An important feature in the method was the rate at which the disc revolved. The disc turned so slowly that the vibrations, instead of being spread out as a harmonic curve, were closely crowded together. This had two great advantages; the measurements were not so laborious, and the intensity changes were much more definitely seen than in the elongated form of record. Each syllable had an intensity form, as a 'box,' 'spindle,' 'double spindle,' 'truncated cone,' 'cone,' etc. (cf. [p. 446]).
The disc was run, as a rule, at a rate of about one revolution in two minutes. The rate could be varied to suit the purposes of the experimenter, and it was perfectly possible to procure the usual form of record when desired. As a result of the low rate, the records were exceedingly condensed. The records of the 300 stanzas measured are on two glass discs of about 25 cm. diameter, and as much more could still be recorded on them.
The diaphragm and the speaking tube were the great sources of error. For measurements of time values the particular component of the tone to which the diaphragm happens to vibrate is not important, but the record of intensities depends on the fidelity with which the diaphragm responds to a given component, preferably the fundamental, of the tone. The speaking tube has a resonance of its own which can be but partly eliminated. For the records here recorded either glass or goldbeater's skin was used as a diaphragm. Goldbeater's skin has the advantage of being very sensitive, and it must be used if the subject has not a resonant voice. It has the great disadvantage of being extremely variable. It is very sensitive to moisture, even when kept as loose as possible, and cannot be depended on to give the same results from day to day. The records marked Hu., Ha. and G. were usually taken with a glass diaphragm, which has the advantage of being invariable. As the phonograph records show, glass does not modify the lower tones of the male voice to any extent.
Psychological Review. Monograph Supplement, 17. Plate X.
Opposite p. 436.
The apparatus is shown arranged for taking parallel records on the smoked glass disc, and on the cylinder of the graphophone. On the left is shown the microscope with which the records on the glass disc were measured.
The speaking-tube used was of woven material, not of rubber, and a pad of felt was kept in the tube near the diaphragm box. As far as possible more damping was used at the other end of the tube, but this had to depend on the voices of the subjects.
The best check on the performances of a diaphragm is the number per second and character of the vibrations. The pitch may be calculated from the rotation rate of the disc, which is very constant, as it is driven at a low rate by the well-regulated high-speed motor of the phonograph. But it is better to place a fork in position to write on the disc and take a parallel record. All the records were taken with the vowel 'a' (sound as in father). This vowel has a very characteristic signature, which is easily seen, even in a very closely packed curve, and the correctness of this is one of the best guarantees that the fundamental of the tone is actuating the diaphragm (though that does not mean that the diaphragm is actually giving the vibration frequency of that fundamental).
Every record was repeated at least twice, and both records were measured. In many of the experiments the intensities were fixed by the conditions of the experiment. There was always the corroborative testimony of the phonograph diaphragm; for the two were not apt to err together. It was easy to determine if the actual intensity relations were preserved in the phonograph (but it could not be taken for granted). Each record was reproduced on the phonograph immediately after it had been taken, and both subject and operator listened for anomalies. In practice it was not hard to get records of the single vowel used (at a small range of pitch which was never more than a third or fourth and was nearly always much less) which represented fairly well the relative intensities. Beside the checks spoken of above, every record was repeated by a number of subjects, and the comparison of the results of different voices shows uniformity.