Audio device distortion includes harmonic distortion, intermodulation distortion, phase distortion, and transient distortion. The most important audio measurement is harmonic distortion, harmonic distortion, simply the extra harmonic component of the sound signal after it is reproduced by the audio device. From the listener's point of view, the sound emitted by different vocal objects is composed of different fundamental waves and harmonic waves. The listener can distinguish the starting sound object according to the characteristics of the sound. If the power amplifier amplifies the musical sound (musical sound consists of fundamental wave and harmonic wave) emitted by a certain musical instrument, the shape, amplitude, and phase of the waveforms of the fundamental wave and each harmonic wave can be distorted without distortion after being played by the loudspeaker. Reproduced, it can be considered a high-quality playback; otherwise, the sound produced by the speaker sounds annoying, awkward, the harmonic distortion has reached intolerable, and even people cannot distinguish the type of vocal instruments. Therefore, harmonic distortion is an important performance indicator of audio equipment.
Harmonic distortion is measured in two ways. One is to input a sinusoidal signal to the device under test, and then analyze the frequency components of the device's response signal to obtain harmonic distortion. Another simpler measurement method is to first use the band-rejection filter to filter out the fundamental frequency component in the response signal, and then directly measure the voltage of the remaining signal and compare it with the original response signal to obtain the harmonic distortion. Obviously, the harmonic distortion obtained by the second method is THD+N. Since the total voltage value of the signal is used instead of the voltage value of the fundamental frequency component, the resulting harmonic distortion is smaller than the actual value, and the actual harmonic distortion is greater, the greater the error.
In the actual audio measurement, usually select a number of frequency points within a certain frequency range, measure the harmonic distortion of each point separately, and then connect each harmonic distortion value with the frequency as the horizontal coordinate to form a curve which is called harmonic distortion curve.
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