Sunday, July 24, 2016

Can you hear me now?

Figuring out the loudness at which an animal is vocalizing is a deceivingly difficult question to answer.  To investigate this question of loudness, we use an equation that, on the surface, is deceivingly simple – the sonar equation. 

SL = RL + TL

Source level (the actual loudness of the call) is equal to the received level (the loudness of the call when it gets to the hydrophone) plus the transmission loss (the amount of loudness that’s lost over the distance between the caller and the hydrophone).  How bad could it be?

Calculating received levels – this involves a few pieces of information.  First, you need to know when your animal is actually vocalizing.  I’ve browsed through dozens of hours of acoustic data from the 2015 season and have marked hundreds of calls from harbor seal males.  Lucky for me harbor seals call almost nonstop; at least finding the calls is easy enough.  I also need to know some information about my hydrophone, mainly the sensitivity.  Different hydrophones have different sensitivity levels, which influences the calls it can pick up.  A more sensitive hydrophone can pick up quieter calls than a less sensitive hydrophone.  Then, after I’ve input these hydrophone parameters, with the flick of my magic acoustic wand, a program called Raven will give me a received level.  Step one is complete.

Pretty picture from Strawberry Island to break from the scienceness of this blog post (Photo: L. Matthews)

Transmission loss is a bit more of a beast.  Back in the 1980’s, Malme and Miles came to Glacier Bay and investigated how sound travels underwater in this environment.  Turns out that sound attenuates at a rate of 15log(r), where r is the distance from the vocalizing animal to the hydrophone.  So if I want to quantify how much loudness is lost between my animal and my array, I need to know from where my animal is calling.  Enter acoustic localization. 

Sound is a pressure wave traveling through a medium.  In water, sound travels approximately 1500 m/s.  Depending on where an animal is located when it calls, the vocalization will travel through the water and arrive on the different hydrophones in our hydrophone array at different times.  The call will get to the closest hydrophone first, and the farthest hydrophone last.  You can see what I mean in the spectrogram below.  Each line is a different hydrophone, and this particular calling animal is closest to the second hydrophone.

I am lucky enough to be collaborating with a group at Cornell University called the Bioacoustics Research Program.  They’re the ones who make the software that I use to do the acoustic localization.  It’s been an up and down process to get this localization up and running, but after a few rounds of troubleshooting, it’s working with a fair amount of consistency.  It’s a slow and time-consuming part of calculating source levels, but so far I’ve managed to locate over 250 harbor seal vocalizations. 

Harbor seals called here.

Then there’s some coding involved to get the actual distances between the caller and the hydrophone, and a little excel spreadsheet organizing, but then it’s just plugging and chugging to get a number for a source level.  Sweet science victory. 

Hello sweet angel.  (Photo: L. Matthews)

I’ve always been intimidated by calculating source levels, and rightfully so.  There are a lot of different pieces that have to fall into place for everything to turn out correctly.  In my downtime on Strawberry Island, I’ve been slowly but surely chipping away at my source level analysis.  The island seems to be a good place to hunker down and focus.  Somehow it’s so much easier to be on my computer and not get distracted when the internet is just a distant memory….

Working hard on the beach.

Now that I’m getting baseline estimates of how loud harbor seals are vocalizing, I can start to answer another question.  Do harbor seals change their source levels when there are vessels nearby?  We see this phenomenon in many species, including humans.  When our environment is loud, we get louder so that other people can hear us.  I’m curious to see if harbor seals do the same thing.

Cruise ships are a regular occurrence during the summer (Photo: L. Matthews)

FUN SCIENCE FACT #36: Sound travels almost 5x faster underwater compared to in air.  From the hydrophones, we can hear a cruise ship coming almost 45 minutes before it gets to the island.

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