I recently sat down in the car, parked in the 4th level of a car park, comfortably listening to the latest episode of Logbuch:Netzpolitik while all of a sudden I noticed a strange and unexpected motion of my surrounding. My seat was shaking! No noise attached, nobody jiggling on the trunk of the car and as quickly vanished as it appeared.

It took me a handful of occurrences after that first disturbing one to realize that the appearance of this phenomenon was correlated with people passing by in front of me.

Fascinating, the footsteps of humans walking along seem to set the whole segment of the parking deck into oscillation.

Fortunately, I had parked close to the ticket machine so there where more observations to be expected. So I mounted my mobile phone firmly to the car and started the phybox app - a pretty awesome mobile application to perform physics experiments on the go developed at RWTH Aachen - and decided to have a look at the acceleration sensor which phybox reported to be a BOSCH BMI160 Accelerometer (Datasheet)

phybox measurement

My phone was mounted upright (display vertical) and the apparently interesting dimension of the accelerometer goes along the y-axis.

Can you spot how many people passed by during the measurement?

Besides that, there a few interesting things to note here:

  1. All data shown here is entirely uncalibrated, which also explains why there is a systematic baseline offset in $a_y$ of about -0.04 m/s^2

    (When choosing phybox's "Acceleration without g" it tries to substract earths gravity from the measurement)

  2. The acceleration is almost entirely oriented along the y-axis (vertical), while all other projections show mostly noise

  3. Even without Fourier analysis one can "see" that the main frequency at which the floor of the parking deck starts to oscillate is about 4 Hz

While all these observations are of course on a very basic qualitative level at best, I still find it pretty neat that phybox enables me to "just pull out my phone and get some measurement done". Modern mobile phones are equiped with plenty of sensor devices... why not make use of them to have some fun from time to time?