Volume 10, Issue 2, October 1938
Index of content:
10(1938); http://dx.doi.org/10.1121/1.1915961View Description Hide Description
Sound absorptionmeasurements in pure at atmospheric pressure and 22°C have been conducted which, together with measurements by others, confirm the collision theory of anomalous absorption as developed by Einstein, Kneser, and others. The absorption coefficient is appreciable at frequencies as low as 2000 cycles, and increases to a maximum of 0.317 per wave‐length at 77,000 cycles. This maximum is higher than values obtained by previous workers, and indicates that both the deformation and symmetrical valence vibrations participate in the exchanges between translational and vibrational energy. Small impurities, as water or alcohol vapors, affect the absorption greatly; the entire absorption band is shifted to higher frequencies. Measurements in mixtures of in and in indicate that neither the nor is appreciably excited by collisions with . Measurements in reveal an absorption similar to that for except that the absorption begins at about 10,000 cycles. In mixtures of and , the observed absorption at frequencies below 10,000 cycles is accounted for by assuming that only the vibration of molecules is excited by collisions with molecules; at higher frequencies the molecules also are excited, principally by collisions with other molecules.
10(1938); http://dx.doi.org/10.1121/1.1915962View Description Hide Description
Serious difficulties have been encountered in attempts to measure the absorption coefficients of sound absorbing ceilings in large offices. An analysis of the sound field is made and it is concluded (1) that the reverberation time formula is usually invalid if the absorption is concentrated on one surface of the room, (2) that the energy density formula is not subject to such a restriction and (3) that under such conditions there is no apparent field method of determining the absorption coefficient that will serve as an accurate check on laboratory measurements.
10(1938); http://dx.doi.org/10.1121/1.1915963View Description Hide Description
10(1938); http://dx.doi.org/10.1121/1.1915964View Description Hide Description
The fundamental problem of absolute soundmeasurements in liquids is discussed. Regarding acoustic determinations, reference is made to certain inherent differences between air and water. An outline is presented of three methods now available for sound determination. A modified radiation pressure apparatus is described which permits a dual check of the basic measurements involved, and at the same time makes possible microphonecalibration for secondary standards.
10(1938); http://dx.doi.org/10.1121/1.1915966View Description Hide Description
A portable tertiary frequency standard, adjustable over a small range, is described. The frequency determining element is a tube‐maintained tuning fork, adjustment of whose frequency over a relatively small range is accomplished by changing the position of movable weights on the prongs. Calculations are simplified by the design of the fork, which permits the assumption of lumped constants.
The weights may be moved while the fork is running without interfering with its operation. Indication of their position is by means of a pointer reading on a scale graduated directly in frequency units. A precise method of calibration is given. Mechanical arrangements are described for obtaining a uniformly spaced scale, and the possibility of using such a scale in correcting for the temperature coefficient of the fork material is discussed.
The relative accuracy of such a fork is greater, the smaller the range of frequency adjustment. In the present development, adjustment may be made over a range of plus and minus three percent of the mean frequency, that is, a musical interval of one semitone, or 100 cents total change. Incorporation of this fork in the chromatic stroboscope has resulted in considerable simplification as well as an increase in the reliability of the instrument.
10(1938); http://dx.doi.org/10.1121/1.1915967View Description Hide Description
10(1938); http://dx.doi.org/10.1121/1.1915969View Description Hide Description
10(1938); http://dx.doi.org/10.1121/1.1915970View Description Hide Description