Step 1: Concept
Sound is classified into musical notes (periodic/regular) and noise (non-periodic/irregular).
Step 2: Meaning
Noise lacks a distinct pitch because it is a complex mixture of many frequencies without a harmonic relationship.
Step 3: Analysis
Regular vibrations produce pleasant musical sounds, while irregular, sudden changes in pressure lead to the sensation of noise.
Step 4: Conclusion
Thus, noise is characterized by irregular and non-periodic vibrations.
Final Answer: (A)