Question

a) Juan is riding his bike eastward at a speed of 15 km/h and heads the siren of an ambulance (700 Hz) that moves west at 60 km/h. Which frequency will the child heard?

b) A sound wave propagates through a 10 cm closed tube. The air temperature is -8 Celsius degrees. Calculate the second and third harmonics.

Answer 1

a) The frequency the boy will hear is

$$f=\frac{330+15·1000/3600}{330-60·1000/3600}·700=747\text{ Hz}.$$

b) The speed of sound is

$$v=\sqrt{\gamma\frac{RT}{M}}=326\text{ m/s}.$$

The second and third harmonics are

$$f_2=\frac{n_2v}{2L}=3260\text{ Hz},\\\space\\ f_3=\frac{n_3v}{2L}=4890\text{ Hz}.$$