Steve Temme discusses the evolution of production line Rub & Buzz measurements in this April 2022 issue of AudioXpress. Starting with  simultaneous analysis of higher order harmonics, he explains the progression of improvements including the greater accuracy offered by normalized distortion measurements, and progressing to the introduction of perceptual metrics. He covers the introduction of the first perceptual distortion algorithm introduced in 2011, and the newest enhancements to this which offer the repeatability necessary for successful end-of-line perceptual distortion measurement, where the reduction in false rejects and resulting higher yields add significant value to speaker and headphone manufacturers.

Full Article

The Evolution of Production Line
Rub & Buzz Measurements

Rub & Buzz defects in loudspeakers, headphone drivers, and microspeakers generate an immediate perception of poor quality. This article follows the development of production line Rub & Buzz detection, starting with the conventional methods that, although developed 25 years ago, are still valuable and well-used today, and ending with the latest perceptual algorithms.

Production line detection of loudspeaker Rub & Buzz defects is important because the harsh sounds created by such faults instantly result in customer perception of poor quality. Such defects are almost always due to manufacturing errors, for example, the loudspeaker spider or surround may not be properly glued, the voice coil lead wires could be the wrong length, or the voice coil may be rubbing. If not immediately identified and corrected, these faults cause high volumes of waste and low yield, or worse still, customer returns.

Not all Rub & Buzz faults detected by conventional mechanisms are audible—the masking effects of the human ear and background noises can render some of them inaudible even though they are clearly present when the signal is analyzed. In the quest for higher yields, it’s valid to question whether a speaker with inaudible faults needs to be rejected. This is particularly true for smaller and lower-margin products (e.g., microspeakers used in mobile phones, laptops, and some smart devices). Over the past decade, this desire for lower costs and higher yields has driven the development of accurate perceptual distortion algorithms that reliably identify whether a fault is audible, rather than simply whether it exists.

This article follows the development of production line Rub & Buzz detection, starting with the conventional methods that, although developed 25 years ago, are still valuable and well-used today, and ending with the latest in perceptual analyses. The trade-offs between each are discussed, and the reasons why both conventional (higher-order harmonic) and perceptual distortion measurements provide valuable data points for end-of-line tests are explained. It should be noted that these days, production line Rub & Buzz measurements are equally likely to be used on headphone drivers or microspeakers as they are conventional loudspeakers, and perceptual measurements are rapidly gaining traction in the quest for higher yields of such lower cost products.

What Causes Rub & Buzz
Rub & Buzz defects are almost exclusively introduced in the manufacturing process. The term “Rub & Buzz” refers to the presence of higher-order harmonics, which create a harsh buzzing sound that often becomes particularly obvious at higher playback levels. Such sounds are created when something abruptly impedes the free movement of the speaker mechanism, for example, loose glue joints, vibrating voice coil lead wires hitting another loudspeaker part (e.g., the cone), or an incorrectly centered and rubbing voice coil.