What is beat frequency in IFC and how is it produced?

• A. Beat frequency equals the difference between two carrier frequencies.
• B. Beat frequency is the average of the two carrier frequencies.
• C. Beat frequency is the sum of the two carrier frequencies.
• D. Beat frequency is unrelated to the frequencies used.

Answer: (A)

Beat frequency is the rate at which the amplitude of two interfering carrier waves modulates. In IFC, two medium-frequency currents with slightly different frequencies are applied to the body. When these two sine waves with frequencies f1 and f2 superimpose, the result can be thought of as a high-frequency carrier with a slow amplitude modulation. Mathematically, the sum of the two waves produces a modulation at the difference between their frequencies, |f1 − f2|. This slow beat (the difference frequency) is what stimulates nerves, while the high-frequency carriers help tissues tolerate the current and penetrate deeper. For example, 4000 Hz and 4100 Hz create a 100 Hz beat frequency. The other options don’t describe how the interference pattern translates into a low-frequency modulation: the beat is not the average or the sum of the frequencies, and it is not unrelated to the frequencies used.