This tutorial explains how to understand spectrograms. An original method is proposed to explain how to understand a spectrogram, starting from a spectrum.
Sound engineers and producers know more or less how to understand a spectrum. A spectrum is a curve that is displayed by spectrum analyzer plugins for example. This curve represents the frequencies horizontally, and the amplitudes vertically. The spectrum curve is animated over the time, to represent the changes of the sound currently playing. A spectrum can be used for example to detect a peak of amplitude corresponding to a narrow frequency range.
A spectrogram is a 2D image which scrolls over the time. This image represents the time horizontally, and the frequencies vertically. The amplitude for a given point of the image is represented by a more or less bright color. For high amplitudes, the colors are brighter, and for low amplitudes, the colors are less bright. For example, very high amplitudes will be displayed with colors close to white, and very low amplitudes (silent parts of the sound) will be displayed with colors close to black. Some other color codes are often used, for example hot colors (like red) for high amplitudes, and cold colors (like blue) for low amplitudes.
We will now see how to go from a spectrum to a spectrogram, by using two rotations. The explanation is given in four examples, from a very simple sound to a real world example. Each example shows the explanation step by step.
Example 1: Simple sine wave
This first example shows how to go from a spectrum to a spectrogram, step by step, with a simple sine wave of 500Hz.
Example 2: Sine sweep
This second example shows how to go from a spectrum to a spectrogram, step by step, with a sine sweep. A pure sine frequency moves over the time, from lowest frequencies to highest frequencies, and returns to the lowest frequencies.
Example 3: White noise + low pass filter
Another example to show how to go from a spectrum to a spectrogram, step by step, with a white noise. The sound that is displayed is a white noise with a low pass filter. High frequencies are diminished a lot.
Example 4: A real world example (guitar)
This last example shows how to go from a spectrum to a spectrogram, step by step, with a real world sound. The example sound is a guitar sound. The spectrum is more complex than in the previous examples, and gives a more complex spectrogram.
In this tutorial, we have seen how to understand a spectrogram, starting from the well known spectrum view. Hope this has been helpful! It is advised to try by yourself with simple sounds in real-time, then with more complex sounds. You will see that spectrograms are not so complicated to understand, and after some tests by yourself you should become comfortable with spectrograms!
This tutorial uses three plugins:
- A spectrum analyzer
- The Wav3s plugin for the 3D views
- The Ghost Viewer plugin for the spectrogram views
BlueLab provides several spectrogram plugins that may interest you:
- Ghost-X: spectrogram viewer and editor
- Ghost: free and functional lite version of Ghost-X
- Ghost Viewer: simple spectrogram viewer optimized for visualization
- Infrasonic Viewer: spectrogram viewer dedicated to infrasounds (very low frequencies)
- Wav3s: free sound visualization in 3D using a waterfall plot view, which is very similar to a spectrogram view