Sound!

How are they made? Sound waves are made by vibrations. (simple harmonic motion) Sound waves are made by vibrations. (simple harmonic motion) These vibrations form longitudinal waves These vibrations form longitudinal waves

Longitudinal waves Longitudinal waves have compressions and rarefactions that correspond to crests and troughs. Longitudinal waves have compressions and rarefactions that correspond to crests and troughs.

Characteristics Audible sound waves have a frequency between 20 and Hz. Audible sound waves have a frequency between 20 and Hz. Infrasonic waves have a lower frequency. Infrasonic waves have a lower frequency. Ultrasonic waves have a higher frequency. Ultrasonic waves have a higher frequency. Frequency determines the pitch Frequency determines the pitch

Ultrasound Because ultrasonic waves have a high frequency, they have a small wavelength. Because ultrasonic waves have a high frequency, they have a small wavelength. Therefore, the waves will reflect off of small objects. Therefore, the waves will reflect off of small objects. That’s why we can use ultrasounds to see inside humans, or for sonar That’s why we can use ultrasounds to see inside humans, or for sonar

Speed of Sound The speed of sound depends on the medium. The speed of sound depends on the medium. Because it is a longitudinal wave, it depends on particles hitting each other to travel. Because it is a longitudinal wave, it depends on particles hitting each other to travel. The closer the particles are, the faster it will travel. The closer the particles are, the faster it will travel. It can also depend on temperature. It can also depend on temperature.

Speed of Sound Air (0 o C) 331 m/s Air (25 o C) 346 m/s Alcohol (25 o C) 1140 m/s Sea water 1530 m/s Water 1490 m/s Aluminum 5100 m/s Copper 3560 m/s Iron 5130 m/s Lead 1320 m/s Rubber 54 m/s

3-D Sound waves travel in all three dimensions, as spheres. Sound waves travel in all three dimensions, as spheres. To draw a sound wave, we just draw concentric circles. Each circle represents a compression. To draw a sound wave, we just draw concentric circles. Each circle represents a compression.

The Doppler effect The Doppler effect is when you hear the pitch of a moving sound change as it passes you. The Doppler effect is when you hear the pitch of a moving sound change as it passes you. Remember, pitch and frequency are related. Remember, pitch and frequency are related. If the frequency of the sound stays the same, how can the pitch change? If the frequency of the sound stays the same, how can the pitch change?

The Doppler Effect Since the source is moving, the oncoming waves are closer together, which raises their pitch. Since the source is moving, the oncoming waves are closer together, which raises their pitch.

The Doppler effect The Doppler effect applies with all waves, not just sound waves. The Doppler effect applies with all waves, not just sound waves. This is how radar works, and how astronomers can tell that the universe is expanding—and in which direction it’s moving! This is how radar works, and how astronomers can tell that the universe is expanding—and in which direction it’s moving!

Intensity The energy of a sound wave is called intensity. The energy of a sound wave is called intensity. Intensity is power/area, and can be measured in W/m 2 Intensity is power/area, and can be measured in W/m 2 Relative intensity is measured in decibels (dB) Relative intensity is measured in decibels (dB)

Sound Intensity 1 x dB Threshold of hearing 1 x dB Quiet Whisper Quiet Whisper 1 x dB Normal conversation 1 x dB Vacuum cleaner 1 x dB Lawn mower 1 x dB Threshold of pain 1 x dB Machine gun

Intensity When the intensity goes up by 10 dB, the volume doubles! When the intensity goes up by 10 dB, the volume doubles! Any sound with an intensity over the threshold of pain will cause hearing loss. Any sound with an intensity over the threshold of pain will cause hearing loss.

Resonance When an object starts to vibrate, it makes other connected objects vibrate as well. When an object starts to vibrate, it makes other connected objects vibrate as well. This is called sympathetic or forced vibration. This is called sympathetic or forced vibration.

Resonance When two objects vibrate at the same frequency, the amplitude of the second object will increase as the first’s decreases. When two objects vibrate at the same frequency, the amplitude of the second object will increase as the first’s decreases. This is called resonance This is called resonance

Resonance Resonance is what makes guitars much louder than just their strings—the entire guitars resonates and transmits the sound. Resonance is what makes guitars much louder than just their strings—the entire guitars resonates and transmits the sound. Resonance can also be destructive—opera singers can shatter crystal, and bridges can collapse. Resonance can also be destructive—opera singers can shatter crystal, and bridges can collapse.

Tacoma Narrows Bridge-1940