HELP! Hi-Fi Answers August 1975 TLM dimensions.

[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 (http://www.saturn-sound.com/images%20-%20diy%20projects/build%20this%20monitor%20speaker%20-%20hi-fi%20ans%27%20-%20aug%2075%20-%20fig%27%201.jpg)) 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?


Figure 2 (LINK (http://www.saturn-sound.com/images%20-%20diy%20projects/build%20this%20monitor%20speaker%20-%20hi-fi%20ans%27%20-%20aug%2075%20-%20fig%27%202.jpg)) 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 (http://www.saturn-sound.com/images%20-%20diy%20projects/build%20this%20monitor%20speaker%20-%20hi-fi%20ans%27%20-%20aug%2075%20-%20fig%27%209.jpg)) 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.

If anyone has built a pair of these speakers I would greatly appreciate any clarification, I would rather specify the cuts accurately first time around!

Many thanks.

[peter j]

I had a pair of these.I can't help with the cutting details because I took the project over from someone who had started to make them and gave up half way through.
I do remember painting the insides with some sort of black bitumen paint and stinking the house out prior to filling with the long haired wool.

Two points about your project.I seem to recall that the speakers were a B139,B200 and a T27.I took my speakers from a pair of cabinets that I had just bought but couldn't stand the sound of,(KEF something or other).How easy it would be to obtain these speakers today I'm not sure and if good examples were not available,would it affect the cabinet dimensions when using some other type?

The second point is converting imperial to metric dimensions.Not every measurement works out exactly and coupled to a change in material thickness I can see this being a potential problem.I seem to recall that one of the most likely reasons for the Lens on the Hubble space telescope being made incorrectly was that someone screwed up the conversion when going from metric to imperial.If it can happen to them ..............

Best of luck anyway and do get a decent power screwdriver.I can't remember the exact number of screws to put the whole lot together but lots and lots.

[The Barbarian]

Hi
One thing why do you really need to use 1'' MDF over 3/4'' ?

[awkwardbydesign]

I built these back in the day. I would suggest converting all the measurements (I didn't bother as I cut them myself), cut the panels out of card (can be to scale instead of full size), and jiggle them around until they fit into a standard sheet. Or 2. Remember to allow for the saw width, it can be as much as 5mm on each cut. Don't try to economise on sheets, MDF is cheap!
I would stick with 18mm, 25mm will sound different, but may not be better.
I prefer to taper the line smoothly and curve the bends. Some (Troels Gravesen for one) say it doesn't matter. I suggest you draw it out to scale to get it all to fit.
The top and bottom are shorter because the front is rebated by the same amount. You don't have to do it that way.
Use hardwood fillets along the joins for greater rigidity.
Be prepared to alter the stuffing to suit your room. I always make a hatch at the bottom to get at the parts of the line that are not normally accessible.

[awkwardbydesign]

Actually I lie. I built these! http://p10hifi.net/planet10/TLS/downloads/Pro9TL-Mk2.pdf
But my answers still apply.

[surv1v0r]

Hi
One thing why do you really need to use 1'' MDF over 3/4'' ?In a word, rigidity. Various comments I have read round and about have suggested that this is very important. I will still use battens on the sides just behind the B139 and on the back panel and also dowels to brace the duct panels. My current 48" high, pretty crude cabinets are made of ¾" chipboard and are extensively braced but you can still feel the panels resonating.


I built [the Falcon Rogers Pro 9 TLs (http://p10hifi.net/planet10/TLS/downloads/Pro9TL-Mk2.pdf)] back in the day. I would suggest converting all the measurements (I didn't bother as I cut them myself), cut the panels out of card (can be to scale instead of full size), and jiggle them around until they fit into a standard sheet. Or 2. Remember to allow for the saw width, it can be as much as 5mm on each cut. Don't try to economise on sheets, MDF is cheap!
I would stick with 18mm, 25mm will sound different, but may not be better.
I prefer to taper the line smoothly and curve the bends. Some (Troels Gravesen for one) say it doesn't matter. I suggest you draw it out to scale to get it all to fit.
The top and bottom are shorter because the front is rebated by the same amount. You don't have to do it that way.
Use hardwood fillets along the joins for greater rigidity.
Be prepared to alter the stuffing to suit your room. I always make a hatch at the bottom to get at the parts of the line that are not normally accessible.I have seen this design and realise that it is vaguely similar to the Chris Rogers / Cambridge R50 cabinets. However, the duct seems shorter and exits at the top and the only speaker common to both is the B139. I like the idea of the angled panel at the lower rear of the cabinet, I will probably copy that and perhaps do the same on a much smaller scale at various points in the duct itself.

The idea about cutting shapes out of card and juggling them is an excellent idea, thanks for that. I am trying to get a quote for the cutting from a local specialist woodworking shop and someone has pointed out that B&Q do actually know what they are doing in this area. Likewise I have been warned to allow 5mm for cuts.

[awkwardbydesign]

I have used all the older KEF drivers over the years, and preferred the SEAS mid to the KEF B110. If you can still get the SEAS, of course.
A problem with making cabinets more rigid is that any resonance will be shifted higher, and perhaps to a more audible area. And more mass means more energy storage. I rather like sand filled panels, but even sand transmits some vibration; look at earthquakes.
Oh, and check B&Q's cutting as they go, I use them too, but I always check.