BBC Thin Wall Enclosure - Ported?

Discussion in 'DIY Discussion' started by Tenson, Aug 11, 2011.

  1. Tenson

    Tenson Moderator

    Joined:
    Nov 12, 2003
    Messages:
    5,947
    Likes Received:
    1
    Location:
    Kent, UK
    I've been thinking about thin-wall enclosures that the BBC pioneered and I wonder - does it need to be ported to work right?

    Most of the BBC designs were ported, and at first thought it seems it would be necessary to prevent the thin wall enclosure from being internally pressurized and flexed excessively. However, a port actually only operates as an open 'hole' to sound below its tuning frequency. At or above its tuned frequency it has a high acoustic impedance and is seen by the woofer pretty much as a closed box. So in such a speaker with a 50Hz tuned port, at 100Hz the enclosure will still be pressurized regardless. Does that seem right?

    If this is indeed the case then it makes one wonder if it really matters that the cabinet is ported since it would only reduce flexing of the walls at very low frequencies. Maybe that matters though?

    Comments and thoughts?
     
    Tenson, Aug 11, 2011
    #1
    1. Advertisements

  2. Tenson

    felix part-time Horta

    Joined:
    Dec 13, 2003
    Messages:
    757
    Likes Received:
    1
    Location:
    dead
    I doubt it makes much difference either way. The point of the BBC thin-wall was to minimise energy storage in the walls and provide a damped shell, really to deal with colouration in the midrange. It's BBC RD paper 1977-03 you want, 'Factors in the design of loudspeaker cabainets' , from here:

    http://www.bbc.co.uk/rd/
     
    felix, Aug 11, 2011
    #2
    1. Advertisements

  3. Tenson

    RobHolt Moderator

    Joined:
    Oct 24, 2004
    Messages:
    4,123
    Likes Received:
    5
    Rogers built both and had both ported and sealed within the same range. The Export Monitor I have here had a slightly smaller Domestic Monitor alongside and that was sealed.

    My gut feeling having read the BBC papers (and heard some BBC designs) is that the principle works better with larger boxes, i.e. not small bookshelf sized cabinets.
     
    RobHolt, Aug 11, 2011
    #3
  4. Tenson

    UK Duty Paid

    Joined:
    Aug 17, 2009
    Messages:
    155
    Likes Received:
    0
    AFAIK the design used dissimilar panels with differing resonances & damping & they used layers of them

    thin walls can be made to be very stiff to prevent flexing so the size of enclosure is not a restriction
     
    UK Duty Paid, Aug 12, 2011
    #4
  5. Tenson

    DSJR

    Joined:
    Sep 1, 2007
    Messages:
    96
    Likes Received:
    0
    Harbeth's current interpretation of "thin-wall" is a rather more "solid" and well damped affair than my 1974 vintage Spendor BC2's (same box as the venerable BC1) I have to say and the "knuckle-rap" test seems to back this up to a high degree I found, on ALL their models....
     
    DSJR, Oct 8, 2011
    #5
  6. Tenson

    Markus S Trade

    Joined:
    Jun 20, 2003
    Messages:
    1,527
    Likes Received:
    0
    Location:
    Nether Addlethorpe
    Is thin-wall construction still relevant (leaving marketing considerations aside)? There has been considerable research into making cabinets acoustically opaque (bracing, damping, materials etc.). Why would you want to make your cabinets flex and introduce a secondary, time-distorted, non-linear-transfer element into your design mix?
     
    Markus S, Oct 10, 2011
    #6
  7. Tenson

    Tenson Moderator

    Joined:
    Nov 12, 2003
    Messages:
    5,947
    Likes Received:
    1
    Location:
    Kent, UK
    Depends how you brace it. A lot of speakers use bracing that attempts to make the panel stiffer. That results in the resonances being low level, but also higher frequency where they seem to be more audible.

    However, I do like panel to panel cross-bracing. This doesn't tend to stiffen the panel much, but does massively reduce the main resonance.

    I think there is mileage to be had from the BBC research on the best materials for lowering the audibility of resonances. So I'd like to see a speaker with damped 9mm birch plywood panels, together with cross-bracing. Those two should produce an extremely low resonance cabinet.
     
    Tenson, Oct 10, 2011
    #7
  8. Tenson

    RobHolt Moderator

    Joined:
    Oct 24, 2004
    Messages:
    4,123
    Likes Received:
    5
    ...... and screwed in front and back.
     
    RobHolt, Oct 10, 2011
    #8
  9. Tenson

    DSJR

    Joined:
    Sep 1, 2007
    Messages:
    96
    Likes Received:
    0
    Please remember folks that things have moved on, even at Harbeth, in the last twenty years or so. I'm leaving Spendor out here because their currently promoted range has little to do with the ranges that built the company apart from the name - not a criticism, but the classic series was positively discouraged when I sold the brand just a few years ago.
     
    DSJR, Oct 10, 2011
    #9
  10. Tenson

    RobHolt Moderator

    Joined:
    Oct 24, 2004
    Messages:
    4,123
    Likes Received:
    5
    I'd say the cost/performance issue is the major reason to still use the methods pioneered by the BBC.
    I'm sure you can do some wonderful things with new materials, or even old materials such as carbon fibre combined with new methods, but the cost escalates quickly for small volume production.
    I think thin-wall really sits as the best of the relatively affordable construction techniques.

    One thing that did surprise me recently was talk from Alan on HUG that the thin wall cabinet added welcome tone to the sound. That runs completely counter to the aims of the BBC research which was to minimise the contribution from the cabinet.

    Then again there are the surprises. I was playing my old Heybrook HB2s the other day, which use thick chipboard, bracing and lots of mastic damping, followed by foam lining.
    Very heavy, and very very dead.
     
    RobHolt, Oct 11, 2011
    #10
  11. Tenson

    DSJR

    Joined:
    Sep 1, 2007
    Messages:
    96
    Likes Received:
    0
    Alan S is "playing" to his devoted far eastern audience I think, where the "warm tones" of BBC Legacy style speakers are revered. In actual fact, his boxes usually are pretty inert these days (and heavy too.)

    You know, good old chipboard has its faults, but properly lagged and damped I think it's still good. I believe there's a sort of laminate that Peter Comeau started using in the WD speaker designs that seems better than all of them, especially in the upper midrange - I'll have to look it up again..

    HB2's eh? Loved these and the pipe with holes in the side acting as rear port. One way of ensuring rigidity was to put a glob of blu-tac betwixt bass driver and rear of port pipe I seem to remember - or was it a brace and driver, can't remember now. needless to say, the midrange was sublime with a decent source and amp and using some EAR 509's to drive a pair made the sound almost etherial.....
     
    DSJR, Oct 12, 2011
    #11
  12. Tenson

    YNMOAN Trade - AudioFlat

    Joined:
    Jul 16, 2007
    Messages:
    674
    Likes Received:
    0
    I think the thin wall BBC construction is an interesting one. Employing such a construction method will definitely alter the resonant frequency of the cabinet (lower it); it may also reduced the amplitude but spread it over a broader range. It also seems likely (to me) that screwing in the front and/or rear is likely to create a more chaotic resonant structure.

    As stated earlier, a lot of bracing, and stiffer construction materials, effectively raise the first breakup mode of the structure (cabinet), they should lower the amplitude though. It seems possible that the cabinet resonant frequency may lie uncomfortably close to the X-over point between mid/bass and tweeter. Lowering that frequency may make it less subjectively obvious (however, part of that may be that it is moved to an area of the spectrum that lacks the same level of definition as other areas).

    I Believe Harbeth still use this construction in some, but not all, of their speakers, and my limited experience of their speakers has proved to be rather variable.
     
    YNMOAN, Oct 12, 2011
    #12
  13. Tenson

    DSJR

    Joined:
    Sep 1, 2007
    Messages:
    96
    Likes Received:
    0
    I've read that NOT having the front and back baffles screwed in these cabinets can cause the very problems you alude to Mark, as having these glued defeats the object of a "lossy" cabinet.

    Like I've repeated above, the latest thin-wall boxes are VERY different - and superior IMO - to what was being made in the 70's. Maind you, I was surprised at the hollow-box Tannoy Turnberry's I heard a while ago. The sound was fine and IMO not at all "wardrobey." They didn't screech either..
     
    DSJR, Oct 12, 2011
    #13
  14. Tenson

    YNMOAN Trade - AudioFlat

    Joined:
    Jul 16, 2007
    Messages:
    674
    Likes Received:
    0
    I believe they used to be 6mm ply with hardwood battens. I think they are now MDF - I'm not against MDF, but I can't help but think that cost is a significant aspect in this change.

    You don't really want the cabinet to have clearly defined breakup modes that are of a high amplitude
     
    YNMOAN, Oct 13, 2011
    #14
  15. Tenson

    Darren

    Joined:
    Oct 20, 2003
    Messages:
    88
    Likes Received:
    0
    Location:
    West lancs
    Does anyone remember the Wharfedale 708 with its very stiff and light foam laminate cabinet?

    Its odd that dfferent cabinet materials never really took hold in the mainstream.
     
    Darren, Nov 25, 2011
    #15
  16. Tenson

    RobHolt Moderator

    Joined:
    Oct 24, 2004
    Messages:
    4,123
    Likes Received:
    5
    I remember the 708, and IIRC it also featured bayonet fit driver mounting.

    Celestion had the aluminium honeycomb cabinet for the 600 & 700.
     
    RobHolt, Nov 25, 2011
    #16
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.