Pressure wave reflections in an organ note channel

Abstract

In this investigation, measurements of windchest pressure, organ pipe sound output, and air velocity into the foot of a small pipe are displayed as functions of time, using a long windchest note channel with a hand‐operated pallet valve. For rapid opening of the pallet, the pressure signal advancing along the channel consists of (a) a negative pulse due to the pallet displacement, followed by (b) a rising pressure step of height p, typically 80% to 95% of the supply pressure. When the pressure step is reflected from the rigid far end of the channel, a pressure pedestal of height 2p and duration equal to the double passage time is observed there. With further reflections, the pressure changes continue as damped oscillations which depend in detail upon position in the channel. These results indicate that a long channel must be considered as an acoustic transmission line. An extension tube was also added to the end of the note channel. The pressure attained at the end of the extension tube was nearly 3p (theoretical limit 4p) as a result of reflection from its end in addition to the reflection at the impedance discontinuity between the tube and the note channel.