Impacts of Impulsive Sound

Marine mammals are continuously exposed to anthropogenic sound sources, but not all acoustic signals are received equally by the animals. Mammalian hearing is sensitive and susceptible to damage from intense impulsive sound events; impulsive sounds, such as the soundwave accompanying underwater explosions, airguns, and impact hammer pile driving, are thought to have a greater potential for adverse impacts on marine mammal hearing than non-impulsive sound of equal sound energy (Hastie et al., 2019). 

Estimating the potential impacts of a sound source on marine mammals is no simple task. Sound qualities such as intensity, frequency, duration, rise time, and impulsivity, as well as the hearing sensitivity of the receiver all factor into the way the sound might impact hearing (Ketten, 2012). In practice, metrics such as the peak sound pressure level (L_pk) and time-integrated sound exposure level (SEL) are used to determine if a given sound will have adverse effects on marine mammals by inducing behavioral changes, as well as temporary or permanent changes to hearing sensitivity. In some cases, exposure to a sound source can induce hearing loss by overstimulating the auditory system. One component of the system, hair cells, (Ketten, 2012) are important transducers of sound that are susceptible to damage that may cause temporary or permanent hearing loss (hearing threshold shifts). 

Most studies focusing on the impacts of impulsive sounds have been done on terrestrial mammals. These have shown that exposure to impulsive sounds induced more loss of hair cells than exposure to non-impulsive sounds with a similar total SEL and showed that smaller animals are subject to more damage than larger mammals (Hamernik and Qiu, 2003; Hamernik et al., 1987; Henderson and Hamernik, 1986; Melnick, 1991; Luz and Hodge, 1970; Sulkowski and Lipowczan, 1982)). Similar physiological studies on the impacts of impulsive sound on marine mammals are limited, with few key studies focusing on observations of behavioral changes of certain species in response to controlled playbacks of impulsive sound (Nachtigall et al., 2018; Klopfer, 2005; Finnernan et al., 2000, 2002, 2015; Kastelein, 2015). These studies indicate that marine mammals, like humans and other terrestrial mammals, are vulnerable to temporary or permanent hearing loss and are more susceptible to impairment from impulsive sounds compared to non-impulsive sounds (Hastie et al., 2019).

Metrics for assessing and predicting the impacts of a specific sound source on an animal are derived from the “Equal Energy Hypothesis,” which predicts the degree of noise-induced hearing loss is constant with respect to cumulative SEL (cSEL) to which the animal was exposed over a given duration. Studies on the impacts of impulsive sound on fish, however, have challenged this hypothesis, providing evidence that how the sound energy is delivered to the animal will influence the effects of sound exposure (Hawkins and Popper, 2017). Impulsive sounds deliver sound energy with short rise and rapid decay times and with more energy in a concentrated period of time than continuous sounds. 

References:

Finneran, J. J., C. E. Schlundt, D. A. Carder, J. A. Clark, J. A. Young, J. B. Gaspin, and S. H. Ridgway. 2000. Auditory and behavioral responses of bottlenose dolphins (Tursiops truncatus) and a beluga whale (Delphinapterus leucas) to impulsive sounds resembling distant signatures of underwater explosions. Journal of the Acoustical Society of America 108:417431. 

Finneran, J. J., C. E. Schlundt, R. Dear, D. A. Carder, and S. H. Ridgway. 2002. Temporary shift in masked hearing thresholds in odontocetes after exposure to single underwater impulses from a seismic watergun. Journal of the Acoustical Society of America 111:2929–2940.

Finneran, J. J., C. E. Schlundt, B. K. Branstetter, J. S. Trickey, V. Bowman, and K. Jenkins. 2015. Effects of multiple impulses from a seismic air gun on bottlenose dolphin hearing and behavior. Journal of the Acoustical Society of America 137:1634–1646.

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Hawkins, Anthony D., and Arthur N. Popper. “A Sound Approach to Assessing the Impact of Underwater Noise on Marine Fishes and Invertebrates.” Edited by Howard Browman. ICES Journal of Marine Science 74, no. 3 (March 1, 2017): 635–51. https://doi.org/10.1093/icesjms/fsw205.

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