Michelle Greene: Acoustics Team…Do You Hear What I Hear?

NOAA Teacher at Sea

Michelle Greene

Aboard NOAA Ship Gordon Gunter

July 19 – August 3, 2018

 

Mission: Cetacean Survey

Geographic Area: Northeast U.S. Atlantic Coast

Date: July 24-25, 2018

 

Latitude: 40° 2.629″ N

Longitude: 67° 58.954″ W

Sea Surface Temperature: 23.3° C (73.9° F)

Sailing Speed: 1.80 knots

 

Science and Technology Blog:

Today I had the opportunity to shadow the acoustics team in the dry lab.  The acoustics team uses a linear array or a prototype tetrahedral array of hydrophones to listen to the sounds that whales and dolphins make under the water.  So far in this journey, the team has only used the linear array.  The array has been towing behind the ship with the “line” of hydrophones parallel to the surface of the water about 10 meters below the surface.

When the array is deployed, the acoustics team uses a computer software called PAMGuard to record the sounds and track the clicks and whistles of whales and dolphins.  PAMGuard can be programmed to record sounds in any frequency range.  On this cruise, acoustics is looking at sounds up to about 100,000 hertz.  A human being can hear from about 20 Hz to about 20 kilohertz with normal human speech frequency between 1,000 Hz and 5,000 Hz.  The optimal hearing age for a person is approximately 20 years of age and declines after that.

Beaked whales click at a frequency too high for human hearing; however, PAMGuard can detect the clicks to help the acousticians possibly locate an animal.  PAMGuard produces a real-time, time series graph of the location of all sounds picked up on the array.  A series of dots is located on a continual graph with the x-axis being time and the y-axis being bearing from the ship. The array picks up all sounds, and PAMGuard gives a bearing of the sound with a bearing of 0° being in front of the ship and a bearing of 180° being behind the ship.  The ship creates noise that is picked up by all the hydrophones at the same time, so it looks like a lot of noise at 90°.  The acousticians must sift through the noise to try to find click trains.  Rain and heavy waves also create a lot noise for the hydrophone array.  The acoustician can click on an individual dot which represents a sound, and then she can see a Wigner plot of the sound which is a high resolution spectrogram image of the sound.

Scientists have determined what the Wigner plot image of a beaked whale sound should look like.

 

When a Wigner plot image looks to be a possible Mesoplodon, the acoustician starts tracking a click train on the time series graph in hopes of getting the sound again.  If the acoustic signal repeats, the acoustician then adds it to the click train.  Each time the acoustician adds to a click train, the bearing to the new click is plotted on a graph.  The array cannot calculate the actual location of an animal, so a beam of probability is plotted on a chart.  Then the acoustician uses the angle of each click in a click train to determine a possible location on the port or starboard side of the ship.  If the click train produces a sound that can be localized with the convergence of beams to a certain point, the acoustician can call the visual team to look on a particular side of the ship or ask the bridge to slow down or turn in a certain direction.  Mesoplodons have average dive times of between 15 and 20 minutes and foraging dive times of up to 45 minutes, so there is a time delay between getting the clicks and seeing an animal.

The objective of this cruise is to find the occurrence of beaked whales, but PAMGuard does not record just beaked whale clicks, so several other whales and dolphins are heard by the array.  Sperm whales (Physeter macrocephalus) have clicks that can be heard by the human ear with an average frequency of 10 KHz.  Sperm whales have a synchronized click train.  It can be thought of as “click click click click…” with about 0.5 to 1.0 second between each click.  Scientists believe the clicks are used for echolocation.  Since it is very dark in the ocean and light does not travel far underwater, sperm whales use their clicks as sort of flashlight for locating food which usually consists of squid.  When a sperm whale senses the location of food, it produces a rapid series of clicks called a buzz.  After the buzz, the animal makes a dive.  If the dive is not successful, in other words the whale did not get food, then clicks return to their normal pattern until another attempt is made.  Clicks are also used for social interaction between sperm whales.  Sperm whales have been very vocal on the cruise so far.

Personal Log

I have been spending my days rotating between the visual sighting team and the acoustics team.  Even when I am not scheduled to be there, I am in acoustics.  I find listening to the sounds very interesting.  I had no idea whales made clicking sounds.  I knew dolphins whistled, but clicking is not a term I was familiar with until this cruise.  We have had several episodes where many dolphins will go by the ship.  When that happens, the whole plot in PAMGuard almost turns black from all of the dots on the screen.  It is amazing to hear all of the clicks and whistles from the dolphins.  My favorite whales right now are sperm whales.  I can now look at the screen and see the clicks and know it is a sperm whale.  I get so excited.

Getting a Mesoplodon click train is like watching a whale lover’s version of Storm Chasers.  When a possible Mesoplodon click train is detected, everybody gets excited in hopes of seeing a beaked whale.  I can really understand how the visual sighting team relies on the acoustics team to find a location.  We have two people on big eyes and two people on binoculars, and the ocean is all around us.  We have a high probability of missing a Mesoplodon, so having the acoustics team getting a click train with convergence in a certain direction helps to focus the visual sighting team in sighting an animal.  The reverse idea is also true.  When the visual sighting team sees a Mesoplodon, they call down to acoustics to see if a click train can be detected.

Life aboard the Gordon Gunter has been a real classroom for me.  I think I learn something new about every five seconds.  Since I have been out of college, I have not dealt with biological sciences much, so this math teacher is relearning some key information about marine animals.  I have really enjoyed seeing the passion in everyone’s eyes for the beaked whales.  When we get a sighting of a beaked whale on the flybridge, everyone rushes to that side of the ship in hopes of just getting a glance at the elusive creature.  When we get a Mesoplodon click train, the acousticians get really excited.  One evening, we got a sustained click train for a Sowerby’s beaked whale (Mesoplodon bidens).  One of the acousticians was not in the dry lab, so I went to try and find her with no luck.  She was really upset when she returned, because she had not been there to see it.  I hope to develop that kind of passion in my students, so they can become great thinkers about life in their futures.

Did You Know?

1. Even though Moby Dick was a fictional sperm whale, real life event inspired Herman Melville to write the novel.  Check out this page on those events:  https://oceanservice.noaa.gov/facts/mobydick.html.
2. Sperm whales use an organ in the front of their head, something called the spermaceti organ, to make their clicking sounds.  Check out this PBS article: http://www.pbs.org/odyssey/odyssey/20010809_log_transcript.html.

Animals Seen

1. Sperm whales (Physeter macrocephalus)
2. Fin whales (Balaenoptera physalus)
3. Cuvier’s beaked whale (Ziphius cavirostris)
4. Risso’s dolphins (Grampus griseus)
5. Manta ray (Manta birostris)
6. Whale shark (Rhincodon typus)

Vocabulary

1. (Ocean) Acoustics – the study of how sound is used to locate whales and dolphins and how whales and dolphins communicate
2. Bridge – the room from which the boat can be commanded
3. Click train – a series of whale clicks
4. Dry lab – a lab that primarily uses electronic equipment such as computers
5. Echolocation – a process used by whales and dolphins to locate objects.  A whale will emit a pulse, and the pulse then bounces off an object going back to the whale.  The whale can then determine if the object is food or something else.
6. Flybridge – an open platform above the bridge of a ship which provides views of the fore, aft, and sides of a ship
7. Hertz – a measure of sound frequency.  For example, when you hear someone singing in a low (or bass) voice, the frequency of the sound is low.  When someone is singing in a high (or soprano) voice, the frequency of the sound is higher.
8. Hydrophone – a microphone that detects sound waves under water
9. Spectrogram – a visual representation of a sound
10. Wigner plot – a high resolution spectrogram