Overblowing is the manipulation of supplied air through a wind instrument that causes the sounded pitch to jump to a higher one without a fingering change or the operation of a slide.
In some instruments, overblowing involves the direct manipulation of the vibrating reed(s), and/or the pushing of a register key while otherwise leaving fingering unaltered.
With the exception of harmonica overblowing, the pitch jump is from one vibratory mode of the reed or air column, e.g., its fundamental, to an overtone.
Another type of overblowing is that used on instruments such as the transverse flute, where the direction of the airstream is altered in order to sound higher notes.
Among Highland pipers, the term more often refers to a problem affecting the steadiness and reliability of the pitch and tone caused by an excess of air pressure.
Often, a piper will over-squeeze the bag while still exhaling, causing a pipe to cease to sound or to vary its tone and pitch.
It does not induce the sounding reed to sound a higher overtone – free reed overtones do not even begin to approximate the harmonic series nor are they particularly musical – nor does it induce a higher vibrational mode in air in a pipe or other resonator – harmonicas generally have no such resonator.
Notable practitioners of overblowing are Howard Levy, a founding member of the Flecktones, Paulo Prot, Adam Gussow, Otavio Castro, Chris Michalek, Jason Ricci, and Carlos del Junco.
As for a flute, which does not have a reed, but rather is a reedless cylindrical instrument open at both ends, the pitch similarly increases by an octave.