Measurements of the stability of an underwater acoustic channel were made at long range (500 km) and relatively low acoustic frequency (206 Hz). The source depth and depth of the two receiving hydrophones were carefully chosen to limit the number of totally refracted acoustic paths. In addition, a discrete spectrum coded waveform was transmitted to allow time domain resolution of refracted and reflected arrivals. The results show considerably less channel time spreading than is predicted by recent theoretical models, and indicate that coherence bandwidths of 100 Hz are obtainable with proper source and receiver geometry. Received pulses wander systematically with changes in acoustic path length strongly influenced by energy at the frequency of the semidiurnal tide. The spatial coherence of signals propagated to two receivers separated by 250 m indicates that acoustic paths are similarly modulated by ocean processes with frequencies below the Brunt–Väisälä internal wave cut‐off. Above the Brunt–Väisälä frequency, coherence is small indicating that processes with space scales much less than 250 m are responsible for path length fluctuations.