organ pipes are made from either wood or metal and produce sound ("speak") when air under pressure ("wind") is directed through them. as one pipe produces a single pitch, multiple pipes are necessary to accommodate the musical scale. the greater the length of the pipe, the lower its resulting pitch will be. the timbre and volume of the sound produced by a pipe depends on the volume of air delivered to the pipe and the manner in which it is constructed and voiced, the latter adjusted by the builder to produce the desired tone and volume. hence a pipe's volume cannot be readily changed while playing.