A windchest is a wooden box that holds air to be delivered to organ pipes, which are inserted in valved holes. These two examples were used in lectures to demonstrate the different characteristics of organ pipes of varying sizes and shapes.
A tonometer consists of a series of steel cylinders that resonate at specific frequencies upon being struck with a metal hammer. They are used as standards for high frequencies, in the same manner as tuning forks. The transverse vibrational frequencies (i.e. fundamental and harmonics) of a given cylinder depend on the length, elastic modulus, and linear density of the metal.
The fork-clock was first described by N. Niaudet in 1866. It is used to determine the frequency of a fork to high accuracy. The vibrating fork drives the clock in the same way as a pendulum in a pendulum clock–by way of an escapement mechanism. This Max Kohl clock uses a 100 Hz fork to drive a tiny escapement. Energy is provided by an enclosed wind-up spring. The three dials record the total number of vibrations. The absolute frequency of the fork can then be determined by comparison with an astronomical time standard.
William Stern invented the tone variator in 1897 to study human sensitivity to changes in pitch, going beyond the traditional psychological research of studying the sensitivity to differences in discrete tones. The instrument consists of an adjustable brass resonator (see Helmholtz resonator for more information), which is supplied with a constant flow of air across an opening at the top.
The double siren is an instrument that produces sounds from two sources in a way that allows the phase of one sound to be changed relative to that of the other. It is capable of producing everything in the range of a half-tone to an octave. A double siren consists of two coaxial Dove sirens—disks rotating at high speed. As air flows through a series of holes in each disk, a distinct tone is produced, the frequency of which depends on the rotational speed of the disk, and the spacing of the holes in the disk.