Audio Cable High-Frequency Rolloff

Formula

Description

Long audio cables form a low-pass RC filter between the source output impedance and the cable capacitance, causing high-frequency rolloff that dulls the sound. This is particularly problematic for high-impedance sources like passive guitar pickups (5-15 kΩ) and some microphone circuits. The -3 dB point is where the signal is attenuated by half power. Cable capacitance typically ranges from 30-100 pF per foot, so a 20-foot guitar cable can have 2000 pF or more of capacitance.

Variables

• f — -3 dB rolloff frequency (Hz)
• R — Source output impedance (Ω)
• C — Total cable capacitance (F)

Practical Notes

A passive guitar pickup with 8 kΩ impedance driving a 20-foot cable at 80 pF/ft (1600 pF total) rolls off at about 12.4 kHz, audibly dulling the high end. Buffer pedals and active pickups solve this by presenting a low source impedance (typically < 1 kΩ). For professional audio, keep cable runs short or use balanced low-impedance connections (150Ω mic level or 600Ω line level).