Changing the Spectrogram Magnitude Display

A spectrogram represents the evolution of the magnitude of the spectrum of a signal versus time (and possibly versus RPM). This magnitude, also known as the level, can be displayed in two different ways in Sound: Analysis and Specification.

  1. As a logarithmic level, in which case the dB level is calculated from the linear magnitude and then displayed as a color scale. The dB level is calculated as:

    where:

    • LdB is the level expressed in dB
    • t is the time
    • f is the frequency
    • is the magnitude of the (one-sided) spectrum with time t and frequency f
    • P0 is the reference value associated with the signal unit (for example P0=2e-5 when the unit is Pa) - see Managing Physical Units.

  2. As a linear level, in which case the unit of the displayed magnitude is the same as the unit of the original signal. For example, if your original signal is an acceleration signal in m/s2, the linear level will be expressed and displayed in m/s2. The displayed quantity is then .

Use the View > Amplitude display menu option to change the spectrogram display between a linear and logarithmic scale.

Tip: Amplitude display is also available by right-clicking the spectrogram display.