Would be interesting to analyze the mechanical construction with some finite element package to see if these frequencies are in the audible area and what to do in order to suppress them... I might be able to help here.
Within itself I am all open to this (plus I can estimate your capabilities on this terrain !).
However, as usual I'm a bit differently oriented - also in this area. Could be too ignorant, but let's say good things may spring from it.
Ok, here is my latest experience, which is only from last night - that's why this specific example; It should tell us that as always other things are first, when it is about audio :
With this topic in mind, I thought "why not put the material under the NOS1 you see in the below picture". And so I did;
My own NOS1 is plain on the floor for "years" by now, with the main reason of comfortably testing. The cover is not on, which may matter for my today's little story.
There is carpet on the floor with some percentage of wool (could be something like 80%). Under the four legs of the NOS1 there's this pink anti static material, known to those who bought the original NOS1 (we always delivered four pieces with it; in a later stage they became white).
This material was my explicit choice because of its structure - never mind everybody will think it is cr*p (which it is by itself of course). Now :
During playback, under each leg I put two of those white pieces from the picture below on to the pink pieces (the latter are ~10x10cm). Of course I imagined an immediate difference, but I learned to almost explicitly not pay attention to it; in a week or so I would know the result (let unconsiousness flow).
After I had applied this, I maybe played for another hour, then dinner was ready and sound was shutoff.
"Hey dad, there's a buzz, man !"
What the f... ?
It was so loud I really had to disconnect the interlinks before any dinner attempt.
After dinner I continued on it, and there was a clear 50Hz mains rattle. What the heck did I do to incur for this ? I didn't do a thing !
To the left of my NOS1 there was another NOS1 connected, which I tested earlier the afternoon. It was still under power, so I shut that off. Didn't help a thing. Removed the interlinks to the analyser. Didn't help. Checked the other nest of cabling - nothing. I just couldn't get it. This never happened before.
So, that other NOS1 (a black one), did not have any feet under it yet (yes, these days we ship with cheap normal little feet), so it could be shifted more towards the left, and away from my blue NOS1. What ? that helped ! Huh ?
When the power is off (which it was), both neutral and phase are cut, but protective earth keeps connected. So, something must be radiating or working as a(n unfinished !) antenna.
Now I took out the black NOS1 to another aera in the room, and ... hum got less again. But still not gone.
This didn't fit, because dozens of NOS1's had been there. Left, right, in front, more to the left. And in various colours too. Never there's hum. I don't want hum and I have officially banned it.
Aha.
I rearranged all the cabling once again (moved interlinks away from power cords etc.), but it didn't help a thing.
In beteen lines again : this is all about paying attention. I may be fairly good at it inherently, but it is also a matter of training yourself. Never let a change just go. Sort out what really happened, and try to explain the why of it. I know, for audio this should be somewhat more easy for me, but for everybody should count : be more explicit about it.
Ok, so this left that added white material under the legs for a reason. It already was suspicious to me, but first I had to be certain that all was in the state I'm used to, and nothing was overlooked so combinations of happenings/arrangements could be counted out;
With all under normal power (music not playing) I started with lifting one leg, and find the two white pieces under there. This is a kind of being careful operation because without cover the chassis is not as stiff of course, and PCBs might get wrenched.
The two white pieces where shoveled away from under there, and I carefully let sink the leg onto the pink material.
BANG. The sound of a small (very transient) firecracker emerged. Hmm ...
Did the second leg, and exactly the same happened. The third leg (now as carefully sinking the leg as I could do it) - same thing. Only at the fourth leg nothing happened anymore.
It was clear. This was electric discharge. But, not sure how it actually worked, because I sank the legs onto the ESD protective material. But, this material by now was some 4mm thick, originating from the ever 10mm. It flattends throughout time. Also, it is known that over time this (pink) material looses its protective capabilities to some extend (possibly all). Still it didn't feel like a discharge through it towards the woolen carpet. I merely envision a kind of lightning flash travelling over the underside of the bottom towards one of the other legs. And mind the last leg not "flahsing" anymore ...
Of course I can only try to think of what could have happened; I see some giant electric field under the bottom, there circling some 12 mm above the floor, caught by flat steel plate above it. How the white material encurs for it, apart from creating this ~8mm more distance compared to normal, I don't know. However it looks like a giant impact on how loaded (charged) electricity *was* guided away, and that this means changed completely (up to not working at all) with the white stuff under it.
Of course I don't need to tell you that the hum was gone after this ...
The moral :
How the chassis of an electrical apparatus connects to its surface is of vast importance. What the surface is, is therewith of evenly great importance;
When this is not done right (and sadly again this will differ per each of your situations), you close to *will* be confronted with a thing like my story above. And again (also see Paul's topic about similar), see the complexity in order. An *other* device - put next to the device in question - creates most of the hum, and it is not even powered on. It will do something to the antenna working though, and antennas will be everywhere in your room (sadly). While in this case that other NOS1 made it worse, it can just as well work out for the better; think transmitting antennas need a receptor, or otherwise your interlinks (etc.) will be just that.
This is why audio feet are important for quite another reason, and don't ask me the general guideline (I don't think it will exist). If this is not solved adequately first, I don't think there will be much sense in preventing sonic impacts as talked through in the before post. In the end, yes of course, but :
But you may wonder what it actually is when "a" material presents you better sound. I mean, nobody needs to tell me that my hum will have created better sound, and might one doubt : I tell you that it will have made it worse. But what was there to notice ? For you ? most probably a sheer nothing, because you wouldn't have heard the hum anyway, or you accepted it long ago. Besides you may use a preamp, and it won't be a problem ever.
Ah, is that so ...
Peter
