Volume 7, Issue 4, April 1936
Index of content:
7(1936); http://dx.doi.org/10.1121/1.1915837View Description Hide Description
7(1936); http://dx.doi.org/10.1121/1.1915839View Description Hide Description
7(1936); http://dx.doi.org/10.1121/1.1915840View Description Hide Description
A loudspeaker may be tested by removing its horn and loading it acoustically with an air column of variable length in a closed tube. In this way one can measure the acoustic impedance of the horn and the sound power output of the speaker, under the assumption that the air chamber in front of the diaphragm can be regarded as an addition to the tube of an equivalent length r determined by experiment. Under this assumption the air load impedance ZA at the diaphragm is given by where A 1 is the diaphragm area, A 2, the cross section of the tube, and x the total length, i.e., actual length plus r. An analytical treatment leads to the formula . Eq. (1) leads to correct results for the measurement of acoustic impedance of the horn and sound power output, but cannot be extended to determine the mechanical impedance of the diaphragm. Eq. (2) is the basis of a new treatment which is more general. It is expressed in terms of a parameter r which can be calculated theoretically at low frequencies, or determined empirically at higher frequencies. The theory has been verified experimentally.
7(1936); http://dx.doi.org/10.1121/1.1915841View Description Hide Description
High quality reproduction of recorded music is possible only if the speeds of recording and reproducing machines are maintained constant within very narrow limits. As improvements are made in these machines greater refinement in measurement of the variations is required in order that the progress may continue. The development of an improved “Wow‐meter” for such measurements is described, and various alternative schemes are discussed. The securing of a standard record, itself sufficiently free from imperfections, presents one of the most serious difficulties. For testing turntables a toothed steel wheel was built, in which special precautions were taken to minimize indexing and other possible errors. The current generated by means of this wheel is supplied to an electric circuit so arranged that variations in frequency cause deflections of a galvanometer, which are registered on a photographic film. The principle employed, namely, a resonant circuit slightly off tune, has been used in earlier devices. In the meter described a push‐pull arrangement has been adopted to reduce the likelihood of errors due to variations in voltage. A frequency meter of this type has certain inherent limitations in its ability to register very rapid fluctuations. Certain measures have been adopted to make the meter satisfactory in this respect, and the possibility of going further in this direction if necessary is given consideration.
7(1936); http://dx.doi.org/10.1121/1.1915842View Description Hide Description
7(1936); http://dx.doi.org/10.1121/1.1915844View Description Hide Description
The American Standards Association Sectional Committee Z‐24 on Acoustical Measurement and Terminology has under it a Subcommittee on Fundamental SoundMeasurements. The Subcommittee during the past few years has been considering definitions and test procedure pertaining to the calibration of microphones. The following report is the result of its work. At a recent meeting of the Sectional Committee it was voted to submit this work to the interested scientific bodies for publication in their journals with the thought of promoting constructive criticism and discussion. The Subcommittee made use of considerable material from the tentative report of the I.R.E. Standards Committee on Electro‐Acoustic Devices, and this is gratefully acknowledged.