Question

1. The lowest frequency that we can hear is about 20 hertz. If the speed of sound in air is 340 meters/second, what is the wavelength of that low sound?

(1 point)

17 m

6,800 m

0.059 m

170 m

2. A stationary source produces a sound wave at a frequency of 100 Hz. The wave travels at 1125 feet per second.
A car is moving toward the sound source at a speed of 200 feet per second.
What is the wavelength of the stationary sound source and the wavelength that a person in the car perceives?

(1 point)

wavelength of the stationary source: 13.25 ft; perceived wavelength: 11.25 ft

wavelength of the stationary source: 11.25 ft; perceived wavelength: 9.25 ft

wavelength of the stationary source: 11.25 ft; perceived wavelength: 13.25 ft

wavelength of the stationary source: 9.25 ft; perceived wavelength: 11.25 ft

Answer 1

1) 17 m.
Calculation: λ = v/f = 340 m/s ÷ 20 Hz = 17 m.

2) Choose: wavelength of the stationary source: 11.25 ft; perceived wavelength: 9.25 ft.
Work:
- stationary λ = v/f = 1125 ft/s ÷ 100 Hz = 11.25 ft.
- moving observer toward source compresses the wavelength: λ' = (v − v_obs)/f = (1125 − 200)/100 = 9.25 ft.