510 BELL SYSTEM TECHNICAL JOURNAL 



Many observations have indicated a relation between swishes and 

 the quasi-musical sound in the range between 500 and 1500 c.p.s., 

 which in an earlier paper has been called "intermediate frequency 

 noise." ^ Frequently this noise is first observed as a subdued jumble 

 of hollow rustling or murmuring sounds. It often increases regularly 

 in intensity for some time, after which faint swishes may begin to 

 appear in the same frequency range. The swishes may increase in 

 intensity and length, eventually submerging the murmuring sound. 

 Occasionally the murmuring has continued for a short time after the 

 swishes have reduced in amplitude or have disappeared. As a general 

 rule the murmuring is not audibly prominent although it seems to 

 be rather continuous in character. As a result it may considerably 

 increase the atmospheric intensity in the intermediate voice range. 



On a few occasions musical high frequencies similar in general 

 character to the murmuring have been observed. This sound appears 

 as a continual chirping or jingling in the vicinity of 3200 c.p.s. The 

 amplitude is usually low and the duration short. Like the murmuring 

 sound, it appears to accompany periods during which swishes are 

 present, and probably is composed of large numbers of short, over- 

 lapping, high-frequency swishes. 



These types of atmospherics appear to have no connection with the 

 time of day, or with local weather conditions and there is no indication 

 of any correlation with the time of year. During some periods they 

 have been observed frequently during days and nights for possibly 48 

 hours or longer. They have been found at times to persist steadily 

 through the early morning, bridging the transition period when the 

 more common forms of atmospherics rapidly change character. At 

 times several weeks of daily observation have passed with practically 

 no appearance of swushes or related sounds. 



During periods of prominent swishes the variation of intensity is 

 usually gradual with maxima and minima spaced at irregular intervals 

 of possibly a few minutes. At maxima, the swish may approximate 

 the intensity of the usual audio night-time atmospherics. The in- 

 tensity which swishes may attain is evidenced by their occasional ob- 

 servation without use of amplifying apparatus. A twelve-mile tele- 

 graph line free from power interference has been found a satisfactory 

 antenna, and with a telephone receiver between the line and earth, 

 swishes of remarkable clearness have been observed. Tweeks have 

 been heard with the same equipment. 



In the short time during which the sound recording apparatus was 

 available in Ireland, swishes were very infrequent with the exception 

 of one day when all swishes were of the descending frequency type. 



