Beat frequency is calculated as the absolute difference between the two carrier frequencies f1 and f2. Which equation represents this?

• A. f1 × f2
• B. |f1 - f2|
• C. f1 + f2
• D. |f1 + f2|

Answer: (B)

When two carrier tones are close in frequency, their waves interfere and the overall amplitude rises and falls at a rate equal to how fast one wave slips past the other. That rate is the beat frequency, and it comes from the difference between the two frequencies. Since frequency is a magnitude, we take the absolute difference so the beat rate is always a positive value, regardless of which frequency is higher.

The correct expression is the absolute value of f1 minus f2. For example, if f1 is 1000 Hz and f2 is 990 Hz, the beat frequency is 10 Hz, meaning the envelope of the combined signal modulates 10 times per second.

The product f1 × f2 or the sum f1 + f2 (or its absolute value) would describe different phenomena (mixing products or a sum-frequency component), not the rate at which the amplitude modulates due to beating.