SpeakerTalk

[neddy]

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

[ColinR]

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 .

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]

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.

[SaSi]

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.

[iso]

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

[dsmith]

[iso]

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

[dsmith]

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

[audiolabtower]

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?

[neddy]

[iso]

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

[speakerguru]

[audiolabtower]

[neddy]

[speakerguru]