Harmonic Distortion Measurement: The effects of sampling rate and stimulus frequency on the measured harmonic frequency (including THD and Rub & Buzz)

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Author: Steve Temme

Harmonic distortion measurements can be confusing to interpret, and it’s important to understand the data that you are looking at, particularly the relationship between the stimulus frequency and the measured harmonic frequency. This brief guide explains the relationships between sampling rate, stimulus frequency and measured harmonics for both normalized and conventional harmonic distortion measurements.

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Practical Impedance Measurement

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Author: Steve Temme
Loudspeaker impedance measurements are made for many reasons. In the R&D lab, these range from the simple task of identifying a speaker’s resonant frequency to more complex functions such as calculating the speaker’s Thiele-Small parameters. On the production line, impedance measurement is a key quality control parameter that verifies that the speaker’s motor properties are correct, that the magnet is charged correctly, the voice coil number of turns is correct and that the moving mass (cone and voice coil) is within specification.
There are two basic methods of making impedance measurements on loudspeakers, micro-speakers and headphones using sound card and software based systems. These are basic single channel measurements, and more complex, but more accurate, dual channel methods. Both methods are implemented in SoundCheck, and with some additional hardware these tests are simple to carry out.

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HarmonicTrak Algorithm for Fast and Accurate Swept Sine Measurement

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Author: Steve Temme

The sine generator in SoundCheck®, operates on the same principles as a traditional swept sine measurement system consisting of a sine generator, voltmeter, tracking filter, and level recorder. The main difference is that it is implemented in the software as a virtual instrument. The SoundCheck sine generator sweeps in discrete steps; each frequency step contains numerous cycles that are synchronously averaged to minimize the effect of background noise. Transducer settling (transient ringing) is also minimized by discarding the first few cycles at each new frequency step and providing a phase continuous transition between frequencies. This provides higher frequency, amplitude, and phase accuracy, as well as excellent noise suppression.  Instead of using a tracking filter that can only measure one harmonic at a time, SoundCheck uses a proprietary FFT-based algorithm, called HarmonicTrakTM, which is equivalent to a parallel bank of individual tracking filters that measure all selected harmonics simultaneously. This parallel analysis technique saves considerable measurement time over the traditional serial analysis method.
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Audio Distortion Measurements

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Author: Steve Temme

In the never ending quest for better sound transmission, reinforcement, and reproduction, the electronics have been extensively analyzed for distortion. Distortion in the electroacoustic transducers, while typically several orders of magnitude greater, has often been neglected or not even specified because it has been difficult to measure and interpret. With a basic understanding of transducer limitations, some knowledge of human hearing, and the application of different distortion test methods, electroacoustic transducer distortion becomes easier to measure and assess. Note: Although this paper was written over 20 years ago, the principles are still true today.
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Time Selective Measurements with a Logarithmically Swept Sine

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Author: Martin Rung

A time selective measurement of a frequency response is (directly or indirectly) based on a measurement of the impulse response, where a well-defined time window is applied to the impulse response. The frequency response is simply the Fourier transform of the impulse response. Time selective measurements are often used in electroacoustics to make simulated free-field measurements of transducers. This is to isolate the directly transmitted, “freefield” sound from reflections due to the surroundings. By using a time window applied to the impulse response, it is
possible to obtain results similar to those obtained in a non-echoic environment.
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ABOUT LISTEN INC

Listen is a global leader in audio measurement and analysis, with over 30 years’ experience developing innovative test methods and algorithms. SoundCheck, our powerful software-based audio analyzer, pairs with our test interfaces, precision microphones, and accessories to offer a complete solution for testing loudspeakers, headphones, microphones, smart devices, communication systems, hearing aids, automotive audio, and more. Trusted by engineers worldwide, Listen delivers the accuracy and flexibility essential for modern audio testing, from the R&D lab to the production line.

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