Teschler on Topic
Leland Teschler • Executive Editor
On Twitter @ DW_LeeTeschler
Back in the 1980s, I wrote an article about hard drives as the PC revolution brought them into the hostile environment of factories. They weren’t faring well. Most hard drives of the time were designed to sit in pristine mainframe computer rooms. Putting them into industrial settings typically caused havoc. Ordinary safeguards such as head carriage locks were optional–not all disk drives had them, and the locks on some units could only be actuated manually. Drive makers also had varying ideas about how to safeguard against vibrations. One drive maker I talked to described an elaborate configuration of elastomer spacers and rubber grommets designed to give the disk components a natural resonant frequency well below the range where most severe vibration amplitudes arise.
Shortly after the article published I got a letter from a mechanical engineering professor who had read it. He informed me that much of what the drive maker had said about vibration mitigation was nonsense, and he provided a few equations to prove it.
This episode exposed an unappreciated aspect of the electronics industry back then: In companies dominated by electronics engineers, the mechanical engineering on products often wasn’t first rate.
Of course, the electronics industry has changed since then. And you would think that by now, resonance problems would be simulated and resolved long before hard drive designs got to the prototype stage. So I was surprised to find recently that resonant frequencies can still be a problem on disk drives, though in what you might say is an off-beat way.
The issue I refer to was that playing the music video for Janet Jackson’s “Rhythm Nation” would not only crash certain laptop models, but also crash laptops sitting nearby that weren’t playing the video. It turns out the song contains one of the natural resonant frequencies for the 5,400-rpm laptop drive. The drive manufacturer eventually solved the problem by adding a custom filter in the audio pipeline that detected and removed the offending frequencies during audio playback.
Still, the scenario seems weird: If a frequency in a song causes problems, wouldn’t the same frequency be present in a lot of other songs and do likewise long before now? This conundrum bugged NYC-based bass player and composer Adam Neely enough to explore it in a YouTube video.
Neely’s short answer to the riddle is that “Rhythm Nation” has a bass line that contains the frequency that causes the problem. In music theory, a bass line is the foundation upon which all other melodies and chords are built. It is also the song’s lowest-frequency melody and is usually played on a bass instrument like a bass guitar.
Neely thinks the 2.5-mm laptop hard drives in question exhibit a strong resonant peak at 87.5 Hz. Analyzing “Rhythm Nation” for resonant peaks reveals one at 84.2 Hz. What’s interesting is that “Rhythm Nation” is written in the note of E, 82.4 Hz. Neely also notes that “Rhythm Nation” isn’t tuned as you might expect. Normally, standard tuning is A –440 Hz, meaning the A above middle C on a keyboard will vibrate at 440 Hz. Instead, Rhythm Nation is tuned to roughly A—450 Hz.
The different frequency comes out of an effect called varispeed—-basically a slight speeding up of the recording to make the music sound more exciting. This process also boosts the pitch by a small amount. In the case of “Rhythm Nation” the increase in pitch from varipeeding made the low E match the resonant frequency of the drives.
You might wonder if other songs might have similar resonant peaks. Neely says the closest he’s found is Metallica’s “For Whom the Bell Tolls” with a peak at 84 Hz.
So if your next laptop doesn’t rattle itself to pieces playing pop music, you may have the creators of “Rhythm Nation” to thank for getting manufacturers to pay closer attention to resonances in the disk drives they design. DW
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