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Thinking of Stiffening 105.1 BassEnclosure with Ceramic Tile
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neddy
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PostPosted: Wed Apr 18, 2012 1:20 pm    Post subject: Thinking of Stiffening 105.1 BassEnclosure with Ceramic Tile Reply with quote

I am thinking of externally cladding the bass enclosure of my 105.1 with 8mm 500x500mm ceramic black polished tiles, cut to size. My cabinets are a bit tatty so I am not worried about looks too much. I thought 8mm would add stiffness without too much mass. I plan to use polyurethane adhesive to get a strong bond, namely Gorilla glue: http://www.handyhardware.ie/product/33662/Gorilla-Glue-500-ml

Any thoughts out there ? Not a good idea at all ? Better adhesive ? Will I need additional higher frequency internal damping ?

cheers
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ColinR
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PostPosted: Wed Apr 18, 2012 1:45 pm    Post subject: Reply with quote

For your foray into the field of constrained layer damping make sure that you beef up the bottom panel too and DON’T remove the castors unless you really want a bad back Sad .

The internals need not be altered as your modification to rigidify a flexible sided adiabatic system will only generate a small extra amount of heat for the damping foam/enclosure to absorb.
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dsmith
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PostPosted: Thu May 10, 2012 12:26 pm    Post subject: Reply with quote

You really should start by reading the Harwood paper on the subject.

http://www.bbc.co.uk/rd/pubs/reports/1977-03.pdf

He finds that lowering the level of cabinet resonances in the midrange is key to reducing cabinet colorations. He also finds that increasing cabinet stiffness is not the answer. It will tend to push resonances into a more audible frequency region. It will also make damping less effective as the general mechanical impedance is rising with greater wall thickness (both mass and stiffness increase). What is wanted is cabinet wall damping and the easy way to get that is to use thinner cabinet walls and a greater thickness of some damping material.

Your ceramic tiles may be a form of constrained layer damping (an especially efficient damping approach) but only if they are adhered by the right lossy adhesive. This will not be a hard glue but more some type of soft damping material. Search out the car audio sources. I used a Swedish material modestly called "noise killer yellow" on Snell products and it worked well.

David S.
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SaSi
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PostPosted: Fri May 18, 2012 7:16 pm    Post subject: Reply with quote

I'm going to try something along the lines of the Harwood paper.

Build an enclosure out of sandwiched parts of 4mm plywood alternating with 3mm bitumen sheet, for a total of 4 plywood and 3 bitumen sheets per panel.

I have a pair of Kefkit baffles for a B200 and T27, so I'm going to size this for that kit. Will post pics and measurements, compared with a similar thickness birch plywood enclosure.

A stiff enclosure results in higher peaks at higher frequencies - moving the resonance up brings it up in the audible frequency range.
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iso
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PostPosted: Sun May 20, 2012 12:55 am    Post subject: Finish Birch Plywood Reply with quote

In my opinion Harwoods idea to use birch plywood is based on stiffness of plywood, especially when using birch plywood made here in Finland. Young´s modulus of birch plywood is roughly 3 times higher than modulus of chip bord. Harwood does not underline this fact too much in his report.

From 1977 BBC report you can find that stiffness of 9mm plywood is similar than 25mm chip board (Young´s modulus of MDF is quite similar to chip board). This means that you can build speaker enclousure of similar stiffnes but only 35% mass, when using plywood. Bitumen damping is much more effective on low mass enclousure.

Usually 25 mm chip board cabinet is considered to be heavy duty. For some odd reason 9 mm plywood cab is considered to be light duty, even their stiffness is similar.

Best Regards,

Kimmo
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dsmith
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PostPosted: Wed May 23, 2012 6:44 pm    Post subject: Re: Finish Birch Plywood Reply with quote

iso wrote:
In my opinion Harwoods idea to use birch plywood is based on stiffness of plywood, especially when using birch plywood made here in Finland. Young´s modulus of birch plywood is roughly 3 times higher than modulus of chip bord. Harwood does not underline this fact too much in his report.

Kimmo


I think the reason he doesn't dwell on relative stiffnesses is that he is finding that stiffness is not the crucial parameter, rather the damping to stiffness or damping to mass ratio is. Increasing stiffness can make the task of damping the resonances harder.

I did some recent research on the acoustics of tents or air pressure supported buildings. It seems that architectural acoustics is well familiar with an ideal of the "limp mass" for issolation walls. That is, all walls have transmission loss (TL) roughly tied to their mass per unit area, except where marred by the structural resonances that allow narrow bands of sound to go through unimpeded. The exception would be a membrane of limp mass, i.e. having no stiffness. It is resonance free and would have the best possible issolation properties.

There is no difference between a room with good issolation and an ideal loudspeaker cabinet (okay, maybe the sizes are different). A heavy flexible membrane (say lead filled rubber sheet) would be the ideal wall construction. Since this is impractical, a thin walled cabinet with a high mass of damping compound would be the next best thing. Increasing the wall stiffness beyond the minimum needed degrades rather than improving matters.

David S.
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iso
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PostPosted: Thu May 24, 2012 12:32 am    Post subject: Stiffness Reply with quote

Does this mean that stiffness of 9 mm plywood or 25 mm MDF is close to optimal in medium sized cabinets.

Harwood found out that undamped 18 mm plywood cab was more coloured compared to 9 mm plywood cab. Also 18 mm cab with standard damping was inferior to 9 mm cab. His conclusion was that 18 mm cab would probably have similar performance with double thickness damping, but it would not be wise to use it if criterion can be met with less effort.

This makes 25 mm MDF or chipboard cabinets quite unpractical to be damped with bitumen glued inside of cabinet. According to study 3 or 6 mm layer of bitumen is not enough to damp encousures made of 18-25 mm material.

However, Harwood ignored cab coloration below 200 hz, as it was considered to be unimportant. There 18 mm cab had 20 dB less unvanted output, with and without damping. These frequencies are foundation of the music. I really do no not know if they are not important.

This is very interesting topic.

Best Regards

Kimmo
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dsmith
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PostPosted: Fri May 25, 2012 2:15 am    Post subject: Reply with quote

Hi Kimmo,

Yes, very interesting indeed. I am struck by how Harwood's findings are so at odds with perceived audiophile notions. As to the optimal stiffness of 9mm ply or otherwise, I think the takeway notion is that stiffness isn't really a desirable attribute, certainly not something that should be highly pushed for. Mass and damping are desirable, stiffness makes damping less effective so a cabinet should have adequate stiffness but no more.

I wouldn't say he ignored resonances below 200. His own threshold graph was based on all the resonances invarious cabinets he had listened to, and is just the dividing line between those that were audible and those that weren't. Resonances below 200 are audible, just progressively less so.

David
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audiolabtower
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PostPosted: Fri May 25, 2012 11:31 am    Post subject: Reply with quote

As I understood it a lot of producer feedback was involved - this direct comparison between live singers, orchestras etc (even symphony orchestras) and the control room sound. This type of feedback was not available to even the best manufacturers at that quantity during design process. The mid band colouration was deemed paramount in importance. I suspect bass colouration perception falls away similar to Fletcher Munson curves?
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neddy
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PostPosted: Fri May 25, 2012 1:57 pm    Post subject: Re: Thinking of Stiffening 105.1 BassEnclosure with Ceramic Reply with quote

neddy wrote:
I am thinking of externally cladding the bass enclosure of my 105.1 with 8mm 500x500mm ceramic black polished tiles, cut to size. My cabinets are a bit tatty so I am not worried about looks too much. I thought 8mm would add stiffness without too much mass. I plan to use polyurethane adhesive to get a strong bond, namely Gorilla glue: http://www.handyhardware.ie/product/33662/Gorilla-Glue-500-ml

Any thoughts out there ? Not a good idea at all ? Better adhesive ? Will I need additional higher frequency internal damping ?

cheers


Thanks for your thoughts guys. To clarify this discussion from my point of view:

(a) I wanted to attempt something that would be impractical for a commercial manufacturer. As I know the drive units for the KEF are superb I figured the cabinet was an area to look to improve.

(b) the bass cabinets of the KEF 105.1 reproduce up to 400Hz (24db/octave crossover).

(c) I had been under the impression that critically damped thin-walled construction was the most cost-effective approach for superior midrange though that this was necessarily at some expense of bass reproduction.

(d) I tried to think of the stiffest material I could use that was not prohibitively expensive. Regarding stiffness to mass ratio I thought 8mm ceramic tile was a good compromise. Even these tiles will add approx. 19kg to each bass cabinet and the bass cabinets are heavy in the first place.

(e) Regarding whether to use a lossy glue or not and I now thinking of using Evostik Timebond. Stiffening the structure is my main aim; hopefully resonances that do exist will be raised beyond the pass band of the bass cabinet.

The general discussion has given me lots of food for thought for when I consider if and how to improve the midrange enclosure, and thanks again for that.
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iso
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PostPosted: Fri May 25, 2012 6:31 pm    Post subject: Octave between 100 and 200 Hz Reply with quote

I have tried to understand why Harwood considered this part of spectrum less important.

Slightly later Linn and Celestion tried reduce cabinet coloration especially in this region in products like Sara and SL700. It is difficult to improve every parameter same time, especially if you have limited budget. Linn and Celestion concentrated on articulation... or as Linn said on timing. Due to the same reasons, these products did not perform too well where BBC designs have traditionally been pretty good.

Maybe Harwood was not too interested on this aspect of sound. In my opinion lack of articulation or timing is one kind of coloration, and should not be overlooked. Most often it seems that large speakers do have better performance on this respect. Maybe Linn and Celestion tried to produce small speakers with good imagining capabilities, but wanted them to have slam of bigger speakers.

I have also wondered, if the drive units that were used were colored in this band. If so... cab resonance might not degrade sound, if drive unit is the worst performing part of the system. Maybe this has something to do what Kef did, when they redesigned B300 for 105.2.

Best Regards

Kimmo
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speakerguru
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PostPosted: Fri May 25, 2012 10:30 pm    Post subject: Re: Thinking of Stiffening 105.1 BassEnclosure with Ceramic Reply with quote

neddy wrote:
... hopefully resonances that do exist will be raised beyond the pass band of the bass cabinet.


Just in case you thought it was simple or easy; don't forget that the midrange driver can excite the woofer cabinet and vice versa, by both mechanical and acoustic paths.......................
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audiolabtower
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PostPosted: Fri May 25, 2012 11:06 pm    Post subject: Re: Octave between 100 and 200 Hz Reply with quote

iso wrote:
Maybe this has something to do what Kef did, when they redesigned B300 for 105.2.


The B300B is exactly the same drive unit, just the traditional BBC heavy cast chassis on screwed baffle (a deliberate "lossy" design intent) was swapped for a cheaper pressed 3 point fixing with rubber grommets for the "lossy" fixing to the simpler one piece bass enclosure construction to reduce mechanical coupling.
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neddy
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PostPosted: Sat May 26, 2012 2:18 am    Post subject: Re: Thinking of Stiffening 105.1 BassEnclosure with Ceramic Reply with quote

speakerguru wrote:
neddy wrote:
... hopefully resonances that do exist will be raised beyond the pass band of the bass cabinet.


Just in case you thought it was simple or easy; don't forget that the midrange driver can excite the woofer cabinet and vice versa, by both mechanical and acoustic paths.......................


Adding mass should minimise acoustic breakthrough, though I find it difficult to think that this is that important. Critical to when and if and how I try to improve the midrange enclosure will be the mechanical coupling between the bass cabinets and the midrange enclosure, which I think is much more important than acoustic breakthrough (coupling). One thing at a time though...
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speakerguru
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PostPosted: Sat May 26, 2012 8:25 am    Post subject: Re: Octave between 100 and 200 Hz Reply with quote

audiolabtower wrote:
The B300B is exactly the same drive unit,...

Not exactly. I already said in this thread that I changed the voice coil from the BBC design.

audiolabtower wrote:
... just the traditional BBC heavy cast chassis on screwed baffle (a deliberate "lossy" design intent) was swapped for a cheaper pressed 3 point fixing with rubber grommets for the "lossy" fixing to the simpler one piece bass enclosure construction to reduce mechanical coupling.

Both B300s had a cast chassis. The original 6 point fixing one was a sand casting. The later 3 point fixing was a die casting fitted with anti-vibration mounts (not grommets). The BBC LS2/1 original was on a Celestion pressed steel chassis.
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