Minimum Sound Energy for Audition for Tones of High Frequency

Minimum Energy for Audition for Tones above 2000 p.p.s.---Using the new tone generator developed by Hewlett as the source of sound, experiments were carried on at night and out of doors on a platform raised 5 meters above the ground and with the listening ear 150 cm. from the source. The amplitude of vibration of the diaphragm of the generator was calculated from the electrical input, and from it the corresponding sound intensity at the ear of the listener was computed, assuming the amplitude of the sound wave at the diaphragm the same as that of the diaphragm and correcting for the concentration of sound around the axis of symmetry as experimentally determined. No correction for the effect of the listener's head, however, was made. Sensitivity frequency curves for eight normal ears, ages 19 to 35 years, are reproduced. The minimum audible intensity was found to remain practically constant at 7 ×{} 10^-8 ergs per cm.² for frequencies from 2,000 to 14,000 p.p.s., but above 14,000 the sensitivity decreased more and more rapidly until at between 18,000 and 20,000 p.p.s., depending on the individual, an intensity of 0.1 erg per cm.² was inaudible. A variation with age was observed, the upper frequency limit being considerably higher for children and lower for older persons than for normal young adults.Distribution of Sound Intensity around a Hewlett Tone Generator for 2,000 to 15,000 p.p.s. was determined roughly by studying the minimum amplitude for audition as a function of the angular position of the listener with reference to the axis of symmetry. The sound is largely concentrated in a cone of about 20^∘{}, the angle decreasing as the frequency increased, so that for 10,000 p.p.s., for instance, the intensity 150 cm. along the axis was fifty times as great as if the distribution had been spherical.