Draft Acoustic Guidance FAQs
What is the purpose of the Guidance?
The Guidance provides acoustic threshold levels for assessing the effects of anthropogenic sound on marine mammal species under NOAA Fisheries' jurisdiction. It identifies the received levels, or thresholds, above which individual marine mammals are predicted to experience changes in their hearing sensitivity (either temporary or permanent) for all underwater anthropogenic sound sources.
This is the first time NOAA Fisheries has presented acoustic threshold levels for marine mammals in a single, comprehensive document in order to improve consistent implementation across the array of relevant laws that protect marine mammals.
What are the acoustic threshold levels?
We have compiled, interpreted, and synthesized the best available science to produce updated acoustic threshold levels for the onset of both temporary and permanent hearing threshold shifts. The acoustic thresholds in this document identify the levels of sound, which after they are exceeded, NOAA anticipates (after evaluating and interpreting all available science) changes in auditory sensitivity (temporary or permanent threshold shift).
It is important to note that these updated acoustic threshold levels do not represent the entirety of an impact assessment, but rather serve as one tool to help evaluate the effects of a proposed action on marine mammals and make findings required by our various statutes.
Who should use the Guidance?
This Guidance is intended to be used by NOAA analysts and managers and other relevant user groups and stakeholders, including other Federal agencies, when seeking to determine whether and how their activities are expected to result in particular types of impacts to marine mammals via acoustic exposure.
Activities with the greatest potential to affect marine mammals by noise include:
- Seismic airguns
- High-energy sonars (military)
- Explosive detonations
- Certain construction activities (impact pile driving)
What has changed between the updated acoustic threshold levels and the thresholds NOAA currently uses?
It is important to note the Guidance updates best available science, and does not change our regulatory application (e.g., issuing permits, conducting consultations, etc.). NOAA Fisheries is proposing updated acoustic threshold levels; however, our application of these thresholds under relevant statutes (Marine Mammal Protection Act, Endangered Species Act, and National Marine Sanctuaries Act) will remain consistent with past NOAA practice.
The current acoustic threshold levels, used for most sound sources, consist of a single threshold for cetaceans and a single threshold for pinnipeds regardless of sound source (i.e., they do not take into account the hearing ability of different marine mammal groups or the differences among sound sources in terms of auditory impacts). The updated acoustic threshold levels consist of several thresholds and when finalized will replace those currently in use by NOAA Fisheries. These thresholds reflect the best available science on the potential for noise to affect auditory sensitivity by:
- Dividing sound sources into two groups based on their potential to affect hearing sensitivity:
- Impulsive sound sources (e.g., airguns, impact pile drivers): are transient, brief (less than 1 second), broadband, and typically consist of high peak pressure with rapid rise time and rapid decay, and based on these physical characteristics have a greater potential to affect hearing sensitivity
- Non-impulsive sound sources (e.g., sonar, vibratory pile drivers): can be broadband, narrowband or tonal, brief or prolonged, continuous or intermittent and typically do not have a high peak pressure with rapid rise time (typically only small fluctuations in dB level) that impulsive signals do
- Choosing metrics that better address the impacts of noise on hearing sensitivity:
- Peak sound pressure level: better reflects the physical properties of many sound sources, especially impulsive sources, to affect hearing sensitivity
- Cumulative sound exposure level: accounts for not only level of exposure but also durations of exposure
- Dividing marine mammals into functional hearing groups and developing auditory weighting functions based on the science supporting that not all marine mammals hear and use sound in the same manner (For reference, humans hear from 20Hz to 20kHz):
- Low-frequency cetaceans (hearing range 7 Hz to 30 kHz): large, baleen whale species
- Mid-frequency cetaceans (hearing range 150 Hz to 160 kHz): dolphin species
- High-frequency cetaceans (hearing range 200 Hz to 180 kHz): porpoise species
- Otariid pinnipeds (hearing range: 100 Hz to 40 kHz): eared seals (e.g., sea lions)
- Phocid pinnipeds (hearing range 75 Hz to 100 kHz): true/ earless seals (e.g., harbor seals)
NOAA is working on a companion "User Guide" (including a spreadsheet calculator) to be released with the final Guidance document that will assist applicants and other members of the regulated community to apply updated acoustic threshold levels correctly.
What do we know about the effects of anthropogenic noise on marine mammal hearing?
We are using the best available science to develop our updated acoustic threshold levels, but we do not have a lot of information, especially from marine mammals in the wild. The data we have on hearing loss from noise exposure comes from laboratory studies on a few species (e.g., bottlenose dolphins, harbor porpoises, California sea lions, and harbor seals) and a limited number of individuals within those species. Where limited or no data are available, surrogate data from other marine mammal or terrestrial mammal species has been used. The threshold levels will help identify data gaps and encourage further research.
How will application of the acoustic threshold levels in the Guidance compare to what NOAA has used previously?
Given the specific nature of the guidance updates (e.g., different metrics, auditory weighting functions, etc.), it is not possible to directly compare the updated acoustic threshold levels with the thresholds previously used by NOAA.
The updated acoustic thresholds are more complex than those previously used by NOAA. For example, the new threshold levels include metrics for not only the level of exposure but also the duration of exposure, while the previous thresholds only account for level of exposures. Additionally, the updated acoustic threshold levels take into account that not all marine mammals hear and use sound in the same way (i.e., functional hearing groups and auditory weighting functions), while previous thresholds only divided marine mammals into two basic groups (cetaceans and pinnipeds). NOAA is working on a companion "User Guide" to be released with the final Guidance document that will assist applicants and other members of the regulated community to apply updated acoustic threshold levels correctly.
In some situations (e.g., depending on sound source, species, and duration of exposure), updated acoustic thresholds may result in more exposures than previously applied thresholds, while in others they may result in less exposures. However, these updated acoustic threshold levels reflect the best available science on the potential for noise to affect auditory abilities of marine mammals.
Why is Guidance for behavioral response of marine mammals to sound not included in this document?
NOAA is continuing our examination of the effects of noise on marine mammal behavior and will focus our work over the next year on developing guidance regarding the effects of anthropogenic sound on marine mammal behavior. Behavioral response is a complex question and we determined we still need time to research and address it appropriately.
What is the review process for the Guidance?
The Guidance is classified as a Highly Influential Scientific Assessment by the Office of Management and Budget. Independent peer review is required prior to broad public dissemination by the Federal Government. These updated acoustic threshold levels have been peer-reviewed by independent scientific experts during an initial peer review, a second peer review will be done to evaluate a new methodology proposed by the U.S. Navy, and a third peer review will be done to evaluate our updated methodology for defining threshold usage based on sources being characterized as impulsive or non-impulsive. The initial Guidance document has also been shared with our partner Federal agencies for their informal input. To complete the review process for the initial Guidance, NOAA solicited input from the stakeholders and the pubic via the public comment period and public meeting/webinar. After the additional peer reviews are complete, another public comment period will be held. More details on the review process are available on our website.
When will the Guidance be finalized and become effective?
NOAA Fisheries will publish our final acoustic Guidance document after we review and incorporate all public comments on the proposed draft. We anticipate having the Guidance finalized in late 2015.
Updated: April 17, 2015