Cable Capacitance

Since the capacitance is in parallel with the amplifier and load, as frequency increases it will eventually form a short circuit around the loudspeaker. The required capacitance for the roll-off to just become audible is:

How much wire is required to accumlate this much capacitance. At 50 pF/ft (a high figure for loudspeaker wire), the required length becomes:

The effects of cable capacitance in a loudspeaker circuit are not likely to produce an audible effect.

Cable Inductance

Wire also has inductance. A series inductance will form a low-pass filter between the amplifier and loudspeaker. The highest frequency of interest (20 kHz) will be most affected by the presence of an inductance. The inductance required to just make an audible change is:

Calculating the inductance of a straight piece of wire is not trivial, but can be approximated by(1) :

Assuming a wire radius of 0.035” (AWG16) we can perform a few iterations to find how long the wire would need to be to form a 3.2 uH series inductor.

This represents a worst case. In practice other factors can affect the results, such as conductor spacing, coiling, etc. It does serve to make the point that if wire length is kept short and wires are not coiled cable inductane can Neglecting the effects of capacitance and inductance, the circuit of Figure 2 simplifies to the following:

Cable Resistance

The resistance of the wire increases with length, decreases with increased conductor cross-sectional area, and is independent of frequency. As the resistance of the wire increases, the current flowing in the circuit is reduced. This reduces the current flow through the loudspeaker which reduces the voltage drop across the loudspeaker and results in a drop in sound level. The effect is commonly referred to as line loss. Those familiar with electronics will recognize the circuit as a voltage divider. In this voltage divider circuit the losses due to wire resistance can be minimized by keeping this parasitic resistance of the wire small relative to the impedance of the loudspeaker.

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