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Bib Design

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Ok, here is what I have gathered on the design of BIB enclosures, I have compiled this from www.diyaudio.com, conversing with other people, and building mine.

BIB Design Criteria

o It is a 1/4 wave enclosure with a line length of 1/2 wavelength of tuning freq (a 1/2 wavelength enclosure has to have both ends open)

o Normally set to Fs or just below. With a really low Fs you can tune a bit higher

o Bad idea to tune lower than .707Fs

o Vb is 20Vas x (Qt^1.25)

o Width and Depth are selected to preserve the conical horn profile

o Depth is 1.414 times width, room for experiment though

o Terry Cain’s original cabinet had a 1:2.2 ratio

o Internal baffle goes from top front corner to termination in bottom center of cabinet equal distance from bottom, front, and back.

o Above driver needs a light layer of stuffing, carpet internal base to absorb HF waves and damp reflections.

o Deflector panels are bad, bass doesn’t care, but we don’t want HF so keep the abrupt corners to eliminate HF out mouth of horn.

Driver Positioning (From Scottmoose on DIY Audio)

Driver placement is an interesting issue. For most BIBs, for practical purposes ~0.217 line length is used. According to MJKs worksheets 0.416 is ~ideal, though there is a likelihood that in-room a driver positioned much closer to the horn mouth will in fact work better. A pipes' harmonic modes are independent of the driver you see: you know where they'll occur without ever needing a driver mounted in them. A straight pipe will only have a fundamental resonance, and harmonics occurring at even multiples of the fundamental. A horn, which unlike a straight or negative taper pipe, or even a TQWT (which is negatively tapered toward the throat -horns are positively tapered toward the mouth, which sounds the same but isn't in physics) has expanded sufficiently for 1/2 wavelength action to occur, with a 1/4 wavelength fundamental, has both odd and even harmonic structure. The position of the driver in the line determines the level of excitation of both the fundamental, and the higher harmonic resonances. If positioned at the throat of a pipe for example, all modes, fundamental and harmonic, will receive the maximum possible excitation. Move the driver along the pipe / horn / whatever you like to call it, and you will reduce the excitation of some harmonic modes (and the fundamental to an extent).

Second driver for low frequency boost.

Yes, adding a second driver mounted through the internal baffle is an interesting idea for a bit of tapped-horn boost. Note that Vb will have to double though -something of a downside when a cabinet is already rather large. ;^) Wish I'd thought of it, but GM takes credit for that one. No, it wouldn't necessarily have to be at the same height as the forward-firing unit, though it it would be advantageous IMO. Overall, I suspect it would end up being somewhat design-specific as to where it will best be positioned. I can't see there being a major problem of cancellation occurring, assuming the correct placement of the drivers so that the wavefronts do not encounter something in the reverse-phase.

Links that should be read if you are interested in BIB and associated enclosures.

Quarter Wavelength Loudspeaker Design

Vas and TQWT - diyAudio

Terry Cain's BIB -why does it work and does anyone have those Fostex Craft Handbooks? - diyAudio

Link for an Excel sheet that does the calculations

http://www.filehosting.org/file/details/96524/BIB_Calculator.xls

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I built a pair of BIB's for some Pioneer full range 8's a long time ago. Bass heavy is an understatement and are even more impressive in a larger room. They are great party speakers!

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Great post. I have noticed many people make Fostex the BIB choice for smaller drivers. Did anyone happen to see the new Tang Band FR?

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Parts Express: Featured Categories

That one?

Interesting design, with a phase plug and a whizzer...judging by the specs, obviously aimed at the Fostex market.

Funny combination too. Excel phase plug, diyma style mags, white fostex cone and whizzer.

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I just remembered I have some full range speakers needing some BIBs...........have to wait til later.

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