The alteration of an audio signal where additional frequencies (harmonics) are introduced as integer multiples of the fundamental frequency, resulting in a modified sound character.

Harmonic Distortion

Harmonic distortion is a fundamental concept in audio signal processing that occurs when a system modifies an input signal by adding or emphasizing harmonic frequencies. These harmonics are integer multiples of the original (fundamental) frequency, creating changes in the signal's timbral characteristics.

Types of Harmonic Distortion

Even-Order Harmonics

Occur at 2x, 4x, 6x, etc. of the fundamental frequency
Generally perceived as "musical" or "warm"
Common in vacuum tube amplification systems
Add fullness and richness to the sound

Odd-Order Harmonics

Occur at 3x, 5x, 7x, etc. of the fundamental frequency
Often described as "aggressive" or "harsh"
Typical in solid-state electronics
Create more obvious distortion characteristics

Measurement and Analysis

The total harmonic distortion (THD) is measured as a percentage, representing the ratio of harmonic content to the fundamental frequency. Modern audio equipment typically specifies:

THD figures under normal operating conditions
THD+N (Total Harmonic Distortion plus Noise)
Frequency response curves showing distortion behavior

Applications

Musical Applications

guitar amplification use in music production
analog synthesis tone shaping
audio compression effects in recording

Technical Considerations

signal chain integrity in professional audio
impedance matching equipment interfacing
headroom operating levels

Sources of Harmonic Distortion

Electronic Components
- transistor behavior
- transformer saturation
- capacitor limitations
Physical Systems
- speaker excursion limits
- acoustic resonance constraints
- room acoustics factors

Mitigation Techniques

To minimize unwanted harmonic distortion:

Proper gain staging
High-quality components
balanced audio connections
Adequate power supply design
grounding system grounding

Creative Uses

While often considered a defect in high-fidelity applications, harmonic distortion has become an essential creative tool in:

music production
sound design
audio effects processing
analog modeling emulation of analog equipment

The intentional use of harmonic distortion has shaped many musical genres and continues to influence modern production techniques.

Impact on Audio Quality

Understanding harmonic distortion is crucial for:

Equipment design and evaluation
audio mastering decisions
monitoring systems accuracy
audio restoration work

The presence and character of harmonic distortion can significantly impact the perceived quality and emotional impact of audio content.