Why Expensive power Cables when Romex behind Wall?

Bwhite wrote:
"Make sense?"

Not to me. In fact, IMO mechanical resonances account for very little that is audible in most systems. Certainly if you put your components on a thin steel shelving unit, you might get some mechanical resonance effects, but most systems do not experience this. If anything, the electrical resonance of longer cables has a more pronounced effect. You are taking the snake-oil literature out there too seriously. Here are some papers that are not snake-oil:
Technical Papers and Audio FAQ at:
http://www.empiricalaudio.com

OK I’m an electrical engineer who has never tried to play with power cords. This means I know just enough to be dangerous but not enough to really know what I’m talking about. I am not going to claim that power cords do not make a difference because I suspect they do. That said, here’s my 2 cents.

Electrical power cord as filter: I’m not convinced but… The parallel is a fuel or water filter. Water travels for miles in dirty pipes but the filter at the end is able to clean it up. This argument does not work with power cords. A power cord is the last section of pipe, not a filter at the end. As such the analogy would be miles of dirty pipe with a clean section at the end. Not adding dirt for the last few feet isn’t going to help. A power conditioner is a good analogy to the water filter. IF the power cord has some form of filtering built in, that would be different. A power cord might include a high frequency filter to clean RF noise or something. Even if the designer didn’t intend it, the geometry of the wire could make a filter of some sort (I wouldn’t put money on it). Basically, I don’t by the filter part unless some form of RF filter was deliberately added.

Electrical power cord as the last few feet of pipe: Using the water analogy
1. The original pipe was too small (ie wire gauge too small). If the power cord was too small to handle the current you could get voltage drops. My 240wpc Bryston amp came with a heavier power cord than my Bryston preamp. Thanks to V=IR loading on the power lines and cord the amp might see a significant reduction in line voltage. I would expect this to hurt amplifier performance. If your system is plugged into a power strip this voltage drop might be shared with the other components in your system.
2. The pipe has flashing around the fittings (ie the plugs don’t mate well). Maybe the issue isn’t the last few feet but the connections at the plug. Just like the issue with small gauge wire, a bad connection can cause a voltage drop. This would explain why high quality hospital plugs can make a difference.
3. The last few feet of pipe is where all the dirt is! Perhaps the power from the wall isn’t that dirty but all your equipment is really making the noise. Shielding the cables as they near this big rack of electronics may make all the difference. Perhaps the worst dirt is the dirt generated by your own system feeding back into itself. This could explain why shielding may help.

Take it for what it’s worth. Personally I think cables make some difference but I would rather spend the big money on components rather than cables.

Audioengr. I knew you would reply! That's why I mentioned the steel or brass outlet cover. Its a sure fire way to prove mechanical resonance. Come on man, go get yourself a steel or brass outlet cover - they are cheap 2 or 3 bucks. Replace the cover you currently use with it and then let us know what you hear. Before you write this off as snake-oil you ought to experience what other people are hearing.

Here are quotes from Audioengr's "Technical Papers" - seems he is contradicting himself in claiming that resonance is for the most part not audible - and discounting it as snake-oil here on Audiogon.

Conclusions:
Resonance is a very real behavior in most speaker cables. The nature of the speaker load will vary both the amplitude and frequency of the resonance. The fundamental resonant frequency is primarily a function of the cable length, but
different loads can move this frequency. Resonances can become a problem if high-bandwidth amplifiers are driving high-impedance speaker loads.

Empirical Audio takes resonance seriously, so we design anti-resonant terminations into our cables to eliminate the problem. We believe that the audible "Haze" or "Veils" that often overlay music in high-end systems is in part a result of cable resonance.

What he fails to mention is that there is no such thing as no resonance. Everything resonates. His "Anti-Resonant" terminations just change the frequencies at which the cable resonantes hence making it sound different - Did someone say SNAKE-OIL?

Empirical Audio has taken steps to deal with resonance in our cables. We put anti-resonant terminations in our cables to reduce this effect. To understand the effect of the anti-resonant termination, we first examine a cable without the termination.

Hmmm... so this confirms, you don't remove the resonance (you couldn't anyway) - you just change it. Isn't that what I said in my earlier post? -- Changing the resonant frequencies is one way that different cables sound different.

Bwhite's comments may be describing some of the differences I hear with premium power cables.

Nikkidanjo may be onto something where he describes cable shielding:

"Shielding the cables as they near this big rack of electronics may make all the difference. Perhaps the worst dirt is the dirt generated by your own system feeding back into itself. This could explain why shielding may help."

It may be a long wait before technology is developed that "proves" how some power cords work in our systems. Having heard significant improvements with several power cables, I don't intend to wait for a piece of paper telling me what I already know is true.

I have not tried metal outlet plates as Bwhite describes. I would not prejudge it effectiveness, until and UNLESS I tried it for myself.

I would likely believe in Mr. White conclusion, even if it failed to improve my system. Every system is different and it would be foolish to pass up an opportunity that at least one member has been successful with, especially one that costs about what some of us spend on a large soft drink.

I intend to purchase a couple of plates next time I visit Home Depot. Worse case I return them a few weeks later.

Thanks Albertporter! Please - don't expect the plates to make an improvement. They will however add a noticable and somewhat annoying brightness to your system. Hopefully this will help folks understand how mechanical resonance can play a significant role in what we hear in our systems.

Also, while you're at it, and if you feel like playing... the tightness of the screws which hold the outlet to the wall make a change as well.

After trying this with several outlet covers I ultimately decided to leave them off all together in my system. Having no outlet cover so far seems better than any other material.

Now if I can just figure out how to firm up that giggly junction box in the wall without tearing my sheetrock to bits.