Why a song by Janet Jackson could crash hard drives

Singing wine glasses to pieces is not that easy, as various videos on YouTube show. Only a few singers, mostly men with a very powerful voice, have managed this so far.

As far as I know, Janet Jackson was not one of them, but she did manage another feat.

A manufacturer of notebooks had discovered that the song "Rhythm Nation" by Janet Jackson could crash the hard drives of laptops.

The search for the cause must have given those responsible sleepless nights at the time, until it turned out that the song contains a natural excitation frequency that becomes a problem for hard drives running at 5,400 rpm. It was interesting that the problem was only observed in laptops, while desktops with the same hard disks had no problem. At the time, the manufacturer didn't know what else to do but install a filter to the audio line that filters out the critical frequency.

So far so good. From their engineering studies, most people know the video of the collapse of the Tacoma-Narrows Bridge in 1940, which was attributed to vortex-excited resonant vibrations. The whole thing is not really comprehensible to the layman either. But even the experts are no longer quite sure, as can be read in Physics Today (https://physicstoday.scitation.org/doi/10.1063/PT.3.2991).

Therefore, Janet's song with the subsequent hard disk crash is the simpler problem from the eyes of a computational engineer. After all, in addition to the structural dynamics, he does not have to deal with flow effects and detachment vortices that influence each other. In technical jargon, this is called a strong fluid-structure interaction (FSI). Not so easy, even with simulation.

I prefer the path from the simple to the complex model. If every step is understood, you can make everything as complex as you like. Unfortunately, this often fails with the basic understanding of what a natural frequency or resonance is. At least this is shown by the basic questions we at Merkle CAE Solutions GmbH ask potential employees in the job interview.

But back to the actual topic. The hard disk crash is therefore a structural-mechanical problem that can be calculated using FEM alone.

To the basics. What is a natural frequency? If a body, be it a piano string, a gearbox housing or a hard disk is struck with a hammer (although the hard disk usually does not survive this), it vibrates at a certain frequency.

This is usually the first natural frequency, and the unit is oscillation per second. The natural frequency can be inferred directly from the acoustic sound produced. Once struck, a body could vibrate at this frequency for any length of time if there were no friction.

At a natural frequency, kinetic energy changes into tension energy and then back into kinetic energy, and so on. This is like a child's swing that is pushed. It (the body, not the child!) gets a package of energy with it, so to speak, through the hammer blow.

Since energy is never lost, but only converted into other forms of energy, you can imagine friction or damping in such a way that a small portion of the energy packet is converted into heat with each oscillation. That is why a piano string does not vibrate indefinitely, but gets quieter and quieter. It slowly loses energy after being struck until the body has stopped vibrating.

A natural frequency is therefore a typical property of an elastic body. The softer and the heavier the body, the lower the first natural frequency. The forms of vibration are called natural modes.

However, every body has not only one natural frequency, but an infinite number of them. Although almost all of them (at least a finite number) can be calculated, the smaller natural frequencies usually play the most important role.

If a body is now bombarded by energy packets at exactly one natural frequency, it pumps itself full of energy more and more, the oscillations become greater and greater until the deformations and stresses are so high that the material fails.

Like our children's swing, which always gets a bump at the highest point. At some point the child flies off the swing, which of course can't happen with our safety-fanatic helicopter parents. Who is already afraid of Winnetou...!

If the energy packets supplied are somewhat larger, the small friction packets that take energy away play less and less of a role.

Let's look at what happens with the hard disk. A laptop is quite stiff compared to a desktop computer. Vibrations of the loudspeaker and the case are therefore transmitted directly to the hard disk. So larger energy packets arrive than with a softer desktop.

So when the hard disk crashes, there must be vibrations that are higher than the gap between the read-write arm and the magnetic disk. The gap, by the way, is much smaller than the thickness of a human hair.

This results in contact of the arm on the surface, which is not good for it.

So what does the speed have to do with it? The magnetic disks of a hard disk are quite thin. The centrifugal force increases the natural frequencies.

If a natural frequency were to occur at the rotational frequency of 5,400 rpm, which corresponds to 90 Hz, the hard disk would quickly be ruined.

How does a calculation engineer, for example at Merkle CAE Solutions, go about finding the damage? He determines the natural frequencies of the hard disk in the rotating state at 5,400 rpm. Then he analyses the excitation frequencies of the song and compares them with the natural frequencies of the spinning hard disk including all components. The cause of the disk crash is where a direct excitation of either the magnetic disk or the arm is found.

But how can the system be optimised?

So let's assume that Janet or other artists don't want to be told what the rhythm of their songs should be. So you have to change something about the hard drive or the natural frequency.

We are talking about primary and secondary measures.

The primary measure gets to the root of the problem: shifting the natural frequency. How? By changing the stiffness or the mass or the speed.

The secondary measure tries to make the excitation somewhat smaller. An acoustic filter dampens the critical excitation frequency, thus making the incoming energy packets smaller. But will the song sound the same then? That was the solution chosen to get to grips with the problem at the time. Probably also the cheapest. Change the construction because of one song and then there are problems with another song ...?

It is always best to know the natural frequencies and excitation frequencies of a device, machine or building at the design stage and make sure they are sufficiently far apart.

How? That's what Merkle CAE Solutions GmbH is for!

And what if the baby has already fallen into the well or off the swing? Then the engineers at Merkle CAE Solutions will tell you what measures you can take to "repair" damage or avoid it in the future. Just between us: These are, for example, machine halls where the floor vibrates, dryers where shafts break or compressors that are too loud or vibrate too much.

PS: If you want to listen to the song "Rythm Nation": https://www.youtube.com/watch?v=OAwaNWGLM0c
PPS: My audio spectrum analyser app on my iPhone tells me that the peak frequency of "Rythm Nation" is 198 Hz. There's something about walking past machines with this app that have vibration problems 😊
PPPS:My computer and hard drives survived it. It's not a laptop 😉