Section Meeting Report
by Dan Mortensen, PNW Section Treasurer
April 17, 2000 The Truth About Audio and Other Cables
Steve Lampen, Belden
Our April meeting featured a presentation by Steve Lampen, Technology Specialist/Multimedia Products with Belden Wire & Cable, titled "The Truth About Audio and Other Cables."
Steve started by making a distinction between things we can measure in wire (resistance, capacitance, inductance), and those things we can't measure (soundstage, "detail", "directionality", and other things you can "hear"). There is rarely a correlation between what you can measure and what you can hear.
For best electron flow, you want to use a metal that has low resistance. In circular-mil ohms per foot at 20°C., silver leads the way at 9.9, copper is next at 10.4, gold is 14.7, aluminum is 17, nickel is 47, and steel is 74. Although silver is the best conductor, it has several disadvantages: it tarnishes, which then interferes with connection; it is pretty expensive; and it cannot be annealed. Wire is made by repeatedly pushing metal through ever-smaller dies, until it is the size you want. This process is really boring to watch, and now is all done by robots. After going through the dies, the wire is very brittle and easy to break. Copper wire can be heated to 700 and annealed, which lines up the crystalline structure and removes the brittleness, making it very useful for cable purposes.
Directionality, or the idea that electricity flows better in one direction through a cable than the other, is a common concept among certain self-identified audiophiles. Belden did a double-blind test for cable directionality in conjunction with an audiophile magazine. The end result was perfectly random. Belden is still happy to manufacture and sell directional cables to enthusiasts. They make up a long length of cable, cut it in segments, identify the ends of the segments so they know how it came off the spool (length A->B, length B->C, length C->D, etc), and then let the customer identify by careful listening which direction is "better". Over thousands of cables sold, the chosen "best" signal flow is random, for segments cut from the same spool!
Steve said that even though copper is the most useful metal for most cable purposes, other metals are used for a variety of reasons. Cable TV coax is frequently made from aluminum; they buy millions of feet of it, and it works well enough at cable TV frequencies. Steve said, though, that when you see aluminum cable, you should think "crap". The molded RCA cables that come free with your Hi-Fi equipment are made with aluminum, but, hey, they're free! Steel wire is used where great strength is required, and because of the skin effect (where higher frequencies are conducted closer to the exterior skin of the wire), steel is OK for some uses.
As an aside, he mentioned that in the early days of the telegraph, they used a single steel wire attached directly to wood poles, run alongside railroad tracks because that was a pretty good way to go long distances and be able to get to the cable to maintain it. Wire sizing using the American Wire Gauge system (AWG) was invented around then, in the 1840's/1850's. As the coal-burning locomotives went down the tracks, their soot coated the wooden poles with carbon, shorting the steel wire to ground, stopping the operation of the telegraph. The insulator was invented in response to this phenomena, a design that reduced the coating of carbon. Nowadays fiber optic cable is being buried next to railroad rights-of-way, creating a whole new industry for the railroads.
By the way, what's the difference between wire and cable? A cable has more than one conductor.
Cable has three electrical characteristics: Capacitance, Inductance, and Impedance.
Capacitance stores a charge in the cable, and is determined by the distance between conductors, and by the type of insulation (dielectric) between conductors. Signals with very high frequencies (like digital signals) want to be in low-capacitance cable, especially for long runs, otherwise the signal won't make it to the other end. Running electricity through a wire creates a magnetic field, and the ease with which the magnetic field flows through a wire encased with a dielectric depends upon the dielectric material; those qualities are measured by the Dielectric Constant (K) and the Velocity of Propagation (Vp). For direct current, a lower number is better; Vp percentages will be in (%), and for Vp, a higher number is better. The DC for air is 1; Teflon is 2.1 (70%); Polyethylene is 2.25 (66%); Polypropylene is 2 to 3 (64%); PVC is 3 to 5 (50%); rubber is 5+; chemically foamed polyethylene or Teflon is (78%); nitrogen gas-injected foamed polyethylene or Teflon is (83%). (He didn't tell us Vp for air or rubb er, or the DC for the chemical foams). Vp is relative to the speed of light (100%). CAT 5 cable, as used between computers, has a foamed dielectric; using regular microphone cable wouldn't work as well.
Inductance is the inverse of capacitance, and has a very tiny effect in cables. You have to make the cable into a coil to really get any appreciable inductance. Impedance is the hardest spec to understand; for all intents and purposes, most audio users will never have an impedance issue in the cable itself, so manufacturers don't even tell you the impedance of cable so you won't get confused by it.
Steve told a long and fascinating story (too long to retell here) about the guy who invented Polyvinyl Chloride (PVC), using waste products from the petroleum industry. He also told a good story about rubber: when the Spanish Conquistadors came to the New World in the 1530's, they found the Natives playing a game with a funny bouncing ball. They took the ball back home, but nobody really paid attention to it or how it was made. Vulcanization wasn't discovered until 1839 by Charles Goodyear, so how did the Indians do it? In 1988, a grad student discovered that the Indians took latex from the plant, added morning glory juice (which is high in sulfur), and did this on the hottest day of the year. They made vulcanized rubber.
For those wishing to learn more, Steve's book, "Wire, Cable, and Fiber Optics for Video and Audio Engineers" is published by McGraw-Hill. The Belden Web Site (www.belden.com) has over 5000 pages of information available to the public.
Thanks to Steve Lampen for a fascinating look at a very important part of our world.
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Last modified 11/4/2001.