An ambulance siren emits a sound of frequency 1800 Hz. The speed of sound in air is 330m/s. If the ambulance moves toward a stationary observer at a constant speed of 50m/s. What is the frequency heard by the observer?
Correct answer: C. (1800 x 330) / 280
- A. (1800 x 290) / 330
- B. (1800 x 330) / 370
- C. (1800 x 330) / 280
- D. (1800 x 330) / 380
Explanation
Formula for this case, when source is moving and observer is stationary, is: f' = f (v) / (v - vs) where: f = frequency of the sound emitted by the source v = speed of sound in air = 330 m/s vs = speed of the source Plugging in the values, we get: f' = 1800 x (330) / (330 - 50) = 1800 x 330 / 280
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The speed of sound depends on the properties of the medium, including its elasticity and density, and in gases it varies mainly with temperature. Questions connect speed with frequency and wavelength, compare sound in solids, liquids and gases, and distinguish the speed of propagation from the oscillating particles' motion.
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