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Interesting read I found on Lossy Cabinet designs by Harbeth
Interesting read I found on Lossy Cabinet designs by Harbeth
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Old 26th November 2012, 05:49 PM   #101
Juhazi is offline Juhazi  Finland
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My previous link was not a good one, here is a better one

And when British engineers decide to minimize reflections they end up with something like this
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Last edited by Juhazi; 26th November 2012 at 05:51 PM.
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Old 26th November 2012, 05:55 PM   #102
Baseballbat is offline Baseballbat  Germany
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Originally Posted by speaker dave View Post
The strongest drive to the box is the reaction force from the chassis driving the baffle. That is why compliant woofer mounting works well. Secondarily is the internal acoustical drive from the air pressure.
David, I don't agree here. They can be even, or in case of internal resonance, the air driven movement can be stronger. However, the internal volume resonances can be usually handled by proper stuffing.

A fairly extreme example was a 90 cm tall cabinet made of aluminum producing considerable output at 380 Hz. The result was a quite huge bump in the overall frequency response, nearly 2 dB IIRC. Stuffing reduced the resonance, but was not applicable, because it reduced bass output too much. The solution was bracing at half of the cabinet height.

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Originally Posted by markus76
Have we ever seen data that shows a cabinet resonating at exactly the same frequencies the knock-test suggests?
A "knock" is (close to) an impulse. An impulse contains every frequency. The response to an impulse fourier transformed gives the frequency response.

An engineer with insight in signal theory would not even consider making this additional test.
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Old 26th November 2012, 06:02 PM   #103
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An engineer with insight in signal theory would not even consider making this additional test.
no, ofcourse not
its like kicking your car tires to see if there air in them
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Old 26th November 2012, 06:03 PM   #104
Baseballbat is offline Baseballbat  Germany
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Originally Posted by Cal Weldon View Post
I can't think for a minute that it's a positive thing to have the cabinet walls moving...
Because they move less...

OK, I admit, it's weird, and I'm sure that's why heavy, stiff cabinets are so popular among audiophiles. How can they know? Most of them has no degree in acoustics, mechanics or electronics. And I know guys with such a degree who still promote heavy*, stiff cabinets.

*heavy by itself is not bad. It's the stiffness, which shifts the resonances up to audible frequencies. And most of the time, the weight is achieved by thickness. And thickness increases the stiffness much more than the weight.
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Old 26th November 2012, 06:13 PM   #105
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one thing I have never liked very much is the coupling of opposite sides, be it with a stick or bracing
but quite funny, I still like to use to the old method with real wood lists to support all glued edges inside
I suppose its the old british way, and probably also still practiced by Harbeth, Spendor, etc
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Old 26th November 2012, 06:41 PM   #106
Juhazi is offline Juhazi  Finland
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Lists in corners are just holding the pieces together, they have minimal effect to vibrations. Crossed studs prevent ballooning effect well, the placement has some effect too (uneven distance to edges).

The magnitude of vibration is important but so is duration (damping). Both must be handled.

I am about to make two big closed subwoofer boxes (259L) of spruce plywood. It has lower density than birch, which usually means shorter ringing when thickness is same. The bethod of bracing is open. The sub enclosure is a much easier task than full range because frequencies are so low.
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Old 26th November 2012, 06:58 PM   #107
markus76 is offline markus76  Germany
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Quote:
Originally Posted by tinitus View Post
yes, thats one way to determine plate ressonance
300year old method
A plate is not a box and a speaker driver is not a knocking finger.

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btw, did you try to put one ear against the speaker box ?
please do, and tell us what you hear
I hear a strange voice talking backwards, saying "sutinit"
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Old 26th November 2012, 07:03 PM   #108
markus76 is offline markus76  Germany
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A "knock" is (close to) an impulse. An impulse contains every frequency. The response to an impulse fourier transformed gives the frequency response.

An engineer with insight in signal theory would not even consider making this additional test.
Thanks for the answer but that was not what I was asking.
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Old 26th November 2012, 07:19 PM   #109
Baseballbat is offline Baseballbat  Germany
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That was exactly what you were asking.

You wanted to know if a cabinet is resonating at the frequencies identified by a "knock" test. If you excite a panel by a knock, and record the frequency content of the response, then this is close to the frequency response. From this point on it does not matter where and how you excite the panel, you can be sure that all those resonance frequencies seen in the "knock frequency response" will get excited, supposed that the new excitation is sufficiently broadband (which music is).

The only question is how much every resonance will get excited, when you use a driver instead of a knuckle at a complete different position. With this test, it can happen, that a few resonances will not get excited, because the driver is placed at a node of these resonances. But this is very unlikely.
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Old 26th November 2012, 07:23 PM   #110
Juhazi is offline Juhazi  Finland
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The Wilson way to knock Nanometers - YouTube
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