Total Harmonic Distortion (THD) Explained in Microphones and Audio Systems

Content Summary

This article provides a detailed introduction to Harmonic Distortion and Total Harmonic Distortion (THD), explaining their causes in accordance with the national standard GB/T 2900.86-2009, and analyzing their specific manifestations in dynamic microphones, condenser microphones, and wireless microphones. By integrating content from “Detailed Explanation of Distortion,” “Detailed Explanation of Frequency Response,” and “Detailed Explanation of Fundamental Frequency,” this article explains the impact of harmonic distortion on audio quality and how to reduce distortion through technical means to enhance the overall performance of audio systems.

Detailed Explanation of Harmonic Distortion (HD) and Total Harmonic Distortion (THD)

Harmonic Distortion (THD) Explaine

In audio engineering, “harmonic distortion (HD)” is a common form of Nonlinear Distortion. It not only affects sound clarity but also causes tonal distortion, potentially compromising the overall quality of recordings or live performances.

According to the national standard GB/T 2900.86-2009:  

“A form of distortion caused by the generation of numerous higher-order harmonic components when a signal passes through a transmission system.”  

It further states: “The ratio of the root mean square (RMS) value of all added harmonic components to the RMS value of the total output signal is referred to as Total Harmonic Distortion (THD).”

This article will delve into the types, causes, and measurement methods of harmonic distortion from an electroacoustic perspective, and combine knowledge from “Distortion Explained,” “Frequency Response Explained,” and “Fundamental Frequency Explained” to explore its role and impact in dynamic microphones, condenser microphones, and wireless microphones.

1. What is harmonic distortion? What is its basic principle?

1. Definition of harmonic distortion

When a pure sine wave signal (e.g., fundamental frequency f₀ = 1 kHz) enters an audio system, if the output signal contains integer multiples of that frequency (e.g., 2 kHz, 3 kHz, 4 kHz, etc.), these are harmonic components, and the resulting waveform changes are referred to as harmonic distortion.

For example:  

A singer produces a low note at 100 Hz;  

If the sound picked up by the microphone also contains additional components at 200 Hz, 300 Hz, and 400 Hz;  

These are harmonic distortions introduced by the system;  

2. Calculation of Total Harmonic Distortion (THD)  

THD is an important metric for measuring the degree of harmonic distortion, typically expressed as a percentage (%). The formula is as follows:

THD (%) = (√(V₂² + V₃² + V₄² + …)) / V₁ × 100%

Where:

V₁ is the original signal voltage (fundamental frequency)

V₂, V₃, V₄… are the voltages of each harmonic order

The lower the THD value, the better the system's linearity and the higher the sound fidelity.

2. Analysis of the main types and causes of harmonic distortion

1. Even-order harmonics vs. odd-order harmonics

Even-order harmonics (e.g., 2f₀, 4f₀): sound “warmer” and “softer,” commonly found in tube amplifiers.

Odd-order harmonics (e.g., 3f₀, 5f₀): These sound more “harsh” and “sharp” and are commonly found in transistor devices or microphone systems operating in overload conditions.  

2. Primary Causes  

a) Nonlinear response of the microphone diaphragm/circuitry  

This is one of the most common sources. When a microphone is subjected to excessive Sound Pressure or an input signal exceeds its Dynamic Range, clipping occurs.

For example:  

Dynamic microphones experiencing magnetic saturation when recording drum kits;  

Condenser microphone preamplifiers without clipping protection;  

Resulting in a large amount of harmonics in the output signal;  

b) Aging or low-quality components in audio circuits  

Low-quality capacitors, resistors, operational amplifiers, and other components can introduce nonlinear distortion.

c) Compression encoding issues in wireless microphones

Some wireless microphone systems use digital compression algorithms to transmit audio, which may result in the loss or incorrect reconstruction of high-frequency harmonics.

3. Manifestation of harmonic distortion in different types of microphones

Different types of microphones exhibit varying degrees of harmonic distortion due to their structural differences and operating principles.

1. Dynamic microphones

Dynamic microphones are known for their durability, but they are prone to harmonic distortion in high-sound-pressure environments.

Mr Senma S-ONE Dynamic Vocal Microphones

a) Clipping distortion caused by magnetic circuit saturation

The diaphragm of a dynamic microphone is connected to a coil and operates within a magnetic field. When the input sound pressure exceeds a certain threshold, the magnetic field may reach saturation, preventing further movement of the coil.

Results:

The output signal is “clipped”;

resulting in a noticeable “clicking” sound or harshness;

b) Harmonic accumulation due to frequency response limitations  

Dynamic microphones typically have good mid-frequency response but limited ability to capture high-frequency overtones.  

For example:  

The Shure SM58 has a frequency response upper limit of 15 kHz;  

for vocals or string instruments with rich overtones, this may result in partial harmonic loss or imbalance;

2. Condenser Microphones

Condenser microphones have high sensitivity and a wide frequency response, but they are more susceptible to electronic noise and overload.

Mr Senma A7-Small Diaphragm Condenser Microphones

a) Clipping distortion caused by preamp overload

Condenser microphones rely on internal preamps to amplify signal strength. If the input signal is too strong (e.g., plosive sounds, close-miking), the preamp may clip.

For example:

Recording vocals without a pop filter;

Causing sudden airflow to impact the microphone capsule;

Resulting in noticeable distortion;

b) Harmonic enhancement effects caused by high-frequency response

Condenser microphones can capture higher-frequency overtones, but if not handled properly, they may also introduce unwanted harmonics.

For example:

When a female singer sings high notes;

If the microphone is overly sensitive to high-frequency response;

which may make the sound harsh;

3. Wireless Microphones

Wireless microphones must not only address traditional distortion but also deal with interference and compression issues during wireless transmission.

Mr Senma f7 portable Wireless microphones

a) Signal distortion caused by radio frequency interference

Wireless microphones operate in the UHF or VHF frequency bands. If there are other wireless devices nearby (such as Wi-Fi, Bluetooth, or walkie-talkies), this may cause signal interference.

For example:  

At large-scale events;  

Multiple wireless microphones used simultaneously;  

If channels are improperly configured;  

This may result in frequency interference or signal dropouts, manifesting as fluctuating volume levels, hoarseness, or intermittent sound;  

b) Dynamic range compression distortion caused by compression algorithms  

To conserve bandwidth, some wireless systems use compression encoding, which may cause sound to become “flat.”

For example:  

When recording podcasts using a Bluetooth microphone;  

If the encoding quality is low;  

Some dynamic details may be lost;  

Resulting in a lack of layered sound quality;  

4. How to measure and control harmonic distortion?  

1. Measurement tools and methods  

Use an audio analyzer (such as APx500, SoundCheck) to accurately measure THD;  

Input a single-frequency sine wave (such as 1 kHz);

Analyze the energy percentage of each harmonic order in the output signal;  

Derive the THD percentage value;  

2. Control Strategies  

a) Select microphones with low THD  

Review product specifications and prioritize professional-grade microphones with THD < 0.7%.  

b) Use pop filters and shock mounts  

Avoid direct impact of plosive sounds on the microphone capsule to reduce the risk of instantaneous overload.

c) Set appropriate gain and PAD switch  

Avoid overloading the microphone preamplifier, especially when recording high-pressure sound sources, by enabling the PAD (attenuation) function.  

d) Optimize the wireless environment  

Use radar frequency scanning to avoid interference bands and ensure stable transmission.  

5. THD is one of the key indicators for evaluating audio quality

Whether it's a dynamic microphone, condenser microphone, or wireless microphone, harmonic distortion is a significant factor affecting sound quality.

From the stability of the fundamental frequency to the integrity of the frequency response, and the accuracy of the sound power level, every step can be a source of harmonic distortion. Only through scientific selection, proper use, and good maintenance can harmonic distortion be minimized to the greatest extent, restoring true, clear, and natural sound.

As a professional microphone factory manufacturer, we have thoroughly considered THD control mechanisms during the product design phase to ensure that every product delivers exceptional audio performance across various usage scenarios.