Descending frequency swept tones have lower thresholds than ascending frequency swept tones for a harbor seal (Phoca vitulina) and human listeners.

Many seal vocalizations consist of frequency swept tones. The signal detection thresholds of a harbor seal (Phoca vitulina) and of human listeners were measured using ascending and descending frequency swept tones at 1/3- and 1-oct bandwidths. The swept tones increased or decreased exponentially and traversed the same frequency range. The sweeps were centered on 2, 4, and 8 kHz for the seal study and 0.5, 1, and 2 kHz for the human listeners. The bandwidth of the sweep did not affect signal detection abilities of either the seal or human listeners. The seal had lower signal detection thresholds (1-5 dB) when presented with the descending frequency swept tones (F = 32.04, df = (1,31), p < 0.000001). Human listeners also had lower detection thresholds for descending frequency swept tones (t = -4.78, df = 52, p < 0.0001). Pinniped ascending frequency swept calls may not function as well as descending frequency sweeps for long-distance communicative signals.