Reactance. In line and mic cables, capacitance is more of an issue than resistance. A capacitor is an electrical component consisting of two conductors separated by a space that is often filled with an insulating material. This is precisely how shielded cables are constructed: an inner conductor surrounded by an insulator and a shield. The entire cable is one giant capacitor that resists the flow of an AC current by virtue of its capacitive reactance, which is greatly influenced by the insulating material.

By itself, capacitive reactance diminishes as the frequency of the signal increases. However, when combined with the cable’s resistance, it forms a lowpass filter. As the cable gets longer, the cutoff frequency of this lowpass filter drops, resulting in more attenuation. This is one reason it is advisable to avoid long cable runs.

Another property of all conductors is inductance. As an audio signal travels along a wire, it creates a magnetic field that changes along with the signal voltage. This process is called self-inductance, which impedes the signal by virtue of its inductive reactance. Inductive reactance diminishes as the frequency decreases, but it interacts with resistance and capacitive reactance in complex ways.

Like resistance, this is not a serious concern in line-level connections, because the current is low, resulting in a weak self-induced magnetic field. However, with high-current signals (such as the ones going to your speakers), the magnetic field can be much larger, making self-inductance a cause for concern. Fortunately, high-quality speaker cables can reduce self-inductance, using specific cable geometries to cancel the magnetic field, which in turn can improve the sound quality of your system.

A cable’s magnetic field can also cause inductive reactance in nearby cables in addition to introducing EMI. Again, this is typically only a problem with speaker and power cables, which carry high-current signals. Therefore, you should avoid placing these cables near line and mic cables in your audio system, because doing so can result in an audible hum.

For similar reasons, avoid coiling up excess speaker cable in your studio. Coiling or wrapping the cable forms an inductance coil, in which each coil wrap produces a magnetic field that affects the signal on the other wraps. A better solution for managing excess cable length is to shape the cable into a loose “S” (or similar pattern) on the floor.

Rough Handling
As you flex and move a cable, you may hear what is commonly called microphonic, or handling, noise. Poorly manufactured cables in which the components are loose in the jacket often exhibit this problem. In high-quality cables, tight construction and a good-quality insulator will reduce handling noise. Most good-quality audio cables also include some kind of filler—typically made of cotton, jute, or polyester—to prevent the components from shifting (see Fig. 1 and Fig. 4).

Handling noise is most problematic with guitar cables, mic cables, and any other cable that normally flexes or moves while in use. Therefore, it’s very important to look for tight packing when selecting these kinds of cables. (Many retailers have samples with the ends cut open. If not, you can buy a foot or two of the cable and cut it up right there in the store.) Handling noise also becomes a problem when you use cables meant for other applications as guitar cables. For example, speaker cables and studio interconnect cables are not manufactured with the intention of being moved around while in use and therefore contain little or no filler.

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Reprinted with permission from Magazine, January, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved