# How many $G$'s would a bass drum at 10 feet away impart on a hard drive?

We have a question on superuser about a bass drum possibly affecting spinning disks.

I've been reading some related questions and have the following stats:

• A drive weighs about 430g
• It appears that bass drums have a output of around 20-100Hz though I guess this could be varied a lot by modifications to the drum however we'll stick with this for now.
• The 'rating' for the drive is 30G's of force while running

From another related question it was mentioned that you could work out the 'force' by multiplying the weight of the drive by the force it can undergo giving us around 13.05Kg of force.

Now, I very much doubt a drum can put out anywhere near that much maybe even if the drive was inside the drum however I'm unsure of the methodology I've used, nor if I have all the required information (do I need to know the dB of the drum for example?

I have an answer, but first I want to point out that this is a challenging question for several reasons:

1. For the frequencies of interest we are in the near field, which is commonly defined as distances $d < 2 \lambda = \frac{2 c}{f}$. Even if we consider the main energetic output of the bass drum to be 200 Hz, the far field will begin at $\approx 3.4 m$, or 11 ft. For a lower frequency, it'll be farther. Within the near field the sound pressure and particle velocity are not in phase.

1. Furthermore, because we are indoors and sounds can reflect in the room, we're also in the diffuse field which makes the estimation of the pressure field quite a bit more complex than if things were outside.

2. Even if we could make the near-field and free-field assumptions, the force depends on the physical orientation of the kit and the disk. For example, a plane wavefront perpendicular to the disk's rotation axis would exert a greater force than a wavefront parallel to the rotation axis.

3. The case of the computer or other objects (like a desk, etc) between the drum and the disk will cause reflection and diffraction to occur, changing the pressure that is exerted on the disk.

With all this being said, there is a publically-available paper published in 2017 by Shahrad et al that investigates the possibility of acoustic resonance causing head crash in hard disk drives (where the reader arm scratches the disk.)

The authors found the natural frequency of the hard drive they considered to be ~2.3 - 2.5 kHz, much higher than the frequencies of bass drums. And, significantly, the farthest away they were able to cause damage to the hard drive was at 0.7 m by a sound at 9.1 kHz.

This doesn't actually answer your question about "how many G's" are exerted on a hard drive at 10 ft, but it suggests it's very unlikely that a bass drum would cause damage at this distance.

• That's excellent, thank you. I did read about how the wavelength was around 5o feet meaning that the waves would be rebounding all around the room rather than acting directly on the disk, but wasn't sure how much that matters. – djsmiley2kStaysInside Jan 14 '18 at 12:12
• I asked the original question on superuser, very impressive answer as my math isn't quite as good as my computer. Thanks. I'll let you know if my drives die shortly after drumming. :-) – stu Jan 14 '18 at 16:53