From WebRef.org

- The interval between two frequencies having a ratio of 2:1.
- The range of frequencies between a given point and twice the frequency of the given point. Middle A on a piano has a frequency of 440Hz. One octave higher (the next A) the frequency is 880Hz, twice the original frequency. Source: Church Audio & Acoustics Glossary
- Two frequencies are an octave apart if the ratio of the higher frequency to the lower frequency is two. Source: Glossary of Terms in Noise Control Engineering
- An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Each octave you add on the bottom requires that your speakers move four times as much air! Source: http://www.owenscorning.com/around/sound/glossary.asp

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