Skin effect is a tendency for alternating current (AC) to flow mostly near the outer surface of an electrical conductor, such as metal wire. The effect becomes more and more apparent as the frequency increases. The main problem with skin effect is that it increases the effective resistance of a wire for AC at moderate to high frequencies, compared with the resistance of the same wire at direct current (DC) and low AC frequencies. The effect is most pronounced in radio-frequency (RF) systems, especially antennas and transmission lines. But it can also affect the performance of high-fidelity sound equipment by causing attenuation in the treble range (the highest-pitched components of the audio). In large RF antenna arrays, hollow tubing can be used in place of solid rods with little or no loss of efficiency; in this respect, skin effect is an asset. It also works in favor of the use of copper-clad steel wire for more modest antennas. Such wire is mechanically stronger than solid or stranded copper, because steel has a higher tensile strength than copper. The skin effect causes most of the current to flow through the copper cladding, which is a better electrical conductor than steel. Skin effect occurs with brief pulses of current, for the same reasons it occurs at high AC frequencies. Also see: Ampere per meter squared, current, and radio frequency (RF)