100 Things #21: Normalized Rub & Buzz Measurements

In this short video, Steve Temme explains how SoundCheck’s unique normalized Rub & Buzz measurement method offers greater accuracy, high speed, and excellent noise immunity for production line testing. Additionally, analysis of each harmonic separately yields valuable information to help identify the exact production line fault.

Normalized Rub & Buzz Measurements

Learn more about normalized rub & buzz measurements

This technical note on Harmonic Distortion Measurement, authored by Listen president Steve Temme, explains the relationships between sampling rate, stimulus frequency and measured harmonics for both normalized and conventional harmonic distortion measurements.

Video Script:

Higher order harmonic distortion is an important production line measurement for identifying Rub & Buzz defects introduced in the manufacturing process. SoundCheck uses unique algorithms for measuring Rub & Buzz distortion and  ‘Normalized Rub & Buzz.’  Let’s look at how these differ from conventional methods.

There are 2 commonly used methods for measuring Rub & Buzz. The ‘tracking high pass filter’ method uses a high pass filter to remove the lower harmonics and sums the remainder. This combines all the harmonics plus transient distortion caused by loose particles into a single metric. While this gives an overall indication of distortion level, it reveals nothing about the cause of the distortion. It is also susceptible to background noise, which can result in false rejects on the production line.

A more accurate, but slower method, sequentially measures individual harmonics using a tracking filter that moves from one harmonic to  the next. While this provides accurate and detailed information on each harmonic, it is not fast enough to use on automated production lines.

SoundCheck’s unique Rub & Buzz distortion methods, which date back to the mid 1990s,  offer more accurate measurements, high speed and excellent noise immunity for production line testing. Furthermore, they analyze each harmonic separately which provides a wealth of additional information to help identify the exact production line fault

Let’s take a look at the unique features that make up the SoundCheck algorithms. 

Firstly, our proprietary Harmonictrak algorithm applies advanced filtering techniques to exclude the noise between the harmonics. All the harmonics are measured discretely and simultaneously. This is extremely fast compared to other methods, and also highly immune to background noise. Furthermore, because it measures each harmonic separately, it provides valuable information about the precise reason that the device failed the test. For example, high levels of harmonic distortion in the 10-15th order range usually indicate a rubbing voice coil, whereas distortion at the 50th harmonic and above is frequently caused by vibrating voice coil lead wires hitting the cone. 

SoundCheck also offers an enhanced version of this called Normalized Rub & Buzz. In this version, we compare the harmonic levels to the fundamental level at their measured frequency before their ratio is plotted, rather than the fundamental level at the excitation frequency. This is more accurate, because it removes the effect of the non-flat frequency response from the distortion. This makes it easier to see the peaks in the distortion response independent of the peaks and dips in the fundamental response, and easier to set limits.

Both these measurement options can use a stepped sine wave test stimulus, so they can be measured simultaneously with other end-of-line parameters such as frequency response, THD and even perceptual Rub & Buzz. While the main objective of Rub & Buzz detection is to identify faulty-sounding speakers, the individual harmonic analysis offered by SoundCheck’s algorithms makes them also valuable for continually monitoring production line performance for any drift in characteristics that could lead to product failure with customers.

Although these methods are now 25 years old, they still out-perform all other methods for measuring higher order harmonic distortion on the production line. More details on these unique algorithms can be found in the published papers section of Listen’s website.