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The Adventurer's Guide to Spectrograms

Nothing unlocks the magic of a soundscape recording quite like looking at its spectrogram at the same time as listening to it. Spectrograms show frequency information, or pitch, on the vertical axis and time on the horizontal axis, and the darkness of a point on the spectrogram indicates the loudness of a sound at that frequency. Spectrograms reveal the complex hidden details of sounds and help us understand the structure of animal vocalisations and why things "sound" the way they do.

Spectrogram of a soundscape recording

With just a short amount of practice you'll be identifying animal calls visually from their spectrograms long before you hear them and waiting with anticipation as they scroll across the screen before playing through your speakers.

This also means you can quickly scan over an hours worth of recordings in minutes to find or identify calls of interest, or see something completely new and listen to find out what it was. A distant owl? A Kangaroo splashing through a creek? Or a proverbial tree falling over?

In this guide I'll show you two free tools to get you started with spectrograms:

1. Audacity

Audacity is a free, open source, audio editing software which is easy to use and runs really well.

When you load a file in Audacity it will show the waveform view by default. To change to spectrogram view click on the filename in the top left of the inside window (just above the "Mute" and "Solo" buttons) then select Spectrogram from the drop-down menu, see below:

Next you'll want to change from coloured to black and white spectrograms as it's a lot harder to see patterns amongst the arbitrary changes in colour. To do this click on Edit, Preferences, the select Spectrograms from the list on the left and change the settings to match those shown below:

Spectrogram settings for Audacity

Once you are looking at your spectrogram you can use the [ctrl] + scroll wheel of your mouse to zoom in and out along the time axis and [shift] + scroll wheel to scroll the audio left or right.

So lets look at a spectrogram in action. This one starts off nice and calm with a bar-shouldered dove, an eastern whipbird, but get pretty rowdy 30s in.

Audacity has a lot of other handy features like high-pass or low-pass filters which you can use to get rid of unwanted sounds from your recordings but we'll save that for another day.

2. Raven Lite

Download: (you'll have to register to activate the free license)

The other piece of software I use is called Raven Lite, developed by the Cornell Lab of Ornithology at Cornell University. It's the free version of a more extensive piece of software designed for bioacoustic researchers. It has many useful features including labelling and annotating your spectrograms to name just a few.

By default the spectrograms in Raven are coloured as well but that is easy to change by selecting the colour pallet icon on the left, just above the "Views" window, see below:

I find the interface for zooming and scrolling the around data to be more tedious than in Audacity as there are no mouse scroll-wheel controls and the spectrograms in Audacity seem to have more detail than in Raven (at least in the free version, I haven't used Raven Pro) but it's many other useful features will make you overlook those small complaints.

You can also increase the brightness and contrast of the spectrograms which is handy if you don't want to see the quieter background sounds and just want to focus on finding specific calls more easily when you are scanning through a lot of files.

Here's a short video of me annotating some bird calls but there are lots of good video tutorials over on Cornell's website

Audacity and Raven Lite are both great tools to get started looking at spectrograms. If you've got a job to do and you want to record species lists, annotate and label your spectrograms then Raven can't be beat. If you want to quickly go through and explore your spectrograms for the joy of it then I'd recommend giving Audacity a go.

Of course if you've got thousands of hours of recordings to get through then you're going to need some automated processing software to help you separate the wheat from the chaff first, but that's a topic for another day.

If you know of another program/platform/website that you like to use, let me know and I can add it to the list here.

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