loninappleton said:
Greets!
I haven't been able to keep up with this thread and at this point I can only recall that some of the posts I scanned WRT folding I didn't really agree with, though within reason some error isn't going to be audible, or at least not distractingly so. I mean these things are fundamentally flawed unless coupled to a corner just so to continue its expansion and the room chops it up pretty good no matter what, but thankfully our hearing acuity begins rolling off below ~1 kHz, so in general we're pleased with the results if the average peak response is ~flat in-room.
That said, to minimize line length and expansion errors when folding, there is one set of width/depth dims that preserves both 'close enough' for all but the most anal retentive, err perfectionist.
Ignoring the internal baffle thickness in the bend and the slight loss of length where it comes to a point, if the mouth area is proportioned so that the depth is SQRT(2) longer than the width, then both the expansion and line length is preserved if it's divided in two. If you want even more correct, then put angled baffle boards in the corners and fill the cavities with concrete, but all this does besides making it heavier is that it will cause even more HF out of the mouth that will need attenuating.
For example, a 55"^2 mouth requires:
depth = SQRT(55*SQRT(2)) = ~8.82"
width = 55/~8.82 = ~6.24"
or can be done by using the reciprocal of SQRT(2):
width = 1/SQRT(2)*~8.82 = ~6.24"
So in theory these dims are superior to TC's nominal 11" x 5" not only because they come close to preserving the straight pipe's dims, but it also now has a good acoustic ratio, though without building/comparing the two, your guess is as good as mine as to whether it's audible and to what degree (if any) one is better sounding. I imagine it would be tough to tell with smaller CSA pipes, but larger pipes would definitely need more acoustic power robbing damping (been there, done that). Since it's better to have more Vb than less (within reason), always round up the numbers if needed.
Once you move away from this ratio in either direction, then preserving the simmed values gets increasingly complex since you can no longer just divide the length in two, so I'll leave those to the math whizzes to work out.
If HD in your area is like in mine, you're braver than me to trust them with any cuts other than +/- 1/4" accuracy coarse ripping.
GM
mcgsxr said:
At the moment, they aren't breaking in at all. I'm trying to get the cabinets finished up. It'll take a few days.
My dimensions are:
Height = 60" (that's internal, the bottoms aren't on yet)
Width = 7-5/8" outside (6-1/8" inside)
Depth = 11-1/4" outside (the mouth is 9" deep)
These dimensions were strictly a matter of convenience. 60" is 1/2 of a 10 foot board. 11-1/4" is the width of a nominal 1 x 12 plank, so my sides were a 10 foot 1 x 12, cut in half, and used full width. I ripped 1 x 8 stock down to 6-1/8 for the front, back and inner baffe to make the mouth area come out to 55 in^2.
So, I 'accidentally' wound up within 1/8" of GM's dimensions:
Sometimes life just works out right!
Bill
giantstairs said:
Lolina, Ha.
I'm Lon and I'm in Appleton, WI, USA.
Something clicked when you said half the line length
so I understand that much now (being pretty slow on the
uptake.) So as not to have to cut stuff, what driver has
a line length of 144 in to keep my 72 in untrimmed
board length? Nothing too fancy.
GM said:
HDs are all the same. A good heads up. I went to a Stock lumber yard
and the gave me an estimate on birch ply plus cuts at $1 each.
The veneer core was more than the fiberboard core but what is
the thinking regards construction? Which can be worked better with
a router to do chamfers and such? The weight and density of these should be about the same.
Scott sent me two sims for the Fostex 168S. One is for a straight pipe the other for a BIB. Clearly the pipe is smoother but it's also less efficient and doesnt go quite as deep.
Can someone better explain what's happening from these sims?
Thanks,
Godzilla
PS... both cabinets are just over 5' tall and take up approx the same footprint (pipe is approx 9 x 10 footprint, BIB is approx 9 x 11.5)
Can someone better explain what's happening from these sims?
Thanks,
Godzilla
PS... both cabinets are just over 5' tall and take up approx the same footprint (pipe is approx 9 x 10 footprint, BIB is approx 9 x 11.5)
Attachments
Greets!
Unless the veneer core is very dense (BB, Apple, or marine ply) it's probably not void free, so the fiberboard core will probably machine better if its at least medium density since it won't be prone to splintering or having spurious resonances, though practice safe routing since the dust is to some extent toxic.
GM
Unless the veneer core is very dense (BB, Apple, or marine ply) it's probably not void free, so the fiberboard core will probably machine better if its at least medium density since it won't be prone to splintering or having spurious resonances, though practice safe routing since the dust is to some extent toxic.
GM
Greets!
Not sure what you're asking. I mean the pipe is mass loaded to get a max flat alignment by damping pipe resonances and the horn to get max gain with the attendant undamped pipe resonances. As the driver is moved towards the horn's mouth it will become more like the mass loaded pipe, but at the expense of gain. No 'free lunch here'.
BTW, how did you do the FR overlay?
TIA,
GM
Not sure what you're asking. I mean the pipe is mass loaded to get a max flat alignment by damping pipe resonances and the horn to get max gain with the attendant undamped pipe resonances. As the driver is moved towards the horn's mouth it will become more like the mass loaded pipe, but at the expense of gain. No 'free lunch here'.
BTW, how did you do the FR overlay?
TIA,
GM
Hi GM!
I’m trying to understand why the BIB sounds different by just looking at the graphs. I guess it’s pretty easy to see what’s happening. After building and listening I can better understand what the graphs mean and how the sound changes. So far BIB builders confirm midrange coming thru the top of the BIB and using some kind of fiberfill to damp this down. These resonances that ripple thru the graph need to be damped to get the best from these designs. But at the same time taking advantage of maximum gain! My SI amp or any low powered tube job will appreciate the added gain… but our ears may not like the other additions to the overall sound.
I want to post a drawing of how to stuff a BIB to reduce the added resonances… at least provide a strategy and starting point so we can tame these beasts.
Peace,
Godzilla
PS… the overlay was done in Photoshop. Cut and paste one graph, then add another layer and paste the other. I changed the color of one of the graphs to a purpleish blue. Then change the opacity of the top graph so the other shows thru. Flatten the image (combine the two layers to one) and fine tune the brightness/contrast.
I’m trying to understand why the BIB sounds different by just looking at the graphs. I guess it’s pretty easy to see what’s happening. After building and listening I can better understand what the graphs mean and how the sound changes. So far BIB builders confirm midrange coming thru the top of the BIB and using some kind of fiberfill to damp this down. These resonances that ripple thru the graph need to be damped to get the best from these designs. But at the same time taking advantage of maximum gain! My SI amp or any low powered tube job will appreciate the added gain… but our ears may not like the other additions to the overall sound.
I want to post a drawing of how to stuff a BIB to reduce the added resonances… at least provide a strategy and starting point so we can tame these beasts.
Peace,
Godzilla
PS… the overlay was done in Photoshop. Cut and paste one graph, then add another layer and paste the other. I changed the color of one of the graphs to a purpleish blue. Then change the opacity of the top graph so the other shows thru. Flatten the image (combine the two layers to one) and fine tune the brightness/contrast.
Hi Jeff.
Because the damping of harmonics rather depends on their position in a room, and the resonant character of that room itself, there's going to be no definitive.
Starting point: stuff the point above the driver, and a 1in layer of stuffing on the base. Lining the front wall of the internal baffle will also help absorb unwanted highs, you can also line the rear wall of the internal baffle. A pad of damping on the internal rear wall right up near the terminus will also help.
Because the damping of harmonics rather depends on their position in a room, and the resonant character of that room itself, there's going to be no definitive.
Starting point: stuff the point above the driver, and a 1in layer of stuffing on the base. Lining the front wall of the internal baffle will also help absorb unwanted highs, you can also line the rear wall of the internal baffle. A pad of damping on the internal rear wall right up near the terminus will also help.
GM said:Greets!
True, just not in this case unfortunately.
GM
The 11.25 was outside, 6.125 was inside.
The inside dimensions are 9/6.125 = 1.469x.
The ratio is different at every point down the pipe. Does the ratio at the outlet have a disproportionately large influence over the total response?
Bill
Greets!
Oops! This one's fine too.
True. Yes, since the mouth represents an impedance mismatch, so any standing waves across it will have a tendency to radiate back to the throat, modulating the driver. The ratio at the driver's location is the other critical point, but with a tapered line this isn't much of an issue, which can usually be resolved by damping one parallel wall. On problematic mouth ratios, putting an open cell foam 'cap' of the right density or similar around its perimeter takes care of it.
GM
Oops! This one's fine too.
True. Yes, since the mouth represents an impedance mismatch, so any standing waves across it will have a tendency to radiate back to the throat, modulating the driver. The ratio at the driver's location is the other critical point, but with a tapered line this isn't much of an issue, which can usually be resolved by damping one parallel wall. On problematic mouth ratios, putting an open cell foam 'cap' of the right density or similar around its perimeter takes care of it.
GM
Godzilla said:
Greets!
Thanks! Bummer though, I was hoping there was a quicker/less tedious way.
Any damping in the horn will affect gain, but sometimes you have to think outside the box, literally. Remember in this post http://www.diyaudio.com/forums/showthread.php?postid=904863#post904863 an acoustic tile ceiling ~solved the problem?
GM
Greets!
Not really, this would make it an echo chamber, which is horrible sounding. If enough speakers were used to create a truly diffuse soundfield over the entire audible BW though, then in theory our internal processor would overload and sum it as a holographic sonic event, making all positions in the room the 'sweet spot'. Talk about big 'headphones'...........
GM
Not really, this would make it an echo chamber, which is horrible sounding. If enough speakers were used to create a truly diffuse soundfield over the entire audible BW though, then in theory our internal processor would overload and sum it as a holographic sonic event, making all positions in the room the 'sweet spot'. Talk about big 'headphones'...........
GM
Confused
For me the BIB page is confusing. 'Zillas home page
has tabs across the top for access. I was fiddling
around the BIB page a long time before I figured out
that clicking on logos was supposed to do something.
A tabbed layout would be better with maybe the
logos down to the size of those fave-icons we always
see in the address bar.
For me the BIB page is confusing. 'Zillas home page
has tabs across the top for access. I was fiddling
around the BIB page a long time before I figured out
that clicking on logos was supposed to do something.
A tabbed layout would be better with maybe the
logos down to the size of those fave-icons we always
see in the address bar.