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. _________________________________________________________ For more information contact:- Bruel & Kjaer Tel: +44(0) 1438 739000 E-mail: Website: October 2001

Home - Search - Suppliers - Links - New Products - Catalogues - Magazines
Problem Page - Applications - How they work - Tech Tips - Training - Events