People vary in their ability to detect the pitch of a sound. For example, in tuning a guitar, some are able to distinguish easily between two notes of similar pitch while others can't hear the difference. A more objective measure than pitch is in terms of frequency, measured in hertz (Hz).
The frequency of any vibration is defined as the number of complete vibrations made each second:
The frequency (in hertz) is equal to the number of vibrations per second
Sometimes a complete vibration is referred to as a cycle, so the frequency (in hertz) is also equal to the number of cycles per second.
You can picture one vibration or cycle of the speaker cone being completed as it starts from its most forward position, moves backwards, and then moves forwards to return to its original position.
The frequency value not only specifies the number of complete vibrations made by the source each second, but also the number of regions of high (or low) density produced in the medium each second. In other words, it gives the frequency of the actual sound.
High pitch corresponds to high frequency, low pitch to low frequency. There is a whole spectrum of sounds arranged along the frequency axis.
Quite often the numbers get large, and the frequency is referred to in terms of kilohertz (kHz): 1000 hertz = 1 kilohertz.
For example, a hi-fi loudspeaker might have a range of 20 hertz to 20 kilohertz and so be capable of producing sounds over our whole range of hearing.
In 1939 musicians moved to an agreement on standard pitch. Middle C (called that because it is a note around the middle of the piano keyboard) is 256 hertz and concert A (the note the orchestra tunes to before the show begins) is 440 hertz.
The frequency doubles for every octave upwards.