Another type of experiment often used to measure acoustic coupling and the response of a biological clock is to present a period stimulus and determine the degree to which the clock can phase lock to the stimulus. This is a single example of an entrainment paradigm presented to a male snowy tree cricket. The males natural period is taken as the average period of the chirps prior to the stimulus. Then a stimulus is presented having some entrainment period, T_s. Because the entraining stimulus is regular it is convenient to use the stimulus as a timer and to measure the cricket’s phase relative to the entraining stimulus. If the cricket’s phase remains constant from one entrainment period to the next then the cricket is said to have phase locked to the stimulus. The value t is the ratio of the entrainment period to the natural period and represents the degree to which the two periods differ.