than in addressing 250 in the Physical Theatre of the Sorbonne. In the large theatre of the Sorbonne, holding 3000, the resonance extended almost to 3 sec. empty, but was only 1 sec. or less when full. The acoustic properties of this hall are considered very good. Four other halls are studied and details given.

619. "The author concludes (1) in agreement with Sabine, that the resonance serves to characterise the acoustic properties of a hall. (2) The duration of the sound varies with the quality, the pitch, and the intensity of the primary sound, hence a hall good for a speaker may be bad for an orchestra. (3) With the formula before given we can determine the duration of the resonance as a function of the number of auditors. (4) For a hall to be good acoustically, the duration of the resonance should be practically constant at all parts of the hall, vowels, and fall between sec. and 1 sec. (5) If the duration of the resonance much exceeds 1 sec., the speaker can make himself understood only by speaking very slowly, articulating distinctly, and avoiding giving to the voice too much energy."

and for all

CHAPTER XI.

RECORDERS AND REPRODUCERS

620. The Phonautograph.—In this chapter we shall treat of various instruments which have been devised to record or reproduce sounds. Some of these involve applications of electricity to the subject of acoustics. Hence, for the full appreciation of these parts of the chapter it is necessary that the reader should possess some conversance with electrical theory and practice. But as it is clearly outside the scope of this work to teach elementary electricity, it will be assumed that the student has acquired from other sources a sufficient electrical knowledge to enable him to follow intelligently the descriptions and qualitative actions of the instruments dealt with.

For convenience' sake, however, a few notes will be given at the end of the chapter on those parts of more advanced electrical theory which are applicable to the subject in hand and are needed for its quantitative study.

We commence with the phonautograph of Scott and Koenig. This instrument was introduced in 1864, and yields a record of sounds in the form of a wavy trace on smoked paper. The paper is fixed on a rotating drum, and the curves are made by a style attached to a membrane. This membrane is set in vibration by the sounds which it receives after concentration by passage through a conical or paraboloidal funnel. By means of this instrument many

curves have been obtained due to single sounds or to a combination of sounds. Its interest is now chiefly historical. For examples of curves originally due to the phonautograph, see Fig. 13, Art. 31.

621. The Phonograph.-The phonograph was invented. by Edison in 1877. It not only records sounds as the phonautograph did, but also reproduces them. Thus it may be held to solve in the domain of acoustics a problem closely analogous to that achieved by photography in the domain of optics. For in each case a record is made, and then an imitation of the original facts may be in a sense reproduced from that record at will and after the lapse of time. But in the case of an ordinary photograph the form and light and shade are imitated, but the colour is lost. With the phonograph, however, not only may the pitch. and relative intensities be correctly repeated, but even the quality also is reproducible and with astonishing though not perfect fidelity.

622. The essentials of the first instrument are briefly as follows:-The original sound falls upon a diaphragm whose consequent vibrations move a style which appropriately indents a sheet of tinfoil wound on the surface of a rotating drum. This indented tinfoil constitutes the record of the sound in question. To reproduce it the drum is adjusted to the starting-point, and again rotated at the same speed as before. The style and diaphragm are thus caused to repeat approximately their original motions, and thereby reproduce what is recognisable as an imitation of the original sound.

The details of the apparatus may be understood by reference to Figs. 101 and 102.

623. Supported upon a suitable base and bearings we have the horizontal axle aa, Fig. 101, carrying the drum or cylinder cc. Both axle and cylinder have a screw thread of the same "hand" and the same pitch. Thus, on turning the handle h, a point in one of the threads of the drum will

remain in the same thread although the drum is advancing. The thin diaphragm d, made of mica, animal membrane, or metal, is placed just below the mouthpiece m, see Fig. 102. Both are carried on the bar b pivoted at pp, and adjustable by the screws, so as to make right contact with the tinfoil round the cylinder. At the centre of the diaphragm is cemented a small plate 1, Fig. 102, which carries a little style t. This presses on the steel spring, which bears a little rounded steel point n, which indents the tinfoil. After taking a record the screw s is unfastened, and the

barb swung up so that the point n clears the tinfoil. The handle is then turned backwards until the cylinder is brought to its original position. The bar b is then brought down again, the screw s adjusted, and the funnel ƒ fitted on to the mouthpiece. The handle is now turned at the same speed as at first, and the original sound is thereby approximately reproduced.

The original design of the instrument just described, and in which the record was made upon tinfoil, was followed by a modified form in which a wax cylinder replaces the tinfoil. Further, in the improved forms of the instrument uniformity of rotation is secured by clockwork, or an electro

motor, both in taking the record and in reproducing the sounds from it.

624. We have already referred to the use of the phonograph by Hermann to settle the question as to whether vowels are characterised by a fixed or a variable pitch. We may now notice in detail how the phonograph lends itself to this determination. Suppose, for example, the vowel ōō to be sung into the phonograph at the pitch F,

b

m

FIG. 102.-SECTIONAL DETAIL OF PHONOGRAPH.

the chief resonance being ƒ an octave higher. Then the second partial ƒ would be specially reinforced, and this special reinforcement would be registered in the phonograph's record. Now, if the vowel ōō is characterised by a special reinforcement of its second partial, the phonograph when reproducing the sound at any speed would retain and reproduce correctly this reinforcement of the second partial, the whole compound tone being raised in pitch if the handle was turned quicker than in taking the record, and the whole lowered in pitch if the handle were turned slower.