The periodic rise and fall in the intensity of sound produced whn two notes of nearly equal frequencies are sounded together is called?

Step 1: Analyze the question The question describes two sound notes with nearly equal frequencies played together. The result is a periodic rise and fall in loudness (intensity). We need the name of this phenomenon.

Step 2: Explain the concept When two waves of slightly different frequencies, f₁ and f₂, travel together, they alternately reinforce each other (constructive interference) and cancel each other (destructive interference). This produces a regular "waa-waa-waa" pulsing in loudness. Each pulse is called a beat.

The beat frequency is:

f_beat = |f₁ − f₂|

For example, if two tuning forks of 256 Hz and 260 Hz sound together, you hear 4 beats per second.

Step 3: Check each option

• A. Doppler effect: An apparent change in frequency due to relative motion between source and observer (e.g., an ambulance siren changing pitch as it passes). This is not the case here.
• B. Beat: The periodic rise and fall in loudness from two nearly equal frequencies. This matches the description.
• C. Interference: This is the general superposition of waves. It is the underlying cause of beats, but it does not specifically describe the periodic variation in intensity from two close frequencies.
• D. Resonance: This occurs when an object vibrates at its natural frequency because of an external driving vibration of the same frequency. This is not the case here.

Step 4: Answer The correct option is B. beat.

Exam tip: Whenever you see "two nearly equal frequencies" together with "rise and fall in loudness," think beats. Remember the formula: beat frequency = difference between the two frequencies.