Octave
In music, an octave (sometimes abbreviated 8ve or 8va) is the interval between one musical note and another with half or double the frequency. For example, if one note is pitched at 400 Hz, the note an octave above it is at 800 Hz, and the note an octave below is at 200 Hz. The ratio of frequencies of two notes an octave apart is therefore 2:1. Further octaves of a note occur at 2n times the frequency of that note (where n is an integer), such as 2, 4, 8, 16, etc. and the reciprocal of that series. For example, 50 Hz and 400 Hz are one and two octaves away from 100 Hz because they are 1 / 2 (1 / 21) and 4 (22) times the frequency, respectively, however 300 Hz is not a whole number octave above 100 Hz, despite being a harmonic of 100 Hz.

Octave band
A frequency range with an upper limit that is twice the frequency of its lower limit. The audible frequency range is often divided into bands of frequencies because sound transmission through solid barriers can vary dramatically with the frequency of the sound. The broadest bandwidth commonly used is an

octave. An octave is any band where the highest included frequency is exactly two times the lowest included frequency. For example, the frequency band that covers all frequencies between 707 Hz and 1,414 Hz is an octave band. The band is identified by its centre frequency, which is defined as the square root of the product of the highest and lowest frequency. In the above case the centre frequency is 1000 Hz. The entire frequency range of human hearing can be covered in the following 10 standard octave bands: 31 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz, and 16000 Hz

One-third octave band
A frequency band that has a width (in Hz) that is only 1/3 of the width of an octave. It takes 31 one-third octave bands to cover the entire frequency range of human hearing. One-third octaves are used when octave band analysis does not provide adequate resolution in the frequency domain.

Overtone
A whole-number multiple of the fundamental frequency of a tone. The overtones (also called harmonics; 1st overtone is 2nd harmonic) define the harmonic spectrum of a sound. E.g. a fan with a rotating frequency of 45 Hz (=60*45=2700 rpm) can generate vibrations at 45 Hz and overtones in 90, 135, 180 Hz.