Arctic Spectral Parameter Guide

 

When you first open an LTSA file, you’re default Plot and Control windows look like the following:

 

 

 

 

 

Always adjust the Brightness (40 dB) and Contrast (200 %dB) settings so that the intensity of sound is represented by many colors:

 

 

 

Using the parameters above, the same Plot window now looks like the following:

 

 

Notice the Color Bar to the right of the spectrogram. The scale of colors can help you to determine where there are acoustic events on the screen. Dark blue is low energy (or “quiet”) while dark red is high energy (or “loud”). We measure the power of sound in units called Decibels.

 

 

 

 

 

Change the Plot Length and display six hours in one ltsa window:

 

 

Scan through time in the LTSA using the green toggle buttons:

 

Beginning                   Manual                                 Manual                             End

of File               Rewind                                Forward                          of File

           Fast                                                                                Fast

         Rewind                                                                 Forward    

 

 

The LTSA is ready for you to explore. Use the expand button   to select a time in the spectrogram. Triton will open the XWAV file below at the specified time.

 

 

When you first open an XWAV file, you’re default Plot and Control windows look like the following:

       

Start by changing the Plot Length from the default (only 1 second) to somewhere between 30 to 60 seconds. The spectrogram shouldn’t look “grainy” if you are displaying longer durations of time in the data.

 

 

Always adjust the Brightness and Contrast settings so that the intensity of sound is represented by many colors. Start with a brightness of somewhere between 10 – 15 dB and a contrast of about 150 %dB.

 

 

 

Using the parameters above, the same Plot window now looks like the following

 

 

Notice the color bars to the right of the spectrograms. Its good practice to always change the plot length, brightness, and contrast settings whenever opening a new spectrogram.

For exploration shifts, you will be expected to analyze data at specified times throughout the day. For example, if your shift begins on May 1, 2007, at 1200, you should type the following into the Plot Start Time box:

 

 

You can begin scanning through the XWAV using the blue toggle buttons:

 

For scanning mode, keep the Plot Length between 30–60 seconds (the x-axis of the spectrogram).

 

When you have discovered an event in your spectrogram, zoom-in by changing your Plot Length accordingly so the event fills the window like shown below:

 

Notice this spectrogram is displaying 15 seconds in time (x-axis) during a call.

The setting that controls the y-axis of the spectrogram is the Plot Frequency box. The default values in the Control window are 0 Hz (Start) and 16000 Hz (end).

 

 

 

Broadband is a term that refers to all frequencies in the data band. So when you are analyzing data from 0 – 16 kHz, you are in broadband mode. If you are scanning through your exploration shift and are unsure what to look for, then it is probably best to keep broadband frequency settings.

 

When you are investigating events in the spectrogram that only appear within a narrow frequency band, try adjusting the Start and End frequencies in the Plot Frequency box. This will zoom-in and display only frequencies of interest. Shown below is an example for a call between 5–10 kHz.

 

 

 

 

In general, when you are searching for ice seals and beluga whales, their calls will be found in higher frequencies between 1–16 kHz.

 

 

This is not the case for all marine mammal vocalizations though. Walrus knocks are usually found in mid-frequencies between 400 Hz – 2 kHz. You can refer to the Arctic Marine Mammal Guide to learn more about the different frequencies that Arctic mammals vocalize at.

 

 

For example, in order to find bowhead whale calls, change the Plot Frequency box to 0 – 1 kHz. When you find a call, zoom-in on the appropriate frequencies, as shown below:

 

 

 

 

 

 

Band Pass Filter

 

Turning on the Band Pass Filter allows you to display sound only in the frequency range you specify. Set the frequency parameters for the Low End and High End in the Band Pass Filter box. Data will be discarded below the Low End and above the High End frequencies, and you will only see the frequencies in between. The example shown below has a band pass filter set to 5 – 10 kHz.

 

 

 

Do not use the Band Pass Filter for scanning mode. Only use for spectrogram analysis (zoom-in mode) when investigating an acoustic event of interest. This is a good method to get rid of unwanted noise in the spectrogram and keep only spectral content with a clean signal.

 

 

 

 

 

 

 

Spectral Parameters

 

Think of FFT Length as the quality of data resolution in frequency (or the y-axis), using Fourier Transform analysis. The default value for FFT Length is 1000.

 

 

 

In most cases for Arctic marine mammal vocalizations ranging in frequency between 1 – 16 kHz, this setting is appropriate.

 

This is not the case for all marine mammal calls though. For sounds found at low frequencies below 1 kHz like Walrus knocks or Bowhead whale calls, you should decrease the resolution of the FFT Length by adjusting the parameter to about 500. An example of a spectrogram with an FFT Length of 500 is shown below:

 

 

 

 

Think of % Overlap as the quality of data resolution in time (or the x-axis), using a time averaging method. The default value for Time Overlap (%) is 0.

 

 

Do not adjust the Time Overlap (%) for scanning mode. Only use for spectrogram analysis (zoom-in mode) when investigating an acoustic event of interest. In that case, start by adjusting the % Overlap box between 50-75%. Never exceed 90% or else data is not reliable. Shown below is an example using 75 % overlap for a spectrogram in order to properly display clicks and buzzes.

 

 

 

Audio Playback

 

If you’re not sure what it is, LISTEN TO IT!

 

                      Speed             Volume                Play

 

Adjusting the speed of playback is useful for low-frequency sounds that are difficult for the human ear to hear. Try adjusting this setting by increasing the speed to a parameter >1. For example, 2 would be twice the speed and 3 would be three times the speed. Alternatively for high-frequency sound, you can set the speed to <1. For example, 0.5 would be half the speed. Slowing down the audio is not really necessary for the Arctic data set.

 

Get in the habit of turning up the Volume and pressing Play in order to listen to the spectrogram representation in the Plot window.

 

Hint – If the audio playback is not working properly, first check to make sure that the computer’s volume is turned up. If you still can’t hear anything, try using a Band Pass Filter around the frequencies you want to listen to. The audio playback should now work while the Band Pass Filter is turned on.