Also, what does EMI noise sound like? Is it hum?

I hope not!!! I just made 4 brand new quad mic cables (Neutrik gold plated ends/mogami cable). I am anxious to hear the answer to this as well. I have always heard that quad cables have more noise rejection, but have not heard anything about reduction of high freqs...

I was intrigued by this question, so I posted it on the SOS forum. The following answer is from Hugh Robjohns, one of the Technical Editors from SOS:

Are you talking about 'Star-Quad' cable?

This is a special balanced cable design used primarily in television and theatre applications where there may be strong sources of interference placed close to the mic cables (eg DMX-controlled lighting cables running alongside).

Instead of just two inner cores with a screen, there are four, all twisted together in a tight twist. When wiring to an XLR plug the opposite pairs are wired together -- normally the red and blue pairs to pin 2 and the white and green pairs to pin 3.

With very close sources of interference to a conventional dual-wire balanced cable, there is a likelihood that one core will be significantly closer to the interference source than the other, and so there will be unequal amounts of interference in the two cores, and so it won't all cancel in the balanced line receiver.

The idea of the Star Quad arrangement is that if one wire in the pair is closer to the interference, the other coupled wire in its pair will be further away. Averaging the interference signal colllected by both wires in each pair is more likely to produce equal interference, and so the cable provides better immunity to interference FROM VERY CLOSE SOURCES.

There is no point in using Star-Quad unless you regularly run mic cables next to other cables carrying nasty interference signals.

The down side, of course, is that the extra wires increase the cable capacitance quite substantially, and that can result in greater loss of high frequency signals over extended cable runs.

Hope that helps

Capacitance loading, or lack of; is one of the advantages of low impedance balanced line. Due to the low impedance source, capacitance loading has little effect unless you start to run extremely long lines; perhaps over 300 feet you might start to measure a difference, but you probably wouldn't hear the difference yet in a subjective listening test. In many concert venue situations a 500' snake cable for mic lines isn't uncommon, and it doesn't seem to pose any problems in terms of audio quality.
Bill Y.