[kyrill]

"Over a dozen years ago the late Doug Dunlop, designer of the Concordant range of valve preamplifiers, demonstrated something to me so forcefully that I had to re-evaluate my whole audio system. He took the chassis of the Excelsior pre-amp he was evaluating out of its steel sleeve case and played the same tune through it. It was a dramatic and repeatable improvement in sound quality, "Chalk & cheese" as he put it. Then he put the chassis into a specially made hardwood sleeve. There was a further tiny improvement on the no-box condition.
Doug repeated this demonstration often enough with enough different models of pre-amp to convince me that the observed phenomenon was the same every time: repeatable empirical evidence.

Being the inquisitive type I had to find out whether this phenomenon applied only to Doug's designs; or whether it was universal; or whether it changed with different materials; or whether it changed with different circuits, technologies, wiring methods etc. Consistently, undamped steel or aluminium cases are bad for sound quality, like really cheap circuit componants.
The difference is like that of replacing all the cheap capacitors in a circuit and power-supply with Black Gtaes, Wondercaps and Musicaps. Talking to other audio experimenters and amplifier builders, they commented that the breadboard circuit often sounds worse when put into a case. But few had tried to find out why, or gone as far as Doug with a production item. Some tube amp builders expressed a preference for steel over aluminium. A solid-state builder expressed that aluminium is better, arguing that eddy currents were preferable to magnetic-hysteris.
Denis Morecroft's DNM designs have been in acrylic cases for as long as I can remember for similar reasons, although they do attract dust. The higher-level Audio-Note products usually feature expensive copper chassis in place of the steel of the lower level products. Many big manufacturers now recognise the problem in particularly vibration sensitive devices (especially cd players) and mechanically damp the chassis or copper plate aluminium parts.

Describing DAC tweaking, one writer commented "A chance discovery manifests itself in an optional 3" circular hole cut in the Chorus chassis directly over the DITB chassis "[it] blew my mind", Mike Vans Evers (The Chorus: A multivocal tube output stage for your "DAC In The Box", Sound Practices Vol 2 #2, Summer 1994). He was describing just one experience of removing some sheet steel that was parallel with an audio pcb.
I have tried casework various materials around hard-wired and pcb wired amplification circuits.

In my experiments wood wins consistently. Hardwood, softwood, plywood all sound fine. ( spruce seems to be the best sounding, kyrill) I always have suitable bits of timber (lumber) remaindered from speaker projects. Real woods are nicer to have around than fibreboard or chipboard type materials, and there have been various doubts expressed about the health effects of the composite board adhesives to the extent of a court case in the UK brought against a manufacturer of chipboard and MDF flat-pack furniture (Observer newspaper, 8th April 2001) and a US report on toxicity in the home was mentioned in UK media recently.
I have also tried special audio grade box materials eg Russ Andrews' RATA "Torlyte". Russ Andrews has also noted the phenomenon and has mounted his amplification products in various exotic cases over the years. Torlyte is a lightweight wood based hollow sheet material that is designed to minimise mechanical energy storage. Casework made from this material inexpensive, but arguably justified by the labour involved, to upgrade existing equipment and new-build. For me they work much better than the standard mass-market product steel box or aluminium case or pro 19" rack, but not much better than well designed and made wood cases. You could make them cheaper or commission cases from a local craftsman, or buy any wood box at a crafts store.

The downside of wood as a material for housing gain circuits is their total absence of screening. There is always the necessity of screening the circuit, especially with wideband circuits in our RF saturated city environments. Worst offenders will be phono-stages (especially wideband moving coil circuits), cd players (which will emit as well as pick up copious quantities of RF if inadequately screened) and tuners (obviously).
I was plagued by taxi radio messages from a rehoused moving coil head amp, until I added copper screening. Each wooden box side may be approximately lined with thin copper sheet (star-grounded) to provide adequate RF screening; it doesn't have to be completely lined, just enough to make an effective Faraday cage. ( the copper foil, better than sheet must firmly be glued to the wooden case in order to absorb the wooden resonances kyrill)
Conclusion

Wood is good for amplifiers & cd players. You can work it easily with domestic tools. It's relatively cheap, plentiful and environmentally friendly. It looks better in most homes than black painted tin-boxes. Most importantly, your project will sound better in a well designed and made wooden case than ferrous or aluminum.
So repeat after me:
"Steel & Aluminum: bad
Wood is good".
With apologies to George Orwell

© Copyright 2004 Mark Wheeler - www.tnt-audio.com "

What are your opinions?

[Geoff-AU]

Wood on its own provides an absence of screening and possible effects from "eddy currents" (used loosely in this case, any effect the circuit has on its enclosure will cause a drain on said circuit).  However, you then line the wood with copper to make a Faraday cage.  Since taking the original demo circuit out of its steel enclosure was "dramatic" and putting it in the wood enclosure was "tiny", it's probable that the difference was psychological (nobody likes seeing the guts of a circuit sitting on a table, things always sound better when they look better).

Since wood is also immune to all electrical and magnetic effects, we can conclude it has no real bearing on the circuit's performance anyway.

Why not then simply make the enclosure out of copper?  Would a thicker layer of copper cause the same issues as steel?  That would be my next step.  If having 0.25" copper as an enclosure is not suitable, then it's not that wood is good because it's wood, it's simply an inert material that supports the copper foil, which provides the shielding.  Plastic could equally do such a job.

[kyrill]

hi Geoff

good thinking

But there is another player: mechanical resonance "
 pure mechanical resonance. The copper will resonate with the music in its "ownway"
Think of this
make two identical violins, one of wood and one of copper. which sounds the most musical? Scientifically this you cannot prove why one should sound more beautiful than the other. But the human ear has its "build in references" and these "references" mostly reflective -the ear wants to hear "itself"- prefer the wooden violin.

So wooden resonances of the enclosures ( unless the whole system minus speakers are in a different locked doors room) are absolutely distortions, but they sound better. The wooden resonances are mechanically and structurally coupled to all the metals of the pcb and its components.  If the mass of the wood is big enough and adequately coupled to pcb and  stand alone components, these components and pcb will copy this frequency by suppressing their own nasty little resonances ( never 100%, as it is a battle of masses)  . They will now vibrate in a much more musical sounding way. but  in a complex electromagnetic little universe of their own. These "wooden" vibrations have now however a metallic carrier and because of that the vibrations are transformed in electrical noise and enter the active electrical signal to be amplified and to reach the speakers. 

A copper enclosure will do the same but will introduce in the end  "copper sounding"  electical noise. How beautiful will be a copper sounding violin? But still better than an iron one and much better than an aluminum one and very better than a plastic violin , urgh..)

[Russell Dawkins]

I think this is a potentially important topic that could easily get lost in the mists of (AudioCircle) time just because the subject line is so uninformative.

[kyrill]

Wood o......ay cage.  Since taking the original demo circuit out of its steel enclosure was "dramatic" and putting it in the wood enclosure was "tiny", it's probable that the difference was psychological (nobody likes seeing the guts of a circuit sitting on a table, things always sound better when they look better).
...

no that is not the case. dramatic is the getting rid of the enclosure. so now the new sound is the cageless sound. this can be improved further but tiny: "Then he put the chassis into a specially made hardwood sleeve. There was a further tiny improvement on the no-box condition."  He means on top of the no-boxed condition. Thus it all adds up. The further improvement is the effect of the metallic chassis to be a little forced by the wooden sleeve to incorporate the wooden resonances of the sleeve at the expense of its own metallic resonances.
Why a tiny improvement? I can only speculate. I think the chassis is relatively heavy compared to the wooden sleeve and therefore more difficult to be influenced. Also the coupling is not really firm.
It is well known the more mass equipment has the better it sounds ( also same mechanical reasons). Infinite mass would stop all vibrations and noise is gone from that source.


Russel i changed the title 

[gooberdude]

Unfortuenatly this is an old school discussion & the truth seems to buried in the last 20 yrs of mass production.....weren't most macintosh amps, tuners and preamps in walnut sleeves for decades???  It definitely has a lot to do with mechanical resonances...same reason drum makers & instrument makers in general use wood.    the sympathetic vibrations are musical.

it sounds like music...escpecially the tonewoods.

seems like the only real debate is 'how much does it cost?' when compared to what's easily avail for builders.   


i'd like to hear more opinions on this, but do feel the issue was solved a long time ago...we all just forgot.   


its not as if a crappy amp design can be dropped in a maple enclosure & end up sounding great.  but i do feel a great amp design can be built inside a flimsy metal enclosure and sound shitty.   

its all about the vibes.

[gooberdude]

sorry for the double post, but the title mentions (i think) that designing a musical enclosure might be outside of the scope of knowledge of engineers.

The big picture is probably a resounding YES for much of the gear avail at sane price levels.   the prolific use of 10 cent rubber feet on most, if not all, chassis is an admittance to this.    How would the brass footer salesmen feed their families if the designers of our gear knew it all?

besides, if the engineers knew it all we wouldn't get to spend time pondering such things...  aa

[Russell Dawkins]

Unfortuenatly this is an old school discussion & the truth seems to buried in the last 20 yrs of mass production.....weren't most macintosh amps, tuners and preamps in walnut sleeves for decades???  It definitely has a lot to do with mechanical resonances...same reason drum makers & instrument makers in general use wood.    the sympathetic vibrations are musical.

i'd like to hear more opinions on this, but do feel the issue was solved a long time ago...we all just forgot.   

its all about the vibes.

I don't think it was thought to be all about the vibes by the author - the thinking is it's all about the eddy currents induced in the metal enclosure by the ac signal (music) in the various conductors.

Far from being an old school discussion, I think this has not yet even been considered by the vast majority of manufacturers and hobbyists.

Please note: what is being discussed here is more about electrical interaction between signal conductors and adjacent metal components than mechanical "vibes".

Discussion of the behavior of vibes, on the other hand, is getting to be old school!

[gooberdude]

Russell, don't you think its about all of the above???     Removing a metal enclosure & inserting a wood enclosure would affect the eddy currents differently for sure.   Seems like that aspect should be normal to be able to study though.   ferrous metals, etc should be avoided.

When the chassis is affixed to wood though, the resonance changes...especiallyw hen power transofrmers are attached to the chassis.


from everything i've experienced with damping amp and preamp chassis, and the resulting performace changes, it all counts to some degree.

My power strip is made out of a copper chassis.  that makes sense in terms of RF and all that...

[kyrill]

"eddy currents begin when conductor(s) move across a constant, uniform magnetic field, or when stationary conductor(s) encounters a varying magnetic field."

where does the movement from conductors comes from? vibrations
This little universe is very very complex. But something must move or the magnetic field must "move" fluctuate. But the interactivity is so interdependant. moving cunductors is mechanical vibration of any component including chassis and enclosure if they are from metal.

vibes and eddies are two sides of the same coins, they can be so considered that way
so in a sense  there are musical "wooden" eddies and non-musical "metallic" eddies
  I dont want them metallic

[Russell Dawkins]

When an AC signal passes through a conductor, a fluctuating magnetic field is generated around it - fluctuating because it is AC (music is an AC signal).

This causes eddy currents to be set up in nearby conductors, including metal sleeves.

The eddy currents thus induced create electromagnetic fields of their own which are always of a magnetic polarity such as to oppose the magnetic field that created them in the first place.

This is easily observed if you slide a (strong) magnet down an aluminum ruler. It looks for all the world as if the magnet were sliding through some viscous liquid, like molasses, although aluminum is not magnetic. Although it is not, it is a conductor and thus capable of functioning as an electromagnet, as it does in a speaker with an aluminum voice coil.

[AKSA]

Let me ponder this a bit.

I think there is something to this, but not for the reasons given so far.

To find if eddy currents - the most likely cause - are the problem, we'd need to look for very large currents, as the music signals are pussy stuff by comparison.

For example, the charge pulse on the 100W Lifeforce power supply caps at low volume are up around 50A.  They persist for a tiny portion of the mains AC waveform, perhaps two or three degrees at low volume, but at these amplitudes these currents, circulating through the secondary side of the trafos and the rectifier bridges and caps, are quite capable of exciting eddy currents in any surrounding ferrous metals, and non-ferrous metals for that matter.....

The best proof of this theory would be to enclose the trafos, rectifiers and power caps in another box, and isolate it magnetically from the amp modules and input/output wires.  This has been done many times in the past, and people report mixed benefit, largely because of the longish lines from supply to module which exhibit inductance which delays high current delivery.

An extension of this phenomenon is the Ennemoser C37 lacquer, which Kyrill has tried with some success.  But this is an organic coating, non-conductive, with no effect on eddy current but profound influence over mechanical resonance.  Thus there is another side to the resonance issue;  electrical and mechanical.  I'm confident both are significant, but have no idea on the relative proportions even if it could be measured.  Since music waveforms are so complex, and the processing by the ear/mind interface so unknowable, it might point to significant gains to be had from resonance effects. 

Audiophiles have reported improvements from the strangest things, and while some might be hopelessly subjective (and usually unrepeatable) it should not be dismissed lightly.  When we talk of instrument design, for example pianos and violins, there are huge 'trade secrets' which would make any engineer blanch.  They may not be explainable, but they work....  a Stradivarius commands million dollar prices, so there must be something in it, and musicians, our own Richard Tognetti, head of the Australian Chamber Orchestra, values his medieval Italian violin with his own life it seems.....

The Stuart piano, an Australian instrument made in Newcastle, uses a different plane of attachment for the strings to a Steinway or a Bosendorfer.  It also uses Huon pine extensively, I have heard these pianos and their clarity is unmatched by anything I've heard in the conventional high end.  They are enthusiastically promoted by many classically trained Australian pianists.  This constuction detail would seem a little thing, but the differences are clearly audible.

I quite like the idea of timber cases with copper inlays.  I think this probably does sound better if the power supply is co-located with the modules.  OTOH, I think the layout of the amp, and the way the heavy pulse currents are contained within the supply module, are significant too.

In short, I'm not much help here, but I do think almost everything matters.....

Cheers,

Hugh

[Geoff-AU]

But there is another player: mechanical resonance "

mechanical resonance is ONLY a factor if the audio circuit is microphonic, that is vibrations affect the sound coming out of it.  The main barrier to testing this is you need to be absolutely sure that your vibration generator shaking the circuit is inaudible to the listener.

no that is not the case. dramatic is the getting rid of the enclosure. so now the new sound is the cageless sound. this can be improved further but tiny: "Then he put the chassis into a specially made hardwood sleeve. There was a further tiny improvement on the no-box condition."  He means on top of the no-boxed condition. Thus it all adds up.

I disagree.  If you have two tweaks - one that makes a big difference and one that makes a small difference - and you apply them in any order, the small tweak will always make a small difference and the big tweak will always make a big difference.  The fact that you applied a big tweak first does not somehow make the small tweak more significant.

the sympathetic vibrations are musical.

Again, only if the circuit is microphonic.  I have yet to see a study suggesting they are, but I would be interested if there is one.

"eddy currents begin when conductor(s) move across a constant, uniform magnetic field, or when stationary conductor(s) encounters a varying magnetic field."

where does the movement from conductors comes from? vibrations

I daresay that if vibration was enough to cause signficant movement of case relative to circuit then listening to music would be the last thing on your mind.  The proportional differences in distance will be tiny - that's not to say they make NO difference but you're really talking about the last 0.1% here.  Eddy currents caused by varying magnetic field (PSU charge currents, or amplifier output currents) would be orders of magniture more significant.


I'm going to agree with Hugh on the "everything matters" part, because I realise that sometimes things you have never thought of turn out to be important.  When I started to get into audio the more I learned the more I realise there was to know.  So maybe wood really is the new black.  But since I have a strong engineering background, I'm going to play devil's advocate here* and argue that the difference - at least caused by vibration - is so small it's not worth worrying about.  When you're dealing with a phono pickup or an electric guitar where there ARE microphones and moving coils in the circuit it's a very different story.  But my opinion is for standard circuits, the difference isn't reliably measurable by instruments or humans.  You'd have to have a double-blind test to discount the psychology of a circuit in a wooden box affecting the perception of the sound.


* I relish a good technical debate far more than reading a pissing contest between some of the people on here.

[gooberdude]

Keep it going!!     i know zero about this but am interested.


One more tidbit i'll throw in the ring is in the case of my amp compared to its sibling...

David Belles added padding to the transformer and some damping to the chassis to make the 150A a Hot Rod.   it seems from experience (and what i've read) that the Hot Rod sounds a lot better than the 150A.

I'm sure Mr. Belles made other improvements, but he hasn't listed any done to the circuits (going from memory).

Any idea if this scenario might play into the mechanical resonance aspect???    Its as if he isolated the vibey transformer, and tamed the chassis panels vibes, to allow the amp to work/sound/perform better.

[AKSA]

Now here is one to stimulate discussion......

The placement of the crossover in a speaker is significant.

We found that when we placed the crossover behind the driver in a certain orientation - that was important - the sound seemed slightly enhanced.  I doubt you'd even pick this up on the very best measuring equipment, I really do.

Since the crossover is then nicely out of harm's way this is very convenient, of course.

The reason is the microphonic nature of many caps, particularly non-polar electrolytics, which are used in the shunt elements.  Geoff is dead right - only if the components are microphonic will there be an effect. 

Mostly this effect is to either attenuate or amplify a particular frequency, across a fairly broad band since physical resonances have low Q.  The ear will subliminally respond be either liking the effect or disliking it.  Clearly the physical orientation is important, because it will be driven by a sound pressure wave.

Cheers,

Hugh

[mgalusha]

I had an interesting example of mechanical resonance the other night. I recently built an amplifier for a friend and happed upon some 1" birch drawer pulls at the local home improvement center and thought they might make nice footers and the price was low enough to give them a try. I removed the rubber footer supplied with the chassis and substituted the small hardwood balls. The improvement in sound was quite striking to my ears. Increased low level detail and dynamics and a tightening of the bass.

I was quite happy with the change but of course it could have been in my head. About a week ago I put some thin, about 3/16", sorbothane pieces under the wooden feet and and noticed a reduction in dynamics but it seemed slightly richer in harmonics so I left it alone and didn't think much about it. Last night I pulled them out after several days of feeling the system just wasn't performing as well as normal. Presto, things sounded much better. Dynamics improved and low level detail in spades and the apparent speed of the amp increased. No idea why the amp seemed faster but it left that impression, perhaps this is related to the perception of improved dynamics. The resolution increased enough to cause my wife to comment on the bass players singing/humming along with his playing. I had to agree with her, I'd listened to this recording many times and knew that someone was adding an accompaniment to their playing but it was never clear enough to discern. After pulling the sorbothane out it was very clear that it was the bass player and easy to make out.

I came from the objective measurement camp and I have a strong sense of disbelief for things I can't explain easily but the more years that go by the more I have to agree that everything matters. Ah, this is a SS amp, so the resonance was not due to vibrating tubes. It shouldn't be microphonic but it was mighty tough to deny the difference in what I heard.

As gooberdude says, keep this going. I'm always interested in learning how different folks accomplish their goals, one of the beautiful things about AudioCircle.

mike

[Russell Dawkins]

Hugh,

I think that what may seem arcane and whimsical on the surface of it may, in fact, stand up to "proper" engineering scrutiny.
For example, that part of an amplifier's operating range which determines the way ambiance and "room sound" (reverb) is handled occupies a power range considered by every amp designer I've talked to as problematically subtle in terms of it being under the noise floor of test equipment available to them.

The range I speak of is 50 - 70 dB down from the peaks. This is where reverb can be said to live and, in my mind, has a significant effect on the characteristic "sound" of an amplifier. In fact, I believe a lot of important characteristics that help form our impressions of an amplifier's sound occur around 40 - 60 dB below average program level.
I speak of "air", "texture", "depth of soundfield", and "sense of space" to use my own descriptors.
It is worth noting that even if the music is being played back at realistic levels, this range spans something like .001mW to .1 mW at most.

All distortion vs output level graphs I've seen in amp specs stop short of that. It is well known that we can hear around 20 dB into the noise floor, thanks to our excellent pattern recognition skills, but we can't measure signal that far into the noise.

So I'm thinking these eddy currents need not be large to be significant in this context.

Have you ever heard of "wire cry"? It was a term coined by the designer of AudioNote (can't remember his name). I believe he found that wires in carrying signals could move around in their sheaths in such a way as for this to be audible under test conditions, presumably with the wire in a very quiet room, carrying a strong signal - maybe into a dummy load. Anyway the thought was that if this is true that, again, the movements would induce spurious electrical distortions and contaminate the signal, albeit subtly.

[DSK]

I agree with others here, this is an interesting thread and I suspect one that will reveal a number of different opinions based upon different ears, tastes, equipment, experience, etc etc. This is based upon a number of such threads on AA that I've read and/or participated in over the last few years. These threads were more focussed on the treatment of resonance in equipment racks and component platforms and included views and experiences with a variety of materials (woods, metals, acrylics, rubbers, granite, marble, etc etc) but much of this is relevant to resonance in component chassis too.

The older philosophy seems to be that more weight/mass is always a good thing. More recent thinking is suggesting that extra weight should only be added if it improves the rigidity to weight ratio, typically espoused by the 'light & rigid' proponents. These people suggest that heavier platforms may be more difficult to start resonating, but they will also have far higher energy storage and take far longer to stop resonating. Very light but rigid materials will resonate more readily but with far less energy storage, far less detrimental effect, and will stop far more quickly. After experimenting with bicyle tubes, heavy stone and marble platforms, constrained layer damping combinations etc, I now sit my components on the IKEA Lack table tops. These are very light but quite rigid (and cheap) and performed at least as well as anything else I have tried. I haven't tried Ken Lyons' Neuance platforms but they are based upon much the same premise and have been very widely acclaimed. Light and rigid platforms, in addition to storing less energy, also resonate at a higher frequency and are far more easily damped than heavy, massive platforms. For example, the thicker the metal is in a component chassis, the thicker and more massive the damping material needs to be in order to have any impact.

More for convenience than anything else, I built my GK-1 chassis with hardwood sides (overlaid with 2mm stainless steel), an MDF base, and 2mm stainless steel front, back and top panels. Ages ago I picked up some Dynamat Extreme to apply to the larger stainless steel areas but haven't got around to doing it yet. Just recently, I added some to my PC case and it was very effective in damping the drumming of the PC's thin metal panels. I can now hear more inner detail in the fan noise!    I'm not sure that the effect will be as great on the thicker, stainless steel GK-1 panels but the Dynamat apparently is widely used on car panels with good success and this thread has aroused my enthusiasm again!

Re Hugh's comment on speaker crossover placement ... More due to being anal than previous experience I built my speakers with a separate sealed compartment in the bottom and placed my crossover in there. It screws to the underside of the main cavity floor, compressing a layer of damping material between crossover board and speaker floor. Although the screws do connect the crossover board and speaker floor, the entire crossover board is covered and damped by this damping layer between. I have no idea if this makes an audible difference (didn't try it without) but just in case ...

EDIT: Just an added note on the 'horses for courses' thing ... I hated Vibrapods under a tube CD player (made it sound thick and slow as molasses), but found them quite effective under a solid state one (cleared up a little edginess and improved inner detail and musicality).

[andyr]

Have you ever heard of "wire cry"? It was a term coined by the designer of AudioNote (can't remember his name). I believe he found that wires in carrying signals could move around in their sheaths in such a way as for this to be audible under test conditions, presumably with the wire in a very quiet room, carrying a strong signal - maybe into a dummy load. Anyway the thought was that if this is true that, again, the movements would induce spurious electrical distortions and contaminate the signal, albeit subtly.

Hi Russell,

I don't know if the AN designer's (Peter Quertrup?) description of "wire cry" is the same concept as that which I have (unfortunately) had proven to me ... but my - unintended!    - experiment was as follows:

* when replacing the PS of my phono stage with a Hugh-designed PS, I decided to replace the cheap coax which the mfr had used for the output connection between PCB and RCA sockets with some very thin (30g) solid-core wire ... based on the theory that thinner is better!    I also thought that it would be better to not twist these wires but leave them as individual arcs from PCB to RCA socket.

* the designer of the phono stage agreed that Hugh's PS made it sound much better but, in A-B-ing his original phono stage against mine, we noticed a distinct lack of bass with my phono stage.

* upon taking off the lid, he pointed out my nice new "thin wires" and said they were the culprit!  So he proceeded to whip out his soldering iron and remove them! 

* he replaced them with some more of that cheap coax ... and, lo and behold, the bass was then as low as his! 

* he said that the low mass of very thin wires means that they are able to be "jerked around" by the signals passing through them (mind you, we're talking signal wiring here, not speaker wires!  ) and this degraded the bass!

* having heard the difference, I could only agree with him!    So now for internal signal wiring, I still use thin wires but I use 3x30g for signal, twisted with an earth 24g, to provide greater mass.

Regards,

Andy

[kyrill]

But there is another player: mechanical resonance "

mechanical resonance is ONLY a factor if the audio circuit is microphonic, that is vibrations affect the sound coming out of it.
You state this as a fact why? Because you can measure microphonic fb and interpret the measurements to understandable and therefore accepted cause effect relationships? ONLY then "engineers" accept this as fact? I dont blame you and your brethren. It is your school and teachers how force you to think that way often to create facts.

 The main barrier to testing this is you need to be absolutely sure that your vibration generator shaking the circuit is inaudible to the listener. not necessary. that maybe a too simple "in vitro" approach of the real life complexities in an enclosed amplifier The air moved by the speakers is such a vib generator. it does not only shake your eardrums but everything else in the room, even the walls!! if you put a wineglass with the open end on the wall and the foot on yr listening ear, you can follow a discussion in the other room..

no that is not the case. dramatic is the getting rid of the enclosure. so now the new sound is the cageless sound. this can be improved further but tiny: "Then he put the chassis into a specially made hardwood sleeve. There was a further tiny improvement on the no-box condition."  He means on top of the no-boxed condition. Thus it all adds up.

I disagree.  If you have two tweaks - one that makes a big difference and one that makes a small difference - and you apply them in any order, the small tweak will always make a small difference and the big tweak will always make a big difference.  The fact that you applied a big tweak first does not somehow make the small tweak more significant. Ok, but every perceptible improvement in high end is a victory, so i am very curious for e v e r y  improvement that i can hear, even the tiny ones

the sympathetic vibrations are musical.

Again, only if the circuit is microphonic.  I have yet to see a study suggesting they are, but I would be interested if there is one. This is our biggest difference. i don't need studies to trust my own ears. I need more people who hear the same differences to compliment my ears. as an academic researcher myself albeit in organizational studies, i have long long time ago lost my faith in the objectivity of most researchers and their institutions..for instance the complete non-objective presentations of research data to proof we humans warm up the climate

"eddy currents begin when conductor(s) move across a constant, uniform magnetic field, or when stationary conductor(s) encounters a varying magnetic field."

where does the movement from conductors comes from? vibrations

I daresay that if vibration was enough to cause significant movement of case relative to circuit then listening    why an if? this is always the case, if walls move, surely your case and pcb's move. to music would be the last thing on your mind.  The proportional differences in distance will be tiny - that's not to say they make NO difference but you're really talking about the last 0.1% here.
Maybe 0,01%  in measurable givens like voltage or current relative differences, but that is not my concern. i can hear easily hear a big sound difference in audio because of those too small to measure differences. Your visualmeasurement data is bad to interpret to explain hearing perceptions ( except the all too simple 20hz-20khz range) Ask Hugh why is the LF such a success? the distortions of the AKSA was already very low in the 0.00xx% or lower. Then the LF was tweaked to be even more lower and low and behold it sounds much more better than only "cleaner" try to explain that. Yr ear is a million time more sensitive than scopes or analyzers who can only measure things they are designed for and literally are blind to anything else that happens too at the same time.

For instance set all possible measurements between a Mundorf and an auricap filmcap with the same value next to each other and see from paper why they should  sound different and how they sound different Easy to predict this is impossible. that is why Hugh and friends and all other sound tweakers have to listen 1000 hrs to find out which component is the best musical sounding in that setting. The whole engineering paradign and even academic paradigm rely much to much to text on paper  a very visual experience!! where you find your measurement and research data 

  Eddy currents caused by varying magnetic field (PSU charge currents, or amplifier output currents) would be orders of magniture more significant.


I'm going to agree with Hugh on the "everything matters" part, because I realise that sometimes things you have never thought of turn out to be important. 
nah easy way out, this discussion is more than 12 years old and still your "devil" is not convinced because yòu cannot m e a s u r e  it or find someone elses study who claimed to have measured it. What are you then implying about my experience that i can hear it sooo easily and not only me but Mark Wheeler - www.tnt-audio.com, which i quoted in my first post of this thread and all those thousands of other people. Even in this thread people claim to hear with or without dampers in or under enclosures/equipment, audio differences be it bad or good. Sure there are not all dreaming? my explanation was clear enough your case or enclosure does move and vibe and vibrate and sing and dance with the music, lile the walls around are doing it. They all do.
Many just choose not to ignore this given fact and effects even when they are too complex to understand interrelated eddy behaviour   
When I started to get into audio the more I learned the more I realise there was to know.  So maybe wood really is the new black.  But since I have a strong engineering background, I'm going to play devil's advocate here* haha you ARE forgiven  and argue that the difference - at least caused by vibration - is so small it's not worth worrying about. Oh noo you are NOT forgiven  When you're dealing with a phono pickup or an electric guitar where there ARE microphones and moving coils in the circuit it's a very different story. Ah the measurable story?   But my opinion is for standard circuits, the difference isn't reliably measurable by instruments or humans. 
Ah now, you underline the core of your paradign
You'd have to have a double-blind test to discount the psychology of a circuit in a wooden box affecting the perception of the sound. they have not been done in all those 12+ years??


Signed: the anti-devil's advocate
thanks for the dialogue