Foundations of Physics CPO Science Foundations of Physics Unit 5, Chapter 15

Unit 5: Waves and Sound Chapter 15 Sound 15.1 Properties of Sound 15.2 Sound Waves 15.3 Sound, Perception, and Music

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 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

15.1 Properties of Sound Key Question: What is sound and how do we hear it? *Students read Section 15.1 AFTER Investigation 15.1

15.1 Properties of Sound If you could see the atoms, the difference between high and low pressure is not as great. Here, it is exaggerated.

15.2 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.

15.1 Complex sound When we hear complex sounds, the nerves in the ear respond separately to each different frequency. The brain interprets the signals from the ear and creates a “sonic image” from the frequencies. The meaning in different sounds is derived from the patterns in how the different frequencies get louder and softer.

Common Sounds and their Loudness

15.1 Loudness Logarithmic scale Linear scale Decibels (dB) Amplitude 1 20 10 40 100 60 1,000 80 10,000 100,000 120 1,000,000 Every increase of 20 dB, means the pressure wave is 10 times greater in amplitude.

15.1 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. The Equal Loudness Curve on the right shows how sounds of different frequencies compare. Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. For example, the Equal Loudness Curve shows that a 40 dB sound at 2,000 Hz sounds just as loud as an 80 dB sound at 50 Hz.

15.1 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. The Equal Loudness Curve on the right shows how sounds of different frequencies compare. Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. For example, the Equal Loudness Curve shows that a 40 dB sound at 2,000 Hz sounds just as loud as an 80 dB sound at 50 Hz.

15.1 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.

15.1 Recording sound In modern digital recording, a sensitive circuit converts analog sounds to digital values between 0 and 65,536.

15.1 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.

15.1 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.

15.1 Recording sound The electrical signal is amplified until it is powerful enough to move the coil in a speaker and reproduce the sound.

15.2 Sound Waves Key Question: Does sound behave like other waves? *Students read Section 15.2 BEFORE Investigation 15.2

15.2 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.

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

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 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.

15.3 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

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.

15.3 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.

15.3 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.

15.3 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.

15.3 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.

15.3 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.

Application: Sound from a Guitar