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

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

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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Audio Test Stimuli – An Overview

Stweep Stepped Sine Wave Stimulus

Stweep Stepped Sine Wave Stimulus

Author: Steve Temme

Most audio analysis systems offer a wide range of stimulus option, and selecting the right one can be daunting. This overview explains the different stimulus types, what the waveforms look and sound like (click the speaker icon to hear the sound), the advantages and disadvantages of each and when you would be likely to select each one.

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Headphone Testing – a comprehensive overview

Author: Steve Temme and Brian Fallon

With the headphone market growing towards $10 billion worldwide, and products across the price spectrum from under a dollar up to thousands, there are many and diverse quality expectations and test requirements. Many audio engineers have moved across from the shrinking loudspeaker industry to the burgeoning headphone marketplace. Although many of the characteristics that make for a good in-room listening experience with a loudspeaker – good frequency response, low distortion, no Rub & Buzz or loose particles, etc. – also apply to headphones, and many of the same test principles apply, there are some significant differences and additional issues associated with headphone measurement that need to be taken into account. These include couplers and associated correction curves, acoustic seal, fixturing and additional tests such as L/R tracking. In this paper we outline the issues that are common to testing all types of headphones as well as those specific to particular types of headphones such as Bluetooth and USB headphone testing, noise-cancelling headphones, and Max SPL measurements to prevent hearing loss.

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The Challenges of MP3 Player Testing

Author: Steve Temme

MP3 players have been the ‘must-have’ electronic gadget for the past few years. Over 10 million players were sold in 2005, and this is predicted to rise to more than double this by 2010. But how can manufacturers carry out QA tests on the production line, ensure excellent sound quality and demonstrate their compliance with Sound Pressure Level regulations? MP3 player testing is challenging as it combines traditional audio analysis techniques with some characteristics unique to MP3 players. Here, we examine the equipment and techniques that MP3 player manufacturers can use to test the sound quality of their products, and advise how to overcome some of the challenges inherent to measuring MP3 players.
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HarmonicTrak Algorithm for Fast and Accurate Swept Sine Measurement

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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Are You Shipping Defective Loudspeakers to your Customers?

Author: Steve Temme

Loudspeaker distortion is undesirable. The type and level of distortion, however, can greatly influence the perceived annoyance. In addition, identifying the type of distortion can also help pinpoint the mechanism or mechanisms in the loudspeaker that are causing the distortion. “Rub & Buzz” is a good example of a particularly annoying type of distortion that is very difficult to measure. Pinpointing the cause of the problem from the measurement is an even more difficult task. Understanding why this type of distortion is so annoying and how to measure it is critical in being able to properly test loudspeakers on the production line.
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Evaluation of Loudspeaker Performance at Low Frequencies

Authors: Steve Temme and Christopher J Struck

Evaluation of loudspeaker performance at low frequencies is complicated by long wavelengths, room interaction and cabinet/baffle diffraction. Since low frequency measurements have traditionally required large, impractical testing environments, different techniques have been developed in an attempt to overcome this requirement. Anechoic chambers, outdoor measurements, half-space measurements, ground plane measurements, cepstral liftering, parametric modelling and near field techniques are compared with respect to accuracy, speed, bandwidth and practical implementation.
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Audio Distortion Measurements

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

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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