Man made noise underwater can cover a wide range of frequencies and level of sound, and the way in which a given species reacts to the sound will depend on the frequency range it can hear, the level of sound and its spectrum. Both the sensitivity of hearing and the frequency range over which sound can be heard varies greatly from species to species.
For man, sound is ultrasonic (i.e. above human hearing range) above about 20 kHz. However, for many fish, sounds above 1 kHz are ultrasonic. For a marine mammal, much of the energy of an airgun may be infrasonic, as many cannot perceive sounds below 1 kHz. These considerations indicate the importance of considering hearing ability when evaluating the effect of underwater noise on marine animals. Concerns over the environmental effects of offshore seismic shooting using airguns prompted Subacoustech to develop and propose the dBht(Species) metric as a formal method of evaluating the effects of noise .
The dBht(Species) provides a measurement of sound that accounts for inter-species differences in hearing ability by passing the sound through a filter that mimics the hearing ability of the species. A set of coefficients is used to define the behaviour of the filter so that it corresponds to the way that the acuity of hearing of the candidate species varies with frequency. The level of sound is measured after the filter; the level expressed in this scale is different for each species (which is the reason that the specific name is appended), and corresponds to the likely level of perception of the sound by that species.
The scale may be thought of as a dB scale where the species’ hearing threshold is used as the reference unit; it is identical in concept to the dB(A) scale used for rating the behavioural effects of sound on man. In effect, the dB(A) may be thought of as the dBht(Homo sapiens). One major benefit of the scale is simplicity; a single number (the dBht(Species)) may be used to describe the effects of the sound on that species.
It is anticipated that the eventual use of the dBht(Species) scale will be to provide “species sound level meters” which will enable simple measurements of noise to be made in biologically meaningful units by non acoustically expert users such as MMOs.
The image above shows an audiogram for the pouting (Trisopterus luscus) calibrated for pressure (open diamonds) and particle velocity (closed diamonds) presented at frequencies ranging from 100 Hz to 1000 Hz, with the projector driven in pressure mode (lower curves) and in particle velocity mode (upper curves).