Frequency-Normalized THD Measurement
Introduction
Traditionally when measuring Total Harmonic Distortion (THD), the harmonics are compared to the frequency response at the stimulus frequency. For example when the stimulus is playing back 100Hz, the level of the second harmonic (200Hz) is compared to the level of the stimulus frequency at 100Hz and plotted at 100Hz. One major drawback of this method is that any device nonlinearities will be influenced by the device’s linear response, namely, the fundamental. If the speaker rolls off at low frequencies below 200Hz, the comparison of the 200Hz second harmonic and 100Hz stimulus frequency will also impact the THD calculation.
To avoid this issue, a method called Normalized THD is recommended. Instead of plotting the harmonics at the stimulus frequency, Normalized THD plots the harmonics at the measured frequency, calculates the percent distortion, and then plots it at the stimulus frequency. Using the previous example, the 2nd harmonic at 200Hz would be compared at 200Hz to the fundamental, then plotted at 100Hz. SoundCheck can measure the conventional THD as well as normalized THD in the same step. The harmonics can also be displayed at the measured frequency.
The measured driver rolls off around 500Hz, so using the conventional method of THD (bottom graph in blue) the THD number starts to rise considerably. This has nothing to do with the device’s nonlinearities at low frequencies but is due to the linear response rolling off at low frequencies. By measuring Normalized THD (bottom graph in orange), we calculate THD using the harmonics at the measured frequency (top graph, orange and purple) which removes the effect of the speaker rolling off on the distortion.
For more information on Normalized THD, see the following explanations/articles:
The Value of Normalized Distortion
Software Requirements
- SoundCheck Plus or higher part # 1102 (Version 23 or later)
Hardware Requirements
- Audio Interface – AmpConnect 621 (Part # 4046) or similar
- Measurement Microphone – Listen SCM (Part # 4002) or similar
System Diagram
Hardware Setup and Calibration
- Assign signal paths “Reference Mic” and “Amp 1” to the appropriate hardware channels.
- Calibrate the microphone signal path “Reference Mic” as instructed in the SoundCheck manual.
- Calibrate your amplifier signal path “Amp 1” as instructed in the SoundCheck manual.
You are ready to start the sequence.
Further Sequence Development
Ways in which you could modify or further develop the sequence include:
- This sequence calculates conventional THD and Normalized THD. Conventional Rub & Buzz and Normalized Rub & Buzz could also be added.
- Autosave steps can be added to save your data.
Download Normalized THD (zip file)
Download Normalized THD (PDF)
