Pedal tones are special notes in the harmonic series of cylindrical-bore brass instruments. A pedal tone has the pitch of its harmonic series' fundamental tone. Its name comes from the pedal keyboard pedals of a pipe organ, which are used to play 16' and 32' sub-bass notes. Cylindrical brasses do not naturally vibrate at this frequency.
A closed cylinder vibrates at only the odd members of its harmonic series. This set of pitches is too sparse to be musically useful for brass instruments; therefore, the bells and mouthpieces of brasses are crafted to adjust these pitches. The bell significantly raises all pitches in the series, and the mouthpiece limits the amount to which higher harmonics are raised. The resulting set of pitches is a new harmonic series altogether. This new series has all but one of its members present, instead of only the odd members.
The member not present in the new series is the fundamental. The original fundamental is not raised all the way to the new fundamental pitch, and the original third harmonic becomes the new second harmonic. The new fundamental can be played, however, as a pedal tone. The higher resonances of the new series help the lips vibrate at the fundamental frequency and allow the pitch to sound. The resulting tone relies heavily on overtones for its perception, but in the hands of a skilled player, pedal tones can be controlled and can sound characteristic to the instrument.
Pedal tones are called for occasionally in advanced brass repertoire, particularly in that of the trombone and especially the bass trombone. Although not overly much used, pedal tones can often be played on a didgeridoo.