Sound and the Doppler Effect

Sound is a Mechanical Wave What is a mechanical wave? A mechanical wave is any wave that needs a medium.

Sound is a longitudinal wave created by a vibrating object.

The Nature of a Sound Wave: Sound is a Longitudinal Wave: The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air. Regardless of the source of the sound wave - whether it be a vibrating string or the vibrating tines of a tuning fork - sound is a longitudinal wave.

The frequency of a sound wave determines its pitch. The higher the frequency the higher the pitch and vice versa. The intensity of a sound wave determines its volume.

Sound is broken up into three different ranges: infrasonic, audible, and ultrasonic.

Infrasonic sounds are those with frequencies too low for us to hear. Whales and elephants can communicate at these frequencies. Audible sounds are within the frequencies we can hear. Ultrasonic sounds are higher frequencies than we can hear. Dogs and dolphins can communicate at these frequencies. Ultrasonic waves are also used in medical imaging.

Ever try to talk to your friends underwater in the pool? Sound can move through air but it can also move through liquids and solids. The speed of sound is different in different media. Do you think sound moves fastest through solids, liquids or gasses? Sound moves fastest through solids and slowest through gasses.

The speed of sound also depends on the temperature. Do you think sound travels faster in cold weather or hot weather? Sound travels faster in warmer temperatures.

Every object has a frequency at which it wants to vibrate. This is called the object’s natural frequency. A tree that sways back and forth in the wind is an example of an object that vibrates. A plucked rubber band also wants to vibrate back and forth. These objects will not vibrate back and forth unless there is a force causing them to do so. When a force causes an object to vibrate at its natural frequency, resonance occurs.

During resonance, the vibrating object will vibrate back and forth with a large amplitude. An everyday example of resonance is when you swing on a playground swing. When you pump your legs at the correct time you are matching the natural frequency of the swing so you swing in a very high arch.

Wave Interference Review What happens when two waves meet while they travel through the same medium? What affect will the meeting of the waves have upon the appearance of the medium? Will the two waves bounce off each other upon meeting (much like two billiard balls would) or will the two waves pass through each other?

Wave Interference Wave interference is the phenomenon which occurs when two waves meet while traveling along the same medium.

Constructive Interference

Destructive Interference

Beats When two instruments play at slightly different frequencies the sound waves will interfere, creating beats. When the sound waves are in phase they will produce a sound louder than either individual sound. When the sound waves are out of phase they will produce a sound quieter than either individual sound.

Beats The beat frequency is the difference of the two individual frequencies. Sample Problem: Find the beat frequency when a 256 Hz tuning fork and a 260 Hz tuning fork are sounded simultaneously. (B f = 4Hz)

Beats are a special form of wave interference.

The Doppler Effect nnnnnnnneeeeeeeeeeooooooooowwwwwwww

The Doppler Effect The Doppler effect is the effect produced by a moving source of waves. The Doppler Effect occurs for all waves but it is most noticeable for sound waves.

The Doppler Effect Each wave that is created is closer to the previous wave on the leading side. Each wave that is created is further from the previous wave on the trailing side.

The Doppler Effect If an object creating a wave is approaching you, the waves are closer together than normal. This makes the wavelength smaller and the frequency larger.

The Doppler Effect Sound bite anual/acoustics/effects_of_sound/30_MPH_d oppler.au

The Doppler Effect The Doppler Effect has NOTHING to do with the loudness of the wave. The Doppler Effect changes the frequency and wavelength of a wave. The Doppler Effect cannot change the speed of a wave.

Shock Waves and Sonic Booms The Doppler effect is observed whenever the speed of the source is moving slower than the speed of the waves. But if the source actually moves at the same speed as or faster than the wave itself can move, a different phenomenon is observed.

Shock Waves and Sonic Booms If a moving source of sound moves at the same speed as sound, then the source will always be at the leading edge of the waves which it produces. This is known as a shock wave.

Shock Waves and Sonic Booms If a moving source of sound moves faster than sound, the source will always be ahead of the waves which it produces.

Shock Waves and Sonic Booms A sonic boom occurs as the result of the piling up of waves

If you are standing on the ground as the supersonic aircraft passes by, there will be a short time delay and then you will hear the boom - the sonic boom. This boom is merely a loud noise resulting from the high pressure sound followed by a low pressure sound.

Sonic Booms Do not be mistaken into thinking that this boom only happens the instant that the aircraft surpasses the speed of sound and that it is the signature that the aircraft just attained supersonic speed. Sonic booms are observed when any aircraft which is traveling faster than the speed of sound passes overhead. It is not a sign that the aircraft just overcame the sound barrier, but rather a sign that the aircraft is traveling faster than sound.