Sound Chapter 15

Sound Waves Sound is a longitudinal wave. (medium displaces parallel to direction of the wave) Sound is a longitudinal wave. (medium displaces parallel to direction of the wave) Produced by compression of matter (air) Produced by compression of matter (air)

Sound Waves Vibrating source produces rhythmic vibrations in air pressure. Vibrating source produces rhythmic vibrations in air pressure. Molecules collide transmitting pressure variations away from the source of the sound. Molecules collide transmitting pressure variations away from the source of the sound. Air Pressure effects frequencies of sound. Air Pressure effects frequencies of sound.

Sound Waves The speed of Sound (v s ) - sound depends on the temperature of the air The speed of Sound (v s ) - sound depends on the temperature of the air v s = 343 m/s in Air at room temp. (T = 20 o C) v s = 343 m/s in Air at room temp. (T = 20 o C) v s is greater in solids and liquids v s is greater in solids and liquids v s = 0 in a vacuum (no medium) v s = 0 in a vacuum (no medium)

Sound Waves Examples using Sound Waves Echo - Reflected sound waves Echo - Reflected sound waves Dead Spots – Destructive Interference Dead Spots – Destructive Interference Diffracted waves traveling outside windows Diffracted waves traveling outside windows v s = f v s = f

Doppler Effect Doppler Effect- the change in a wavelength of sound emitted by a moving source. Doppler Effect- the change in a wavelength of sound emitted by a moving source. As the source moves towards the detector the waves crowd closer, the wavelength ( ) decreases. The speed of sound does not change so the frequency (f) increases. As the source moves towards the detector the waves crowd closer, the wavelength ( ) decreases. The speed of sound does not change so the frequency (f) increases. Source towards Detector Source towards Detector decreases  f increases (v Sound is constant) decreases  f increases (v Sound is constant) v s = f v s = f

Doppler Effect The opposite is true when the Source moves away from the Detector The opposite is true when the Source moves away from the Detector As the source moves away from the detector the waves spread apart, the wavelength ( ) increases. The speed of sound does not change so the frequency (f) decreases. As the source moves away from the detector the waves spread apart, the wavelength ( ) increases. The speed of sound does not change so the frequency (f) decreases. Source towards Detector Source towards Detector increases  f decreases (v Sound is constant) increases  f decreases (v Sound is constant) v s = f v s = f

Doppler Effect The Doppler Effect also occurs if Source is stationary and Detector is moving. The Doppler Effect also occurs if Source is stationary and Detector is moving. The Doppler Effect also occurs with Electromagnetic Waves (Light) The Doppler Effect also occurs with Electromagnetic Waves (Light) Radar Gun Radar Gun Light from Galaxies Light from Galaxies Ultra Sound Ultra Sound Bats Bats

Pitch and Loudness Pitch- is related to frequency Pitch- is related to frequency Loudness- is related to Intensity or Amplitude. Loudness- is related to Intensity or Amplitude.

Pitch and Loudness Two notes with frequencies that are related 2 to 1 are separated by one Octave. Two notes with frequencies that are related 2 to 1 are separated by one Octave Hz – 1 Octave Hz – 1 Octave Hz – 1½ Octaves Hz – 1½ Octaves HZ – 2 Octaves HZ – 2 Octaves Ratio determines Musical Interval. Not the frequency itself Ratio determines Musical Interval. Not the frequency itself

Sound Levels Sound level – measures intensity of loudness. Measured in decibels (dB), Sound Level measures the ratio of intensity of a sound wave to the intensity of faintly heard sound. Sound level – measures intensity of loudness. Measured in decibels (dB), Sound Level measures the ratio of intensity of a sound wave to the intensity of faintly heard sound. Lowest level is at 0 dB. This is the softest sound able to be heard. Lowest level is at 0 dB. This is the softest sound able to be heard. A sound ten times larger is 10 dB. A sound ten times larger is 10 dB.

Sources of Sound Sources of Sound are produced by a vibrating object producing molecular motions and pressure oscillations in air. Sources of Sound are produced by a vibrating object producing molecular motions and pressure oscillations in air. Examples, musical instruments Examples, musical instruments

Resonance Resonance is increasing the amplitude of vibration by repeatedly applying a small external force of the same frequency. Resonance is increasing the amplitude of vibration by repeatedly applying a small external force of the same frequency. You can find the points of Resonance of a glass tube filled with water. You can find the points of Resonance of a glass tube filled with water.

Detection of Sound Sound detectors convert sound energy into other forms. (Recorders, Microphones) Sound detectors convert sound energy into other forms. (Recorders, Microphones) Sound energy is kinetic energy from air molecules colliding. Sound energy is kinetic energy from air molecules colliding. The human ear is an amazing sound detector. The human ear is an amazing sound detector.