AV15 slot loaded design (1 Viewer)

[Brian Bunge]

For my own designs that require a vent, if a sub it normally gets PRs, and for every other vented design I don't give two hoots n' a holler what it is since I'm going to 'critically damp' them for best transient response anyway.

Greg, could you go into greater detail about this?

 

[RayJK]

Greg,

I just wanted to post a thank you for the help you provided. I'll let you know how it turns out.

Ray

 

[Greg Monfort]

>Greg, could you go into greater detail about this?
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It's a test that's been around since the beginning of point source driver design. All speaker designs were originally Aperiodic in nature since they didn't have the materials technology to make low Q drivers.

You toggle between an open circuit and 1.5V battery feeding the speaker with a snap switch to determine if the cab's damped enough, then if not you stretch a piece of open weave material across the vent and try again. Once you only hear the switch's CLICK, with no overtones on make AND break then it's critically damped (Qtb = 0.5).

If the click is muted from the get-go then you need to either re-think the design and/or start removing/swapping out insulation used to damp standing waves.

It's mildly interesting to me that it's the final step in tuning a pre-T/S reflex in all the older speaker design books, but all the ones I've seen that were published after T/S omit it. I guess since Small's original formulas are based on a max of ~4.5% mach, it was no longer considered relevant.

GM

 

[Brian Bunge]

So aperiodic damping reduces port air speed huh? Makes sense to me. It's nice to have you guys around that have forgotten more about speaker design/building than I'll probably ever know!

 

[Vince Bray]

Yep, yep.

So it's theoretically possible to use damping material lining the port to suppress the odd harmonics? If so I would go for wool I think, since glass would be airborne, and foam would definitely kill the port area. In a t-line design how does one adjust the port dimensions for the presense of damping, or is it done that way?

 

[Greg Monfort]

>I've actually tried that. I have an old book 'hi-fi loudspeakers and enclosures' which I'm sure you've seen from the heyday of the 50's altec, bozak, etc. which talks about that technique.
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Yep, bought it 'hot off the press', as they say, and still have it.
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> I never really pursued it but you gave me an idea that this could actually be done with a PR design. It would be a pain but you could stretch fabric either over the PRs or (I guess) inside the cab between the PR and the internal volume and control the motion of the PR that way. Crazy? Maybe this would help the PR transient repsonse approach or exceed the ported design. Could this be the holy grail of [small cab/high output/low tuning] subwooferdom? My PRs are recessed into the cabinet quite a bit to allow grills, I may have to try this for S&G... One issue might be the air trapped between the PR and the cloth, although the effect of that would likely be negligible.
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Well, if you think about it, what does the stretched cloth do? It mass loads the air in the vent, so to easily 'critically damp' a PR you add more mass, ergo you design in Qtb, and why now that Tom Danley has shown us how to properly design vented designs using PRs, I prefer them on most subs. At higher Fbs, dealing with their notch an octave below Fb isn't worth the cost IMO.
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>It'd be interesting to test some commercial speakers to see how well they perform this test. Too bad I don't have any of those
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I've done this periodically, and all that I've tested failed miserably. The CLICK sounds like a BONG, or standard T/S Qtb = 0.707.
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>So it's theoretically possible to use damping material lining the port to suppress the odd harmonics?
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Yes, though it damps the fundamental too.
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> If so I would go for wool I think, since glass would be airborne, and foam would definitely kill the port area.
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Stick a sock in it, literally. Experiment with different materials/density, and if a high output vent, then you'll need to retain them somehow as they can be sucked in/blown out, ditto if raw wool is used.
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> In a t-line design how does one adjust the port dimensions for the presense of damping, or is it done that way?
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You don't, though I suppose you could design an adjustable exit. As your old book warns, TLs fall out of tune over time with the change in driver specs, so designing around a well broken in driver is key to maintaining long term performance, though frankly, I doubt you would notice the change since it's gradual and most driver's audible break-in changes after a few hours are actually your hearing adapting to it rather than vice versa. Anyway, play with MK's spreadsheets to see how much changing the variables affect the FR.
GM