SPCR's Hard Drive Testing Methodology

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SPCR HDD TESTING GOALS

There are literally dozens of computer hardware review web sites that run performance benchmarks on hard drives. Some of them do a very credible job. Few measure noise, and even fewer have the expertise to fully profile the noise produced by a hard drive. At SPCR, we are interested in testing how a hard drive sounds; its performance is of secondary concern.

Our basic point of view is that while there are perceivable performance differences between similarly rated drives, they generally tend to be subtle and small, with a 10% difference being considered huge. Noise differences, however, are far more dramatic and obvious. In both measurements and subjective listening, the noise difference between two otherwise similar HDDs can be as large as 100% (10 dBA). This can mean the difference between a computer that's pleasant to work around and one that's difficult to tolerate for any length of time.

Our methodology for testing hard drives highlights acoustics. Our benchmarking tools are hardware-based — an SPL meter, for example — rather than software applications. The metrics by which we compare drives are airborne and vibration noise, rather than throughput and latency.

OUR HDD TEST METHODOLOGY

All acoustic testing is performed in two states: Idle and Seek. If the drive being tested supports AAM (most do), separate seek tests are run with AAM enabled and disabled. These two (or three) possible activity states are reproduced twice: Once for airborne acoustics and once for vibration noise.


SPCR's hard drive test bench.

For all noise measurements, the hard drive is attached to one of our open bench test systems. The system employs a quiet notebook drive in a Smart Drive enclosure for minimal noise, and any fans are stopped temporarily during testing. Extreme head actuator movement (seek/write) noise is produced by using the AAM TEST feature of Hitachi's HDD Feature Tool. The resulting noise is about the worst a HDD will ever sound. (NOTE: Hitachi's downloadable Feature Tool (v1.97) utility works with most modern hard drives, including SATA interface drives. It contains an option to enable the Automatic Acoustic Management feature that reduces the seek noise in exchange for a small decrease in seek performance.)

If the Hitachi Feature Tool does not work with a particular drive, then seek noise is created by defragmenting the drive (if the drive has previously been used), or by copying a large chunk of data to the drive and copying it repeatedly within the disc. This operation ensures maximum seek noise by forcing the drive to read and write different parts of the disc simultaneously.

Assessing Acoustic Noise

Each drive is measured for SPL one meter away from the top of the hard drive. Hard drive noise tends to be directional, the loudest position being directly over the top. SPL readings typically drop by 2~3 dBA/1m when measured from the side of the drive. The drive is placed on a soft foam to ensure that no vibration noise is produced during testing.

SPCR'S SOUND LEVEL METER

The B&K model 2203 sound level meter is a professional caliber SLM that dates back ~20 years, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans over 140 dBA. The unit's absolute sensitivity reaches below 0 dBA, although its real limit may be the internal noise of the mic and electronics, which are said to be at ~15 dB. A quiet environment is a prerequisite to low noise testing; the lab has been measured down to ~17 dBA at night, and a 14 dBA adjacent room is also available for any HDDs that are quieter.

Audio recordings of the direct acoustic noise are made using SPCR's high resolution digital recording system. All recordings are made with the microphone centered 7.5 cm (3") above the top panel of the drive. The HDD itself is placed on soft damping foam atop a tall stool. This setup ensures that only the direct airborne noise from the drive is captured in the recording.


Audio recordings are made with the omnidirectional mic 7.5 cm above the top center of the drive.

If you wonder why the recording microphone is positioned just 3" away when the SPL readings are taken a meter away, it is because some of the drives emit too little noise for it to be audible over the background noise of the recording equipment itself. The 3" recording distance has been the same for every sound recording posted in MP3 format at SPCR, so it is a reasonable de facto reference.

In addition to measuring and recording the acoustic noise, we also evaluate how a drive sounds subjectively. Even though subjective impressions cannot be reduced to simple numbers, we consider this evaluation the most important part of a hard drive review. Measuring the volume of a drive from a particular angle and distance cannot capture the distinct quality of noise that it produces. Often, the MP3 recordings can give a rough impression of how a drive sounds, but even these do not fully capture the complete noise quality of a drive.

Some examples of things we might comment on include differences in noise when listened to from a different angle, head reset noise (typically a "chirping" sound that occurs every few minutes), or harmonics that are especially pronounced. We may also comment if there is an especially large difference between seek and idle noise, or if AAM is especially effective or ineffective for a particular drive.



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