Sound Chapter 12

Sound Waves A pressure variation that is transmitted through matter Sound waves move through air because a vibrating source produces regular oscillations in air pressure Sound is a longitudinal wave

Temperature The speed of a sound wave in air depends on the temperature of the air Sound waves move through air at sea level at 343 m/s at room temperature (20° C) Sound can also travel through liquids and solids at greater speeds than gases Sound can not travel through a vacuum because there are no particles to move and collide

Other properties of waves Reflect off hard objects Echoes Time for echo to return to source can be used to find the distance between the source and the object (sonar) v = λƒ Can be diffracted Can have interference with dead spots

Loudness Physical characteristics of sound waves Frequency, wavelength, amplitude Amplitude is the measure of the variation in pressure along the wave Loudness The human ear can detect a wide range of amplitudes, so they are measured on a logarithmic scale sound level measured in decibels

Pitch Frequency of vibration is pitch Each music note has a different frequency Most people are sensitive to sounds with frequencies between 1000 Hz and 5000 Hz

Intensity Intensity is the rate at which energy is transferred I = power / (4pi)(distance)^2 I = P / 4πr2 Page 415 (1-2)

Doppler Shift (Radar) As a vehicle with a siren moves toward you the pitch changes Frequency is higher when moving toward you and drops to a lower pitch as the source moves away The frequency of the source does not change, but as the vehicle travels toward the listener more waves are compressed in the area More crests reach the ear per second so the frequency of the detected sound increases

Physics of Music Sound (music) is produced by vibrating objects Reeds, surfaces, cone, vocal cords, lips, pipe openings, air columns, strings, wires

Resonance in Air Columns Resonance increases the amplitude of a vibration by repeatedly applying a small external force at the same natural frequency The length of the air column determines the frequencies of the vibrating air Changing the length of the column of vibrating air varies the pitch of the instrument

Resonating tube A hollow tube is placed in water to close one end Closed-pipe resonator A resonating tube with one closed end The length of the air column is changed by adjusting the height of the tube above the water

Lab The sound is loud when the air column is in resonance with the tuning fork A resonating air column intensifies the sound of the tuning fork Page 362

Open-pipe resonator A resonating tube with both ends open that will resonate with a sound source is an open-pipe resonator The sound wave does not reflect off a closed end but rather off an open end The pressure of the reflected wave is inverted

Resonance lengths A standing sound wave in a pipe can be represented by a sine wave The standing waves have nodes and antinodes Nodes are regions of low displacement Antinodes are regions of high displacement 2 nodes or 2 antinodes are separated by ½ wavelength

Resonance Frequencies Closed pipe λ/4, 3λ/4, 5λ/4, 7λ/4, and so on will all be in resonance with a tuning fork Open pipe λ/2, λ, 3λ/2, 2λ, and so on will be in resonance with a tuning fork P. 427 (1-3)

Beat notes Two frequencies that are nearly identical interfere to produce high and low sound levels This oscillation of wave amplitude is a beat When the difference is less than 7 Hz, the ear detects a pulsation of loudness