Harmonic distortion is a type of distortion that occurs in audio signals when a device reproduces or amplifies a signal in a nonlinear way. It produces new frequency components called harmonics that are integer multiples of the original signal’s frequency.
In live sound engineering, harmonic distortion is mostly caused by overdriving or clipping the signal in amplifiers, mixers, or other gear. This happens when the signal level exceeds the device’s headroom, causing the peaks to flatten or “clip.” Even small amounts of harmonic distortion can make a clean sound source sound fuzzy or gritty.
The most common harmonics are at double and triple the original frequency. For example, a 100Hz sine wave clipped by an amp might produce new 200Hz and 300Hz harmonics along with the original 100Hz tone. Higher-order harmonics are also possible but decrease in amplitude.
Too much harmonic distortion is undesirable because it changes the tone and makes it sound unnatural. However, small amounts can sometimes add pleasant warmth or edge, which is why some guitar amps and analog gear intentionally add nonlinearity. In live sound, you usually want to avoid excessive harmonic distortion.
There are a few ways harmonic distortion occurs in live engineering:
- Overdriving the preamp or gain stages on an input channel. Turning the gain too high amplifies the signal beyond the device’s headroom, clipping it.
- Running the main mix outputs too hot into an amp or speakers, causing clipping at that stage. Watch for red flashing overload lights!
- Using low-quality, underpowered amplifiers that distort when pushed hard at high volumes. Upgrade amplifiers to have sufficient headroom.
- Poorly designed or faulty equipment with inherent distortion issues. Use high-quality gear from reputable manufacturers.
To minimise harmonic distortion:
- Leave ample headroom on each channel’s preamp and the main mix output. Keep levels 6-10dB below maximum to avoid clipping.
- Use high-headroom amps designed for live sound. Make sure amp power exceeds your actual needs.
- Insert a limiter on output channels to prevent clipping if levels get too high. Limiting is better than allowing clipping.
- Use EQ and multi-band compression to reduce peaks on problem tracks instead of just turning them down.
- Avoid excessive bass boosts that require cutting master output level. This reduces overall headroom.
- Listen critically at soundcheck for signs of clipping or distortion and correct levels as needed.
Careful gain staging, sufficient amplifier power, and thoughtful use of dynamics processing will help you deliver clean, undistorted sound for any live mixing job. Controlling harmonic distortion takes experience, but leads to better tone and audio quality for the audience.