As shown previously, the spikes in sound occur at 280 Hz, 560 Hz, 840 Hz, and 960 Hz. If the engineer looks at the first three frequencies, they are all wholly divisible by the number of blades on the impeller - seven. Therefore, the 280 Hz, 560 Hz, and 840 Hz frequency spikes correspond to the 7th, 14th, and 21st orders of sound pressure. However, the 960 Hz spike is not wholly divisible by seven. One can surmise that this spike is instead caused by rotor cogging. The frequency of rotor cogging is derived from the least common multiple of the number of stator slots and the number of magnet poles. For this fan model, the least common multiple of the number of stator slots and the number of magnet poles is twenty-four, so rotor cogging occurs twenty-four times per revolution. The 960 Hz frequency is wholly divisible by twenty-four.