time if such an ambient noise results from a stationary noise source such as a motor or from an electric power source generating a hum however it is difficult to preliminarily estimate the spectrum of the ambient noise therefore the speech signal can not be distinguished from the ambient noise even when a plurality of threshold levels are prepared in relation to various different frequencies to monitor each component at the respective frequencies on the other hand the spectrum of the speech signal is nonstationary at the beginning of each speech and therefore exhibits a transient spectrum thereat such a transient spectrum is conspicuous particularly in fricative consonants the transient spectrum does not appear during continuation of single sounds such as vowels in this case it is possible to distinguish between the ambient noise and the beginning of each speech by monitoring the transient spectrum under the circumstances a variation of the spectrum of the input signal in is successively detected in the form of a variation of electric power relating to the spectrum the variation of electric power may be a difference between electric power derived at two adjacent time instants the difference of electric power varies as illustrated in fig1 c and exhibits a steep variation at the start time instant t s thus the steep variation results from the transient spectrum the spectrum of the input signal in namely the electric power relating to the spectrum can be specified at each time instant by each partial autocorrelation coefficient.