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Old 9th June 2012, 01:49 AM   #1
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Default vertical line cabinet design for bass guitar

hi everyone!

I'm trying to design a bass guitar cabinet with the kind of sound dispersion pattern typical of acoustic instruments- not necessarily double basses. I'd like it to sound healthy in 360 degrees, although it doesn't need to sound the same everywhere. I envision using this outdoors with percussion and other omnidirectional instruments, and I'd like it to sound as good from any horizontal direction as the percussion does.

I have a wonderful bass and 440 watt Manley monoblock amp that are working beautifully. I also have 16 (5 1/4") full range drivers that are working nicely. I'm trying to come up with the right approach to the cabinet. I've tried this topic on some bass guitar forums but there's a certain oppressive orthodoxy that isn't helping. I've seen some really interesting things on this forum so I'll try here.

I'm looking at a vertical line of 12 (5 1/4") drivers at 8 ohms, which comes out about 5 1/2 feet tall, in two cabinets, stacked. The amp is 5 ohms nominal, so a 6 ohm load seems like a good bet.

I'm really liking the sound from a 1" wide port that's as tall as the stack. The tone and off-axis response is far superior to the direct output of the speakers, so I've been thinking about putting a small volume and a 1 1/4" slot (about 3/4 of the piston area of the speakers, which have a piston diameter of 3 7/6") in front of the speakers as well.

I've seen some stuff about slots on this forum but I can't follow it. I'm pretty green in the whole area of enclosure acoustics.



I'd appreciate any and all ideas and resources anybody here can contribute. The main issue is providing presence and mid to high frequencies in the off-axis areas.

Thanks!
Ted
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Old 9th June 2012, 02:38 AM   #2
n4vgm is offline n4vgm  United States
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So, Ted, did you build this already and want to modify it or are you just thinking it out at this point?

As a multi instrumentalist I been wondering if this could be done DIY rather than something like the Bose L1 or Fishman Array. From what I've read the issue is the center to center driver spacing and comb filtering which best works with the small drivers.
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Old 9th June 2012, 09:14 PM   #3
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Hi, and thanks for the reply.

I haven't built this yet.

What I'm learning about line arrays, and I'm not sure if this actually would be one per se, is that a very narrow ("focused" was the word actually) pattern results in the highs, and the higher the frequency the narrower. This is actually exactly the kind of beaming I'm trying to get away from. I'm trying to spread the highs out, hopefully over most of 360 degrees, not point them at someone.

The cabs I'm using for some listening tests, which are somewhat poorly braced 7/16th plywood with tolex covering, are two 5 1/4" speakers wide and with both the tall way, 8 high. Something similar in scale to a stack of 10's. I'm finding the high end is rolling off quite abruptly, as soon as I get any distance off axis. That seems quite consistent with the beaming effect of the line array.

However the nearly 4' tall 1" wide port that the two cabinets stacked provide sounds really nice to me, and the port's off axis response is a whole lot smoother and more extensive than that of the drivers. This seems consistent with what I've learned about slot diffraction, as in a diffraction horn, which sets up a cylindrical wave front limited only by the baffle, at frequencies that are small compared to the width of the slot. You've doubtless seen a picture of waves approaching a slit and diffracting out this way, although they were probably described as water waves.

That's why I was thinking a 1 1/4" slot. 1" sounded great, even after the inevitably high end losses from bouncing around inside the cabinet, but I have been wary of reducing the area of the slot opening much below the piston area of the speakers.

Since then though I found an interesting thread here on a slot woofer setup: The Slot Loaded OpenBaffle Project Article By Nelson Pass I've yet to read all 43 pages of the thread, but in the article no mention is made of diffraction but it does use a slot that's 1/3 the size of the piston area and it says this results in 9 times the velocity coming out the slot, compared to coming right out of the speakers without a slot. So now I'm maybe not so timid and I'm thinking of more like a 9/16ths slot, in comparison with which pretty much everything including most of the harmonics and noise coming off a bass will be large in wavelength compared to the slot.

Actually I tried at one point to make a mute this way for an electric guitar amp, thinking the smaller virtual piston area would quiet things down. The unbraced pine cab was not equal to the sound pressure situation and resonated unduly, and the thing was still louder than all get out. I'm thinking now all I did was speed up the sound and provide some more pine to resonate, albeit this time in front of the speaker and thus in phase.

Does anybody know if the height of the slot has the kind of effect the height of a line of speakers does, without a slot? Are there any diffraction slot designs already out there I should be looking at? I'm especially curious about what happens with more than one slot. I'm pulling up a lot of stuff about light but not so far about sound.

I will add that I personally am not now exploring diffraction horns- just a simple slit. Perhaps, dear Moderator, this thread could be retitled with some reference to diffraction slits?

Anyway thanks and I'm encouraged to be in the presence of such a knowledgeable and frequently adventurous community.
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Old 9th June 2012, 09:41 PM   #4
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I'm finding some things- too bad I can't manage links- on Young's double slit experiment. This shows two slits side by side, a situation I've been avoiding for basically the same reason I'm avoiding two cabs or two drivers side by side. Much interference. I've seen illustrations of two cabinets crossfiring at 90 degrees, and this seems far healthier. Specifically what I'm trying to find is what happens when two slits at 90 degrees are both crossfiring, or actually in my case firing away from each other at 90 degree angles. I have a near-superstition based on the smooth transition made my an XY mic array (coincident and at 90 degrees) that this could be quite healthy too, but coincident is not easy to manage with small diameter mics let alone speaker cabinets... I've tried this experiment with two 12" speaker cabinets and found them to sound quite well when stacked in this kind of xy arrangement. Slits though seem like they might behave differently, and I couldn't very well get two vertical lines of speakers angled coincidently. I don't think! So I'm looking for info on two or more slits at 90 degree angles to each other at a healthy distance apart, whatever that might be.

I envision slits cut in 3 or four of the corners of a squarish cabinet \, each all but concealing a vertical line of four speakers, on a stand to bring it all up close enough to ear height.

Anyone?
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Old 9th June 2012, 09:59 PM   #5
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I found this:

Stereo Speaker Interference Experiment | Sounds Amazing | AS/A Level Physics Revision | University of Salford - A Greater Manchester University

Looks like coincidence is very much the ticket... or just one slot. 180 degrees has some serious issues too as the frequency increases- I wonder if this simulation acts differently than a real cab (with a baffle) between the two speakers would act...
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Old 9th June 2012, 11:54 PM   #6
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Now I'm really appreciating how the tall skinny aspect of these commercially available line cabinets would be most appropriate as far as keeping the cabinet out of the way so a cylindrical waveform emerging from a slot could wrap all the way around at all but the highest frequencies.

I read something on a thread here about mini-line arrays where the two cabinets (I notice they tend to come in pairs) combine to create a horizontal image of some kind- I wonder if that's strictly a stereo thing or if mono would be subject to the usual phase anomalies from two separated sources ...
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Old 10th June 2012, 06:31 AM   #7
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You you have 12 drivers, and you want omnidirectional sound, why not put 3 drivers on each side of the cabinet?
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Old 10th June 2012, 04:07 PM   #8
bjorno is offline bjorno  Sweden
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Hi,

FYI:Here is a small PA(+instrumental use..even for bass in smaller venues) array suggestion (calculations) for n x 4-driver' segments. n= 1-4 using the Dayton PA 130-8 driver.

Dayton Audio PA130-8 5" Full Range PA Driver 295-010

b
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File Type: jpg Dayton_PA130_array-section.JPG (551.3 KB, 199 views)
File Type: jpg Dayton_PA130_array-section-Cont.JPG (607.6 KB, 193 views)
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Old 13th June 2012, 01:19 AM   #9
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Thanks for this FYI- it's definitely pushing the limits of my comprehension (OK, exceeding them...) but I've got it printed out and I'll take it home and study it!

Quote:
Originally Posted by Keriwena View Post
You you have 12 drivers, and you want omnidirectional sound, why not put 3 drivers on each side of the cabinet?
My concern is the cancellations that take place when the driver arrays are at a distance to each other. So I have been looking at this approach, trying to get the drivers or their output slots as close together as possible. Judging from the simulations referenced above, I should be able to keep the cancellations up above 300 hz or so with practical dimensions, at least the low end power distribution would be flat. Nothing like having one note disappear or boom out!

At one time I had (4) 10" Rhodes speakers remounted into two cubes with 2 speakers in each at right angles. There doubtless were some cancellations but in practice it worked really well, with the stereo tremelo going all the way across the stage, and plenty of coverage for band and audience alike.

Thanks again everybody, I look forward to anything else y'all come up with.
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Old 13th June 2012, 01:47 AM   #10
sreten is offline sreten  United Kingdom
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Hmmm........

Double split experiments and bass cabinets, do me a favour .....

Its not relevant. Basic physics is. There is no point slot loading a 5" array,
given its unusual for bass, but you can, to reduce throw versus coverage.

Quite frankly most of this thread is ********, because your rig is either backline
where it won't matter so much or frontline where it does, and having a very
different dispersion pattern to the rest of the band will screw things up badly.

Its boring, and not really worth discussing in detail anyway you look at it.

rgds, sreten.
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Last edited by sreten; 13th June 2012 at 01:50 AM.
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