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170 VIII. SOUND, SOUND RECORDING, AND CHARACTERISTICS
as commercial disk and high-quality film recording machines, magnetic recording on tape has a good chance of displacing photographic recording in the making of original records. Such magnetic tape records may be played back immediately, and there is the saving of the cost of photographic development of the original. Many of the features of magnetic recording and its relation to the making of original recordings will be found in a series of papers in the Jan., 1947 (Vol. 48) issue of the JSMPE.
Sound is recorded lengthwise along the film; the distance along the film that represents a wave length for a particular frequency can be calculated readily from the film speed — 36 ft. per min. It will be assumed in the figures shown in the table that the film is unshrunk ; the distance between the centers of adjacent film perforations will be assumed to be exactly 0.300 in.
In order to determine frequency in a practical case, it is necessary to
measure the length along the film of a single wave, and to correct the
measurement by the ratio of the nominal distance between sprocket holes
to the measured distance between sprocket holes :
wave length (corrected) in. = wave length
, ,N . 0.300 in.
(measured) m. x r— —
measured distance
The frequency is then calculated from the relationship V = n\, where V = 7.2 in./sec. and A is the wave length (corrected) in inches.
It is apparent that a film speed of 36 ft. per min. is the equivalent oi 7.2 in. per second. A frequency of one cycle per second, therefore, will
TABLE XII
Frequency Versus Film Length"
Frequency, Frequency, cycles per Length per cycles per Length per cycle,
second cycle, inches second inches
1 7.2000 4,000 0.0018
24 0.3000 5,000 0.00144
50 0.1440 6,000 0.00120
100 0.0720 7,000 0.00103
200 0.0360 10,000 0.00072
500 0.01440 12,000 0.00060
1,000 0.0072 15,000 0.00048
2,000 0.0036 20,000 0.00036
3,000 0.0024
o Standard 16-mm sound speed, 36 ft. per min.