Advances in Impedance Measurement of Loudspeakers and Headphones
Impedance measurement is often the sole electrical measurement in a battery of QC tests on loudspeakers and headphones. Two test methods are commonly used, single channel and dual channel. Dual Channel measurement offers greater accuracy as both the voltage across the speaker (or headphone) and the reference resistor are measured to calculate the impedance. Single Channel measurement methods are more commonly used on the production line because they only require one channel of a stereo soundcard, which leaves the other free for simultaneous acoustic
tests. They are less accurate, however, due to the test methods making assumptions of constant voltage or constant current. In this paper we discuss a novel electrical circuit that offers similar impedance measurement accuracy compared to complex dual channel measurement methods but using just one channel. This is expected to become popular for high throughput production line measurements where only one channel is available as the second channel of the typical soundcard is being used for simultaneous acoustic tests.
Authors: Steve Temme and Tony Scott
Presented at the 135th AES Conference, New York 2013
A perceptual method is proposed for measuring harmonic distortion audibility. This method is similar to the CLEAR (Cepstral Loudness Enhanced Algorithm for Rub & buzz) algorithm previously proposed by the authors as a means of detecting audible Rub & Buzz which is an extreme type of distortion[1,2]. Both methods are based on the Perceptual Evaluation of Audio Quality (PEAQ) standard[3]. In the present work, in order to estimate the audibility of regular harmonic distortion, additional psychoacoustic variables are added to the CLEAR algorithm. These variables are then combined using an artificial neural network approach to derive a metric that is indicative of the overall audible harmonic distortion. Experimental results on headphones are presented to justify the accuracy of the model.
In a previous paper [1], we demonstrated how an auditory perceptual model based on an ITU standard can be used to detect audible Rub & Buzz in loudspeakers using a single tone stimulus. In this paper, we discuss a practical implementation using a stepped sine sweep stimulus and present detailed experimental results on loudspeakers including comparison to human listeners and other perceptual methods.
Manufacturing defects in loudspeaker production can often be identified by an increase in Rub & Buzz distortion. This type of distortion is quite noticeable because it contributes an edgy sound to the reproduction and is annoying because it often sounds separate or disembodied from the fundamental signal. The annoyance of Rub & Buzz distortion is tied intimately to human perception of sound and psychoacoustics. To properly implement automated production-line testing of loudspeaker Rub & Buzz defects, one has to model or imitate the hearing process using a sufficiently accurate perceptual model. This paper describes the results of a Rub & Buzz detection system using a simplified perceptual model based on human masking thresholds that yields excellent results.
Time-Frequency analysis has been in use for more than 20 years and many different Time-Frequency distributions have been developed. Four in particular, Short Time Fourier Transform, Cumulative Spectral Decay, Wavelet and Wigner-Ville have gained popularity and firmly established themselves as useful measurement tools. This paper compares these four popular transforms, explains their trade-offs and discusses how to apply them to analyzing audio devices. Practical examples of loudspeaker impulse responses, loose particles, and Rub & Buzz defects are given as well as a demonstration of their application to common problems with digital/analog audio devices such as Bluetooth headsets, MP3 players and VoIP telephones.
MP3 player audio performance is discussed including measurements of frequency response, phase response, crosstalk, distortion, sampling rate errors, jitter, and maximum sound pressure level with headphones. In order to make these measurements, several measurement techniques and algorithms are presented to overcome some of the challenges of testing MP3 players. We discuss test equipment requirements, selection of test signals and the effects of the encoding on these test signals. A new method for measuring non-coherent distortion using any test signal including music is also presented.
A new approach for measuring distortion based on dual-channel analysis of non-coherence between a stimulus and response is presented. This method is easy to implement, provides a continuous distortion curve against frequency, and can be used with a multitone stimulus, noise, or even music. Multitone is a desirable test signal for fast frequency response measurements and also for assessing system nonlinearities. However, conventional single-channel multitone measurements are challenging because the number of intermodulation tones grows rapidly with the number of stimulus tones and makes it extremely difficult to separate harmonics from intermodulation products. By using dual-channel measurement techniques, only well-known, standard signal processing techniques are used, resulting in simplicity, accuracy and repeatability.
Loudspeaker assembly faults, such as a rubbing voice coil, bent frame, loose spider, etc., have traditionally been detected using experienced human listeners at the end of a production line. Previous attempts to develop production measurement systems for on-line testing typically analyze only low-order harmonics for the primary purpose of measuring total harmonic distortion (THD), and thus are not specifically designed to detect defective rub, buzz, and ticking sounds. This paper describes a new method wherein the total energy of high-order harmonics groups, for example, 10th through the 20th or 31st through the 40th, are measured and analyzed. By grouping high-order harmonics and resolving their respective total energies, distinct signatures can be obtained that correlate to the root cause of audible rub and buzz distortions (Temme, 2000). The paper discusses loudspeakers tested with specific defects, as well as results of a computer-based electroacoustic measurement and analysis system used for detection.
During loudspeaker production, particles may become trapped in the loudspeaker motor and voice coil vicinity, resulting in a distinctive defect that is easily heard, but difficult to detect by traditional test and measurements. We found that a Sine Sweep Stimulus followed by a High Pass Filter and RMS Envelope Analysis efficiently detected Loose Particles and Rub & Buzz defects. The remaining problem is how to reduce the effect of background noise, and get more reliable results. Statistical descriptors such as Crest Factor, Skewness, and Kurtosis are first investigated. Experimental results are given and the different tools are compared. New enhancements are described that increase effectively the overall immunity to background noise and discrimination of the method.