(1) The sounds of aircrafts (a bomber, chaser, reconnaissance machine and a small dirigible) were investigated in the hope of obtain-in fresh light on their nature. (2) The procednre employed is entirely of objective, namely the sounds were recorded as accurately as possible by means of an electrical arrangement consisting of a band-microphone, seven-stage resistance-capacity-coupled amplifier and an oscillograph, and the analysis were carried out upon the records. (3) Besides the sounds emitted by an aeroplane in flight, experiments were also made upon an aeroplane on the ground, with special reference to the directivity of the sound and to the influence of the number of revolution on the nature of the sound. 4. The chief sources of the sound of an aircraft are the revolving airsersw and engine-exhaust; the pitches of the fundamentals of these sounds being given by: N1:(number of hlades ×1/60 r. p. m.)×(reduction gear ratio) and N : (number of cylinders)×1/60 1/2 (r. p. m.), respcctively. The correspondence of the pitches of the recorded sounds to these pitches were examined in all cases. In genera, the sounds of aircrafts are of very complex nature, overtones than the fundamentals being predominant in most capes. At long distance the greater part of the sound originates from the propeller and it consists of the fundamental and second harmonies .N1 and 2 1 while at short distance the xhmst-sound prdlominates. 5. Pifferent aeropkrns epuipped with the same kind of engine gave quim different resords of sounl. 6.On the other hand a llmber with a Xapier-Lion 12-cylinder engine often showed a component having a pitch 1/3 of the fundamental above mentioaed, which undoubtedly due to the fact that the cylinders are arranged in three separate bank. 7. In the case of a chasor equippel with Hispano S-cylinder engine a component Living a pitch 1 2 of the fun-lamental was sometimes observed. 8. Therefore, in c-dculating the pitch of the fundamental of the exhaus-sound it is much more appropriate to use the number of cylindors por bank, inst all of the total number of cylinders. 9. Siveral examples were obmined, which show the existence of the, partioular tune pointel out by Prof. L Prandtl, consisting of only two impalses, i.c the direct impulse and that reflected from the ground. 10. Fluctuation in the intensity of the sounds of aeroplane in flight was also investigate 1.N definite periods are, of course, to be expected, yet one having a priod of one or two seconds seems to occur most frequrntly on the other hand it was often the case that the sound aim ist fade away for several seconds

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