There has always existed considerable doubt as to the magnitude of the periodic pressure changes, and the concomitant velocity and temperature changes which the air in a sounding organ pipe undergoes. The difficulty of following these rapid changes has prevented the success of many attempts, and results obtained by a few successful experimenters have not been in agreement. Kundt, and, later, Dvorak, using a manometer provided with a valve, which opened and shut with the frequency of the air oscillations in the pipe, measured the cumulative pressure of the condensations. Töpler and Boltzmann used an optical method. That part of a stopped pipe close to the node was fitted with glass windows and placed between the plates of a Jamin interferometer, so that part of the light which produced the interference bands in the instrument passed through and part outside the pipe. The interference bands appeared to be broadened when the pipe was sounded; from the extent of the broadening, the change of density or of pressure at the node due to the vibration was calculated. Raps obtained actual photographs of the oscillating bands, which the optical fatigue of the eye made to appear widened in the earlier experiments. A summary of their results is given in Table I. Of course, the oscillatory change of pressure will depend on the form of the pipe and the pressure at which it is blown, but with the average stopped diapason, blown at a few inches of water, the discrepancy between the results by different methods is still too great. Against the valve-manometer the criticism may be levelled that the motion of the air in the pipe is seriously affected by the motion of the valve pallet, which will cause additional variations of pressure in the pipe. Kundt's values are the highest (his maximum recorded amplitude was 1/16 atmosphere); Töpler and Boltzmann recorded 1/60, Raps 1/22. Interference methods have the advantage of leaving the air entirely undisturbed, but are troublesome both int their adaptation to existing pipes and in their interpretation, while slight vibration of the walls under the action of the sound waves or the blast produces shifts of the bands of the same order as those sought for in the air motion.
Scattering of Sound Waves at a Periodic, Pressure‐Release Surface