We studied it about ways to detect damage of a tire about randomness ultrasonic signal which occurs repeatedly while rub a tire of driving car and a road surface. The signal randomness is decreased through the preprocess of short-time energy calculation and the average value of coherence function is used by the normalization expression of the signal randomness. The process limit that can be decide on the dominant period of a signal using the coherence threshold is analyzed and the algorithm to decide the dominant period is proposed by setting up the -3dB threshold of the maximum value on the power spectrum.