Acoustic interferometers operate by dividing the signal into two parts, which are sent on paths of different lengths and then recombined. If the path difference is an even number of wavelengths, the waves interfere constructively when they rejoin, and a loud sound is heard. However, if the path length difference is a half-odd number of wavelengths, the signals are 180 degrees out of phase when they combine, and a minimum amplitude of sound is the consequence.

   The three interferometers below are similar. The acoustic signal is introduced at one side, and travels to the other side through the tubes on the right and the left. One of these tubes telescopes; when the tube is pulled out by a half wavelength, the path length increases by an entire wavelength. The sound can thus go from a maximum to a maximum, or a minimum. Pulling the tube out by a quarter wavelength increases the path length by a half wavelength, thus changing a maximum to a minimum or a minimum to a maximum. The sound can be produced by a tuning fork (of known frequency), and picked up by a funnel connected to the apparatus by a rubber tube. The maxima and minima are detected by using a rubber tube to lead the signal to the ear of the experimenter. The Kohl instrument at Miami University cost 27 marks in 1900.
 

St. Mary's College in Notre Dame, Indiana

Miami University in Oxford, Ohio

University of Cincinnati

The apparatus at St. Patrick's College (Maynooth, Ireland - at the left) and the College of Wooster (at the right) was developed by  Rudolph Koenig  and is listed at 250 francs in the 1889 Koenig catalogue, and 220 marks in the 1900 Max Kohl catalogue. Sound of a known frequency is supplied to one side by a tuning fork or a Helmholtz resonator, and detected on the other side by a manometric flame.      Return to Acoustics Home Page  Return to Home Page