Measurements of the rate of decay of sound in a reverberation room, first with no fog or smoke in the room and then with fog or smoke of known concentration and particle size added to the room, show that the attenuation of sound in a number of aerosols is in approximate agreement with values predicted by the theories of Sewell, Epstein, and Oswatitsch. In a water fog having a concentration of 2.0×10−6 grams/cm3 and average droplet radius of 6.25×10−4 cm, the attenuation owing to the fog increased from about 5 db/sec. at 500 c.p.s. to 13 db/sec. at 8000 c.p.s. In a quite similar fog of mineral oil, the corresponding attenuation increased from 1.6 db/sec. at 500 c.p.s. to 21 db/sec. at 8000 c.p.s. At very low frequencies, a fog of water is much more absorptive than is a fog of oil; the difference is ascribed to a “relaxation” effect of evaporation from and recondensation the droplets, which is much greater for water than for oil. The attenuation of sound in smoke may become rather high, amounting to 58 db/sec. at a frequency of 6000 c.p.s. for a moderately dense smoke of NH4Cl (180 g in a volume of 6080 cu. ft.).
VOL.XIX.J THE AMERICAN PHYSICAL SOCIETY. 261 between films and but one of many types of instruction,-the demonstration lecture. Comparisons of films with laboratory methods, recitations, or problem solving types of instruction which are the most important teaching methods in science, would be obviously absurd.A TELEPHONE receiver energized by a current from a vacuum tube oscillator is used as a source of sound for determining the sensibility of the ear to small differences of loudness and pitch. Tones varying in frequency from 30 d.v. to 20,000 d.v. are produced by the oscillator used in these experiments. By means of a divided resistance circuit the intensities of the tones can be varied by any desirable and measurable intervals from the threshold values up to very high values.Some auxiliary experiments showed that the acoustical energy developed by the receiver diaphragm is a linear function of the electrical energy which actuates it. The electrical energy is therefore a convenient measure of the relative acoustical energy at any fixed frequency.Data that have been taken thus far on a limited number of ears show that the sensibility of the ear to small differences in intensity, measured by the ratio of the smallest perceptible increment in energy to the whole energy, AE/E, is -.1. Dependent upon the intensity. The ratio t AE/E decreases as the intensity increases. For a wide range of moderate and high intensities the sensibility is nearly constant. Its value for ordinary frequencies is roughly 0.10.2. Nearly independent of the frequency between 100 d.v. and 4,000 d.v. 3. Nearly the same for all normal ears. The sensibility of the ear to small differences in frequency measured by the ratio of the smallest perceptible increment to the whole, is -1. Dependent upon the intensity. The law of variation with intensity closely resembles the law of variation of sensibility to small intensity differences.2. Dependent upon the frequency. The sensibility ratio decreases from about 0.01 at 50 d.v. to about 0.002 at 1,000 d.v. For higher frequencies the ratio increases.3. Only approximately the same for different ears.UNIVERSITY OF CHICAGO.
As the listener-observer hears the successive groups of three words, he writes them on a special form. The list is then graded and the * Within -½3 db from 100 to 7000 cycles. a The details of the laboratory are given in J. Acaus.
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