15.2 Sound Waves

Chapter 15 Objectives  Explain how the pitch, loudness, and speed of sound are related to properties of waves.  Describe how sound is created and recorded.  Give examples of refraction, diffraction, absorption, and reflection of sound waves.  Explain the Doppler effect.  Give a practical example of resonance with sound waves.  Explain the relationship between the superposition principle and Fourier’s theorem.  Describe how the meaning of sound is related to frequency and time.  Describe the musical scale, consonance, dissonance, and beats in terms of sound waves.

Chapter 15 Vocabulary Terms  acoustics  beats  cochlea  consonance  decibel  dissonance  Doppler effect  Fourier’s theorem  frequency  spectrum  microphone  musical scale  note  octave  pitch  pressure  reverberation  rhythm  shock wave  sonogram  speaker  stereo  subsonic  supersonic

Inv 15.2 Sound Waves Investigation Key Question: Does sound behave like other waves?

15.2 Sound Waves We know sound is a wave because:  Sound has both frequency and wavelength.  The speed of sound is frequency times wavelength.  Resonance happens with sound.  Sound can be reflected, refracted, and absorbed and also shows evidence of interference and diffraction.

15.2 Sound Waves A sound wave is a wave of alternating high- pressure and low-pressure regions of air.

15.2 Amplitude of sound  The amplitude of a sound wave is very small.  Even a loud 80 dB noise creates a pressure variation of only a few millionths of an atmosphere.

15.2 The wavelength of sound

15.2 The Doppler effect  The shift in frequency caused by motion is called the Doppler effect.  It occurs when a sound source is moving at speeds less than the speed of sound.

15.2 The speed of sound  The speed of sound in air is 343 meters per second (660 miles per hour) at one atmosphere of pressure and room temperature (21°C).  An object is subsonic when it is moving slower than sound.

15.2 The speed of sound  We use the term supersonic to describe motion at speeds faster than the speed of sound.  A shock wave forms where the wave fronts pile up.  The pressure change across the shock wave is what causes a very loud sound known as a sonic boom.

15.2 The speed of sound  The speed of a sound wave in air depends on how fast air molecules are moving.  The speed of sound in materials is often faster than in air.

15.2 Standing waves and resonance  Spaces enclosed by boundaries can create resonance with sound waves.  The closed end of a pipe is a closed boundary.  An open boundary makes an antinode in the standing wave.  Sounds of different frequencies are made by standing waves.  A particular sound is selected by designing the length of a vibrating system to be resonant at the desired frequency.

15.2 Sound waves and boundaries  Like other waves, sound waves can be reflected by surfaces and refracted as they pass from one material to another.  Sound waves reflect from hard surfaces.  Soft materials can absorb sound waves.

15.2 Fourier's theorem  Fourier’s theorem says any complex wave can be made from a sum of single frequency waves.

15.2 Sound spectrum  A complex wave is really a sum of component frequencies.  A frequency spectrum is a graph that shows the amplitude of each component frequency in a complex wave.