Exploring Waves and Sound: Properties, Perception, and Music
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Physics of Sound Chapter 12
Waves and Sound Chapter 12 Sound • 12.1 Properties of Sound • 12.2 Sound Waves • 12.3 Sound, Perception, and Music
Chapter 12 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 12 Vocabulary Terms • pressure • frequency • pitch • superposition principle • decibel • speaker • acoustics • microphone • fundamental • wavelength • stereo • Doppler effect • supersonic frequency • spectrum • shock wave • resonance • node • antinode • dissonance • harmonic • reverberation • note • sonogram • Fourier’s theorem • rhythm • musical scale • cochlea • consonance • longitudinal wave • beats • octave
Properties of Sound Key Question: What is sound and how do we hear it?
Properties of Sound • If you could see the atoms, the difference between high and low pressure is not as great. Here, it is exaggerated.
The frequency of sound • We hear frequencies of sound as having different pitch. • A low frequency sound has a low pitch, like the rumble of a big truck. • A high-frequency sound has a high pitch, like a whistle or siren. • In speech, women have higher fundamental frequencies than men.
12.1 Loudness • Every increase of 20 dB, means the pressure wave is 10 times greater in amplitude.
Sensitivity of the ear • How we hear the loudness of sound is affected by the frequency of the sound as well as by the amplitude. • The human ear is most sensitive to sounds between 300 and 3,000 Hz. • The ear is less sensitive to sounds outside this range. • Most of the frequencies that make up speech are between 300 and 3,000 Hz.
How sound is created • The human voice is a complex sound that starts in the larynx, a small structure at the top of your windpipe. • The sound that starts in the larynx is changed by passing through openings in the throat and mouth. • Different sounds are made by changing both the vibrations in the larynx and the shape of the openings.
12.1 Recording sound • A common way to record sound starts with a microphone. A microphone transforms a sound wave into an electrical signal with the same pattern of oscillation.
Recording sound • In modern digital recording, a sensitive circuit converts analog sounds to digital valuesbetween 0 and 65,536.
Recording sound • Numbers correspond to the amplitude of the signal and are recorded as data. One second of compact-disk-quality sound is a list of 44,100 numbers.
Recording sound • To play the sound back, the string of numbers is read by a laser and converted into electrical signals again by a second circuit which reverses the process of the previous circuit.
Recording sound • The electrical signal is amplified until it is powerful enough to move the coil in a speaker and reproduce the sound.
Sound Waves Key Question: Does sound behave like other waves?
Sound Waves • Sound has both frequency (that we hear directly) and wavelength (demonstrated by simple experiments). • 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.
Sound Waves • A sound wave is a wave of alternating high-pressure and low-pressure regions of air.
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.
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.
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.
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.
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.
Fourier's theorem • Fourier’s theorem says any complex wave can be made from a sum of single frequency waves.
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.
Sound, Perception, and Music Key Question: How is musical sound different than other types of sound? *Students read Section 15.3 AFTER Investigation 15.3
Sound, Perception, and Music • A single frequency by itself does not have much meaning. • The meaning comes from patterns in many frequencies together. • A sonogram is a special kind of graph that shows how loud sound is at different frequencies. • Every person’s sonogram is different, even when saying the same word.
Hearing sound • The eardrum vibrates in response to sound waves in the ear canal. • The three delicate bones of the inner ear transmit the vibration of the eardrum to the side of the cochlea. • The fluid in the spiral of the cochlea vibrates and creates waves that travel up the spiral.
Sound • The nerves near the beginning see a relatively large channel and respond to longer wavelength, low frequency sound. • The nerves at the small end of the channel respond to shorter wavelength, higher-frequency sound.
Music • The pitch of a sound is how high or low we hear its frequency. Though pitch and frequency usually mean the same thing, the way we hear a pitch can be affected by the sounds we heard before and after. • Rhythm is a regular time pattern in a sound. • Music is a combination of sound and rhythm that we find pleasant. • Most of the music you listen to is created from a pattern of frequencies called a musical scale.
Consonance, dissonance, and beats • Harmony is the study of how sounds work together to create effects desired by the composer. • When we hear more than one frequency of sound and the combination sounds good, we call it consonance. • When the combination sounds bad or unsettling, we call it dissonance.
15.3 Consonance, dissonance, and beats • Consonance and dissonance are related to beats. • When frequencies are far enough apart that there are no beats, we get consonance. • When frequencies are too close together, we hear beats that are the cause of dissonance. • Beats occur when two frequencies are close, but not exactly the same.
Harmonics and instruments • The same note sounds different when played on different instruments because the sound from an instrument is not a single pure frequency. • The variation comes from the harmonics, multiples of the fundamental note.
