Sound

Review Sound is a mechanical wave Sound is a mechanical wave Sound is a Longitudinal Wave Sound is a Longitudinal Wave compression rarefaction

Frequency is referred to as Pitch High Pitch sounds: High Pitch sounds: Low Pitch Sounds Low Pitch Sounds

Properties of Sound Waves Frequency of sound waves Frequency of sound waves Humans hear between 20 Hz – 20,000 Hz Humans hear between 20 Hz – 20,000 Hz Ultrasound = Above 20,000 Hz Ultrasound = Above 20,000 Hz Infrasound = below 20 Hz Infrasound = below 20 Hz

Doppler Effect An observe change in frequency when there is relative motion between the source of waves and an observer

Instead of these compressional regions (high pressure regions) reaching you one at a time in consecutive fashion, they all reach you at once. Since every compression is followed by a rarefaction, the high pressure zone will be immediately followed by a low pressure zone. This creates a very loud noise.

Intensity Intensity refers to the Amplitude of the sound wave Intensity refers to the Amplitude of the sound wave Intensity depends on how much energy put into the disturbance/wave. Intensity depends on how much energy put into the disturbance/wave. Measured in Watts/m 2 Measured in Watts/m 2 Relative Intensity measured using the Decibel scale (dB) Relative Intensity measured using the Decibel scale (dB) Logarithmic scale Logarithmic scale

Intensity of a Spherical Wave Intensity = P 4πr 2 Intensity = (W/m 2 ) P = power (W) r = distance from the source (m)

Practice Problem What is the intensity of the sound waves produced by a trumpet at a distance of 3.2m when the power output of the trumpet is 0.2W? Assume that the sound waves are spherical. What is the intensity of the sound waves produced by a trumpet at a distance of 3.2m when the power output of the trumpet is 0.2W? Assume that the sound waves are spherical.

Resonance Forced Vibration Forced Vibration When one object is set in motion, its vibrations are transferred causing other objects to vibrate. When one object is set in motion, its vibrations are transferred causing other objects to vibrate. When frequency of vibration matches natural frequency of object, the amplitude increases – causing resonance When frequency of vibration matches natural frequency of object, the amplitude increases – causing resonance Pushing someone on a swing Pushing someone on a swing

Tacoma Narrows Bridge Collapse Lasted only 4 months! Winds blowing at Fundamental Frequency Caused increased amplitude in bridge vibration An example of constructive interference Engineers now use models to test structures before actual construction

How the Human Hears Eardrum begins to vibrate Vibrations transfer to bones in the middle ear Bones transmit vibrations to Basilar membrane

Back to Harmonics Draw the first three harmonics: Draw the first three harmonics:

Back to Harmonics:

The basis for drawing the standing wave patterns for air columns is that vibrational antinodes will be present at any open end and vibrational nodes will be present at any closed end. The basis for drawing the standing wave patterns for air columns is that vibrational antinodes will be present at any open end and vibrational nodes will be present at any closed end.

Open End Air Column Both ends are open so both ends are antinodes

Closed End Air Column Even numbered harmonics are not possible The closed end must be a node The open end must be an antinode