• Disclaimer: This Vendor's Forum is a paid-for commercial area. Unlike the rest of diyAudio, the Vendor has complete control of what may or may not be posted in this forum. If you wish to discuss technical matters outside the bounds of what is permitted by the Vendor, please use the non-commercial areas of diyAudio to do so.

Modelling the Pensil CHR70 with the Martin King Spreadsheets


2012-12-05 2:54 am
Hi, I'm trying to get familiar with the Martin King spreadsheets and I thought a good place to start would be modelling the Pensil with the CHR70.

I did this using the "open ended with offset driver" spreadsheet and the results look like what I would expect, smooth with an f3 of about 60Hz.

I then tried using the "driver in a mass loaded TL" spreadsheet using the same dimensions but treating the slot as a 3"dia and 0.75" long port. The response for this one is a lot different with a significant peak (+7dB) at about 60Hz.

I would expect the 2 curves to be similar, am I doing something wrong? Any help would be great.


2012-12-05 2:54 am
I have the port entered as 1.5" Radius. I am assuming that the port area should match the area of the slot and the change in form (rectangular to round) has little effect.

Attached (I hope!) is the response.


  • CHR70 freq resp.jpeg
    CHR70 freq resp.jpeg
    27 KB · Views: 122
No you're not. Not at the moment anyway. ;)

Have you modelled the damping correctly? My guess is that you've entered the specified damping quantity into the main data entry section, but this only covers the length / volume from the throat to just below the driver. The damping in the pensil is uniform throughout the enclosure so you need to manually add the appropriate ammount of damping to the rest of the box model (under the open and bottom end of enclosure sections).