Data are presented giving the measured acoustic reactance and resistance for a number of circular orifices varying in diameter from 1 cm down to 0.034 cm, and for a rectangular orifice 1.9 cm×0.075 cm. The measurements were made for various particle velocities, the corresponding Reynolds numbers varying from 0.7 to 3000, roughly. The reactance is found substantially independent of the particle velocity; a formula for computing it is given. The resistance approaches a constant value as the velocity is sufficiently decreased; formulae for computing this “low velocity” resistance are given. At larger velocities the resistance increases with the velocity. This is discussed from the standpoint of a loss of kinetic energy of flow, acting besides viscosity and turbulence.

The minimum audible field (M.A.F.) has been determined from data taken on 14 ears over the frequency range from 100 to 15,000 c.p.s. The observer is placed in a sound field which is substantially that of a plane progressive wave, facing the source and listening monaurally. The M.A.F. is expressed as the intensity of the free field, measured prior to the insertion of the observer. Similar data are presented for binaural hearing, over the range from 60 to 15,000 c.p s., obtained with 13 observers. At 1000 c.p.s. the average M.A.F. observed is 1.9 × 10−16 watts per cm2, corresponding to a pressure 71 db below 1 bar. Included are data showing how the M.A.F. varies with the observer's azimuth relative to the wave front. Another type of threshold data refers to minimum audible pressures (M.A.P.) as measured at the observer's ear drum. The differences obviously to be expected between M.A.F. and M.A.P. values are due to wave motion in the ear canal and to diffraction caused by the head. The M.A.F. data are discussed in relation to the M.A.P. determinations from several sources. Some possible causes of difference between the two, which are due to experimental procedure and may add to the causes already mentioned, are pointed out.

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The absorption coefficients were measured for gases: (A) O2; (B) N2; (C) O2+N2 in normal air proportions; (D) O2+N2+CO2 in normal air proportions; (E) O2+N2+CO2 in normal air proportions, plus H2O vapor at 37 percent Rel. Hum. at 26.5°C; (F) O2+N2 in normal air proportions, plus a CO2-content varied from zero to 1.15 percent by volume. Let R denote the ratio of the measured absorption coefficient to that computed from the Stokes-Kirchhoff equation. For gases (A), (B), (C), and (D), R is roughly 1.5. For gas (E), R is of the order of 10 at 15 Kc.p.s., and approaches 1.5 with increasing frequency. For gas (F) at 89 Kc.p.s., R increases with rising CO2-content, attaining roughly R = 20 at 1.15 percent CO2-content.

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