from the closed box scenario. It should be noted that the efficiency of the ported box can be seen, as the cone excursion is quite a bit less than the closed box when it gets to 20 Hz. I then further took the box design and put it into MJK's MathCAD spreadsheet. The spreadsheet used is the ML TQWT dated 11/07/07. The following are some screenshots of the results. The enclosure parameters are the same as per the drawing design in the previous blog entry.
Basic predicted response. 
Predicted response with Baffle step loss accounted. 
The final result with the BSC added, with the BSC being 20R and 12 mH.
I also modelled the same BSC in Basta and got the following result. It should be noted that in all the modelling, 8V or 8W was inputted as the amplifier power.
Basta simulation of the ported box with the BSC modelled as well to match MJK's spreadsheet.
Currently, I am working on an alternative cabinet using a 24mm solid thick wall, with dual 12mm front panel. This should make construction easier. In both designs, there is an assumption of internal box stuffing. Whilst Basta only allows for a percentage, MJK's spreadsheet allows for a more accurate assessment. In the design, I started at 8 kg per cubic meter. The resultant total weight of insulation was 1 kg and resulted in a reduction in the ripples in the response. I had originally place a thick layer of insulation between the bottom of the port and the base. But in MJK's simulation, it reduced the bass response significantly. And hence, the layer of insulation to the bottom of the enclosure has been removed. It also appears that over stuffing the box will result in reduced bass output.

The dilemma of practical construction will be ensure that the correct amount of stuff is placed, to reduced internal ripples and resonances but also not too much as to reduce the bass output. There will be lots of tweaking and construction sequence issues that will need to be resolved as this build progresses.

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