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Old 8th April 2010, 05:01 AM   #1
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Default Cabinet Resonances

Split off from Mark's new Alpair 12

Has anyone written in these forums on the qualities of various woods? "Dense" doesn't tell us much. I'm interested to know what the natural resonances of the panels in this design are, and how this varies with different wood. Does the frequency shift, and what changes are there in Q?

Bracing would seem to be potentially counter productive as it would reduce panel section sizes and raise the individual resonant frequencies.

I wonder how much bass energy is delivered directly by the enclosure panels in the Pensil series designs?



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Originally Posted by markaudio View Post
Yes, the Japanese Birch are made from the standard plan, no dimensional changes. There's no internal bracing since the Birch is very dense.

At the moment, we have 0.2Kg of micro-fill sheet fibre inside each cab.

Cheers

Mark.
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Old 8th April 2010, 06:05 PM   #2
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A few general points.

1/ The panels are not designed to contribute anything to the sound. You can inject some colouration / character (depending on which way you look at it) via different woods, but that's up to the end user.

2/ That's the entire point of bracing. Fact: you cannot erradicate panel resonance. You can only hope to control it, and push it out of the cabinet's operating BW to a region where it's not going to be significantly excited, and therefore easy to damp if necessary. I'm rushing as I need to go out in a minute, so forgive the semi paraphrasing here:

-stiffness goes up with the cube of panel thickness, and doubling thickness / mass only drops panel Fs ~0.707 that of the original.

-as panel Fs drops, so the amplitude and resonant BW increases. Ergo, it's all but impossible in a modest sized enclosure to push the panel resonance below the box operating frequency. Fine for midrange / HF cabinets, but not practical for bass enclosures. Better is to push the resonant frequency of the panels up, above the cabinet's operating BW, by making them as light and stiff as possible. This is assuming you don't want the panels to contribute anything of course.
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Old 8th April 2010, 10:53 PM   #3
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BBC thin wall design for example intended the panels to be unbraced so that the resonances remained below the critical midrange. It then damped them to reduce Q. Also the lamination of ply is more lossy than solid timber. Alan Shaw of Harbeth once told me in conversation that the panels in this type of design do contribute to the radiated sound, hence the importance of material choice and construction methods.

My interest is in comparisons of different materials in measurement and data. Does anyone have any links or references?
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Old 8th April 2010, 11:21 PM   #4
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Quote:
Originally Posted by gazzagazza View Post
My interest is in comparisons of different materials in measurement and data. Does anyone have any links or references?
Try to find a copy of "Understanding Wood" by Bruce Hoadley. Can't recall exactly what it has pertaining to this (and my copy is currently lent) but I'm fairly certain you'll find it useful if you are considering solid wood for cabinets.

I suspect that instrument building books and forums will have info on wood resonances, but for panels much thinner than what is required for cabinets, so not sure that would help.

Even if you narrow it down to a particular species, it would be necessary to test each piece due to the variations in wood.

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Old 8th April 2010, 11:28 PM   #5
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It then damped them to reduce Q.
A move that makes the resonances more audible. A good thing if you are using the box as a PR.

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Old 9th April 2010, 03:55 AM   #6
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As suggested, a guitar or piano-building forum is probably the best place if you want to cover the subject of different woods as PRs. You'd need to do a lot of testing of specific sized & jointed panels though in order to draw anything but the broadest of generalisations. FWIW, if anyone hasn't seen it, the original BBC RD paper on the LS3/5a is here: http://downloads.bbc.co.uk/rd/pubs/reports/1976-29.pdf Harwood's AES paper is available as per usual from the AES site, for a modest fee.
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Old 9th April 2010, 06:05 AM   #7
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Well I think the point already made is that all boxes will resonate, and therefore will radiate some energy. Better then that the box has low amplitude, broad resonances below the mid-range, than high peaky ones in the mid range.

I recalled also that in the conversation mentioned Alan Shaw said that despite ongoing requests from his dealers and customers he has not built a floor standing loudspeaker because he has not been able to design a box of that size that does not colour the music.

Anyway, I'm well through a build of a pair of Pensil 7s using unbraced ply with bituminous felt damped walls... I'm looking forward to hearing the result.

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A move that makes the resonances more audible. A good thing if you are using the box as a PR.

dave
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Old 9th April 2010, 06:10 AM   #8
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One last question to Dave & Scott. You both have designs published here for various enclosures, and Dave you go to great lengths to tune drive units with esoteric treatments. Do you not run any acoustic tests on the enclosures themselves to determine how they behave and what the spectral signature of their radiation is like?
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Old 9th April 2010, 06:26 AM   #9
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Well I think the point already made is that all boxes will resonate, and therefore will radiate some energy. Better then that the box has low amplitude, broad resonances below the mid-range, than high peaky ones in the mid range.
Research that Toole shows in his recent book, show that low Q resonances are more audiable than high Q ones.

The amount of energy available to excite a resonances decreases as frequency goes up. With music a large enuff amount of narrowband energy to excite a high Q resonance is unlikely.

So if you can push reonances up in frequency, and make them hi Q, then it becomes very unlikely resonances ever get excited, and in the rare event they do they are less audible.

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Old 9th April 2010, 06:33 AM   #10
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Do you not run any acoustic tests on the enclosures themselves to determine how they behave and what the spectral signature of their radiation is like?
I regularily test builds for panel resonances.

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