Frequency is how often something happens in a certain time, for instance the number of times the Praat home page www.praat.org is visited every day.
In Praat, frequency is the number of vibration cycles per second. Although one can sometimes see the abbreviation cps, Praat always uses Hz (short for hertz), which means the same.
Unfortunately, there are two very distinct kinds of vibrations in speech analysis. For pitch, frequency is the number of glottal cycles per second, and for spectral analysis, frequency is the number of sine wave cycles per second. Quite some bit of the training of an acoustic phonetician goes into the understanding of the difference between the ideas behind F0 and F1, and many years can be spent on understanding the influence they have on each other in production, acoustics, perception, or measurement...
In order to prevent confusion, Praat always requires frequency to be expressed in Hz. So if you want to supply a sampling frequency of 20 kiloHertz (20 kHz), you fill in 20000 or 2e4 or 20e3. If you want to switch off pre-emphasis in some spectral analysis, you supply 1 GigaHertz (GHz) for its "from-frequency", by typing 1e9.
In Praat editor windows, frequency usually runs from bottom to top, since time already has to run from left to right. This goes for spectrograms, pitch contours, and formant contours. In spectral slices, frequency runs from left to right, since these have no time axis.
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