Page 56 - Summer 2021

NOISE EXPOSURE CRITERIA
search, consumption, energetics), and reproduction (e.g., mating, parenting). This framework is less qualitatively descriptive of discrete responses and more focused on expected longer term and ultimately population-level consequences of responses affecting these vital functions. Southall et al. (2021) applies the new assessment method to a subset of studies in each of the four specified noise types using multiple independent observers.
based on methods developed by Samuels et al. (2000) for assessing longer term disturbance from wildlife tourism was presented as a potential starting point, but further development is needed. Insightful long-term studies are providing critical measurements for how short-term responses may or may not manifest in population-level consequences (e.g., Thompson et al., 2013). Efforts to link response studies and probabilities to energetic and ultimately to the population consequences of distur- bance offer important new insights and lessons for future behavioral response studies (Pirotta et al., 2021). Finally, major analytical advances are beginning to address the daunting challenges of multiple, interacting stressors and behavioral and physiological effects (e.g., NAS, 2017).
This assessment demonstrates a high degree of con-
currence among assessors for studies where individual
responses to acute noise exposures were directly mea-
sured and individually reported. This includes
increasingly complex controlled-exposure experi-
ments using advanced tag sensors deployed on marine
mammals that measure fine-scale movement, sound “Impulsive” Exposure Criteria and
production, and noise exposure (see Figure 4). Such studies are providing novel, high-resolution data on acute responses, with more of the comprehensive subject, acoustic, and exposure contextual metrics required for behavioral criteria (Southall et al., 2016, 2019b).
Southall et al. (2021) also conclude that fundamentally new perspectives on behavioral-response criteria for data obtained on broader spatial and temporal scales are needed. Their revised response-severity spectrum was limited in considering scenarios (and some well-designed studies) where sustained or repeated disturbance occurs for many animals. A structured synthesis approach
Sound Propagation
Impulsive and nonimpulsive sounds have different audi- tory and behavioral effects. This is expected given what is known about the auditory systems of mammals, includ- ing marine mammals (e.g., Finneran., 2015). However, significant data limitations still exist in terms of under- standing the physical features that cause impulsive exposures to have greater auditory effects at comparable exposure levels. These differences have long been suffi- ciently clear. Southall et al. (2007) established the need for this distinct source categorization for marine noise sources and it continues in the recent criteria (Southall et al., 2019a).
This distinction typically considers parameters associated with the level of “impulsiveness” (e.g., rise time, crest factor, kurtosis) measured in the near vicinity of the source. Southall et al. (2007) categorized discrete events such as an explosions or sonic booms as “single pulses,” repeated seismic airgun shots and pile strikes as “multiple pulses,” and continuous sounds like vessels, drilling, and many sonars as “nonpulses.”
Of course, sound propagation alters the extent to which impulsive signals retain those characteristics at distant receivers. Southall et al. (2007) initially categorized noise sources by source features given the lack of direct audi- tory data for meaningful distinctions, particularly in light of variable acoustic interactions occurring over space, time, and spectra. This was a precautionary approach driven by data limitations and practical complexity. More protective (lower effect onset levels) impulsive criteria
Figure 4. Rissos dolphins (Grampus griseus) in a behavioral- response study. The gold tag affixed to the lead animal has archival fine-scale movement and high bandwidth passive acoustic sensors. Photograph courtesy of A. Friedlaender, taken under NMFS permit #19116.
56 Acoustics Today • Summer 2021