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Sound Power Testing of Hard Disk Assemblies.
Although most modern hard disk drives produce noise well
within the limits established by workplace noise regulations,
the large computer producers who buy the drives are
continuously pressuring disk drive manufacturers to make
them even quieter to meet the demands of discerning customers.
Here, Roger Upton and Niels- Jørgen Jacobsen, noise and vibration
specialists from Bruel & Kjaer, describe how Multichannel Sound Power
for PULSE provides the answer to disk drive sound power testing needs.
The problems - background noise
Disk drive manufacturers have special problems when testing their drive
assemblies. One of these problems is background noise. A typical 2.5in
drive has a sound power level of about 40dB. In a typical sound power
test where the source is measured from a distance of one metre, the
sound pressure level is around 30dB. Since background noise must be
at least 6dB and ideally more than 10dB below the noise of the source,
the only practical place to make such a test is in an anechoic or
semi-anechoic chamber. With the next generation of drives, manufacturers
are talking of typical sound power levels of 30dB instead of 40dB.
Pure tone determination is another problem area. Pure tones are single
frequencies that dominate a sound field and are often irritating to the
listener. The standards for noise measurements on office machines
include testing for pure tones. Since the noise from an idling disk drive
assembly usually contains such tones, this requirement has become
the responsibility of the drive manufacturers. Unfortunately, this
measurement has to be performed using a FFT analyser while the
sound power determination requires 1/1- or 1/3-octave analysis.
A further problem is growing production volume. Although noise testing
only takes place on a sample basis, ever increasing production volumes
put additional pressure on already stretched noise testing resources
if the same percentage of drives is to be tested. Of course, this could
be addressed by adding extra noise testing facilities but building and
equipping a good semianechoic chamber is an investment of hundreds
of thousands of dollars, unwelcome in an increasingly competitive
market with margins constantly under pressure. Clearly, an alternative
method needs to be found.
Testing the old way
The traditional way of testing sound power is to use 9 or 10 microphone
positions and multiplex these into a single-channel analyser, meaning
only one microphone can be used at a time. With 10 microphones and
30 seconds measurement time per channel (to measure down to
100Hz), this requires a minimum of 5 minutes to make a complete test.
This does not include the time required for re-measurement due, for
example, to background noise problems. More time is consumed
making the pure tone determination at one or more microphone
positions. If other operations such as mounting and dismounting the
drive, reporting results etc. are included, it is easy to see how testing
a single drive can take 10 minutes or longer.
The solution - PULSE Sound Power
Enter PULSE and Multichannel Sound Power software. As Niels- Jorgen
Jacobsen, noise and vibration specialist, puts it: "There are a lot of
products in the market for making sound power measurements, all
of them using some variation of the traditional way that consumes so
much time. But, now that we've brought this software on board with
the PULSE system, users can, for the first time, perform measurements
in parallel rather than serially due to PULSE's multichannel and
He continued, "This means PULSE can carry out 1/3-octave analysis
for sound pressure measurements and FFT analysis for pure tone
determination at the same time. PULSE can be configured with up
to 32 channels, so a typical sound power measurement requiring
10 microphones on the measurement surface and two operator
and bystander positions can easily be configured to measure up
to 20kHz in a single pass. Of course, if the PULSE system only
has four channels and one DSP board, the software can easily
be set up to make multiple passes for the same measurement.
And the ability to make batches of measurements during one
session means users can really assembly-line their measurements
and raise productivity."
Needless to say, the disk drive industry has already shown considerable
interest in PULSE and Multichannel Sound Power software. The startling
increase in productivity that PULSE brings, added to the fact it is
PC-based, runs on Windows NT®, is tightly integrated into the Microsoft®
Office suite and is OLE 2.0 compatible, make it an exceptionally
More information from:
Bruel & Kjaer, Bedford House, Rutherford Close,
Stevenage, Hertfordshire, SG1 2ND. UK
Tel: +44(0) 1438 739000, Fax: +44(0) 1438 739099
Web site: www.bksv.com