Abstract: In a method of detecting the presence of speech in a telephone signal, a signal sequence is interpreted as constituting speech if its mean power level exceeds a threshold, the value of which is decreased as the number of passages through zero level in the sequence approaches 1.25 per millisecond (0.25 Hz). A speech detector implementing this method receives pulse code modulated signal samples on its input. It comprises a squaring circuit driving a digital accumulator whose output signal is proportional to the mean power level of each of consecutive 4 millisecond signal sequences. A change of sign detector and a second digital accumulator indicate the number of passages through zero level in each of these sequences. Banks of threshold comparators and a selector circuit are used to choose between the various threshold values. The selected value is lowest when the number of passages through zero level is between 5 and 6 per millisecond.