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Audio analysis: Mel Spectrograms

·3 mins
audio digital signal processing
Matt Moore
Author
Matt Moore
Table of Contents

Waveform
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You’ve maybe seen audio waveforms before. A waveform is essentially a pressure graph over time. It represents the amount of displacement or pressure at any given point in time. It’s a very effective tool for audio editors to use for slicing audio and manipulating volume (pressure).

You can see below in the waveform graph, the X axis is time and the Y axis is amplitude (pressure).

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While useful for general audio editing/mastering, waveforms represent the time domain, and don’t give us any useful information about specific frequencies.

Discrete Fourier Transform
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If we want to see the specific frequencies in a given audio signal, we need to go from the “time domain” of the waveform to the “frequency domain”. We do this with Fourier Transforms. To be exact, we use a “discrete” FFT.

Notice the X axis here represents frequency, and the Y axis represents magnitude of each frequency.

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Spectrogram
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Amplitude
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While the discrete FFT above gave us the magnitude of each frequency, we lost the time information. To preserve as much information as possible for real-world practical audio processing, we need to preserve these 3 dimensions of the audio signal:

  • Time
  • Frequency
  • Magnitude (the amplitude or “strength” of each frequency)

To do this, we can create what is called a “spectrogram”. In a spectrogram the 3 dimensions above are represented as:

  • X dimension for time
  • Y dimension for frequency
  • Heatmap for magnitude

Here’s an example:

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Decibels
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You might notice the spectrogram above is rather hard to see. This is because we’re measuring raw amplitude/magnitude (the “power” of the signal). To make it easier to see, let’s standardize this to decibels, which is the standard way to measure audio volume. Librosa has a function for this called power_to_db which we’ll use:

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Mel Spectrogram
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So far we’ve created a spectrogram to represent the magnitude of each frequency in the audio signal over time. However, there is one final step needed because of a rather interesting neurological phenomenon in humans called “frequency discrimination”.

For humans, whenever the pitch of a sound (frequency) changes, we can generally tell that it has changed. However, our ability to hear the difference in pitch is not the same across all frequencies. We can discriminate changes more easily on the lower end of the frequency spectrum, but on the higher end of the frequency spectrum, our ability to differentiate between pitches is noticeably reduced.

In other words, our ability to perceive differences across frequencies is not linear; it is logarithmic.

What this means then is that we need to normalize (adjust) the frequency representation to something that more closely resembles the human perception of the frequencies and the changes in frequencies. This is done with a “mel-spectrogram”. “Mel” is short for “melody”.

In the spectrogram above (with decibels) we have a lot of frequencies showing up - including many that humans don’t perceive very readily. The mel spectrogram will allow us to adjust to show the frequencies that are more readily perceivable by humans.

Here’s how we do that:

Click here to view this notebook in full screen

Here the dimensions represented are:

  • X axis for time
  • Y axis for frequency
  • Heatmap for magnitude, but here I’ve also converted it from raw amplitude to decibels, the standard for measuring volume.

With the mel-spectrogram you’ll notice more dark areas. This is because we’ve filtered out the frequencies that humans can’t perceive as well, leaving behind the frequencies that are more perceivable by humans.