1/min, as revolutions per minute, or as in rpm. The y axis shows the sound pressure p (sound pressure amplitude). If the graph shows at the x axis the time t , we see the period T = 1 / f .
Strobo RPM Hz Light Free - Free, Faster, Lighter, Cheaper. Can works with RPM and Hz. Range: 0.1 Hz to 80 Hz. ( the maximum depends on the mobile, processor, memory, cam light, etc.) This app can't substitute precision devices or machines.