Is EMI noise in electrical system inaudible?

NO! Any competently designed electronic component has been designed with this factor in mind and will be impervious to radio frequency interference (RFI). There is no need for fancy and expensive mains electricity filtering - contrary to the beliefs of the "golden ears" crowd. The promotion and sale of such AC devices is an attempt to profit from the uninformed and gullible. 

These are the same people that will tell you that you need not only an expensive AC regenerator but also an expensive AC cord - and then a hospital grade AC outlet! All  B*LLS**T!

I think you might be wrong despite your level of enthusiasm.

Possibly.  Kind of depends, but it's rare. My approach is to stick to inexpensive solutions, but here's what I do:

• Use fully shielded interconnects
• Use shielded power cable for analog parts. These can be pretty affordable
• Use iFi power supplies where they matter, on my DAC and streamer to minimize the generation of AC noise.
• Use a power conditioner that starts working down in the audible range.  That includes those Furman with LiFT and SMP features and TrippLite Isobar. 
• Try to keep my network devices (switches, routers, etc) away from my audio, both physically and the power supplies out of the same clean zone.

+100 Jason!

+200 @jasonbourne52!

     Whenever electricity (i.e: an audio signal) passes through a wire, it produces inductance that creates an electromagnetic field.

     Because EMI manifests itself at a high frequency, it is commonly believed not to interfere with the audible range of frequencies. However, all frequencies demonstrate harmonic structures.

     Even if the cause of EMI is outside the audible range, the upper and lower harmonics of a given frequency often present themselves within the audible range.

     Digital equipment (although functioning at extremely high frequencies), unless electrically isolated, can/will affect the presentation of a sound system, in the same home, through the AC lines.

     There are numerous threads on this very platform, regarding the problems some have had with things such as fluorescents and/or faders, making it past those beefy power supplies and causing audible interference in their systems.

     EMI can be picked up outside of the system, via cabling, OR EVEN created by a signal, moving along the traces of a component’s printed circuits or internal wiring, not to mention: a nasty power transformer.

     Of course: much of the effect of EMI, present in a system, may go unnoticed, until it’s gone and the listener experiences what some call, "a blacker background", or: "greater definition, clarity, etc".

                                            So many variables!

ttps://www.vse.com/blog/2021/01/28/how-to-avoid-electromagnetic-interference-emi-in-printed-circuit-boards/

@rodman99999 

+1

When living on a mountain on the south side of Innsbruck, my powered on Proton table radio without prompting would occasionally emanate a garbled broadcast.  When I shared the information with locals, they told me that there were some powerful transmitters located further up the mountain.

Does that qualify as either EMI or RFI?

While my RP7 pre amp was sent in for upgrade, I had in its place a Schiit Valhalla 2.   The Schiit picked up LOTS of EMI noise, perhaps due to lack of shielding.  When I moved my wireless mouse, you'd hear a high pitched whine and buzz, and when there were bright colors on the TV screen you'd hear a squeal, as my computer is near my rig.  The effect was mitigated by plugging my computer into a different oultet, but was still there. It was really annoying, but interesting. There was EMI through the air, and some carried through power cords and the like. Now that the regular pre is back in (Rogue RP9) it seems immune to this EMI, and is completely silent.  

There are many ways high frequency interference can become audible. One way is simply modulation. Two inaudible. signals mixed on non-linear element, like semiconductor junction, will produce new frequencies, that might be audible. The phenomena called "rectification" is less known. Most of op-amps and perhaps many amps suffer from it. Amplifier, being frequency limited cannot amplify very high frequency. They filter it (taking average) resulting in zero output. That is as long as amp’s negative and positive slew rates are identical (almost never are). As a result of it there will be some DC value on the output, that will be proportional to offending interference amplitude. (AM or even FM signals). That way instrumentation amplifier designed by my coworker received local AM station, being bandwidth limited to only 1kHz. We can stop it by using passive filtering at the inputs, but nothing is ever 100%. Wire that is shorter than 1/10 of the wavelength of offending RFI is a very poor antenna. Does it mean. that is not receiving any RFI? Of course not - it will still receive tiny amount. We can reduce it to inaudible levels by using twisted pairs (even exposure to electric and magnetic fields, resulting in cancelation), shielding, filtering, keeping connections short etc.

So what the heck is a harmonic and how does this interfere with good sound?

@jumia . If you mean harmonic distortion it is a deviation from original shape of the signal.

Oh my god really ?