An examination method comprises: generating a received audio signal according to sound generated from rotation of the fan by an audio receiving device; performing Fourier transform on the received audio signal to obtain a reference frequency domain signal; recognizing a plurality of characteristic bands according to the reference frequency domain signal; adjusting the received audio signal according to the characteristic bands respectively to form a plurality of casting audio signals corresponding to frequency components in the characteristic bands respectively; playing the casting signals sequentially by an audio casting device; and recognizing at least one key band among the characteristic bands according to the transmission rate of the hard drive upon playing the casting signals.