RESULTS OF RELEVANT RESEARCH 



223 



ures of the differential thresholds of loudness 

 and pitch as functions of these same vari- 

 ables may also be obtained. 



A number of psycho-physical studies using 

 white noise stimuli have been done by the 

 Psycho-Acoustic Laboratory, Harvard Uni- 

 versity. For example, recorded tests which 

 measure ability to discriminate the pitch 

 and loudness of noises have been developed 

 by Karlin (30). Preliminary results on the 

 use of the loudness discrimination tests in 

 the selection of sonar operators have been 

 reported by Harris (23). Miller (38) has 

 measured sensitivity to changes in intensity 

 over a wide range of intensities. He has 

 found that: 



The just detectable increment in the in- 

 tensity of the noise is of the same order of 

 magnitude as the just detectable increment in 

 the intensity of pure tones. For intensities 

 more than 30 db. above the threshold of hearing 

 for noise the size in decibels of the increment 

 which can be heard 50 percent of the time is 

 approximately constant (0.41 db.). 



He also found that, as for pure tones, just 

 noticeable differences at different intensity 

 levels are not equal in subjective magnitude, 

 the j.n.d. at low intensity levels appearing 

 smaller than at high levels. (See below for 

 discussion of the relation of these results to 

 the phenomenon of masking.) 



Measurement of Physiological Events 



Physio-physical experiments which may 

 be done using white noise stimuli include 

 determination of changes in cochlear micro- 

 phonics and nerve potentials as a function 

 of the different variables of the physical 

 stimulus. 



Effect of Physiological Changes on Hearing 



of Noise Stimuli 



Psycho-physiological experiments which 

 may be performed will be concerned with 

 changes in hearing of noise stimuli as a 

 result of alteration of the physiological mech- 

 anism. For example, absolute thresholds 



of hearing for pure tones and for different 

 bands of white noise can be measured before 

 and after lesions of the cochlea or auditory 

 nervous system. 



To date, there are no reports available of 

 research projects that might be classified 

 under the above two headings. 



Masking Experiments 



Since one of the most important tasks of 

 the sonar operator is the detection of an 

 auditory signal which is more or less masked 

 by a background of sound, studies of mask- 

 ing are of major practical importance in 

 underwater acoustics. Moreover, it is in 

 this area, the study of masking phenomena, 

 that the field research appears most sugges- 

 tive to the experimenter in the university 

 laboratory. 



Beginning with the classical study of 

 Wegel and Lane (60) we have had numerous 

 investigations of the masking of one pure 

 tone by another. With the exception of the 

 experiments on masking of speech by noise, 

 which were carried on for the most part in 

 the Bell Telephone Laboratories, there were 

 almost no studies prior to World War II of 

 the masking effects of noise on pure tones or 

 of noise background on noise signals. The 

 work of the laboratories investigating prob- 

 lems of underwater acoustics and of other 

 laboratories studying the problems of com- 

 munication in noise have emphasized the 

 lack of knowledge in this area. 



Masking of Pure Tones by Noise 



The Harvard Psycho-Acoustic Laboratory 

 has recently published the results of a series 

 of experiments on the masking of pure tones 

 by white noise (26). Tests were made at 

 eight noise levels, ranging from 20 to 90 

 decibels. Results were used to determine 

 two basic functions: 



(a) The critical band width of a masking 

 noise, i.e., the ratio, in decibels, between the 

 level of a pure tone and the level per cycle of 

 the noise that is just able to mask the tone. 



(b) The function relating the amount of 



