1. Acoustics of the Puget Sound
Michael Greiner
Physics 536
University of Washington, March 2019

2. Puget Sound Home to a huge wide variety of marine life
Image Credit: Puget sound institute
Home to a huge wide variety of marine life
Marine invertebrates and shellfish
Salmon and other fish species
Marine mammals
Seals
Sea Lions, (Stellar and California)
Harbor porpoise
Minke, Humpback, Grey, and other whales
Endangered Southern Resident Killer Whale population
Image credit: Metro Field Guide
Img: David Suzuki
Why is it important to study the acoustics and sound levels in the puget sound?
Home to a huge diverse range of wildlife including several marine mammals, most notably the endangered southern resident killer whale population. There is sufficient evidence to conclude that high intensity sound is harmful and on occasion, fatal to marine mammals. And increased ambient noise levels make feeding and communicating difficult for marine mammals.
Image credit: Alaska DFG

3. Large metropolitan area
Recreational Marine traffic
Washington State Ferry System
Second largest in the world
Deep Draft Vessel Traffic
Container ships
Oil tankers
Image credit Wikipedia
The puget sound is also home to a major metropolitan area including a major west coast shipping port of Seattle, Tacoma, who follow only Long Beach, Los Angeles, and Okland for shipping volume on the West coast.
Image: workboat.com
Image: Spectrum.ieee.org

4. Ambient noise from natural sources
Earthquakes < 100 Hz
Wind and waves Hz - 10 kHz
Rain kHz - 21 kHz
Marine Life
Marine invertebrates 2 kHz - 5 kHz
Marine mammal vocalization 10 Hz kHz
Image: Pixels.com
Image: Thomas Dutour
Ambient Noise: Defined as noise in the absence of anthropogenic noise sources
Three sources:
1)Natural Physical process:
Earthquakes up to 100 Hz
Wind dominant source between 100 Hz and 10 kHz
Rain
Acoustic spectra are dependent on droplet size and wind speed
Create sound via two mechanisms, broadband noise due to droplet impact followed by louder resonating microbubble.( kHz, average 15) Typically rates are light to moderate (less than 7.5 mm per hour). Noise level highly dependent on wind speed. If droplet his at normal incidence, 100% microbubble creation probability. Bt with incidence angle of 20 degrees, likelihood of microbubble creation is 20%. So should levels during light rains are decreased with increased wind speed.
Bubble and surface spray 199 Hz to 30 kHz
Image: abc.net.au
Image: folio.ca

5. Anthropogenic Sound Sources:
Explosives
Seismic exploration, airgun arrays
Active sonar
Acoustic deterrent devices
Industrial Activities (pile driving)
Ships
Image: Researchgate.net
Anthropogenic sound sources are the main source of sound in the region. They consist of underwater explosions (usually related to military exercises and exploration) along with seismic exploration done with airgun arrays which are capable of producing sound levels of up to 260 dB! There are also acoustic deterrent devices that salmon hatcheries will use to ward off marine mammals that may be interested in eating the spawning salmon. And especially in this populated area there is noise due to pile driving and construction in the ever growing seattle region. And of course, the most common source of anthropogenic noise is vessel traffic. In the puget sound region, Vessel traffic can be broken down into recreational boaters, washington state ferries, and deep drafe vessel traffic like that of container ships and car carriers.
Image: csmarine.com

6. Anthropogenic Sound Sources:
Interesting to note from this graphic taken from John Hildebrand paper about anthropogenic sound sources in the ocean aree the sound levels of the airgun array (260 dB) the active sonar devices (all above 200 dB) and the cargo vessel (192 dB)
Hildebrand, John A. (2009)

7. How do we measure sound in Water?
Hydrophones
Measure Sound Pressure Levels (SPL) in dB
SPL (dB) = 10log10 (p2/pref2)
Pref in water is 1 μPa
in air pref is 20 μPa
By submerging hydrophones, which are transducers for turning sound waves in water to electrical signals we can get a sense for what the ocean environment sounds like to marine creatures. Keep in mind that not all sound recorded will represent the sound created. Distance from the sound source contributes to the frequency of noise observed by the recorder. In salt water, attenuation rates (measured in dB are approximately four orders of magnitude higher for 10 kHz sounds than for 100 kHz sounds. As a result, low frequency sounds from sources such as shipping vessels can propagate thousands of kilometers in open ocean (also because of the natural wave guide set up by pressure, temperature and salinity levels) while high frequency sounds are rapidly attenuated.
Image: rtsys.eu

8. Example: Admiralty Inlet
Study conducted to investigate ambient noise levels for a proposed underwater turbine project.
Noises detected were sound and pseudo sound
Pseudo sound caused by turbulent water moving passed the hydrophone
(much like rustling of wind in your ears on a windy day)
How to eliminate contribution of pseudo sound to sound level measurements?
Hydrophones measure Two types, Pseudo Sound and actual sound
Example of Pseudo sound is analogous to the sound of wind i your ears, someone standing near, but protected from the wind would not detect the sound, since it is not a propagating wave. Can be caused underwater by current at hydrophone location or turbulence in surrounding water.

9. Two Hydrophones!
Using multiple hydrophones allows for multiple recordings of the same ambient noise.
Compare the recordings and run correlation analysis between the two recordings.
The pseudo sound will have very low correlation since it is random in nature
The propagating sound will have high correlation because both hydrophones are recording the same SPLs
This effect can be reduced by using two hydrophones near each other and comparing their signals. By calculating signal coherence which we can recall is the frequency space measure of correlation of two independent signals. Actual sound propagation near the hydrophones, the signal will have high coherence, but pseudo sound will have low coherence since it is random in nature.

10. Example Data Collection: Admiralty Inlet
Images courtesy of google maps

11. Example Data Collection: Admiralty Inlet
Estimated source levels ( kHz) based on received levels for selected marine traffic transiting Admiralty Inlet in 2011
Bassett, Christopher, et al 2012
Shown here are estimated source levels of sound for several ships that transited Admiralty Inlet over the course of 10 days.

12. Ship noise
Primarily Cavitation of propeller
Broadband noise due to bubble collapse
Tonal components related to blade passage frequency
Image credit: Petr Dalik
All vessels produce noise due to their operation. Through the use of a propeller underwater, the pressure levels along the leading edge of a rotating propeller are reduced to such a degree that the water changes phase from a liquid to a gas and forms bubbles. When those bubbles move away from the propeller, they collapse creating a shockwave of broadband noise in the surrounding water. The amount of noise produced by a ship moving through the water depends on the size and speed of the vessel, but can be near 200 dB for large container ships ad a range of 1 m.
Image credit: Nord Lock

13. Marine mammal sound production
Southern Resident Killer Whale vocalization
Produce a variety of clicks and whistles used for communication and echolocation
Orcas use phonic lips near the blowhole to produce calls clicks and whistles. The sound waves pass through the mellon (rounded region consisting of fats) that acts as an auditory lense, focusing the sound to be directional. The sound reflects off the prey and returns to the orca who can sense the sound through fat filled cavities in the lower jaw that conduct the sound to the middle, and inner ear.

14. Struggling orca population
Orcas must increase their call amplitude in response to vessel noise
S1 whale calls recorded with a background noise level of 100 dB
S1 call recorded in a background noise level of 115 dB
Noise level attributed to vessel traffic in the area
Holt et al 2008
Observations have been made of the SPL of orca calls made in the presence of low level background noise as well as with increased noise levels showing that in a noisy environment, Orcas must communicate at higher volume levels indicating that the background noise makes communication more difficult.

15. Southern Resident Orca Whale Recovery
Gov. Jay Inslee announced investment of $1.1 Billion to help the Orca population
Increase abundance of Chinook Salmon
Decrease disturbance posed by vessel traffic
Reducing toxic pollutants
Ensure adequate funding for recovery efforts moving forward

16. References:
Bassett, Christopher, et al. "A vessel noise budget for Admiralty Inlet, Puget Sound, Washington (USA)." The Journal of the Acoustical Society of America (2012):
Dahl, Peter H., et al. "Underwater ambient noise." Acoustics Today 3.1 (2007):
Hildebrand, John A. "Anthropogenic and natural sources of ambient noise in the ocean." Marine Ecology Progress Series395 (2009): 5-20.
Holt, Marla M., et al. "Speaking up: Killer whales (Orcinus orca) increase their call amplitude in response to vessel noise." The Journal of the Acoustical Society of America (2009): EL27-EL32.
Southall, Brandon L., et al. "Underwater Noise from Large Commercial Ships—International Collaboration for Noise Reduction." Encyclopedia of Maritime and Offshore Engineering (2017): 1-9.
Also Worth noting: A march 7 article in the washington post describes a hearing in which a Trump official is being questioned by a representative of south carolina about the use of Airgun arrays off the coast in an effort to explore for oil in the region. As an example for the official, the SC representative blasted an air horn in the hearing and explained that the airgun produced a sound more than 16,000 times louder in an effort to protect the marine mammals in the region.