More ways to remove unwanted noise - Noise reduction and Pass filters

Much of the noise that you’ll want to remove from your recordings will occur across a limited frequency range, so can’t simply be cut out as demonstrated in this post. In this recording of a grey wagtail in flight, you’ll notice that the bottom half of the sonogram (the lower frequencies) is quite dark compared to the upper half (the higher frequencies). I made this recording not far from a busy road, so much of the low frequency noise will be that of cars passing by in the background.

There are three ways to reduce or eliminate this sort of noise from a recording, and all use features in the Effect menu. The tools to use are: Noise reduction, Pass filters, and the Equalisation tool. 

Noise reduction

Highlight a part of your recording that contains the noise you want to remove, but does not contain any of the important stuff you want to retain. Then select Noise reduction from the Effect menu. In the window that appears, click on the ‘get noise profile’ button.

When this is done, highlight the whole of your recording (click and drag) and then return to the Effect menu, selecting Noise reduction again. This time, just press OK and Audacity will reduce the sounds that you ‘profiled’ and reduce them across the whole recording. 

You will see in the image below that the darker area at the bottom of the sonogram has been greatly reduced.

Pass Filter

To reduce unwanted low frequencies, as with this example, choose the High pass filter from the Effect menu. 

A window will appear, and in the box labelled ‘Frequency’, type the frequency below which you want to reduce the levels. In this example I’ve typed 5000, as this is roughly where the background noise disappears, and crucially, is lower than the lowest frequency shown by the calls you’re interested in. Press OK and the High pass filter will reduce much of your unwanted noise.

You’ll be able to hear the difference in your recordings of course, as well as seeing it on the sonograms. Beware however that over editing of this sort can produce rather unnatural sounding recordings - with too much editing of this nature making recordings sound down right unpleasant. It's an effective way of cleaning up sonograms though, so if you're submitting a sonogram and a recording for a description, for instance, consider 'cleaning' the sonograms but sending a different, less edited r