Energetic and informational masking of complex sounds by a bottlenose dolphin (Tursiops truncatus).

With few exceptions, laboratory studies of auditory masking in marine mammals have been limited to examining detection thresholds for simple tonal signals embedded in broadband noise. However, detection of a sound has little adaptive advantage without the knowledge of what produced the sound (recognition) and where the sound originated (localization). In the current study, a bottlenose dolphin's masked detection thresholds (energetic masking) and masked recognition thresholds (informational masking) were estimated for a variety of complex signals including dolphin vocalizations, frequency modulated signals, and a 10 kHz pure tone. Broadband noise types included recordings of natural sounds and computer generated sounds. Detection thresholds were estimated using a standard go, no-go adaptive staircase procedure. The same dolphin learned to associate whistle-like FM sounds with specific arbitrary objects using a three alternative, matching-to-sample (MTS) procedure. The dolphin's performance in the MTS task was then tested in the presence of the same masking noise types used in the detection task. Recognition thresholds were, on average, about 4 dB higher than detection thresholds for similar signal-noise conditions. The 4 dB difference is likely due to additional cognitive demands of recognition, including attention and pattern recognition.

Time and frequency metrics related to auditory masking of a 10 kHz tone in bottlenose dolphins (Tursiops truncatus).

Metrics related to the frequency spectrum of noise (e.g., critical ratios) are often used to describe and predict auditory masking. In this study, detection thresholds for a 10 kHz tone were measured in the presence of anthropogenic, natural, and synthesized noise. Time-domain and frequency-domain metrics were calculated for the different noise types, and regression models were used to determine the relationship between noise metrics and masked tonal thresholds. Statistical models suggested that detection thresholds, masked by a variety of noise types at a variety of noise levels, can be explained with metrics related to the spectral density of noise and the degree to which amplitude modulation is correlated across frequency regions of the noise. The results demonstrate the need to include time-domain metrics when describing and predicting auditory masking.

Auditory masking patterns in bottlenose dolphins (Tursiops truncatus) with natural, anthropogenic, and synthesized noise.

Auditory masking occurs when one sound (usually called noise) interferes with the detection, discrimination, or recognition of another sound (usually called the signal). This interference can lead to detriments in a listener's ability to communicate, forage, and navigate. Most studies of auditory masking in marine mammals have been limited to detection thresholds of pure tones in Gaussian noise. Environmental noise marine mammals encounter is often more complex. In the current study, detection thresholds were estimated for bottlenose dolphins with a 10 kHz signal masked by natural, anthropogenic, and synthesized noise. Using a band-widening paradigm, detection thresholds exhibited a pattern where signal thresholds increased proportionally to bandwidth for narrow band noise. However, when noise bandwidth was greater than a critical band, masking patterns diverged. Subsequent experiments demonstrated that the auditory mechanisms responsible for the divergent masking patterns were related to across-channel comparison and within-valley listening.

Directional properties of bottlenose dolphin (Tursiops truncatus) clicks, burst-pulse, and whistle sounds.

The directional properties of bottlenose dolphin clicks, burst-pulse, and whistle signals were measured using a five element array, at horizontal angles of 0°, 45°, 90°, 135°, and 180° relative to a dolphin stationed on an underwater biteplate. Clicks and burst-pulse signals were highly directional with directivity indices of ~11 dB for both signal types. Higher frequencies and higher amplitudes dominated the forward, on-axis sound field. A similar result was found with whistles, where higher frequency harmonics had greater directivity indices than lower frequency harmonics. The results suggest the directional properties of these signals not only provide enhanced information to the sound producer (as in echolocation) but can provide valuable information to conspecific listeners during group coordination and socialization.