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RE: Olsen Modified BLH monster

Posted by Paul Eizik (A) on June 22, 2007 at 10:20:30

In Reply to: RE: Olsen Modified BLH monster posted by freddyi on June 22, 2007 at 06:36:07:

Hi Fred

This project was a bit off the beaten path for me, but since my rig has been down since I started remodeling my listening room in Nov., I gotta live horns vicariously. You can see the scale of the various internal dimensions from the sketch. John is to be congratulated for transforming this rough sketch into reality. I suggested to John that he construct the cabs so the top panel could be easily removed for tuning. Note the chamber to the rear of the 15"er, which was made large for tuning options. Also note the rectangular holes down line in the 'W" top manifold which connect this manifold to the lower "S" bend of the bass horn. The size of these is another tuning option (thanks to Steve Schell for some thoughts on this, and some observations from his RCA studio monitors designed by The Master himself). Note the dashed lines in the top down view which picture some panels which also are another tuning option. Note that the horn mouth makes maximum use of the front cabinet space. The original sketch pictured the edges angled at 45 degrees to the sides of the direct radiator 15" front for baffle-step control. John built the speaker as a box (which is the easiest way to do it), but he could easily cut the triangular chambers off there. Some freq. resp. measurements would indicate if this is worth the trouble.

John originally wanted to use the BM-15CX38 as a driver, and at a certain point in the discussions it occurred to me that we were talking apples and oranges here. A web search turned up with three different spec sheets for this driver, all with different TS specs! This says either that the specs are rather loose, or that the manufacturer has changed the specs without changing the model number. I think the later is most likely, as the mid horn freq. resp. graphs were reasonably consistant between the different sheets. I was concerned that this driver needed a tweeter, as it's rather rough in the highs, going by the graphs. John seems quite happy with the Tannoy Golds as an alternative though, which look alot better going by the graphs for them.

This was a fun project, and a bit frustrating at times as John lives in Melbourn Australia and I live in Chicago. I would have started by testing the BM's TS specs to see just where they are, but John does not have any test equpt, so quite a lot of the project was "armchair engineering" guess work on my part. I tried my best to discourage John from building these monsters, and even sent him a birds-eye perspective drawing and stern WAF warnings, but he has pressed ahead regardless! I am genuinely impressed by his enthusiasm! This project is also a WAF engineering feat on par with the concrete and brick horn John Crabbe built in his dining room while his wife was pregnant in the 1950's. The stuff of legends!

Comments on the design anyone?

Paul

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