Magnetics Business & Technology - January/February 2022 - 20

PRODUCT NEWS
Audio Precision Raises the Bar for Detecting Rub & Buzz Defects
the level of distortion in a typical speaker is far greater, and more
diverse, than in a typical electronic device and can mask these
defects. This latter point means the standard audio measurements
of frequency response and distortion will only detect the
most severe cases of rub & buzz.
If magnet material chips loose during the manufacture of speakers
it can cause what audio designers call a " buzz defect " . This
can be an onerous problem to detect but new software from
instrument maker Audio Precision raises the bar for doing so.
The improvement comes in the latest update to the company's
APx500 audio measurement software, version 6.1, for production
test of loudspeakers, headphone drivers, and microphones. Most
importantly for designers and manufacturers of speaker drivers,
and products incorporating them, the software now covers all the
standard techniques for detecting " rub and buzz " defects.
" Simply put, APx audio measurement software now offers the
broadest set of rub & buzz defect detection methodologies available, "
said Daniel Knighten, general manager of Audio Precision.
The term is commonly used to describe any one of several
possible manufacturing defects that cause undesirable noises to
be emitted during speaker operation. Rub-related noises occur
when the speaker cone is off-center or canted and mechanically
rubs when it moves to create sound. Buzz defects occur when
loose particulate matter such as chipped magnet material or bits
of glue are trapped in the gap between the speaker's voice coil
and the cone.
As an audible defect in a speaker driver, or device incorporating
a speaker, rub & buzz presents several challenges to device
manufacturers, the company explains. First off, it is a subjective
issue, with little broad-based agreement on how much is too
much - the noises created by a specific defect can be perceived
very differently by different listeners. Compounding the issue is
So, with rub & buzz defects, you have imperfections in a speaker
that can negatively impact a listener's perception of the speaker's
quality, but those defects are not necessarily detectable by
traditional audio measurements. To address this issue, several
different rub & buzz detection methodologies have been developed
within the industry: various approaches using high-pass
tracking filters, distortion-oriented methods that focus only on
high-order harmonics, and techniques applying human hearingbased
algorithms to the distortion signal. While all these methods
are applicable to rub & buzz detection, no single method has
achieved uniform adoption in the industry.
Rub & Buzz - Previously available in APx software, this
method uses high-pass tracking filters to remove the fundamental
signal and detect defects as excursions in the residual
*
waveform's crest factor or peak ratio.
*
SoneTrac (new with v6.1) - A Bose-developed improvement
to Rub & Buzz, this method filters the residual signal and
ratios it to the RMS of the total signal to reduce the noisiness of
the measurement result.
*
High-Order Harmonic Distortion (HOHD; new with v6.1)
- A classic method for rub & buzz detection, HOHD uses the THD
(Total Harmonic Distortion) ratio but only of harmonics above the
10th, 10-35, 20-200, etc., which is a simple way to account for
frequency masking effects.
*
Rub & Buzz Loudness (new with v6.1) - Applies a
psycho-acoustic loudness model to the residual signal to calculate
the perceived level of the rub and buzz based on well-established
models of the human perception of sound.
Audio Precision is a brand of electronic audio and electroacoustic
test instrumentation made by Axiometrix Solutions,
based in Beaverton, Oregon. Other major brands of the company
are GRAS Sound & Vibration and imc Test & Measurement. For
more info, see www.ap.com.
All the rub and buzz detection methods: Rub & Buzz (upper left), SoneTrac (upper right), High-Order Harmonic Distortion (lower left), and Rub
& Buzz Loudness (lower right)
20 Magnetics Business & Technology * January/February 2022
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