14.3 Doppler Effect and Sonic Booms

• The Doppler effect is a change in the observed pitch of a sound, due to relative motion between the source and the observer.
• An example of the Doppler effect due to the motion of a source occurs when you are standing still, and the sound of a siren coming from an ambulance shifts from high-pitch to low-pitch as it passes by. The closer the ambulance is to you, the more sudden the shift. The faster the ambulance moves, the greater the shift.
• sound spreads out from the point where it was emitted.
• Therefore, the wavelength is shorter in the direction the source is moving, and longer in the opposite direction
• the frequency at which they receive the compressions changes.
• If the source exceeds the speed of sound, no sound is received by the observer until the source has passed, so that the sounds from the source when it was approaching are stacked up with those from it when receding, creating a sonic boom.
• A sonic boom is a constructive interference of sound created by an object moving faster than sound.