Speaker wire

Thicker wires reduce resistance. The resistance of copper 16-gauge or heavier speaker connection cable has no detectable effect in runs of 50 feet (15 meters) or less in standard domestic loudspeaker connections for a typical 8 ohm speaker.[4] For aluminum or copper-clad aluminum wire, 14-gauge or heavier cable is needed to support this claim due to higher resistivity.[4] As speaker impedance drops, lower gauge (heavier) wire is needed to prevent degradation to damping factor – a measure of the amplifier's control over the position of the voice coil.

Insulation thickness or type also has no audible effect as long as the insulation is of good quality and does not chemically react with the wire itself (poor-quality insulation has occasionally been found to accelerate oxidation of the copper conductor, increasing resistance over time). High-power in-car audio systems using 2-ohm speaker circuits require thicker wire than 4 to 8-ohm home audio applications.

Most consumer applications use two conductor wire. A common rule of thumb is that the resistance of the speaker wire should not exceed 5 percent of the rated impedance of the system. The table below shows recommended lengths based on this guideline:

^* While in theory heavier wire can have longer runs, recommended household audio lengths should not exceed 50 feet (15 m).[4]

The gauge numbers in SWG (standard wire gauge) and AWG (American wire gauge) reduce as the wire gets larger. Sizing in square millimeters is common outside of the US. Suppliers and manufacturers often specify their cable in strand count. A 189 strand count wire has a cross-sectional area of 1.5 mm² which equates to 126.7 strands per mm².[5]

Use of copper or copper-clad aluminum (CCA) is more or less universal for speaker wire. Copper has low resistance compared to most other suitable materials. CCA is cheaper and lighter, at the expense of somewhat higher resistance (about the same as copper two AWG numbers up). Copper and aluminum both oxidize, but oxides of copper are conductive, while those of aluminum are insulating. Also offered is Oxygen-free Copper (OFC), sold in several grades. The various grades are marketed as having better conductivity and durability, but they have no significant benefit in audio applications.[4] Commonly available C11000 Electrolytic-Tough-Pitch (ETP) copper wire is identical to higher-cost C10200 Oxygen-Free (OF) copper wire in speaker cable applications. Much more expensive C10100, a highly refined copper with silver impurities removed and oxygen reduced to 0.0005 percent, has only a one percent increase in conductivity rating, insignificant in audio applications.[4]

Silver has a slightly lower resistivity than copper, which allows a thinner wire to have the same resistance. Silver is expensive, so a copper wire with the same resistance costs considerably less. Silver tarnishes to form a thin surface layer of silver sulfide.

Gold has a higher resistivity than either copper or silver, but pure gold does not oxidize, so it can be used for plating wire-end terminations.

Capacitance occurs between any two conductors separated by an insulator. In an audio cable, capacitance occurs between the cable's two conductors; the resulting losses are called "dielectric losses" or "dielectric absorption". Capacitance also occurs between the cable's conductors and any nearby conductive objects, including house wiring and damp foundation concrete; this is called "stray capacitance".

Parallel capacitances add together, and so both the dielectric loss and the stray capacitance loss add up to a net capacitance.

Audio signals are alternating current and so are attenuated by such capacitances. Attenuation occurs inversely to frequency: a higher frequency faces less resistance and can more easily leak through a given capacitance. The amount of attenuation can be calculated for any given frequency; the result is called the capacitive reactance, which is an effective resistance measured in ohms:

${\displaystyle X_{c}={\frac {1}{2\pi fC}}}$

where:

• ${\displaystyle f}$ is the frequency in hertz; and
• ${\displaystyle C}$ is the capacitance in farads.

This table shows the capacitive reactance in ohms (higher means lower loss) for various frequencies and capacitances; highlighted rows represent loss greater than 1% at 30 volts RMS:

The voltage on a speaker wire depends on amplifier power; for a 100-watt-per-channel amplifier, the voltage will be about 30 volts RMS. At such voltage, a 1 percent loss will occur at 3,000 ohms or less of capacitive reactance. Therefore, to keep audible (up to 20,000 Hz) losses below 1 percent, the total capacitance in the cabling must be kept below about 2,700 pF.

Ordinary lamp cord has a capacitance of 10–20 pF/ft, plus a few picofarads of stray capacitance, so a 100-foot run (200 total feet of conductor) will have less than 1 percent capacitive loss in the audible range (100 ft * 20 pF/ft = 2000 pF, and 2000 pF < 2700 pF). Some premium speaker cables have higher capacitance in order to have lower inductance; 100–300 pF is typical, in which case the capacitive loss will exceed 1 percent for runs longer than as little as 10 feet (10 ft * 300 pF/ft = 3000 pF, and 3000 pF > 2700 pF).

All conductors have inductance, which results in an inherent resistance to changes in a current. That resistance is called inductive reactance, measured in ohms. Inductive reactance depends on how quickly the current is changing: quick changes in current (i.e., high frequencies) encounter a higher inductive reactance than do slow changes (low frequencies). Inductive reactance is calculated using this formula:

${\displaystyle X_{i}=2\pi fL}$

where:

• ${\displaystyle f}$ is the frequency in hertz; and
• ${\displaystyle L}$ is the inductance in henrys.

Audio signals are alternating current and so are attenuated by inductance. The following table shows the inductive reactance in ohms (lower means lower loss) for typical cable inductances at various audio frequencies; highlighted rows represent loss greater than 1% at 30 volts RMS:

The voltage on a speaker wire depends on amplifier power; for a 100-watt-per-channel amplifier, the voltage will be about 30 volts RMS. At such voltage, a 1% loss will occur at 0.3 ohms or more of inductive reactance. Therefore, to keep audible (up to 20,000 Hz) losses below 1%, the total inductance in the cabling must be kept below about 2 μH.

Ordinary lamp cord has an inductance of 0.1–0.2 μH/ft, likewise for shielded cord,[6] so a run of up to about 10 feet (20 total feet of conductor) will have less than 1% inductive loss in the audible range (10 ft * 0.2 μH/ft = 2.0 μH, which is at or below the proximate threshold of 2 μH given above). Some premium speaker cables have lower inductance at the cost of higher capacitance; 0.02-0.05μH/ft is typical, which at the worst end means that a run of up to about 40 feet will have less than 1% inductive loss (40 ft * 0.05 μH/ft = 2.0& μH).

Skin effect in audio cables is the tendency for high frequency signals to travel more on the surface than in the center of the conductor, as if the conductor were a hollow metal pipe.[3] This tendency, caused by self-inductance, makes the cable more resistant at higher frequencies, diminishing its ability to transmit high frequencies with as much power as low frequencies. As cable conductors increase in diameter they have less overall resistance but increased skin effect. The choice of metals in the conductor makes a difference, too: silver has a greater skin effect than copper; aluminum has less effect. Skin effect is a significant problem at radio frequencies or over long distances such as miles and kilometers worth of high-tension electrical transmission lines, but not at audio frequencies carried over short distances measured in feet and meters. Speaker cables are normally made with stranded conductors but bare metal strands in contact with each other do not mitigate skin effect; the bundle of strands acts as one conductor at audio frequencies.[7] Litz wire – individually insulated strands held in a particular pattern – is a type of high-end speaker wire intended to reduce skin effect. Another solution that has been tried is to plate the copper strands with silver which has less resistance.[8]

Regardless of marketing claims, skin effect has an inaudible and therefore negligible effect in typical inexpensive cables for loudspeaker or other audio signals.[9] The increase in resistance for signals at 20,000 Hz is under 3%, in the range of a few milliohms for the common home stereo system; an insignificant and inaudible degree of attenuation.[7][10][11]