Sound Waves Sound is longitudinal AND it is mechanical Sound needs a medium to travel. No sound in a vacuum. Produced by vibrations in matter, such as vocal chords, strings, or air columns Sound Waves Frequency of vibrations determines the pitch Amplitude is related to the energy and the loudness Pressure wave travels, but air particles only move side to side Speed of Sound Depends on the medium (elasticity, density, temperature) Sound through air @ 20C = 343 m/s (warmer air increases speed slightly) 𝑣𝑠𝑜𝑢𝑛𝑑 < 𝑣 𝑖𝑛 𝑎𝑖𝑟 𝑠𝑜𝑢𝑛𝑑 𝑖𝑛 𝑙𝑖𝑞𝑢𝑖𝑑 < 𝑣 𝑠𝑜𝑢𝑛𝑑 𝑖𝑛 𝑠𝑜𝑙𝑖𝑑 Speed of Sound Light (3x108 m/s) travels much faster than sound (343 m/s) through air This is why there’s a delay with fireworks or lightning far away Frequency of Sound Average human can hear between 20 Hz 20,000 Hz Below 20 Hz is infrasonic Above 20,000 Hz is ultrasonic range of frequencies test Examples: dog whistle, dolphins, ultrasounds, the mosquito ring tone on your phone Amplitude of Sound (Loudness measured in decibels) Example 1 You see a lightning strike and then hear the thunder 7.5 seconds later. How far away was the lightning strike? Example 2 In a concert hall, the orchestra tunes to the oboe playing a frequency of 440 Hz. What is the period of this sound wave? Example 3 In a concert hall, the orchestra plays middle C at 256 Hz. What is the wavelength of middle C at 256 Hz? If you are sitting 75 m away, how much time does it take the sound to get to you? Example 4 A submarine sends out a sonar signal of 7000. Hz to detect an unknown object. If the speed of sound in water is 1500. m/s, and the reflected wave comes back 3.0 seconds later. What is the wavelength of the sound wave? How far away is the unknown object from the submarine? Interference of Sound Overlapping crests of a wave will result in… ...an increased amplitude. This is constructive interference. Overlapping a crest and a trough results in... ...a decrease in amplitude. This is destructive interference. Beat Frequency Beats - the periodic variation in loudness of two sounds played together Beat frequency = 𝑓2 − 𝑓1 What is the beat frequency when a 256 Hz and a 260 Hz tuning fork are sounded together? Resonance Resonance – when a driving frequency matches another object’s natural frequency and causes an increase in amplitude • Pushing a friend on a swing • Resonance boxes • Speaker shattering wine glass • Tacoma Narrows Bridge • Low bass notes rattling car windows Standing Waves continuous constructive and destructive interference of waves at certain frequencies Nodes—where total destructive interference occurs (no movement) Antinodes—where constructive interference occurs (maximum displacement) Doppler Effect If the object or observer are moving towards each other, the observer will hear a higher frequency. If the object or observer are moving away from each other, the observer will hear a lower frequency. Sonic Boom If an object moves faster than the speed of sound in air (343 m/s), the object is supersonic The object continuously drags a shock wave of pressure behind it that is the sonic boom Sound videos No sound in a vacuum Sound videos Resonance and beat frequency Sound videos Resonance of a wine glass Sound videos Doppler Effect Sound videos Sonic Boom Example Doctors can broadcast focused sound waves and cause kidney stones to shatter while inside the body. This is an example of a) b) c) d) reflection interference beats resonance Example The natural frequency of a wine glass is 337 Hz. In order to shatter the wine glass, a loudspeaker would have to play a) A frequency that is greater than 337 Hz b) A frequency that is less than 337 Hz c) A frequency that is exactly 337 Hz Example The natural frequency of a wine glass is 337 Hz. In order to shatter the wine glass, a loudspeaker would have to play a) A frequency that is greater than 337 Hz b) A frequency that is less than 337 Hz c) A frequency that is exactly 337 Hz Example One violin is playing a frequency of 200 Hz and another violin is playing 205 Hz. The beat frequency that you will hear is a) b) c) d) 202.5 Hz 405 Hz 5 Hz 2.5 Hz Example One violin is playing a frequency of 200 Hz and another violin is playing 205 Hz. The beat frequency that you will hear is a) b) c) d) 202.5 Hz 405 Hz 5 Hz 2.5 Hz Example An F-18 jet is flying at 686 m/s. This plane is a) b) c) d) ultrasonic infrasonic supersonic subsonic Example An F-18 jet is flying at 686 m/s. This plane is a) b) c) d) ultrasonic infrasonic supersonic subsonic Example A police car is driving away from you while the siren sounds at 500 Hz. You will hear a sound wave with a a) b) c) d) Velocity greater than 343 m/s Velocity less than 343 m/s Frequency higher than 500 Hz Lower than 500 Hz Example A police car is driving away from you while the siren sounds at 500 Hz. You will hear a sound wave with a a) b) c) d) Velocity greater than 343 m/s Velocity less than 343 m/s Frequency higher than 500 Hz Lower than 500 Hz Example The following diagram is of a standing wave. If L is 6 meters, then the wavelength of this wave is a) 2 m b) 4 m c) 6 m d) 12 m Example The following diagram is of a standing wave. If L is 6 meters, then the wavelength of this wave is a) 2 m 1.5 waves fit inside 6 meters, so b) 4 m 1 wavelength is 4 meters. c) 6 m d) 12 m Example The following diagram is of a standing wave. This standing wave has a) 3 nodes and 4 antinodes b) 6 nodes and 4 antinodes c) 3 antinodes and 4 nodes d) 6 antinodes and 4 nodes Example The following diagram is of a standing wave. This standing wave has a) 3 nodes and 4 antinodes b) 6 nodes and 4 antinodes c) 3 antinodes and 4 nodes d) 6 antinodes and 4 nodes Example A car’s horn produces a sound wave of constant frequency. As the car speeds up going away from a stationary spectator, the sound wave detected by the spectator a) b) c) d) decreases in amplitude and decreases in frequency decreases in amplitude and increases in frequency increases in amplitude and decreases in frequency increases in amplitude and increases in frequency Example A car’s horn produces a sound wave of constant frequency. As the car speeds up going away from a stationary spectator, the sound wave detected by the spectator a) b) c) d) decreases in amplitude and decreases in frequency decreases in amplitude and increases in frequency increases in amplitude and decreases in frequency increases in amplitude and increases in frequency
