Description
This apparatus, reminiscent of a trombone, is for experiments and demonstrations relating to the interference of sound waves. From an original design by Sir John Herschel (1792-1881) this example was built by the Parisian physicist and acoustical scientific instrument maker Rudolph Koenig.

The apparatus consists of two metal tubes that bend in a U shape. A sound of definite pitch is produced by means of a tuning fork and Helmholtz resonator, which is then introduced to the two tubes by means of another Y shaped tube. The sound is then split in two identical sound waves, which travel along each main tube. The two sound waves may then be recombined at the other end. One of the tubes may be varied in length by means of sliding, rather like a trombone slide, and this variation can be measured precisely with the ruler fixed in place. As the length of the tube is changed constructive and destructive interference is observed at the opening by means of the ear or by using a manometric flame apparatus.

Interference occurs whenever two sound waves are combined. Sound waves are regular variations in air pressure that travel outwards from sound sources. The pressure at a given point, say at the ear of the observer, varies from higher than atmospheric to lower than atmospheric as the surrounding air is in turn compressed and rarefied. When two sound waves combine, the resulting air pressure is the sum of the two pressures at a given time and location. Sound waves may therefore act to reinforce each other (constructive interference) or to cancel each other out (destructive interference) depending on their frequency and phase (phase being the measure of where the wave is between its maximum and minimum pressures). By making two sound waves of identical frequency travel distances differing by one half of the wavelength it is possible to produce perfect destructive interference. This results in no sound being emitted.

Koenig’s apparatus can be used to determine the velocity of sound in air by using the equation, velocity = frequency x wavelength. If one introduces a sound of known frequency f and achieves destructive interference with a tube length difference of d, then we can find the velocity v by v=2fd (d being half a wavelength). The apparatus can also be used to determine the velocity of sound in other gases by filling the tubes with the gas under investigation.

The idea behind Koenig’s apparatus has been used to develop sound reduction systems in all kinds of areas. In particular, the idea of splitting and recombining out of phase sounds has been deployed in building silencers (mufflers) for automobiles.
28/05/2008
Created by: Dr. Torben Rees on 28/05/2008