SpeakerTalk

[surv1v0r]

I hope someone who has built Chris Rogers' Transmission Line Monitors from the August 1975 Hi-Fi Answers notes can help me here. I am trying to get MDF cut at a timber merchants but am not 100% certain that I entirely understand the dimensions.

Based on Figure 1 (LINK) I can see that the top (5) and bottom (6) fit inside the sides (3 & 4) and that the front and back (1 & 2) fit inside the resulting box. What I don't understand is why the top and bottom are half an inch less deep than the sides, surely they too should be 18 inches deep? The front and back (1 & 2) are the same height, 1½" less than the sides, presumably to allow for the ¾" thickness of the top and bottom.

I have now come up with the following dimensions:

From 25 mm MDF:

4 off 875 x 300 (front & rear - 5 mm undersized all round)
4 off 930 x 470 (sides)
4 off 305 x 470 (top & bottom)

From 18 mm MDF:

4 off 305 x 305 (front two port panels)
2 off 250 x 305 (B110 box base)
2 off 260 x 305 (B110 box top)
2 off 450 x 305 (B110 box rear and rear port panel)

I will probably position the front port panel vertically.


Figure 2 (LINK) is equally confusing. What is going on with the baffle board, it seems to be "notched" at the top and bottom panels but this isn't mentioned anywhere in the text. The various dimensions at the top of the cabinet add up to 18" assuming that the port panels (9, 10 & 11) are ¾" thick but I am somewhat confused by the exact positioning of the port panels. Having spent time trying to understand Figure 2 I get the impression that the depth of the B139 "duct" goes from 5" (bottom of 11) to 4¼" (top of 11) to 4" (top of 11 & 10) to something undefined (bottom of 10 & 11) to something else undefined (bottom of 10 & 9) to 2¾" (top of 9 & 10) to 3¼" (top of 9 & rear of baffle) to 3" (bottom of 9 & rear of baffle)
i.e. 5" => 4¼" => 4" => ? => ? => 2¾" => 3¼" => 3"
This despite the comment in the middle column of page 1 of the notes that: "The duct starts with an area of 66 sq.in. at the drive end and is reduced by 50% to finish at 30 sq. in. at the duct mouth."

Does it actually matter that the duct doesn't consistently reduce in size? Would it cause a problem if panel 9 was vertical, making the duct the same size for the whole length of the final inverted "U" section?


Figure 9 (LINK) also appears to be missing a dimension or two.


I need to convert all these dimensions into metric and in addition I aim to use 25 mm MDF rather than 18 mm (¾") for the external panels and I have now come up with the following dimensions:

From 25 mm MDF:

4 off 875 x 300 (front & rear - 5 mm undersized all round)
4 off 930 x 470 (sides)
4 off 305 x 470 (top & bottom)

From 18 mm MDF:

4 off 305 x 305 (front two port panels)
2 off 250 x 305 (B110 box base)
2 off 260 x 305 (B110 box top)
2 off 450 x 305 (B110 box rear and rear port panel)

I will probably position the front port panel vertically.


If anyone has built a pair of these speakers or can understand the plans I would greatly appreciate any clarification, I would rather specify the cuts accurately first time around! I realise that this may not result in the world's finest speakers but since I have two working sets of the relevant drive units and the Falcon Acoustics R50 crossovers it does seem to be the obvious solution based on my starting point.

Many thanks.

[habrune]

I've built some pairs, and my answer is:
i) You're right, if you want a plain box the top and sides have the same depth. Rogers'design probably allows for insertion of a frame in front that falls within the extending sides, but extends as high as the sides, thus: falls from the front to the top and bottom panel.
ii) In the plans I have, the dust starts with 140mm (behind B110, panel 9 low end), 102 on both sides from top of panel 9; panel 10 bottom: resp 85mm (to 9), 76 (to , 76 (to 11). Top of panel 1: 70 to 10 and front, 76mm to top. End of duct: 60mm tot front panel. In my experience it will not make much differenc if you mould the end of the line into two parallel ducts of equal size, as long as the pipe in general tapers; this lowers the lower cutoff frequency. My speakers had benefits from removing some sheep from these last two ducts. Rogers recepy for wool stuffing is rather dense on the end. But this is personal, depending from room acoustics.
iii) i have cutting plans in mm, but do not know how to upload these, if they can be at any service. An e-mail? (New on this forum)