# Terminology: spectrum, spectrogram, spectrograph, sonogram, etc

In term of proper or accepted naming conventions of DSP graphics or instrumentation output, what is the difference between the words spectrum, spectrogram, spectrograph, and similar terms, and what type of chart, graph, CRT display, or etc. does each best describe.

ADDED: Also I found the term sonogram used in a couple books for spectrum-vs-time graphics. When might thus be appropriate in preference to one of the above terms, or vice versa?

It depends on context.

In signal processing, a spectrum (plural is spectra) shows the frequency content of an entire signal. It's a 1-dimensional function of amplitude (vertical axis) vs frequency (horizontal axis):

Spectra are often shown with a logarithmic amplitude axis (such as dB), but this isn't necessary.

A machine that produces a spectrum is usually called a spectrum analyzer. In other fields, the machine is called a spectrograph or spectrometer.

A spectrogram shows how the frequency content of a signal changes over time. It's a 2-dimensional function of amplitude (brightness or color) vs frequency (vertical axis) vs time (horizontal axis):

Sometimes this is called a sonogram. The time and frequency axes are sometimes swapped. If amplitude is shown as a 3D surface rather than using color, it's called a waterfall plot.

Confusingly, a machine that produces a spectrogram is also called a spectrograph, or spectrograph is used as a synonym for spectrogram.

Also the line is kind of blurred, because if you view the spectrum of a live signal on a spectrum analyzer, it's displaying the spectrum of small chunks of the signal and you're seeing how it changes over time, which is essentially the same thing as a spectrogram.

I think the important distinction is just the way they're displayed: A spectrum is a 1D plot and a spectrogram is a 2D plot.

• Your graph of spectrum has frequency on the X axis and amplitude on the Y axis. That seems like two dimensions rather than one. (Your signal graph also has two dimensions, time and amplitude.) Mar 4, 2021 at 0:40
• @LarryEngholm Maybe I should say "one-variable" math.stackexchange.com/q/918740/2206 Mar 5, 2021 at 2:51

Here a freeware for the spectrogram/sonogram extraction--> https://github.com/Christoph-Lauer/Sonogram

• Your program might be useful, however it does not (yet) qualify as an answer. Jul 14, 2021 at 10:21