interpreted in terms of direction; it is possible to predict an apparent change in direction from the mere varying of the intensity of the sound.

Other things being equal, the fainter sound is judged to be more distant.

Timbre is the characteristic quality of a sound and depends upon the presence and reinforcement of overtones. The difference between the tones of the various musical instruments is chiefly a difference in timbre. As a rule, the richer the tone the better it can be localized. The human voice in normal speech can be localized better than any other sound. The localization of the sound of a tuning-fork is very much inferior to that of the voice; and some contend that a simple pure tone cannot be localized at all. At any rate, the richer the tone the more "ear-marks" of direction it can have.

The longer a sound lasts within certain limits, the better it is localized.

By introducing a funnel into the ear and turning it in different directions, one can make the same sound seem to come from different places.

The resonance and similar modifications of the sound by the environment are familiar sources of disturbance to localization. If the experiments are made indoors, they should preferably be made near the center of a relatively empty cubical room.

There are exceedingly interesting phenomena of sound fusions. No matter how many sounds are produced simultaneously, no matter from what different directions they may come, provided the sounds are of

the same kind and pitch, they will be heard as one. A very pretty experiment is to connect two telephone receivers to the same tuning-fork so that the same tone is produced in both receivers. The two tones will invariably be heard as one, and this one is located according to definite laws. If the two receivers are pressed against the ears, the fused sound will be heard at the root of the tongue-a very weird experience. The laws for the localization of fused sounds are among the most valuable criteria for testing a theory of sound localization.*

To revert, then, to the questions at the beginning of the chapter, we may sum up the evidence in the form of a reply. The following points may be considered in favor of the intensity theory:

(a) The general variation in the delicacy of localization; that is, the form and distribution of the sensory ellipses.

(b) The existence of confusion-points.

(c) The difficulty of localizing sounds radially in the median plane.

(d) The introspections which reveal our dependence upon intensity.

(e) The inferiority of monaural localization.

From experiments of this kind we may therefore conclude that intensity is the primary factor in the local sign of hearing.

But intensity operates in two distinct ways: (1) through the binaural ratio and (2) through monaural

*This may be made a very effective demonstration exercise to be developed by two or three members of the class.

ratios. With two ears we get simultaneous impressions in which the intensity for one ear stands in a certain ratio to the intensity for the other ear.

But, for each ear or with a single ear, a sound in one direction has a certain intensity ratio to the same sound in another direction.*

We have also found distinct evidence of another factor, namely tone quality, or the modification of the character of the sound by the form of the outer ear. Among the evidences for this are:

(a) Complex sounds can be localized more accurately than relatively pure tones.

(b) The possibility of improvement with practice, especially within the median plane.

(c) The inability to localize pure tones by one ear

alone.

(d) The curve of monaural localization.

(e) The introspective evidences; i.e., consciousness of quality difference.

Hence we shall conclude in favor of the fifth theory, the intensity-quality theory: sound is localized by means

* An interesting corroboration of this twofold nature of the local sign of direction of sound is found in the "teeth" of the curve in Fig. 10. There are five prominent teeth, namely, 15° RF, 50° RF, 90° R, 60° RB, and 25° RB. The more accurate the measurement the more distinct these teeth become. Now, the explanation is to be found in the fact that there is a change at these points in the means of localization. At 15° RF and 25° RB there is a transition from the simple balancing of the relative intensity of the two ears to the recognition of quality changes for one ear. At 50° RF and 60° RB there is a transition from this double standard to the monaural localization on the basis of the character of the sound. The large peak at 90° R is due to the fact that this represents a sort of zero-point for both modes of localization.

of binaural and monaural ratios of intensity and characteristic differences in quality. *

Is the ear a space-sense organ? It is. As we have in two eyes a means for the perception of distance, we have two ears for the perception of direction. We also have on each side, where the two ears cannot well cooperate, ear-funnels which change the quality and the intensity of the sound with reference to the direction in which the vibrations impinge upon one ear.

Movement, tendencies to movement, images of movement, visual or auditory images, tactual sensations or images, and other factors may have been observed as secondary factors which influence localization.

What are some of the laws of localization? The graphic representation in Fig. 11, for example, embodies a whole system of laws.†

*Quality is here used in the popular sense, as synonymous with timbre. Strictly, the quality of the tone is its pitch.

A good survey of this topic is found in Pierce, "Studies in Space-perception." The present chapter is based mainly upon Starch, "The Localization of Sound," The University of Iowa Studies in Psychology, Nos. 4 and 5.

CHAPTER VI

TACTUAL SPACE

For Two.

1. Tactual Localization of a Point.-Mark off an area 50 millimeters wide and 100 millimeters long from the wrist upward, on the volar (inner) surface of the forearm by ink-dots at the corners of the parallelogram. Make a similar plot in the note-book. Working within this area, touch O, who sits with eyes closed, lightly with the point of a pencil and require him to try to locate this spot, eyes opened, by touching with another pencil.* Mark the relative positions of the two spots by dots in the note-book plot, and connect them by a light line to indicate the magnitude of the error. Label the dots S (stimulus) and L (location) respectively. Make 15 trials. Record O's analysis of what constitutes the "local sign"-the terms in which he recognizes the location of a touch.†

There are three factors in the record: the measurement of accuracy of localization in terms of the magnitude of the error; the direction of the error; and the analysis of the local sign.

Accuracy of localization varies for different parts of the skin; in general, the portions of the skin which are

* Keep the results secret from O.

For definition of local sign see Ch. IV, p. 45.