The overtone series can be produced physically in two ways – either by overblowing a wind instrument, or by dividing a monochord string. If a monochord string is lightly damped at the halfway point, then at 1⁄3, then 1⁄4, 1⁄5, etc., then the string will produce the overtone series, which includes the major triad. If instead, the length of the string is multiplied in the opposite ratios, the undertones series is produced. Similarly, on a wind instrument, if the holes are equally spaced, each successive hole covered will produce the next note in the undertone series.
String quartets by composers George Crumb and Daniel James Wolf,[citation needed] as well as works by violinist and composer Mari Kimura,[3] include undertones, "produced by bowing with great pressure to create pitches below the lowest open string on the instrument."[4] These require string instrument players to bow with sufficient pressure that the strings vibrate in a manner causing the sound waves to modulate and demodulate by the instruments resonating horn with frequencies corresponding to subharmonics.[5]
The tritare, a guitar with 'Y' shaped strings, cause subharmonics too. This can also be achieved by the extended technique of crossing two strings as some experimental jazz guitarists have developed. Also third bridge preparations on guitars cause timbres consisting of sets of high pitched overtones combined with a subharmonic resonant tone of the unplugged part of the string.
Subharmonics can be produced by signal amplification through loudspeakers.[6]