THD Calculator for Audio Signals

Calculate Total Harmonic Distortion (THD) for audio signals. Essential tool for audio engineers and audiophiles.

Manual Input
Signal Generator
Upload Data

THD Formula: THD = √(V₂² + V₃² + V₄² + ... + Vₙ²) / V₁ × 100%

Where: V₁ = Fundamental frequency amplitude, V₂...Vₙ = Harmonic amplitudes

Amplitude of the fundamental frequency (Volts)
Frequency of the fundamental tone
Audio sampling rate
2nd V
3rd V
4th V
5th V
6th V
7th V
Enter amplitudes for harmonics (2nd through 7th). Higher harmonics are usually smaller.

Signal Generator: Generate a test signal with controllable harmonic distortion for THD analysis.

Upload Signal Data: Upload a CSV file containing time-voltage pairs for THD analysis.

CSV file should contain two columns: time (seconds) and amplitude (Volts)
Expected fundamental frequency of the signal
Calculating THD...

Understanding Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is a measurement of the harmonic distortion present in a signal. It's defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.

THD Formula (Voltage): THD = √(V₂² + V₃² + V₄² + ... + Vₙ²) / V₁ × 100%

THD Formula (Power): THD = √(P₂ + P₃ + P₄ + ... + Pₙ) / P₁ × 100%

Where: V₁/P₁ = Fundamental, V₂...Vₙ/P₂...Pₙ = Harmonics

THD Classification for Audio Equipment

THD Range Classification Audio Quality Typical Applications
< 0.01% Excellent Virtually indistinguishable from perfect High-end studio equipment, precision measurement
0.01% - 0.1% Very Good Extremely high fidelity Professional audio equipment, high-end amplifiers
0.1% - 1% Good High fidelity, slight coloration possible Consumer hi-fi equipment, quality amplifiers
1% - 5% Acceptable Audible distortion, reduced clarity Budget audio equipment, small speakers
> 5% Poor Significant distortion, poor audio quality Low-quality equipment, not recommended for music

Harmonic Distortion in Audio Systems

Harmonic distortion occurs when an audio system introduces frequencies that are integer multiples of the original signal. These additional frequencies are called harmonics and can significantly alter the sound quality.

Even vs. Odd Harmonics:

  • Even Harmonics (2nd, 4th, 6th...): Often described as "warm" or "musical" distortion
  • Odd Harmonics (3rd, 5th, 7th...): Typically described as "harsh" or "metallic" distortion

Factors Affecting THD in Audio Systems

1

Amplifier Design: Class A amplifiers typically have lower THD than Class AB or Class D

2

Component Quality: High-quality capacitors, resistors, and transistors reduce distortion

3

Operating Level: THD typically increases as equipment approaches its maximum output

4

Frequency Response: Some equipment has higher THD at frequency extremes

5

Power Supply Quality: Clean, stable power reduces distortion in amplifiers

Measurement Standards and Methods

  • IEC 60268-3: International standard for sound system equipment
  • IEEE 219: Standard methods for measurement of audio equipment
  • SMPTE RP-120: Test method for differential gain and phase
  • AES17: AES standard method for digital audio measurements
  • THD+N: Total Harmonic Distortion plus Noise - more comprehensive measurement

Measurement Note: THD measurements should be performed at multiple frequencies and power levels for complete characterization. The human ear is most sensitive to distortion in the 1-2 kHz range.

Frequently Asked Questions

THD measures only harmonic distortion, while THD+N (Total Harmonic Distortion plus Noise) includes both harmonic distortion and noise. THD+N is generally considered a more comprehensive measurement of audio quality as it accounts for all unwanted signals in the audio path.

The audibility of THD depends on many factors including frequency, type of harmonics (even vs. odd), and listener experience. Generally, THD below 0.1% is considered inaudible for most listeners under normal conditions. Trained listeners in controlled environments might detect THD as low as 0.01% with certain signals.

Tube amplifiers typically produce more even-order harmonics (2nd, 4th, 6th) which are often perceived as "warm" and "musical." Solid-state amplifiers tend to produce more odd-order harmonics (3rd, 5th, 7th) which are perceived as "harsh." The specific harmonic profile, not just the THD percentage, determines the sonic character.

Different amplifier classes have different typical THD characteristics:
  • Class A: Typically 0.1-1% THD, simple design, low crossover distortion
  • Class AB: Typically 0.01-0.1% THD, more efficient than Class A
  • Class D: Typically 0.01-1% THD, very efficient, requires output filtering
  • Class G/H: Typically 0.01-0.1% THD, variable power supply for efficiency

Not necessarily. While low THD is generally desirable, it's only one factor in audio quality. Other factors like frequency response, transient response, noise floor, and the specific harmonic profile may be more important. Some equipment with slightly higher THD (like tube amplifiers) is preferred for its specific sonic character. THD below 0.1% is generally sufficient for high-quality audio reproduction.