PSYCHOLOGICAL ACOUSTICS. The least notice able intensity of auditory sensation is measured by some form of acoumef'r. The stimulus is given by the dropping of a tiny hammer upon a steel bar, by the fall of a cork o• pith ball upon a glass plate, etc. Either the sound is kept con stant, and the observe• notes the distance at which it can be heard; or the distance is kept emmstant and tl e height of fall varied until a sound can must be sensed, Our discrimination of scumd intcusities is measured by the sound ilum, or the full photionietcr. The principle f 'HI it is the same—two sounds. of -1i,litly different intensities. are produced by the eping of a hall or pendulum-bob against a ...ird•nood block, and the observer is required to say at what point the difference becomes notice able. The highest audible tone is determined either by of a series of tiny tuning forks, r steel cylinders. or by means of (lallon's whis tle, a piston whistle of small bore actuated by a rubber bulb, whose length can be varied from that required for a shrill tone to a length that gives a sharp hissing noise, with no trace of tonal quality. The lowest audible tone is deter mined by a very large tuning fork, giving tones between the limits 16 and 35 vibrations in the second; by tuning forks of wire. loaded at the
tips of the prongs; by a steel tongue, Appunn's lamella, which can be set in vibration between the limits 4 and •L4 in the second; or by the pro duction of deep Ilitrereiwe tones. (See At.InTioN.) Discrimination of tonal pitch and the phenomena of clang-tint and tonal fusion (see FusioN) are studied by the aid of delicately graduated tuning forks; reeds, with their appropriate bel lows tables; organ pipes: blown bottles; siren; sonometer; etc. For the study of rhythm. the ticks of a metronome are employed. or the puffs of tone from a tuning fork placed behind a rotat ing disk of cardboard, pierced at regular inter vals. For the study of localization of sound, i.e. of the apparent distance and direction of a given source of sound. the sound cage is used. This consists of two graduated semicircles of wire, the otc turning about a horizontal and the other about a vertical axis. The observer sits with his head at the centre of the semicircles.. and attempts to localize the click of a telephone re ceiver, which is phieed by the experimenter at some point upon time circumference of the sound sphere.