Spectra of sound‐pressure amplitude versus frequency in the range 1.1 to 2.4 kc have been obtained at regular intervals in shallow (10 ft) water during two periods of extended observation. The acoustical data have been presented in the form of contour diagrams of sound‐pressure amplitude versus frequency and time as coordinates and compared with water conditions.The form of the contour diagrams permits differentiation between field‐structure variation and variation in attenuation although differentiation between the associated processes has not been possible. Temporal variation in the vertical sound‐velocity profile produced only small changes in the gross features of the field structure, as inferred from the contour diagram, for up to the first three modes of propagation. For frequencies above 1.6 kc (i.e., when three or four modes were stimulated) the variations in field structure were prominent. Attempts have been made, with little success, to explain periods when the received signal was strongly attenuated in terms of vertical sound‐velocity structure.