A siren having a ring of 200 hole makes 132 rev/min. A jet of air is directed on the set of holes. Calculate…

Analyze the question: A siren disc has 200 holes and rotates at 132 revolutions per minute. Each hole passing the air jet produces one puff of air, so the frequency of the note depends on the number of holes and the rotation rate. We need the frequency and then the wavelength, using v = 350 m/s.

Concept:

• Frequency = (number of holes) × (revolutions per second)
• Wavelength: λ = v / f

Solution:

1. Convert the rotation rate to revolutions per second: 132 rev/min ÷ 60 = 2.2 rev/s

2. Frequency: f = 200 × 2.2 = 440 Hz

3. Wavelength: λ = v / f = 350 / 440 = 0.795 m (to 3 s.f.)

Check against the options: 0.795 m matches option A.

Answer: Frequency = 440 Hz, wavelength ≈ 0.795 m, so the correct option is A.

Tip: For siren questions, always convert rev/min to rev/s first, then multiply by the number of holes. Remember that 440 Hz is the standard musical note A, which is a good sanity check. Then use λ = v/f.