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444 CINEMATOGRAPHIC ANNUAL
the recorder, the amplitude of the wave on the wax at a frequency of 200 cycles will be twice as much as the amplitude for 400 cycles, and five times as much as the amplitude for 1,000 cycles, etc. Since it also happens that the energy of speech lies principally in the lower frequencies, it is obvious that the heaviest waves on the wax and therefore the greatest tendency for two adjacent grooves to cut into each other, occurs at the lower frequencies.
Some idea of che precision with which this stylus of the recorder must operate may be gained from a consideration of this frequency characteristic and some of the dimensions involved. Since a pitch of 92 is normally employed, the center to center spacing of the grooves on a wax record is about .011 inch. The width of the groove itself is about .006 inch, so that about .005 inch is available for lateral motion of the stylus — half of this amount to either side of the mean position. Since the maximum amplitudes occur at the lower frequencies because of the constant velocity characteristic of the recorder (above 200 cycles) the amplitudes of the higher frequencies will be exceedingly small. Assuming a full cut wax having roughly equal levels of a variety of frequencies present the 200 cycle amplitude will be about .002 inch to either side of the mean. The 1,000 cycle amplitude will be about .0004 inch, 2,000 cycles .0002, and 4,000 cycles about .0001 inch. Assume then that the volume drops about 20 db — a not uncommon range in talking picture work — and the amplitude of the 4,000 cycle wave becomes .00001 inch, or about ten millionths of an inch. It is because of these small amplitudes that the microscope seen at the left of Fig. 4 becomes a useful accessory to a wax recording machine, affording a ready means of determining the character and general level of the record.
The overall characteristic of wax recording must take account also of the electrical characteristics of the reproducer. The curve of an average reproducer is shown in Fig. 11. When combined with the characteristics of the recorder, shown in Fig. 10, a gradual droop toward the high frequency end results. An additional, but smaller, downward trend toward the high frequency end results from a mechanical effect which is analogous to film transfer loss. This effect results from the relation between the finite size of the needle point, which must be used in practice, and the length of the waves in the groove representing the higher fre