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Old 21st September 2003, 04:39 PM   #21
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Quote:
Originally posted by SimontY
Hmm, that sounds bad. But don't we want a lower frequency to resonate at? Like commercial speakers...resonance in the midrange sounds like a disaster. My idea of bracing is not to increase the resonant frequency, but to reduce resonances!
The idea with bracing is to raise the frequency a panel resonates at, up to the point where there is not enuff energy to excite the resonance... we will effectively never get rid of the resonances.

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Old 21st September 2003, 04:54 PM   #22
GM is offline GM  United States
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>roofing felt or dynamat will pull the resonant frequencies of the panels back down so to some extent negates what you are trying to do with the bracing.

====

If you increase the thickness of a low resonance panel by using the same/similar material, you are 'chasing your tail' since to get it really well damped requires its resonance to be pushed below the design's Fb. Might as well use concrete cause that's where you're going to wind up WRT the amount of net mass required if Fb is below ~125Hz.

OTOH, if the panel resonance is raised, its amplitude falls due to its energy falling and the mass of the panel further attenuates it as it appears increasingly massive with increasing frequency. The damping material continues it rather than lowering its resonance by increasing mass/other 'lossy' properties through the impedance mismatch of very dissimilar materials, like when you (highly damped) grab a resonating tuning fork.

I long ago came to the conclusion that there's no such thing as too massive/rigid a cab until its resonance is either below or above the speaker's BW for excellent acoustic efficiency/damping cab colorations to below the -35dB S/N ratio of the average person's hearing acuity.

Since it's more practical in most cases to make a rigid cab than a massive one, using the most practical rigid material (13ply Baltic Birch in most designs) combined with good structural design and bracing it through triangulation to further raise its resonance will yield sufficient mechanical/acoustic efficiency for HIFI/HT apps. All that's left then is to use a very dissimilar material (high DF) to attenuate any audible HF 'ringing' that occurs due to any standing waves.

In average size cabs, lining one wall, back, and top with fiberglass is normally sufficient, and cheap self stick linoleum tiles or similar on larger ones that will have standing waves down in the lower mids/midbass BW. Using non parallel wall designs further reduces standing wave amplitudes, making the need for additional damping material moot in some designs.

Bottom line is that unless I want the cab to 'enhance' the driver's output by 'singing along' over a narrow BW, then before any damping material is added I don't consider it stiff enough until it rings ~uniformly at a pitch similar to one made from sheetmetal when tapped all over with a ballpeen hammer.

GM
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Old 21st September 2003, 07:41 PM   #23
SimontY is offline SimontY  United Kingdom
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GM, thanks for helping to clear up my ideas about cabinet damping, and I'm sure others needed to know too

btw, I did try the soft foam/rubber type of carpet underlay in some speakers and it sounded much better than the white foam wadding stuff. I won't embarass myself by guessing why...


-Simon
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Old 21st September 2003, 08:26 PM   #24
Vikash is offline Vikash  United Kingdom
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Quote:
If you increase the thickness of a low resonance panel by using the same/similar material, you are 'chasing your tail'....at a pitch similar to one made from sheetmetal
This is quite interesting...So having super thick MDF is not necessarily a positive thing. And how about using 5mm thick steel? or is the lack of metal cabinets around related to stored energy or something?
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Old 21st September 2003, 09:02 PM   #25
octopus is offline octopus  England
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Stainless or alumium would be better. Just imagine the interference on a TV when you have steel speakers

I've read that laminated boxes are better than single sheet, like 2x10mm instead of 20mm. Hardwork though...
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Old 21st September 2003, 09:14 PM   #26
SimontY is offline SimontY  United Kingdom
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Quote:
is the lack of metal cabinets around related to stored energy or something?
I'm obviously not so confident in this area of knowledge but...

Metal rings, because it has poor 'self-damping', and I think has more than one area of resonance. This is partly why metal racks tend to introduce a nasty and un-natural character to the sound.

The best practical material may be some sort of hard wood, which resonates over a broader band than stuff like metal, and MDF. Also, some people seem to think using two or more types of wood/material is better than one, becuase the two characters help to null the effects of each other.

The 'best' dynamic speakers all seem to be large and heavy. When a cabinet is super-stiff and heavy, the waterfall/time response plot of the speaker is good.

However, from reading the previous replys there seems to be a slight lack of clarity in identifying what is ideal. There must be a goal, if you were making, say, an 'ultimate speaker', like the andromeda for example - that is heavy. The guy making that used bitumen between two layers of MDF I believe! Why is that (not?) a good idea?


-Simon
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Old 22nd September 2003, 01:02 AM   #27
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Quote:
Originally posted by octopus
I've read that laminated boxes are better than single sheet, like 2x10mm instead of 20mm. Hardwork though...
13 1mm layers = 1/2" plywood :^)

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Old 22nd September 2003, 01:13 AM   #28
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Quote:
Originally posted by planet10


13 1mm layers = 1/2" plywood :^)

dave

But what if a 14th layer of mdf was added (different resonance ? )

Cheers

Rob

btw, I always thought the idea of mixing 2 different types of material together was that when 1 was resonating, the other was damping, and vice versa.....
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Old 22nd September 2003, 01:18 AM   #29
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Quote:
Originally posted by RobWells
But what if a 14th layer of mdf was added (different resonance ? )
There is a product you can buy that is plywood sandwiched with mdf. Haven't tried it in speaker cabinets thou.

I have used plywood (& hdf) sandwiched both sides with aborite/formica.

dave
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Old 22nd September 2003, 01:31 AM   #30
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Quote:
Originally posted by planet10


There is a product you can buy that is plywood sandwiched with mdf. Haven't tried it in speaker cabinets thou.

I have used plywood (& hdf) sandwiched both sides with aborite/formica.

dave

Hi Dave,

What observations compared to 'straight' ply or mdf ?

Cheers

Rob
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