Noise testing of hard disks.
Bruel & Kjaer noise and vibration specialists ROGER UPTON and
NIELS-JORGEN JACOBSEN describe how Multichannel Sound
Power for Pulse provides an answer to sound power testing
for disk drives
AIthough 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 constantly pressurising disk drives
manufacturers to make them even quieter.
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 1m, 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.
Single frequencies
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 an FFT analyser, while the
sound power determination requires 1/1- or 1/3-octave analysis.
A further problem is growing production volumes. 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 semi-anechoic 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 is needed.
The traditional way of testing sound power is to use 9 or 10 microphone
positions and multiplex these into a single-channel analyser, meaning
that only one microphone can be used at a time.
With 10 microphones and 30s measurement time per channel (to
measure down to 100Hz), this requires a minimum of five minutes to
make a complete test. This does not include the time required for any
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 are included, such
as mounting and dismounting the drive and reporting results, it is
easy to see how testing a single drive can take 10 minutes or longer.
Enter Pulse and Multichannel Sound Power software. 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. With the new multi-channel and multi-analysis capability,
users can, for the first time, perform measurements in parallel rather
than serially.
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 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.
Pulse is PC-based, runs on Windows NT, is tightly integrated into
the Microsoft Office suite and is OLE 2.0 compatible.
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For more information contact:-
Bruel & Kjaer
Tel: +44(0) 1438 739000
E-mail: info@bkgb.co.uk
Website: www.bksv.com
October 2001
