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Jan. 2018
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BBC-style thin-wall cabinets. Why so special?

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  • #31
    Panel damping

    Dear Mr Shaw,

    As illustrated earlier in the 9mm damped vs 18mm undamped panels graph, the 9mm panel has resonance around 100Hz whereas the 18mm panel has resonance around 500Hz. However in a cabinet using the 18mm panels usually there would be some felt/wool/fiberglass/acoustic foam/polyester damping which should tame the 500Hz resonance to some extent and also smaller peaks above that. On the other hand it would be very hard to minimize the 100Hz resonance in the 9mm panel even with more damping. I feel its unfair to compare a damped thin panel to an undamped thicker panel because for all practical reasons there would be always some form of damping added to the thicker panel.

    I am really eager to know whether the thinner panel still hold an advantage over the thicker panel with appropriate conventional damping.

    Again, why are we not using a bitumen damped thin wall for the front baffle?

    Sorry for digging an old thread but I found the discussion is incomplete without these two questions.

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    • #32
      Audibility

      Originally posted by DrWest View Post
      Dear Mr Shaw,

      As illustrated earlier in the 9mm damped vs 18mm undamped panels graph, the 9mm panel has resonance around 100Hz whereas the 18mm panel has resonance around 500Hz. However in a cabinet using the 18mm panels usually there would be some felt/wool/fiberglass/acoustic foam/polyester damping which should tame the 500Hz resonance to some extent and also smaller peaks above that. On the other hand it would be very hard to minimize the 100Hz resonance in the 9mm panel even with more damping. I feel its unfair to compare a damped thin panel to an undamped thicker panel because for all practical reasons there would be always some form of damping added to the thicker panel.

      I am really eager to know whether the thinner panel still hold an advantage over the thicker panel with appropriate conventional damping.

      Again, why are we not using a bitumen damped thin wall for the front baffle?

      Sorry for digging an old thread but I found the discussion is incomplete without these two questions.
      Aren't the panels of both the 9mm and 18mm cabinets beneficially treated with internal-surface sound damping panels? That said, our cabinet maker recently commented that almost all his production (for various speaker brands) comprised 18mm panels as a minimum thickness, and none of them had any surface damping treatment applied.

      We have to think about the Q of the various structural/panel resonances and their likely audibility. Is there any point adding cost and complexity to any engineered product beyond what is absolutely necessary, in this case, to suppress issues to below audibility? And we know that human acuity of LF detection is very poor compared with in the middle frequencies.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

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      • #33
        Different thicknesses?

        Mr. Shaw, Thanks for your reply.

        Originally posted by A.S. View Post
        That said, our cabinet maker recently commented that almost all his production (for various speaker brands) comprised 18mm panels as a minimum thickness, and none of them had any surface damping treatment applied.
        I was under the impression that none of the commercial speaker cabinets would come without proper internal surface damping unless its a subwoofer where the mid-frequency colorations are out of its pass band. Cost cutting or poor designing?!

        I remember reading somewhere that you use a thicker and undamped panel for the front baffle unlike the side cabinet panels. So, if I am not wrong, would you please illustrate the advantage of this deviation.

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        • #34
          Counter-layers

          Originally posted by DrWest View Post
          I was under the impression that none of the commercial speaker cabinets would come without proper internal surface damping unless its a subwoofer where the mid-frequency colorations are out of its pass band. Cost cutting or poor designing?!

          I remember reading somewhere that you use a thicker and undamped panel for the front baffle unlike the side cabinet panels. So, if I am not wrong, would you please illustrate the advantage of this deviation.
          Few contemporary speaker cabinets have bitumen/rubber (or equivalent) panel treatments bonded to the internal panel walls as Harbeths have. In fact, one speaker cabinet maker who approached us recently was issued a set of drawings, and upon studying them said that he needed to understand where to obtain the bitumen counter-layer: he'd never been asked for such a think before.

          18-24mm wall thickness on "tall boy" speakers, very much the vogue, seem to have little or no panel damping, even from the well known UK specialist brands.

          Front panel (baffle) thickness: consider how much of the panel is milled-out for the drive units. What remains has to support them so needs to be adequately strong, hence a little thicker than the other panels.
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

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          • #35
            Screwed on backs

            Originally posted by Miles MG View Post
            I discovered that the early BBC LS3/5a had a screwed-on back panel, but later ones including Harbeth's, didn't.
            Does this indicate it is less effective on a smaller cabinet ? Mind you, the modern Stirling Broadcast LS3/5a has a screwed-on back panel, as does the Harbeth P3ESR. My old ( 1993 ) HL-P3s don't. I suppose it all depends on the particular design, as Stirling's '3/5a and the 'ESR don't have the same drive units as the original LS3/5a.

            I have recently made up some LS3/5a clones for a bedroom system, as I can't justify another pair of P3-ESRs ! A pair of ex. BBC 9mm cabinets with screw on backs, somewhat rare items, were found. The front baffles were glued and screwed in.
            These are usually just screwed in on sealing strip.

            They sound very good, as I expected, but how much the removeable backs contribute I don't know. They may be compromised by the glued in baffles.

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            • #36
              Cabinet bracing?

              I note from a review photo that the M30.1 has a kind of timber T-brace inside the rear of its cabinet. Is this for additional resonance control?

              Comment


              • #37
                Originally posted by A.S. View Post
                Re: BBC-style thin-wall cabinets. Measurements and observations (1)

                The reason that the BBC began to investigate alternatives to the thick wall enclosure used in the massive floor standing LSU/10 was simple:


                >
                Alan, kinda curious, how exactly did you made these measurements and how did you manage to "isolate" the sound from the driver itself?
                Attached Files

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