Just found the BIB design

bvbellomo

Member
2013-03-08 2:31 pm
I just found Terry Cain's BIB design. While I've found a ton of information, most of it references zillaaudio.com, which seems to be dead links.

Am I correct that this is a TQWT designed to be 1/2 wavelength instead of the standard 1/4?

If so, does that mean there is an opening? That would make sense, but most of the designs look like there isn't one.

Where is the driver placed? Is it the standard TQWT 1/3" the length of the pipe? Is the only reason to do an inverted BIB to place the driver closer to ear level?
 
The original BIB is indeed a 1/2 WL pipe horn with the driver off set 21.7%, though of course it doesn't have to be a 1/2 WL of the driver's Fs if you want it longer or shorter for whatever reason.

GM
 

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Thanks to those who helped me get the spreadsheet. Maybe others will find it here since it isn't online at zillaaudio anymore.

Is the entire 'terminus' area supposed to be open on top? What if I invert this design?

The reason for looking at this design is that I wanted to minimize space taken up by surround speakers (WAF) but need to raise the drivers up off the floor (no stands, behind a couch). The "Z Driver" puts this almost in the center vertically, but I want it on top. Maybe I can play with the way this is folded.

The spreadsheet calculates the "Z Driver" as 0.217 * the unfolded length. Does anyone know the reason .217 was chosen? This seems like an arbitrary number.
 
Depends how far you space them off the floor, the floor construction / covering, and the internal damping.

Re the driver tap location (Zd/Zdriver), 0.217*L was what Terry Cain used in his RS 40-1354 pipe, which was itself derived from a box for the FF125K in the Fostex craft handbooks. It's close enough to the correct point to help kill the 5th harmonic, while as far as the original box at least was concerned, it also placed the driver at a reasonable ear-height. So no, it's not arbitary. Other tap locations that will provide decent results can be used as far as the pipe is concerned; how practical they would be physically is another matter -depends on the box size and configuration.
 
Is the entire 'terminus' area supposed to be open on top?

What if I invert this design?

Does anyone know the reason .217 was chosen?

Yes.

Floor loading the terminus can create a vent, lowering its tuning [Fp], but not having MJK's latest software I don’t know if it can be simmed.

Some say 0.21, others 0.217 is the point along the acoustical path-length where the first 5th is, but it varies depending on the pipe’s effective end correction once all boundary conditions are accounted for, so I’m not ‘wedded’ to either, especially since stuffing density will shift it also.

Frankly, I’ve never built a BIB per se, only ML-horns, i.e. restricted [vented] pipe horns [AKA Voigt pipe] based on a different acoustic path-length with the driver location much further down the pipe and the vent area [Av] = driver effective piston area [Sd]; neither did I use them for ‘full-range’ apps, only up to 500-750 Hz depending on the XO point, so how they might impact the driver’s mids/HF didn’t concern me too much.

That said, after seeing that MJK had calculated a 50% driver offset for his original ML-TQWT as opposed to my simple rule-of-thumb, I plugged in a few different ‘FR’ driver ML-horn alignments into his original freeware software and was pleasantly surprised at the results, so if one did a bit of fiddling with line length using a restricted terminus [vent] in his current software, up to a ~62% offset can be realized with good in-room results, especially if corner loaded.

For max practical pipe loading though, around 20-25% is required.

GM
 

bvbellomo

Member
2013-03-08 2:31 pm
I don't really understand the physics behind a TQWT. I would think we could think of a speaker as a 'room'. It reinforces even 1/4 wavelengths (1/2, 1, 1 1/2) and suppresses odd ones (1/4, 3/4, 1 1/4, etc). But that would mean a quarter wavelength tube would be the WORST length to choose, because it would suppress fundamental frequency. A BIB, being ½ wavelength, would be ideal.

Continuing our room analogy, our driver is either our speaker or listener (physics are similar). Again, we'd reinforce wavelengths at even 1/4 distances to the wall and cancel odd ones. So placement to cancel 1st harmonic would be 1/4, 1/2, 3/4 and 1 of our fundamental wavelength, and reinforcement (worse position) would be at 1/8, 3/8, 5/8 etc. Our fifth harmonic is 1/5 the length of our fundamental frequency. So good (canceling) position would be odd 1/20ths (1/20, 3/20, 1/4) etc and worst would be even 20ths (1/10, 1/5th, 3/10ths, etc). 0.217 is between 1/5th (worst) and 1/4th (good) but closer to 1/5th so this is a bad position to kill 5th harmonics. It isn’t good for 3rd harmonics or 7th. Using my logic, it is possibly the worst possible place to put the driver.
 

bvbellomo

Member
2013-03-08 2:31 pm
If waves are reflected from closed surfaces with 180 degree phase shift, and from open boundaries without phase shift, then an opening at 1 end of the tube should behave the same as a closed tube ½ wavelength longer. This makes a TQWT effectively a ¾ wavelength room, which is still bad. A BIB becomes 1 ½ which is still good. So my logic doesn't change much.

Driver placement is not affected at all, as the end we are closest to is not open.
 

bvbellomo

Member
2013-03-08 2:31 pm
Try looking through the website I linked to, and these:

Standing Waves
Resonances of open air columns

Good links, but they seem to agree with what I wrote. If a TQWT is a valid design (and I am sure it is) than I missed something somewhere, and need a good link that does not agree with what I wrote.


One point I did find that might be it:

"A conical air column will produce the same fundamental frequency as an open cylinder of the same length and will also produce all harmonics."

I've been thinking of these as a tube open at one end and closed at the other. Should I be thinking about this as a tube open at both ends? Or is this just because the site is primarily concerned with instruments, and the end you blow into behaves differently?

If a TQWT really behaves as a cylinder open on both ends, it has the same modes as a 1 1/4 wavelength closed cylinder, which still suppresses the fundamental frequency, so that doesn't help.