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Multi-Way Conventional loudspeakers with crossovers

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Old 8th May 2013, 08:13 PM   #8721
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I should also mention the open-back box filled with Bonded Logic is surprisingly much less prone to internal standing-waves than an equivalent box with a solid back. If I recall right, Gary actually put a microphone in the box and measured what's going on, and opening the back nearly dropped the box modes to zero. (Remove the filling, though, and the improvement is considerably less, and a lot more sound zips out the rear.)

Since the back looks like an open window to the internal standing waves (unlike a hard, almost perfectly reflective surface), and the box is filled with a highly lossy material, the modes are de-Q'ed and store less energy. Subjectively ... well, it doesn't have as much box sound (you saw that coming, didn't you?). The enclosure walls are quieter, too, which is a nice benefit, and relaxes the requirements for internal cross-bracing (although still a good idea, you don't need as much).

I think of these as short transmission lines, aided by wideband high-loss filling. No deep bass, considering the size, but really good upper-bass and midrange.

I encourage the readers of this forum to try the internal microphone measurement for themselves, comparing:

1) Closed-back with no filling

2) Closed-back with Bonded Logic filling

3) Open-back with no filling

4) Open-back with Bonded Logic filling

Should be interesting.

Last edited by Lynn Olson; 8th May 2013 at 08:24 PM.
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Old 8th May 2013, 08:19 PM   #8722
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Would 1" acoustic ceiling tiles have a similar effect?
Another member uses these acoustic tiles to line his boxes to help dampen internal waves.
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Old 8th May 2013, 08:33 PM   #8723
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Almost no effect at all. The ceiling tiles might quiet down vibration in the side or top panels, but they probably don't have enough mass to couple to the dense plywood or MDF of the enclosure walls. It takes a heavy material, like asphalt tile, to damp plywood or MDF enclosure walls.

The effect of ceiling tiles on internal standing-wave modes would be very slight, probably too small to measure. Acoustical ceiling tiles, to the extent they do anything at all, work from 500 Hz on up, and are most useful giving a moderate degree of isolation from the acoustical plenum where the air-conditioning ducts are in a typical office space.

Gary and I like Bonded Logic because it's the most broadband and effective acoustical absorber we know, far more pleasant to work with than awful fiberglass, and is not that expensive (if you buy it in bulk instead of Home Depot). The traditional long-fiber wool is good too, but it really has to be moth-proofed and is also prone to sagging. Wads of Bonded Logic, by contrast, are self-supporting, which is very convenient for filling enclosures.

Now if you wanted to be a little kinky with the ceiling tiles, you could try making the sidewalls from them, instead of using plywood or MDF. They'd be pretty lossy, which is a good thing in an open-back box. The open-back box will still need to filled with Bonded Logic, though, since that's what makes the open-back box work in the first place ... otherwise it's just a resonant old U-box, like an old-school TV or table radio.

If you want to get really weird, the sidewalls could be open-framed with one-by-twos, like a miniature house, with acoustical ceiling tiles glued on the inside and outside of the framing, and the space in between filled with (of course) Bonded Logic. Fill the interior of this little house with more Bonded Logic and leave the back open. Not exactly the most beautiful enclosure, but it would sound different.

Last edited by Lynn Olson; 8th May 2013 at 08:53 PM.
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Old 8th May 2013, 08:51 PM   #8724
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I believe I remember BudP talking about Gary Pimm(I presume) using the bonded logic in his woofer setup. THanks for the comments.
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Old 8th May 2013, 08:55 PM   #8725
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You want out-of-the-box thinking, you've come to the right place.
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Old 8th May 2013, 09:03 PM   #8726
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Just when I thought it was safe to come out and make a little joke, something a lot weirder than this discussion comes out. Try this on for size: plants communicate with each other via sound. Here's the links:

BMC Ecology | Abstract | Love thy neighbour: facilitation through an alternative signalling modality in plants
and
http://www.biomedcentral.com/content...6785-13-19.pdf
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Old 8th May 2013, 09:27 PM   #8727
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Already got that going on in my garden. Got a book about it somewhere. You can even underplant cover crops to follow that will both shade the plants and give nice head start for next planting. THey sya that Basil helps both tomatoes and peppers, in growth, flavor, and pest control. I ahve a feeling some salty dogs have forgotten more about gardens than we will learn in the next couple years.
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Old 8th May 2013, 09:34 PM   #8728
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Quote:
Originally Posted by Lynn Olson View Post
I encourage the readers of this forum to try the internal microphone measurement for themselves, comparing:

1) Closed-back with no filling

2) Closed-back with Bonded Logic filling

3) Open-back with no filling

4) Open-back with Bonded Logic filling

Should be interesting.
Also very important to note is the material density. The 3.5" R-13 cotton batts are about 2 pcf density; R-13 batts of fiberglass are generally around 1.5 pcf.

As is widely known, as stuffing is added to a box, effective box volume and damping increases while Fs decreases. As one would suspect, at some point any increase in density becomes a volume-robbing solid, and the Fs trend reverses. This turning point is typically somewhere around 1-1.5 pcf density (for a sealed box). Damping may sometimes continue to increase for a bit longer, but will also reverse as density increases.

For an open-back box, 2 pcf cotton might be nearly optimum for damping, but is also rigid enough function as moving mass (somewhat evidenced by the cotton "walkout").

Empirically, it's pretty easy to see what's going on with woofer-tester software, but it would be interesting to also correlate with mic measurements.

The most interesting possibilities lie in mixing densities. Whereas what we typically think of as stuffing only serves to de-tune a system, a layer of semi-rigid (like those fiberglass ceiling tiles, typically 3 or 6 pcf density) would enable resonance tuning, and bass extension like a passive radiator.
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Old 8th May 2013, 11:49 PM   #8729
BudP is offline BudP  United States
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Quote:
Noncoincident arrival time would certainly account for the mixture of blur and glare in commercial PA systems ... it's loud, all right, but you can't understand a word, thanks to a hash of non-coincident arrivals from the large arrays. Since the arrivals in the 0-3 mSec range are particularly critical for perception of intelligibility and tonal coloration, that's right where the line arrays will fall down, while still measuring fairly decent in the frequency domain.
Made me think of migeO's work with wavelet analysis. What he provided gives a very clear picture of why even individual drivers can be confused sounding. The repeated patterns within the first 4ms are clearly shown. I suspect wavelet analysis could find and help in the elimination most horn difficulties too.

Bud
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Old 9th May 2013, 03:04 AM   #8730
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Default Wavelets and the older engineer

Quote:
Originally Posted by BudP View Post
Made me think of migeO's work with wavelet analysis.
Being lazy, can anyone recommend a primer or text book on wavelet analysis - I understand the concept but it post-dates my time at school.
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