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Old 29th November 2013, 06:28 PM   #11
system7 is offline system7  United Kingdom
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You will find this interesting I think, DrBoar.

It was always the BBC tradition to NOT time align drivers, but do flat baffle.

In fact the woofer was often recessed, as in this Harbeth model:

Click the image to open in full size.

If you know your filters, you will know that 4th order acoustic Linkwitz-Riley should be time-aligned on the drivers' acoustic centres and WIRED IN PHASE for good lobing and phase alignment. So this arrangement really looks hopeless. But it isn't.

Interestingly, with real world filters (like second order electrical bass and third order electrical tweeter), if you move the tweeter half a wavelength forward, which is about 5cms at 3kHz, and WIRE OUT OF PHASE, you can get some very good phase alignment too. My PREDICTION would therefore be that your own splendid Dorspen speaker is wired out of phase. Along with the BBC LS3/5A and the Rogers LS5/9. And they ARE!
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Old 30th November 2013, 12:53 AM   #12
sreten is offline sreten  United Kingdom
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Hi,

I haven't seen the bitumen loaded felt multilayer pads available for
a very long time, for 12mm thick they were hardly what you'd call
lightweight, but nothing like as as heavy as 12mm of solid bitumen.

rgds, sreten.
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Old 30th November 2013, 01:04 AM   #13
system7 is offline system7  United Kingdom
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I feel the basic idea is to add some mass and damping to the sides and top and bottom of the cabinet.

I was lucky enough to be working with some acoustic engineers who were improving room acoustics. They laid down some stuff called Regupol stuck with aerosol carpet adhesive, made from old car tyres and glue if I understood correctly. They gave me a load of offcuts too.

Click the image to open in full size.

It's about 1/4" thick and quite easy to apply. You then lay a thin wooden floor on top of it.

Click the image to open in full size.

Must do the same thing really? Glue went everywhere though!
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Old 30th November 2013, 03:08 AM   #14
gk7 is offline gk7
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There are serveral Regupol types according to the manufacturer:
Regupol | BSW Company Profile

Which is the one you use ?
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Old 30th November 2013, 10:40 AM   #15
tvrgeek is offline tvrgeek  United States
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Adding mass lowers the panel resonance. Adding bracing raises the panel resonances, but makes it harder to excite. Some mass treatments also do absorb some of the resonances ( conversion to heat).

I find well braced ( no panel larger than about 40 sq inches) lined with 1 inch OC 407 compressed FG t work well for me. It has been 30 years since I mixed undercoat and sand to mass load my boxes.

Look in the car audio suppliers. I have used a generic ( cheap) form of Dynamat in very small boxes.
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Old 30th November 2013, 11:29 AM   #16
system7 is offline system7  United Kingdom
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Quote:
Originally Posted by gk7 View Post
There are serveral Regupol types according to the manufacturer:
Regupol | BSW Company Profile

Which is the one you use ?
I just have it because I was given some offcuts. So I really can't say it's the most suitable.

It looks like Cork and Rubber bonded together. I'd guess it's the Regupol K225:
Regupol Underfloor Impact Sound Insulation - BSW GmbH

Sorry I can't be more helpful. Just 1/4" thick rubbery stuff to me. I have doubled it up on occasion on the smaller top and bottom panels. The BBC did that on the LS3/5A.
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Old 30th November 2013, 04:06 PM   #17
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Dear All,

Thank you for your advice and opinions so far. Everything said here (and, often by the same people, in other threads) certainly needs to be considered. I will have to calculate the WAF in to some degree as well.

Yesterday, I had the opportunity to "slam" one of those Audax TWO34 tweeters into a box and nothing could better demonstrate what tvrgeek and others said about loose experiments and advancing technology. Wasn't my idea, my friend Fred wanted to replace a blown 'pink' Wharfedale tweeter (in an old Dovedale 3 box). Cross-over at 5 KHz, 2nd order if I'm not mistaken. The lack of both balance and smoothness was remarkable, to say it politely. At least the hiss from that old tube amplifier was now clearly audible.

So, I realize I better do a lot of homework first, then invest my efforts in something not too deceiving. Maybe a BBC-style speaker, but not necessarily an active one along the lines of the LS5/8 (and I always had my doubts about the somewhat veiled, "behind the curtain" character of the Quad 405, even after refreshing it's circuits).

For now I'm tempted by the Seas offerings (also considering the Seas Mimir as an exercise, my son keeps requesting a reasonable speaker set while his girlfriend insists on a cabinet in the same colour as their walls), but I could also fall for one of Troels Gravesen's open baffles (tribute to Gilbert Briggs -- aah!, again suffering from nostalgia). Definitely low WAF, this one.

It's definitely going to be interesting. Thank you all so far for your valuable thoughts.
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Old 14th February 2014, 09:09 PM   #18
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I have wondered this myself about the LS 5/8 design wondering if there were any suitable 12" bass drivers out there these days for this kind of project
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Old 3rd March 2014, 12:28 PM   #19
hbc is offline hbc  United Kingdom
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Just heard a pair of the LS5/8s in Lodgesound's reel to reel tape recorder room at the Hifi wigwam show. I am of course perplexed and troubled (by all that 70s/80s opamp electronics and retro plastic drivers etc) but they did sound good, and some including myself would say they were one of the better sounds at the show…
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Old 3rd March 2014, 04:36 PM   #20
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Quote:
Originally Posted by hbc View Post
Just heard a pair of the LS5/8s in Lodgesound's reel to reel tape recorder room at the Hifi wigwam show. I am of course perplexed and troubled (by all that 70s/80s opamp electronics and retro plastic drivers etc) but they did sound good, and some including myself would say they were one of the better sounds at the show…
I only had a quick listen but they were pretty impressive. I was surprised they were only a 2-way with a large (12"?) midbass driver. It sounded like there was a large subwoofer (a good quality one) hidden in there.
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