Measured relative frequency response, r_a(f), obtained from oceanic in-situ acoustical survey measurements of s_v on Atlantic mackerel schools. Data from within an observation depth interval of 30 m to 110 m are included. Each point (x or o) is based on measurements from a whole school. (a) 120 kHz measurement frequency, using 250 W and 1000 W electrical power settings in survey operation (alternating), and 250 W during calibration. (b) 200 kHz measurement frequency, using 120 W and 1000 W electrical power settings in survey operation (alternating), and 120 W during calibration.