THD vs. IM

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THD vs. IM

When we look at audio specs we typically see distortion as a goal to keep as low as possible. After all, distortion is the opposite of perfection.

But there are multiple types of distortion, so which ones are the worst?

The most commonly rated distortions are what's known as harmonic distortion, or THD. This kind of distortion isn't all that bad. In fact, some types of THD are thought to be a bit beneficial if it adds warmth.

THD occurs when an audio component adds harmonic frequencies to the signal it processes. These harmonics are multiples of the original frequencies. For instance, if the original sound has a fundamental frequency of 100 Hz, harmonic distortion might add frequencies like 200 Hz (2nd harmonic), 300 Hz (3rd harmonic), and so on. The even ones are considered "beneficial", and the odd ones grating.

These harmonics are added tones—they weren't there in the music.

Intermodulation Distortion (IMD), on the other hand, is a more complex beast. It occurs when two or more different frequencies interact within an audio component, generating additional frequencies that are not just simple multiples of the originals but rather sums and differences of all combinations. For example, if tones at 300 Hz and 800 Hz are played simultaneously, IMD might add frequencies like 500 Hz (800-300 Hz) and 1100 Hz (800+300 Hz) to the mix.

IMD is particularly problematic because it can create frequencies that are far removed from the original tones, leading to a more noticeable and often harsh form of distortion. This can significantly alter the audio's timbre and clarity, especially in a system that is supposed to deliver high-fidelity reproduction of complex, layered music.

So, if we're going to stress over distortion numbers, best to focus on IM vs. THD.

We want perfection but, since that's never possible, the next best thing is to understand where the sonic boundaries are that matter.

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Paul McGowan

Founder & CEO

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