1958 (ISO Recommendation R 31) 5 1 Hz is the frequency of a periodic phenomenon of which the periodic time is 1 s.
1966 Sept. 50 (advt.) In order to extend the flat response below 40 Hz the rear of the bass driver is loaded with an acoustic transmission line.
1969 Spring–Summer 8 Frequency response of 35 to 18,000 Hz. Crossover is at 2500 Hz.