Monday, 20 February 2017

Refining your sonograms with Window size and type

Tucked away in the Audacity menu, hiding in the preferences tab, are two ways of refining the appearance of your sonograms. These involve changing the size of the window (it doesn't actually change the size of the window as it appears on the screen) or changing the window type. This changes the way in which the sonogram is generated and can therefore have some effect on its appearance. 

Changing the window size effects how much frequency detail is revealed. The general rule is the larger the number, the more frequency detail one gets at lower frequencies. The payoff is that there is less temporal resolution, but I find that this is less critical when viewing sonograms of birdcalls. How the window size effects the appearance of the sonogram can be illustrated nicely with this rather poor recording of a distant tawny owl (that has been ‘cleaned’ with a bit of noise reduction). To alter the size, open the 'Audacity' dropdown and select 'Preferences'. Select the 'Spectograms' tab, then, experiment with the window size options in the relevant dropdown.

The first sonogram shows a sonogram with a window size of 512, and the second with a window size of 2048 - see how the frequency detail is so much easier on the eye at 2048.

Changing the window type usually has a less radical effect on how your sonogram appears, but it can be worthwhile experimenting with different options if you have a sonogram that could look better. Again, choose the 'Prefefences' from the 'Audacity' menu, and experiment with the different window types in the 'Spectograms tab. Here, this firecrest call is displayed with the default window type, which is called ‘Rectangular’. 

Not very helpful! Changing it to the second option, called Bartlett, helps thins a little, but still, doesn’t really produce anything that you can really work with. 

Choosing the option called Gaussian(a=4.5) however, produces something much more useful. I don’t know of any hard and fast rules regarding how choosing different window types might improve your sonograms, so it’s always worth just experimenting with a few options in order to get the best results.

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