Transient distortion, or ‘loose particle’ measurement, is an important loudspeaker production line quality control metric that identifies and facilitates troubleshooting of manufacturing issues.
This paper introduces a new enhanced loose particle measurement technique that discriminates more accurately and reliably than current methods. This new method introduces ‘prominence’ after envelope detection, a new metric for audio measurements, that effectively isolates transient distortion in the presence of periodic distortion. This technique also offers the unique ability to listen to the isolated transient distortion waveform which makes it easier to set limits based on audibility and has widespread applications.
Authors: Steve Temme, Rahul Shakya and Jayant Datta, Listen, Inc.
Presented at 155th AES Conference (October 2023) New York, NY
Transient distortion, or ‘loose particle’ measurement, is a valuable quality control metric because it identifies non-periodic distortion, for example, rattling parts, separately from periodic distortion such as rubbing or buzzing parts. This facilitates troubleshooting of manufacturing issues. This paper introduces a new transient distortion measurement technique that is more accurate and reliable than current methods. In addition to improved performance, this new algorithm also aids understanding of the correlation between measurement results and audibility, since it is possible to isolate and listen to just the transient distortion artifacts. Although this analysis method was developed for measuring loose particles in loudspeaker drivers, it is also valuable for measuring rattling parts such as buttons, fasteners, and loose wires on various audio devices, and measuring impulsive distortion or Buzz, Squeak and Rattle (BSR) in automotive audio applications .
What is Transient Distortion? Why does it matter?
Transient distortion is caused by random clicking, popping, and other noises in the time domain. In a speaker or headphone driver, this might be caused by foreign particles such as glue or magnet fragments trapped in the gap behind the diaphragm or dust cap. In a device such as a smart speaker, transient distortion might come from a loose volume control button on the device that rattles when sound is played. In an automotive application, it could be characterized as buzz, squeak and rattle from loose wires, screws or fasteners in a car door that the loudspeaker is mounted in. In all cases, the sound is undesirable, so devices that exhibit such faults should be identified and rejected.
In the recorded time waveform, transient distortion faults appear as impulsive noises added on the stimulus wave. These impulses are not related to the frequency of the stimulus, but rather to the vibration caused by the displacement amplitude of the diaphragm. The transient distortion is more frequent and significant when the speaker is driven near or below its resonant frequency, where the displacement of the diaphragm is the greatest.
Although the sound – a random clicking, buzzing or popping noise – can sometimes sound similar to higher order harmonic distortion (Rub & Buzz), such defects are not clearly reflected in the frequency spectrum of the waveform. Figure 1 shows a waveform with transient distortion, and the corresponding frequency spectrum. The vertical black line represents the stimulus frequency and the orange broadband noise spectrum indicates the transient distortion. Transient distortion is best identified at the time the transients occur, unlike Rub & Buzz distortion which is best identified by the frequency at which it occurs .
The entire paper also covers:
Prior Measurement Methods
The new algorithm – comparison and results