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An accurate method of measuring small frequency differences between pure tones of short duration

Pairs of pure tones, each tone lasting 1 s, the first of a pair being separated from the second by about 1/4 s, were reproduced through a loudspeaker. The frequency of the first of each pair was always 440 Hz, and the second was higher or lower by a small amount. When the difference in frequency was below about 4 Hz it became imperative to have a check on the magnitude of the differences. The check had to be made on the signals actually played in case any errors had been introduced in the recording process. The authors have devised such a check and since the method developed is suitable for signals of duration down to 1/4 s, and for frequency differences from about 1/2 Hz up to 10 Hz, they consider that it might be of interest to investigators in other fields.