How to Measure VST Plugin THD and Distortion

When processing audio, it's critical to know exactly what a plugin is doing to your signal. Many analog-modeled plugins introduce saturation, character, and color—but how much, and at what frequencies? This is where measuring Total Harmonic Distortion (THD) comes in.

What is THD?

THD stands for Total Harmonic Distortion. It is a measurement that quantifies the amount of harmonic distortion present in a signal. In simpler terms, when you feed a pure sine wave into a plugin, the plugin might add additional frequencies (harmonics) above the fundamental tone. THD is the ratio of the sum of the powers of all these harmonic components to the power of the fundamental frequency.

Why Should You Measure THD?

• Gain Staging: Find the exact input level where an analog-modeled EQ or compressor begins to saturate.
• Plugin Comparisons: Compare the harmonic signatures of two different saturation plugins to see which one adds odd vs. even harmonics.
• Aliasing Detection: Heavy distortion can cause digital aliasing. Measuring the spectrum lets you see if fold-back frequencies are ruining your high end.

How to Measure THD with Plugin Freq

While you can set up a sine wave generator and a spectrum analyzer in your DAW to do this manually, Plugin Freq Pro acts as a dedicated test bench for your entire plugin folder.

1. Open Plugin Freq and select the plugin you want to test.
2. Navigate to the Measure tab.
3. The test bench will automatically run a pure sine sweep through the plugin.
4. View the real-time THD+N graph, and see exactly how distortion changes based on frequency and input level.