When two similar sound sources vibrate with slightly different frequencies, a regular rise and fall in the loudness of the combined sound is heard. What is this phenomenon called?

Introduction / Context:
In acoustics, the interference of sound waves can produce striking effects in what we hear. When two sounds of nearly equal frequency are played together, the combined sound does not have a constant loudness. Instead, the listener hears a periodic waxing and waning of intensity. This question asks for the name of that specific phenomenon.

Given Data / Assumptions:

• Two similar sound sources (for example, tuning forks or musical notes) are sounding together.
• Their frequencies are close but not exactly equal, such as 250 Hz and 252 Hz.
• Both sounds reach the listener at the same time and interfere.
• The listener detects a regular rise and fall in loudness over time.

Concept / Approach:
When two waves of slightly different frequencies superpose, the resulting waveform has a periodically varying amplitude. This variation in amplitude produces alternating loud and soft sounds, known as beats. The beat frequency is equal to the absolute difference between the two frequencies. Other phenomena such as Doppler effect, resonance and echo involve different mechanisms: Doppler effect is the apparent change in frequency due to relative motion, resonance is large amplitude response at a natural frequency, and echo is the reflection of sound from a distant surface.

Step-by-Step Solution:
Step 1: Consider two sound waves with frequencies f1 and f2, where f1 and f2 are close but not equal. Step 2: When these waves superpose, their displacement values add according to the principle of superposition. Step 3: The mathematical result is a wave whose amplitude varies with time, creating alternate constructive and destructive interference. Step 4: When interference is constructive, the combined sound is loud; when it is destructive, the combined sound is soft. Step 5: This regular fluctuation in loudness is called beats, and the number of beats heard per second is |f1 - f2|.

Verification / Alternative check:
If you strike two tuning forks of slightly different frequencies and listen, you will hear a throbbing sound where the loudness rises and falls regularly. The frequency of this throbbing matches the difference in their frequencies. Experiments in physics labs and music tuning practices use this effect; musicians listen for beats when tuning instruments and adjust until the beats disappear, meaning the frequencies have become equal. This matches the textbook definition of beats, not Doppler effect, resonance or echo.

Why Other Options Are Wrong:
Doppler effect: This is the apparent change in frequency when the source or listener is moving; it does not produce the regular rise and fall in loudness described here.

Resonance: Resonance refers to large amplitude oscillations when a system is driven at its natural frequency; it does not involve two slightly different frequencies producing alternating loud and soft beats.
Echo: An echo is a reflected sound heard after a delay, not a continuous modulation in loudness.

Common Pitfalls:
Students sometimes confuse beats with resonance because both involve amplitude changes. The key difference is that beats arise from the superposition of two similar frequencies, producing a single combined sound whose loudness varies periodically, whereas resonance is a strong response at one specific frequency. Keeping the phrase "slightly different frequencies" in mind helps you recognise the beats phenomenon.

Final Answer:
The regular rise and fall in loudness due to two slightly different frequencies is called Beats.