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Status of Audio Technology • Signal Path’s Impact on Audio • Infinite Baffle • Searchingfor “Good Sound” Meter for Loudspeakers • EOL Testing of Acoustic Devices• ViscoelasticModels • Advanced Distortion Analysis Methods • GrapheneQ • In-Vehicle Audio • VoiceControl Analysis System • Q&A with Industry Leaders • And Much More

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INDUSTRY FEATURESEOL Testing of Acoustic Devices
A Truly Multidisciplinary Task
My job at NTi Audio enables me
to support numerous acoustic
device manufacturing places
worldwide. While many things arehandled individually, there arealso a lot of common procedures
and challenges found in almost everyacoustic Quality Control (QC) environment. This article summarizes
the major considerations for running a single or multiple-line
acoustic QC test that is worthwhile, reliable, and reproducible.
By

Gregor Schmidle
NTi AudioK

eeping up consistent yield and quality ina high-volume acoustic device production
environment requires well-designed endof-line (EOL) manufacturing and test
processes. However, there are many possible
reasons for variations in measurement results.
Within a single production line, variations
can be caused by fluctuations in the qualityof subcomponent material, changes in the
manufacturing and assembly processes, as wellas variance in environmental influences.
To ensure accuracy, the test system mustbe periodically calibrated or characterized in asystem calibration. This is especially importantwhen several production lines of the same typeare operated in parallel, introducing line-to-line
deviations. In many cases, these production lines
are also spread across multiple facilities. Whencommon limits are in use on different productionlines, the mechanical differences between the test
jigs introduce additional deviations in acoustic
behavior. Further reasons for unwanted deviations
are operator handling errors and wear and tear oftest system components (e.g., electrical contacts).
Another important topic is a recovery plan in caseof an IT disaster (e.g., the loss of a hard drive).

32

2017 Loudspeaker Industry Sourcebook

What and How to Measure

Defining appropriate measurement parametersand functions for the EOL test lays out thegroundwork for an efficient test system. There
are two fundamental, yet conflicting requirements
to fulfill:• The test must be as detailed as necessary toreliably test the desired quality.
• The test must be fast enough to ensure nobottleneck in production.
Typical measurement functions (e.g., in a
passive loudspeaker EOL test) are frequency
re sponse, impedance re sponse, harmonic
distortions in various configurations, soundpressure levels (SPLs) at different frequency
bands, resonance frequency, Thiele-Small (T-S)
parameters, speaker polarity, and Rub & Buzz
measurement.
Technically, all those measurements canbe executed by a single glide-sweep stimulus
signal. However, in many test applications it
is necessary to execute some measurement
functions (e.g., frequency response, SPL, and allelectrical measurements) with the nominal level

Loudspeaker Industry Sourcebook. Reprinted by permission. Entire contents copyright ©2017 KCK Media. All rights reserved. For subscription information, go to www.audioxpress.com, call 800-269-6301, or e-mail audioxpress@pcspublink.com.

INDUSTRY FEATURES

A factory worker atEighteen Sound uses NTi
Audio equipment during
the QC stage.

of the tested device, whereas the Rub & Buzz test
is typically executed at a higher level. Besides
considering the measurement function, it is also
necessary to set the test signal levels high enoughfor a proper signal-to-noise ratio (SNR) above
environmental noise, but also low enough to notdamage the test system operator’s hearing. The
latter can be diminished by suppressing the testlevel at higher frequencies to which the humanear is more susceptible.
Fo r l o u d s p e a ke r s, t h e s t a r t a n d s t o p
frequencies of test signals must not only cover
the loudspeaker’s transmission band but also
adequately include the resonance frequency. If
the driver’s DC resistance is determined by an
extrapolation of the impedance response, it isadvised to set the lower frequency far enoughaway from the resonance. The start and stopfrequency of the Rub & Buzz test signal is typically
set to the loudspeaker’s low-frequency range totrigger any electro-mechanical problems.

Defining Good and Bad

There are various methods and strategies todefine limits when testing an acoustic device.
The first major decision is whether to work withabsolute limits or to use one or several reference
samples. The diagram illustrates the decision
process.
The limit finding strategy should be separately
evaluated for each measurement function.
Typical measurement functions suitable forthe application of absolute limits are frequency
and distortion response, as well as all electricalmeasurements such as impedance response andits derivative results. When using absolute limits
for acoustic measurements, it is important toknow the conditions under which the limits arevalid (e.g., free field). If the conditions in the EOL
test are different, the measurement result must
be corrected accordingly. Reference samples areusually used when no specification is available(e.g., for Rub & Buzz).

work with
Absolutelimits

Q1

work with
Reference
sample pool

Q2

Goldensample

work with
Meanvalues

Min/Maxvalues

Q3

work with
Std Dev
headroom% or dB
headroom

Q4

Test Jig Considerations

The ideal test jig would be an anechoic
chamber. However, in real life this is not feasible.
Nonetheless, the test jig has to fulfill certain
characteristics:• Place the DUT in a defined position relative tothe microphone.
• Tightly attach the DUT to the test jig.
• Create a reproducible acoustic situation.
• Avoid acoustic reflections.

Referencesamples

Apply absolutelimits

Apply standard
deviations
headroom onmean values

Select golden
sample via BMA
calculation
process

Apply frequency
dependant % or
dB headroom onmean values

Apply frequency
dependant % or
dB headroom on
min/max values

Apply frequency
dependant % or
dB headroom on
golden sample

This generic limit calculation decision tree can be used to set up your decision
process.

2017 Loudspeaker Industry Sourcebook

33

Loudspeaker Industry Sourcebook. Reprinted by permission. Entire contents copyright ©2017 KCK Media. All rights reserved. For subscription information, go to www.audioxpress.com, call 800-269-6301, or e-mail audioxpress@pcspublink.com.

INDUSTRY FEATURES• Provide defined front and/or back cavities forthe DUT.
• Shield the microphone from environment
noise.
• Shield the microphone from structure-borne
noise.
• Shield the operator from high sound pressure
exposure.
• Allow efficient loading and unloading of theDUT (manual or automated).
• Allow reliable wiring of the DUT, with correct
polarity (manual or automated).
• Allow to quickly change jig for testing
different models.
Furthermore, the test jig itself should not
introduce noise with resonating walls or rattlingparts.
Taking all the points into consideration, thetypical design is a massive, asymmetric woodenbox shielded with acoustical foam in the inside.
Since test jigs usually are not commerciallyavailable off the shelf, most manufacturers build
their own test jigs.
While building one box is a manageable task,
problems often begin when it comes to building
multiple test jigs. It is recommended to accurately
document all measures (e.g., length, width,
distance, etc.), materials, and building processes,
as even minor variations can produce measurable
differences in the acoustic results.

Multiple Lines and Locations

Frequently, there are several production linesof the same type within one factory. Sometimes,
the production lines are in different factories andeven in different parts of the world. Yet, it is stillnecessary to manufacture and test the productin exactly the same way. The products shall betested with the same test parameters, against
the same limits, and, of course, deliver the same
quality and yield.
Most of the components in different EOL test
systems can be:

About the Author

Gregor Schmidle is the Product Manager at NTi Audio in Schaan, Liechtenstein. He
completed a BS degree at the Interstate University of Applied Sciences of Technology
in Buchs, Switzerland in 1992 and a MS degree at the University of Applied Sciencesin Dornbirn, Austria in 2004. Schmidle has been in the audio and acoustic Test
and Measurement industry for more than 20 years. After working several years
at NEUTRIK as a hardware and software developer, he continued his career at NTi
Audio as an applications support engineer. In his current position, Schmidle is incharge of NTi Audio’s industrial key customers worldwide.

34

2017 Loudspeaker Industry Sourcebook• Easily chosen to be identical (audio analyzer
and accessories, cables and contacts)• Parameterized to behave identically (audioamplifier gain and microphone sensitivity)
However, the test jigs also have to be built
as identical as possible, since they directly
influence acoustic behavior. Although there aremathematical corrections available, these increase
the system’s complexity.
When limit calculation is based on reference
samples, it must be considered that the physicalreference samples are only available on one site.
However, the calculated limits are applied on
multiple lines and sites, mostly without accessto the physical reference.

Data Logging and Traceability

Almost every loudspeaker manufacturer,
especially when using a quality management
system, is required to be able to trace back theEOL test results of their products. For highquality loudspeakers this might be for every
single loudspeaker by using a serial number. On
smaller and cost-effective devices, mostly thebatch number is available. This typically allows
tracing back the factory location, manufacturingdate, and production line number.
For the data logging, this means that theEOL test software must be able to log several
complementary data besides the measurement
re sult s, such as date and time, c alendar
week, project name, operator name, serial orbatch number, and environmental data (e.g.,temperature and humidity). Obviously, those log
files can become quite large over time. Therefore,
it is recommended to periodically start new log
files (e.g., every week or at the start of each newproduction batch).

System Calibration and MaintenanceCalibration of the system ensures its accuracy.
In a device calibration, the measurement resultsof the test device are compared against a highly
accurate reference of that result and, if necessary,correc ted. Device c alibrations are usually
performed annually by the device manufacturer.
These are standards-traceable calibrations under
ISO or other quality systems. Typical devices fora calibration in a loudspeaker EOL tester are theaudio analyzer and the measurement microphone.
The sensitivit y of a microphone can becalibrated by the system operator by using
a microphone calibrator. This process can be
executed as part of the system calibration. A

Loudspeaker Industry Sourcebook. Reprinted by permission. Entire contents copyright ©2017 KCK Media. All rights reserved. For subscription information, go to www.audioxpress.com, call 800-269-6301, or e-mail audioxpress@pcspublink.com.

INDUSTRY FEATURES

system calibration is a procedure that is executedby the operator with a DUT connected to the test
system. It consists of:• Calibrating the test voltage at the DUT
terminal, considering the internal or external
amplifier gain setting• Compensating for the electrical frequency
response of said amplifier, thereby ensuringthat the test voltage is set over the entire
tested frequency band
• Calibration of the microphone sensitivity witha microphone calibrator• Consideration of microphone to DUT distance(when calibrated for a certain SPL)

Wear and Tear on Parts

Some parts of an acoustic device EOL test
system are vulnerable to wear and tear, and shouldbe periodically checked or renewed. Parts thatneed to be checked include:
The electrical contacts that connect the DUTto the test system often show wear from friction,
resulting in increased resistance or total loss ofconnection.
The measurement microphone membrane canbe contaminated by dust and dirt, especially when
it is facing upward. This can cause changes in
microphone frequency response and sensitivity.
An easy prevention is to use a dust cap andperiodically exchange it.
The mechanic rest on which the DUT is
positioned and fixed for the test is usually madeof a soft material (e.g., foam or cork). Sometimes,
an arrester system is also in use. Because they aremechanically stressed at the very same position
during every unloading/loading process, thoseparts are in danger of wear from friction or even
deformation. This can result in changes of acoustic
and electrical measurement results.

hard disk failure), a regular backup of system andproject settings is recommended. This task can be
automatically handled by test system software.
Again, a documented procedure on how to restore
the data helps to set up a new system quickly andefficiently, thus reducing downtime.

Hardware Handling

Common problems with hardware components
include accidently unplugged cables, turned knobs,
or toggled on/off switches. It is good practice to:• Use amplifiers integrated into the test
instrument with fixed gain.
• If an external amplifier is inevitable, use amodel with fixed gain or protected volumewheels.
• Secure and stash cables.
• Protect on/off switches against accidental
operation (e.g. rack with door).

Fail-Safe Workflow

The DUT must be loaded in the test jig in aclear and unambiguous way. Both the positioning
and the contact process should provide haptic
feedback to the operator. If the operator isrequired to trigger the measurement, the trigger
button should be conveniently located next to thetest jig, so that it can be pushed immediately after
the DUT has been loaded.
Alternatively, the test system can constantly
scan the contacts and automatically start themeasurement once a connected loudspeaker isdetected. The end of the measurement must be

When Disaster Strikes

To guarantee high availability, it is advisable
to be prepared for disasters. Obviously, it is agood idea to have spare parts of all elements ofa test system, especially for the ones that have ahigh risk of being worn out or damaged. But also,
there should be documented procedures on any
configuration steps that need to be taken in caseof an exchange. For example, when exchanging themicrophone, the sensitivity of the new microphonemust be configured in the test system. This
minimizes the system downtime.
To be prepared for an IT disaster (e.g., a PC or

Don’t panic when disaster strikes. Make sure you have a plan in place.

2017 Loudspeaker Industry Sourcebook

35

Loudspeaker Industry Sourcebook. Reprinted by permission. Entire contents copyright ©2017 KCK Media. All rights reserved. For subscription information, go to www.audioxpress.com, call 800-269-6301, or e-mail audioxpress@pcspublink.com.

INDUSTRY FEATURES

Safeguard your system from environmental influences.

clearly displayed to the operator so that he or she
can unload it immediately. When highly optimized,
the next device in line can already be loaded into
a second baffle, alternating with the first one
that is currently being tested. This increases thethroughput of the test system.

Protect Your Settings

To prevent operator errors, the test system
software should have a user role management
system. While a system administrator is a skilleduser with no restriction, an operator is limited
to have access only to the absolute necessarycontrols. If no configuration at all is required by
the operator, interfaces such as the keyboard andthe mouse can be stashed.

Troubles from Outside

Naturally, environmental influences to an EOL
test should be avoided (see Photo 3). However,in reality this is not always possible. Therefore,
it is important to be aware of the consequences
those influences can have, as well as have ways
to deal with them.
Temperature affects mostly the electricalp aram e t e r s of an a co u s tic d evice. Fo r aloudspeaker, this is especially the resonancef requenc y and it s dependent s. Variationsof temperature can be caused by ambient
temperature, but also by manufacturing processes
that heat up the DUT and are executed shortly
before the end-of-line test. For the latter case,before testing have a cooling phase long enoughto bring the DUT to ambient temperature. It36

2017 Loudspeaker Industry Sourcebook

is advisable to log environmental data (e.g.,temperature, humidity, and air pressure) alongwith the measurement data.
Manufacturing noise obviously influences theacoustic measurements. This includes air-borne
as well as structure-borne noise. While somemeasurements are less susceptible to noise(e.g., frequency response), the most criticalmeasurement is Rub & Buzz of loudspeakers.
There are noise cancellation methods available
in loudspeaker EOL test systems. However, thisshould be the last line of defense because they
only deal with impulsive noise and they increasetest time.
The majorit y of noise should ideally bephysically shielded by the test jig. This is especiallyapplicable for all constant background noise(e.g., manufacturing noise and machinery) Suchshielding reduces the noise floor inside the test jig
enabling you to measure Rub & Buzz effects thatwould otherwise be undetectable. All impulsive
noise that only occurs infrequently can be handled
by the noise cancellation algorithms of the EOL
test system.
Electromagnetic influences are best handledby keeping audio cables short and using balanced
audio (XLR cables). This is especially important forcables with low level and high impedance (e.g.,microphone cables).

Wrap Up

Running a worthwhile and reliable EOL quality
control for acoustic devices is by no means trivial.
It requires considerations from several angles
and involves multidisciplinary skills. Reproducible,
reliable results can only be achieved with acombination of reliable test instruments operating
within an environment that either remains
constant or is properly managed. LIS