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Waves73 27th February 2012 09:27 AM

Pensil P70 mathematics
Hi, I'm just about to start building a pair of Pensil 7's, using a set of CHP70 1st generation drivers. Following the plans downloaded from Planet10's archive, I will be using 18mm birch plywood and a piece of 18mm solid walnut for the speaker baffle. And this is where the maths comes in to it.

The walnut is only 144mm wide (the plans state a 146.1mm wide baffle). Although this is a small difference in size it does effect the overall volume of the speaker. By using the plans dimensions I've calculated the internal volume of the 7's to be 38.45litres. After recalculating for the change in width, this becomes 37.93litres.

If I use the 144mm wide baffle and alter the internal depth, by adding 4mm, this brings the internal volume back to 38.45litres.

The question is, can these alteration be used and not effect the sonic presentation of the speakers?

Any advice welcome.

zman01 27th February 2012 11:01 AM

Mark, what you have proposed should be ok, but you might as well wait for a more expert opinion from folks who have a solid understanding of TL theory. :)

I have a pair of these drivers and like them a lot, specially the midrange.

chrisb 27th February 2012 04:15 PM

As long as you're adjusting only the aspect ratio of w x d , but not length, driver location or ports, you should be fine.

Try to rebate the driver cutout 6mm for a flush fit, and don't forget to profile the rear side of cut out before assembly.

planet10 27th February 2012 06:24 PM

What Chris says is accurate, i will only add that you will also need to increase the height of the port so that its area remains the same.


Waves73 27th February 2012 08:50 PM

Thanks guys for the advice. Once I've re-calculated the measurements, and bought the plywood, I'm going to get on with the build and post a few pictures.

Next stop, timber merchants.

skeeter99 6th March 2012 04:53 AM

So how has the build gone? I love my Pensils and really enjoy seeing people fall in love with theirs as well.


Waves73 7th March 2012 07:43 PM

Hi Scott. The build is slow going. Time and money are an unfortunate factor. I've got the drivers and wood for the baffles. Just got to get the plywood for the rest of the cabinets and find the time to build.

I bought three CHP70's. Two for the Pensils and a third to design and build a centre speaker. The plans for the centre speaker are still in development but il draw them up and post them. Will be good to get some feedback on my design.

skeeter99 7th March 2012 07:59 PM

Cool. I understand how slow things can go, just check out my thread on my build! Took FOREVER to get done but I really enjoy them daily.

For a center design, talk to Dave (Planet10). We did a custom Mar-Ken design for my 12's and it blends in with my SuperPensils pretty much perfectly and looks nice also.


malibujeff 8th March 2012 12:28 AM


Get them finished! Your going to be surprised when you listen :D

Waves73 12th April 2012 08:18 AM

1 Attachment(s)
Well I've eventually started the prep for the Pensil 7's build. Started running in the drivers. They are just in their boxes and wired up to an old stereo at the moment but are already sounding pretty good. Also designed and made a router jig and started work on the baffles.

I've also done some initial calculations for the centre speaker. Attached are the plans and dimensions. The design is for a sealed enclosure with an F-3 of 81hz.

A couple of questions about the design are:

As the baffle isn't very high (internal hight of only 109mm) will their be any problems having the driver so close to the wall?

Would I need to consider any form of BSC for the design?

Any feedback would be much appreciated.

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