Bruel & Kjaer - Endevco - PRODUCT NEWS


 
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. Multi-analysis needs 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 multi-analysis capabilities." 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 cost-effective solution. _________________________________________________ More information from: Bruel & Kjaer, Bedford House, Rutherford Close, Stevenage, Hertfordshire, SG1 2ND. UK Tel: +44(0) 1438 739000, Fax: +44(0) 1438 739099 E-mail: info@bkgb.co.uk Web site: www.bksv.com February 2001
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