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Because the audio frequency response of most receivers drops rapidly below a few hundred cycles per second, it is not possible to zero beat two signals exactly by listening to their audio beat note on a receiver's speaker or earphones. A more accurate indication of zero beat can be obtained by measuring either the receiver's a.v.c. voltage or its second detector output voltage with a vacuum tube voltmeter. When the difference in frequency between the two signals is only a few cycles per second, the voltmeter needle will fluctuate with the beat signal. The fluctuations become slower as the frequency difference becomes smaller and a large dip is obtained when the frequencies are exactly equal. However, this dip is extremely sharp and it sometimes requires a steady hand to zero beat the two signals exactly.
D.F. Zawada, K8EMS.