Answer to Question #164353 in Optics for Hanna

Question

(a.) A motorcyclist travels at a constant speed of 72 km/h along a straight level road towards an

observer standing at the center of the bridge over the road. When the motor-cyclist is distant, the

observer hears a sound of frequency 65,0 //z from the engine of the motorcycle. 

(b.) A second motor-cyclist now rides along-side the one mentioned in (b), at the same constant

speed of 72 km/h. The frequency of the sound of the second motorcycle is lower than that of the

first motorcycle. When the two machines are traveling towards the observer, from the same

direction, the intensity of the sound received varies with a frequency of 3.0 Hz.

Name the effect causing the variation of sound intensity and explain how it arises in this case. What

would be the frequency of the variation of sound intensity after the motor-cycles have passed under

the bridge and are moving away from the observer?

Answer 1

The effect is called interference of sound. It arises when two sounds of different frequences are mixed, as it shown onn a picture.

When the motorcycels will pass te bridge the frequency of variation will not change.