Intermodulation in Audio Signals

Intermodulation distortion (IMD) is a critical yet often overlooked phenomenon in audio reproduction that occurs when two or more frequencies interact within a nonlinear system, generating additional, unintended frequencies. Unlike harmonic distortion, which produces harmonics of the original signal, intermodulation creates sum and difference frequencies that are not musically related to the input. These spurious signals can significantly degrade audio fidelity, particularly in complex musical passages with multiple instruments or vocals. When present, IMD manifests as a harsh, gritty, or congested sound that can mask subtle details and reduce the clarity of the original performance, even at relatively low distortion levels. Because the human ear is particularly sensitive to these unnatural byproducts, intermodulation is often perceived as more objectionable than other types of distortion.

Minimizing intermodulation is essential for achieving high-quality audio reproduction, especially in high-fidelity systems and professional audio equipment. Designing audio components with low IMD requires careful attention to circuit linearity, power supply stability, and component matching. Amplifiers, preamplifiers, and loudspeakers are all susceptible to IMD if not properly engineered. Manufacturers often conduct two-tone tests, combining signals such as 19 kHz and 20 kHz, to measure intermodulation products that appear within the audible range. A well-designed component will preserve the integrity of the original audio without introducing dissonant artifacts. In an era where high-resolution audio and precision playback are increasingly valued, reducing intermodulation distortion is not just a technical goal—it’s a fundamental requirement for faithful and enjoyable sound reproduction.

How Intermodulation Occurs

Intermodulation Distortion (IMD) occurs when two or more audio signals pass through a nonlinear system (e.g., an amplifier, speaker, or digital processing chain). This nonlinearity causes the signals to mix and produce new frequencies that are mathematically related to the original ones.

The newly created frequencies are given by:

Where:

Types of Intermodulation Distortion

• Generates new frequencies at
• Less common in well-designed audio systems.

• Generates distortion at
• More problematic because the resulting distortion often falls inside the audible range.

• Produces even more complex artifacts at
• Typically found in extreme cases of nonlinearity (e.g., overloaded amplifiers, poorly designed digital processing).

Here is an example of IMD in a preamplifier using 19 kHz and 20 kHz sine waves as the input signal. You can see the primary peaks at 19 kHz and 20 kHz (the tall peaks), second-ordered intermodulation IMD peaks at 18 kHz and 21 kHz, and higher-ordered intermodulation peaks at 15 kHz, 16 kHz, 17 kHz, 22 kHz, 23 kHz, and 24 kHz.

Why is Intermodulation Bad in Audio?

Common Causes of Intermodulation in Audio

• Overdriving an amplifier can introduce IMD.
• Poor circuit design or improper biasing in transistor-based systems.

• Nonlinear behavior in woofers and tweeters at high volumes.
• Cheap or misaligned drivers often introduce intermodulation artifacts.

• Poor dithering or excessive dynamic range compression can introduce IMD.
• Low-quality sample rate conversion algorithms.

• Nearby radio frequencies mixing within the receiver can generate intermodulation artifacts.

How to Reduce Intermodulation Distortion

1. Use High-Quality Components: Better amplifiers, DACs, and speakers reduce IMD.
2. Avoid Overloading Amplifiers: Stay within the clean gain range of an amplifier.
3. Proper Gain Staging: Maintain healthy signal levels throughout the audio chain.
4. Use Linear Phase Processing: Some EQs and compressors introduce IMD—linear phase options can help.
5. Improve Loudspeaker Design: High-quality drivers and crossovers reduce mechanical IMD.

Example of Intermodulation in Real Life

Imagine two musicians playing 1 kHz and 1.2 kHz tones at the same time. In a perfect system, you should only hear these two frequencies. But in a nonlinear system, you might hear extra tones like:

These “phantom” tones can make music sound muddy, distorted, or less natural.

Intermodulation distortion remains one of the most revealing indicators of a component’s linearity and overall audio fidelity. Unlike harmonic distortion, its non-musical byproducts can severely impact the listening experience, making its reduction a top priority for audio engineers. The constant pursuit of lower IMD has led to innovations in circuit topology, improved component matching, and more precise power supply design. As audio systems advance in resolution and dynamic range, the need to control intermodulation becomes even more critical. Ultimately, minimizing IMD is essential for preserving the purity and realism of recorded music across all types of playback systems.

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