Manatees, Bioacoustics and Boats
Hearing tests, environmental measurements and acoustic phenomena may together explain why boats and animals collide
Living in a World of Sound
Although the audiogram provides a definitive measure of hearing under the quietest of conditions, manatees do not live in quiet habitats. In Florida, ambient noise in manatee habitats typically ranges from 60 to 90 decibels, over a frequency range of 1 to 20,000 hertz, but levels can reach 130 decibels during heavy rain or in industrial areas.
A perceptual phenomenon known as masking takes place when the audibility of one sound (the "signal") is decreased by the presence or occurrence of another sound (the "noise"). When we began our work, we knew that the ambient noise in manatee habitats could conceivably mask the perception of many kinds of signals. Manatee are immersed in a dynamic acoustic landscape filled with a cacophony of sounds, the most prominent biological sound being the continuous crackling from millions of snapping shrimp. Being passive listeners—unlike the echolocating dolphins, which can use active sonar to navigate and detect objects in the environment—manatees are restricted to listening to their auditory landscape. We wanted to understand how noise affects the manatees' ability to hear biologically important sounds, ambient events and the sounds of approaching boats. Toward this end we conducted a series of psychoacoustic investigations using pure tones, complex sounds and samples of typical boat noise and manatee vocalizations. These tests measured the masked thresholds and critical signal-to-noise ratios along the manatees' hearing curve against a background of continuous noise.
The critical ratio compares the intensity of a signal at the moment it is just detectable (the masked threshold) to the intensity of the background noise. For instance, if a manatee can hear a particular signal over 70 decibels of ambient noise when the signal reaches 90 decibels, then the critical ratio is 20 decibels; since decibels are a logarithmic expression, the ratio can be derived by subtracting one sound level from the other. It should be noted that critical ratios are conserved for each frequency regardless of increasing ambient levels, so if the ambient noise increased to 100 decibels, then the signal would have to be at least 120 decibels before the manatee could detect it. The size of the critical ratio has important ecological significance, as high ambient levels could conceivably raise detection thresholds beyond the absolute acoustic energy emitted by many boats.
We conducted hearing tests that used various noise levels representative of typical wild ambient conditions. We tested pulsed and continuous pure tones and broadband noise like that produced by boats. The masking studies showed that manatees have critical ratios that range from 9 decibels above the prevailing background noise for pulsed broadband noise up to 46 decibels for continuous tones. Manatees detected repetitive pulsed sounds at significantly lower critical ratios than continuous wave sounds. This was not an unexpected result, since biological sounds and, in particular, the manatees' own 200- to 500-millisecond vocalizations, are pulsed sounds. Pulsed signals provide additive signal width as well as temporal patterns that manatees may detect from an aperiodic background.