Harvard Psychological Studies, Volume 1 / Containing Sixteen Experimental Investigations from the Harvard Psychological Laboratory.

by Unknown

E.W. Scripture has chosen another type of talking machine from which to obtain transcribed records. The permanent record of the gramophone (which makes a record in the plane of the surface, like the phonautograph) is carefully centered, and a lever attached to a stylus which follows the furrow of the record transcribes the curve on the kymographic drum as the plate is slowly revolved. The method has the advantage of using a record which may be reproduced (i.e. the original gramophone record may be reproduced), and of giving fairly large and well defined curves for study. It is too laborious to be applied to extended research on speech rhythms, and has besides several objections. The investigator is dependent on the manufacturer for his material, which is necessarily limited, and cannot meet the needs of various stages of an investigation. He knows nothing of the conditions under which the record was produced, as to rate, on which time relations depend, as to tone of voice, or as to muscular accompaniments. There are also opportunities for error in the long lever used in the transcription; small errors are necessarily magnified in the final curve, and the reading for intensity (amplitude of the curve) is especially open to such error.

The stylus of such a recording apparatus as is used by the gramophone manufacturers, is subject to certain variations, which may modify the linear measurements (which determine time relations). The recording point is necessarily flexible; when such a flexible point is pressed against the recording surface it is dragged back slightly from its original position by friction with this surface. When the point is writing a curve the conditions are changed, and it sways forward to nearly its original position. This elongates the initial part of the sound curve. This fact is of little importance in the study of a single vowel, for the earlier part of the curve may be disregarded, but if the entire record is to be measured it is a source of error. Hensen[16] first turned the phonautograph to account for the study of speech. He used a diaphragm of goldbeater's skin, of conical shape, with a stylus acting over a fulcrum and writing on a thinly smoked glass plate. The apparatus was later improved by Pipping, who used a diamond in place of the steel point. The diamond scratched the record directly on the glass. The Hensen-Pipping apparatus has the advantage of taking records directly in the plane of the surface, but it does not make a record which can be reproduced; in case of doubt as to the exact thing represented by the curve, there is no means of referring to the original sounds; and it involves working with a microscope.

Fig. 3. Diagrammatic section of recording apparatus. a, diaphragm; s, stylus; g, guide; p, section of plate.

The apparatus which was used in the following experiments consisted essentially of two recording devices—an ordinary phonograph, and a recorder of the Hensen type writing on a rotary glass disc (see Fig. 5, Plate X.). Of the phonograph nothing need be said. The Hensen recorder, seen in cross section in Fig. 3, was of the simplest type. A diaphragm box of the sort formerly used in the phonograph was modified for the purpose. The diaphragm was of glass, thin rubber, or goldbeater's skin. The stylus was attached perpendicularly to the surface of the diaphragm at its center. The stylus consisted of a piece of light brass wire bent into a right angle; the longer arm was perpendicular to the diaphragm; the shorter arm was tipped with a very fine steel point, which pointed downward and wrote on the disc; the point was inclined a trifle to the disc, in order that it might 'trail,' and write smoothly on the moving disc. The stylus had no fulcrum or joint, but recorded directly the vibrations of the diaphragm. In early experiments, the diaphragm and stylus were used without any other attachment.

But a flexible point writing on smoked glass is a source of error. When the disc revolves under the stylus, the flexibility of the diaphragm and of the stylus permit it to be dragged forward slightly by the friction of the moving surface. When the diaphragm is set vibrating the conditions are altered, and the stylus springs back to nearly its original position. The apparent effect is an elongation of the earlier part of the curve written, and a corresponding compression of the last verse written. This error is easily tested by starting the disc, and without vibrating the diaphragm stopping the disc; the stylus is now in its forward position; speak into the apparatus and vibrate the diaphragm, and the stylus will run backward to its original position, giving an effect in the line like a (Fig. 4). If the error is eliminated, the stylus will remain in position throughout, and the trial record will give a sharp line across the track of the stylus as in b.

This source of error was avoided by fixing a polished steel rod or 'guide' at right angles to the vertical part of the stylus, just in front of the stylus; the stylus trailed against this rod, and could not spring out of position. The friction of the rod did not modify the record, and the rod gave much greater certainty to the details of the sound curve, by fixing the position of the vibrating point. This rod or guide is shown in Fig. 3 (g).

The disc was driven directly from the phonograph by a very simple method. A fine chain was fixed to the shaft carrying the disc, and wrapped around a pulley on the shaft. The chain was unwound by the forward movement of the recording apparatus of the phonograph against the constant tension of a spring. When the phonograph apparatus was brought back to the beginning of a record which had been made, the spring wound up the chain, and the disc revolved back to its original position.

A T from the speaking-tube near the diaphragm box was connected by a rubber tube with the phonograph recorder, so that the voice of the speaker was recorded both on the smoked glass plate and on the phonograph cylinder. The advantages of such a double record are that the possible error of a transcription process is eliminated, and yet there is an original record to which it is possible to refer, and by which the record measured may be checked.