The engineer will use a Fast Fourier Transform (FFT) analysis that sweeps across all frequencies of sound for this fan against the sound pressure. One can see that big spikes occur for sound pressure at 280 Hz, 560 Hz, 840 Hz, and 960 Hz. Converting the fan speed from RPM to Hertz once again helps the engineer reveal the reasons behind these spikes in sound. 2,400 rpm is converted to 40 Hz, the 1st order frequency for this fan speed.