Beat frequency in IFC is defined as which of the following?

• A. The sum of the two carrier frequencies.
• B. The product of the two carrier frequencies.
• C. The difference between the two carrier frequencies.
• D. The average of the two carrier frequencies.

Answer: (C)

Beat frequency is the rate at which the interference pattern of two close carrier frequencies fluctuates. When two medium-frequency currents pass through tissue, they interfere and create an amplitude-modulated signal. The envelope of that modulation vibrates at the difference between the two carrier frequencies. So, the beat frequency equals the absolute difference |f1 − f2|. For example, carriers at 4000 Hz and 4100 Hz produce a beat frequency of 100 Hz, which is within the therapeutic range used for IFC. The other ideas don’t describe the modulation that matters clinically: summing gives a higher frequency carrier, averaging gives a midpoint frequency, and the beat concept specifically refers to the difference that sets the modulation rate.