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BBC-style thin-walled cabinets vs. tight bass?

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  • timw
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by fastbike1 View Post
    Pete, wouldn't decoupling the driver from the baffle, then decoupling the baffle work as well and be simpler? I have read about build that have accomplished this via silicone type gaskets.
    Sorbothane works well for decoupling of driver frame mounts.

    The following posts, mostly by "Bushmeister", at the following DIYaudio link are informative for decoupling. Posts #184, 227, 233, 235, 238, and 247. Andrew Jones of Pioneer TAD, designer of the TAD Reference 1 and Compact Reference, drops in for Post #264.

    http://www.diyaudio.com/forums/multi...r-cabinet.html

    Leave a comment:


  • timw
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    I've thought about building an enclosure for the driver that is suspended inside another enclosure to completely decouple the driver from the external enclosure.
    You and Brian Ravnaas, one of the inventors of Green Glue, have a similar idea. See Post #3 at the following link. The whole thread is interesting.

    http://www.avsforum.com/forum/19-ded...y-speaker.html

    Leave a comment:


  • fastbike1
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Pete, wouldn't decoupling the driver from the baffle, then decoupling the baffle work as well and be simpler? I have read about build that have accomplished this via silicone type gaskets.


    I've thought about building an enclosure for the driver that is suspended inside another enclosure to completely decouple the driver from the external enclosure.

    But it seems to me that constrained layer construction will do plenty to reduce transmitted vibration.

    Leave a comment:


  • Pete Schumacher
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    I've thought about building an enclosure for the driver that is suspended inside another enclosure to completely decouple the driver from the external enclosure.

    But it seems to me that constrained layer construction will do plenty to reduce transmitted vibration.

    Leave a comment:


  • andy19191
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by wolfgang View Post
    I've started to realize that and I recently read something in the loudspeaker design cookbook about Q becoming irrelevant as the driver moves out of its piston range, but I still don't fully understand what that means or how to calculate the point at which Q becomes irrelevant base on the driver's T/S parameters.
    The Q is to do with the extension of the low frequency response by using the driver/box resonance. If you high pass the midrange at 300 Hz the details of what the driver is doing below 100 Hz becomes of little importance.

    Originally posted by wolfgang View Post
    Does that just mean using a gasket?
    A thick gasket might work if it had the correct stiffness. Rubber grommets are probably to be preferred. Here is a bit of discussion on the topic.

    Leave a comment:


  • wolfgang
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Thanks for the feedback. After thinking about it for a long while and comparing various drivers to each other I've switched over to a more traditional 3-way design using a 4" mid.

    If you are going to high pass the largish midwoofer covering the midrange the size and Q of the box is going to be irrelevant.
    I've started to realize that and I recently read something in the loudspeaker design cookbook about Q becoming irrelevant as the driver moves out of its piston range, but I still don't fully understand what that means or how to calculate the point at which Q becomes irrelevant base on the driver's T/S parameters.

    , decoupling the driver from the cabinet, etc...
    Does that just mean using a gasket?

    Leave a comment:


  • andy19191
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by wolfgang View Post
    So my uncertainty is, using the driver linked above, placed in a sealed cabinet (0.61 cu ft, Q=.707), playing only midrange frequencies, is the lossy, thin-walled, lowest-possible-resonance-modes design adequate for achieving best midrange detail?
    Using a driver designed for the job of covering the midrange would seem to me the obvious way to get the best midrange detail.

    If you are going to high pass the largish midwoofer covering the midrange the size and Q of the box is going to be irrelevant. This would not be the case if you were to run both midwoofers to cover the low frequencies which would seem the obvious thing to do.

    Will a "BBC cabinet" be adequate for the midrange cabinet? Probably if you get it right but the motion of the cabinet could be reduced further by using a more effective form of damping, decoupling the driver from the cabinet, etc... Having the first resonances at a low frequency is not in itself a recipe for a good cabinet.

    Leave a comment:


  • wolfgang
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    I agree with your statement completely regarding the low frequency cabinet.

    As for the "midrange cabinet," I need to be a little more specific in my design choice. What I want to do is to build a pair of 3-way speakers that are tri-amped and actively crossed-over at line level with MiniDSPs; however, I want to use the same 6.5" woofer for both the lower and middle frequencies, crossed-over around 275Hz. (The 2nd Xover will be around 3khz).

    The LF cabinet can be, as you say, stiff with higher resonance modes because it will be low-passed around 275hz. The HF cabinet will be exactly the same size as the LF cabinet in order to obtain equal Qs and group delays on both drivers, but it will handle the 275hz to 3khz range. This is the cabinet that I want to push down in resonance (ideally to well below 275hz), which is what got me interested in basing my design for that cabinet upon BBC/Harbeth in the first place. I can't remember exactly, but I think Harbeth's have their first mode somewhere just below 100Hz.

    So my uncertainty is, using the driver linked above, placed in a sealed cabinet (0.61 cu ft, Q=.707), playing only midrange frequencies, is the lossy, thin-walled, lowest-possible-resonance-modes design adequate for achieving best midrange detail?

    P.S. I arrived at this design mainly due to aesthics, space considerations regarding the overall width of the cabinet, a desire for near-equal power handling between the bottom-two amp channels (a lot music, on average, has it's energy evenly split right around 275hz), and a desire to keep crossovers outside of the 300hz to 3khz range.

    Leave a comment:


  • andy19191
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by wolfgang View Post
    That makes sense. So how would you improve upon the BBC design, i.e., to make it significantly more inert without shifting the resonance modes higher in frequency? Constrained layer damping? More flexible mass like vinyl?
    Where to place the resonances depends on what you are doing. Having the first few resonances at a low frequency is not always what you want. For example, if you have a separate woofer cabinet that is isolated from the midrange cabinet the best approach is almost certainly to go for a moderate weight but stiff cabinet that pushes the lowest resonance high enough in frequency to be above the passband of the woofer. Such a cabinet will need almost no damping (the difficult bit) and drive no resonances.

    A normal wooden enclosure for the midrange is unlikely to be able to push the first few resonances above the passband of the midrange. It's small size will also mean the first few resonance are going to be higher in frequency than an equivalent large cabinet that includes the woofer. It will normally require effective damping. But the most effective way to reduce the motion of the midrange cabinet is likely to be by isolating the midrange driver itself from the midrange cabinet.

    Leave a comment:


  • carlspeak
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by Jeff B. View Post
    ........ they spent a lot of time tuning it to sound just right too. In fact, in the LS3/5a spec they specify Beech battens and said no other wood sounded right to them. This kind of tweaking is not the easiest thing to do.
    From a prior post in this thread.... " I'm sure, they have 'tuned' to create a symbiotic 'sum of the parts' that benefits their particular designs."

    Leave a comment:


  • Jeff B.
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by andy19191 View Post
    The BBC approach was a pragmatic solution to what was required to push cabinet vibration just below the threshold of audibility for a modest cost and weight. It made use of the fact that cabinet resonances are less easy to perceive at lower frequencies and so placed the largest first few modes relatively low in frequency. It is not an approach intended to produce a "high performance" cabinet but one that is just good enough to do the job.

    To make a significantly more inert cabinet than a "BBC cabinet" following a different approach is not particularly difficult but is very likely to involve more cost and more weight. Simply doubling the thickness of the walls is usually not a good idea because it shifts the first few resonant modes higher in frequency but usually without significantly changing the level and so they become easier to perceive. To be equally loud at higher frequencies the wall displacement needs to be smaller (e.g. equally loud tweeter vs woofer) so the increased stiffness reducing the wall displacement is not necessarily going to help.
    I've read the BBC papers on nearly all of these designs and I agree with your statement here. The BBC had multiple goals in these speaker designs that were very application specific. Often the goal was light weight and low cost, and in some cases they did intentionally use cabinet resonance to add weight to the vocal reproduction of small speakers. But when they did, they spent a lot of time tuning it to sound just right too. In fact, in the LS3/5a spec they specify Beech battens and said no other wood sounded right to them. This kind of tweaking is not the easiest thing to do.

    Leave a comment:


  • wolfgang
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by andy19191 View Post
    The BBC approach was a pragmatic solution to what was required to push cabinet vibration just below the threshold of audibility for a modest cost and weight. It made use of the fact that cabinet resonances are less easy to perceive at lower frequencies and so placed the largest first few modes relatively low in frequency. It is not an approach intended to produce a "high performance" cabinet but one that is just good enough to do the job.

    To make a significantly more inert cabinet than a "BBC cabinet" following a different approach is not particularly difficult but is very likely to involve more cost and more weight. Simply doubling the thickness of the walls is usually not a good idea because it shifts the first few resonant modes higher in frequency but usually without significantly changing the level and so they become easier to perceive. To be equally loud at higher frequencies the wall displacement needs to be smaller (e.g. equally loud tweeter vs woofer) so the increased stiffness reducing the wall displacement is not necessarily going to help.
    That makes sense. So how would you improve upon the BBC design, i.e., to make it significantly more inert without shifting the resonance modes higher in frequency? Constrained layer damping? More flexible mass like vinyl?

    Leave a comment:


  • fpitas
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    To further muddy the waters, most DIY efforts have extensive bracing compared to commercial offerings. That will push any resonances up in frequency.

    Leave a comment:


  • carlspeak
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Originally posted by timw View Post
    Here's another attempt at the link.

    http://www.stereophile.com/content/m...art-two-page-6
    That one worked! Thanks.

    Leave a comment:


  • timw
    replied
    Re: BBC-style thin-walled cabinets vs. tight bass?

    Here's another attempt at the link.

    http://www.stereophile.com/content/m...art-two-page-6

    Leave a comment:

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