Doppler Effect Waves and Sound

Doppler Effect The frequency and wavelength of a wave appears distorted when the source of the waves moves toward or away from an observer. This is distortion is known as the Doppler Effect. When a wave source moves toward an observer the emitted waves appear to have a higher frequency, and the wavelengths appear to shorter. Sound waves will appear to have a higher pitch. Light waves will appear blue shifted (create a slight blue tinting). When a wave source moves away from an observer the emitted waves appear to have a lower frequency, and the wavelengths appear to be longer. Sound waves will appear to have a lower pitch. Light waves will appear red shifted (create a slight red tinting).

Doppler Effect and Sound Waves When sound waves are emitted from a stationary source they radiate outward forming a sphere of sound. One wave follows the next, as shown with the next click of the mouse. When the sound source is centered and the waves move out symmetrically, then the sound source is stationary.

Doppler Effect and Sound Waves However, when the sound source is moving then each new wave starts from a new location. If the sound source (the dot) moves to the right, then the wave pattern becomes The sound source is moving away from this side, causing the wavelengths to become longer. This results in a lower frequency sound heard when the sound source moves away from you. The sound source is moving toward this side, causing the wavelengths to become shorter. This results in a higher frequency sound heard when the sound source moves toward you. λ v λ

Doppler Effect Equation fO Frequency the observer hears fS Frequency emitted by the source V Speed of sound (343 m/s at room temperature) vO Speed of the observer vS Speed of the sourse The plus and minus signs are chosen so that when the source and observer are moving toward each other: fO > fS moving apart: fO < fS

Doppler Effect and Sound Waves Here is an interesting scenario involving a slightly faster sound source. Now the sound waves pile up in front of the sound source. This is known as the sound barrier. For this to occur the sound source must move at the speed of sound, 343 m/s. This is also known as Mach 1.

Doppler Effect and Sound Waves What happens if the sound source moves faster than the speed of sound? Now the sound source out runs the sound waves. This leaves a wake of sound (like a boat wake) behind the source. This wake of sound is known as a sonic boom. The sound source is said to be supersonic.