Most of my time with bioacoustics, thus far, has been with playing sounds – my master’s work with an active acoustic tag – or with identifying odontocete, or toothed whale species, in glider data (typically known as high- or mid-frequency vocalizations).
For my PhD, I’ll be expanding what I know about whale acoustics and looking at baleen whales from glider and float data as well. I started into this the last few weeks and it has been fun, but definitely feels like a step back in time trying to look up literature and see what exactly I am hearing in the data – I’m not used to working with low-frequency sounds.
What do I mean with low- vs high-frequency sounds? These labels are based on human hearing (of course). Humans (babies!) can typically hear from 20 Hz (hertz) to 20 kHz (kilohertz…hertz*1000; 20 kHz = 20,000 Hz). As we get older we start to lose hearing on the higher end. But marine mammals vocalize both below and above our hearing range. The low/high delineation is “generally” accepted at 1 kHz, and typically baleen whales vocalize below this, and toothed whales vocalized above this. But remember, this is just USUALLY. There are always special cases that don’t follow the trend, and its all relative terms when calling things low and high.
This figure from Mellinger et al. 2007 is a great way of see where certain species typically vocalize. (Click he figure to link to the PDF of the paper and zoom in)
Looking at sounds
So since some whales make sounds below my hearing range, and some make sounds above, how do I hear them for analysis? Well first of usually I am identifying sounds by looking at them, at a spectrogram (we’ve posted those before right?).
Then sometimes I need to listen AND look to identify what the sound is, or gather more info about it. Wonderfully there is a work around. For really LOW sounds, you can play them faster, and then that increases the perceived frequency, so you can hear it. Vice versa, for really HIGH sounds, you can play them at half speed, which changes the perceived frequency, and then you can hear them. Does anyone remember Yakbaks? Speeding up your voice makes you sound like a chipmunk, slowing it down makes you sound like…a whale?
If you are interested in hearing some baleen whale sounds, sped up so you can hear them, look here: http://cetus.ucsd.edu/voicesinthesea_org/species/baleenWhales/blue.html
You’ll see that on the spectrogram it says *recording plays at 6 times normal speed for better listening.
But this one (http://cetus.ucsd.edu/voicesinthesea_org/species/beakedWhales/cuviersBeaked.html) is played at 1/10th the speed so you can hear it!
How high can you hear? I lose the signal at about 17 kHz 🙁