AUDIO FREQUENCY ATMOSPHERICS 



503 



TWEEKS 



A tweek consists of a damped oscillation trailing a static impulse. 

 Its audible duration appears to be less than 1/8 second and the initial 

 peak amplitude may approximate that of the maximum audio fre- 

 quency static impulses. 



Oscillographic reproductions of sound records obtained in Ireland 

 disclose that the tweeks practically always start above 2000 c.p.s. 

 and reduce very rapidly toward a lower limiting frequency where a 

 considerable portion of the time of existence is spent. In some cases 

 the highest observed frequency at the beginning of a tweek was in 

 the vicinity of 4000 c.p.s., which was the upper transmission limit of 

 the apparatus. In Fig. 3 is shown an oscillogram of tweeks trailing 



''/••'''■.r^.ff0l^jjfp^\^MW^/*'''-^-''^''^JV^^ 



/v'/,v^~«MV,yjV'^ 



Fig. 3 — Oscillogram of tweeks. Timing impulse frequency, 1000 c.p.s. 



static surges. While in these tweeks, any initial high frequencies are 

 obscured by the prominent static surge, some oscillograms have been 

 made while using electrical filters to suppress the frequencies mainly 

 responsible for the initial impulse. These oscillograms often showed 



4000 



S 3200 



O 2800 



16 



24 



32 



Fig. 4- 



40 48 8 16 



TIME IN MILLISECONDS 



A and B, tweek frequency variation curves. 



C, computed curve. 



initial frequencies as high as 4000 c.p.s. Two tweek frequency de- 

 terminations made from oscillograms are shown in Fig. 4. These 

 illustrate the initial rapid frequency reduction and the subsequent 

 gradual approach to a constant. While not an accurate definition, 



