Eight-foot pitch is a term common to the organ

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

 and the harpsichord

Harpsichord

A harpsichord is a musical instrument played by means of a keyboard. It produces sound by plucking a string when a key is pressed.In the narrow sense, "harpsichord" designates only the large wing-shaped instruments in which the strings are perpendicular to the keyboard...

. An organ pipe, or a harpsichord string, designated as eight-foot pitch is sounded at standard, ordinary pitch. For example, the A above middle C in eight-foot pitch would be sounded at 440 Hz (or at some similar value, depending on how concert pitch

Pitch (music)

Pitch is an auditory perceptual property that allows the ordering of sounds on a frequency-related scale.Pitches are compared as "higher" and "lower" in the sense associated with musical melodies,...

 was set at the time and place the organ or harpsichord was made).

Similar terms

Eight-foot pitch may be contrasted with four-foot pitch (one octave above the standard), two-foot pitch (two octaves above the standard), and sixteen foot pitch (one octave below the standard). The latter three pitches were often sounded (by extra pipes or strings) along with an eight-foot pitch pipe or string, as a way of enriching the tonal quality.

The origin of all these terms is based on the fact that, all else being equal, a pipe or string that is half the length of another will vibrate at a pitch one octave higher.

Why eight feet?

The particular length "eight feet" is based on the approximate length of an organ pipe sounding the pitch two octaves below middle C, the bottom note on an organ keyboard. This may be calculated as follows.

Physics tells us that if a pipe is open at one end, as is true of many organ pipes, its fundamental frequency

Fundamental frequency

The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the lowest frequency of a periodic waveform. In terms of a superposition of sinusoids The fundamental frequency, often referred to simply as the fundamental and abbreviated f0, is defined as the...

 F will be:



If V is assumed to be 1130 feet per second (the speed of sound at sea level, with temperature 70 degrees Fahrenheit), and the pipe length L is assumed to be eight feet, then the formula yields the value of 70.6 hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

(Hz; cycles per second). This is not far from the pitch of the C two octaves below 440 Hz, which (when concert pitch is set at A = 440 Hz) is 65.4 Hz. The discrepancy may be related to historical differing definitions of the length of the foot as well as variations in tuning prior to the setting of A = 440 Hz as standard pitch in the 20th Century.