Section 1: The Nature of Sound

• What causes sound?

  • Each sound is produced by an object that vibrates.

  • All sounds are created by something that vibrates.

• Sound Waves

  • When an object like a radio speaker vibrates, it collides with nearby molecules in the air, transferring some of its energy to them.

    • This process of collisions and energy transfer forms a sound wave.

  • Sound waves are compressional waves.

  • Compressions and rarefactions move away from the speaker as molecules in the air collide with their neighbors.

  • A series of compressions and rarefactions forms that travels from the speaker to your ear.

• The Speed of Sound

  • Most sounds you hear travel through air to reach your ears.

  • Sound waves can travel through any type of matter— solid, liquid, or gas.

  • Sound waves create compressions and rarefactions in any medium they travel through.

  • On the Moon, which has no atmosphere, the energy in sound waves cannot be transmitted from particle to particle because no particles exist.

    • Sound waves can not travel through empty space.

  • The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas

  • Sound travels faster in liquids and solids than in gases because the individual molecules in a liquid or solid are closer together than the molecules in a gas.

  • The speed of sound waves also depends on the temperature of a medium.

    • As the temperature of a substance increases, its molecules move faster.

    • If the particles in a medium are colliding with each other more often, more energy can be transferred in a shorter amount of time.

• Human Hearing

  • Vocal cords and mouths move in many different ways to produce various kinds of compressional waves.

  • Your ears and brain work together to turn the compressional waves caused by speech, music, and other sources into something that has meaning.

  • The ear has three regions that perform specific functions in hearing.

  • The visible part of your ear, the ear canal, and the eardrum make up the outer ear.

    • The outer ear is where sound waves are gathered.

    • The gathering process starts with the outer part of your ear, which is shaped to help capture and direct sound waves into the ear canal.

  • The ear canal is a passageway that is 2-cm to 3-cm long and is a little narrower than your index finger.

    • The sound waves travel along this passageway, which leads to the eardrum.

  • Eardrum: a tough membrane about 0.1 mm thick.

  • When the eardrum vibrates, it passes the sound vibrations into the middle ear, where three tiny bones start to vibrate.

    • These bones are called the hammer, the anvil, and the stirrup.

    • The bones amplify the sound wave.

    • The stirrup is connected to a membrane on a structure called the oval window, which vibrates as the stirrup vibrates.

  • When the membrane in the oval window vibrates, the sound vibrations are transmitted into the inner ear.

  • Cochlea: a spiral-shaped structure that is filled with liquid and contains tiny hair cells

    • It is the cochlea that converts sound waves to nerve impulses.

Section 2: Properties of Sound

• Intensity and Loudness

  • For a compressional wave, amplitude is related to the density of the particles in the compressions and rarefactions.

  • The amplitude of a sound wave depends on how tightly packed molecules are in the compressions and rarefactions.

  • For a sound wave that carries less energy and has a lower amplitude, particles in the medium are less compressed in the compressions and less spread out in the rarefactions. For a sound wave that has a higher amplitude, particles in the medium are closer together, or more compressed, in the compressions and more spread out in the rarefactions.

  • To produce a wave that carries more energy, more energy is transferred from the vibrating object to the medium.

    • More energy is transferred to the medium when the particles of the medium are forced closer together in the compressions and spread farther apart in the rarefactions.

  • Intensity: the amount of energy that flows through a certain area in a specific amount of time.

  • Intensity influences how far away a sound can be heard.

  • Intensity influences how far a wave will travel because some of a wave’s energy is converted to other forms of energy when it is passed from particle to particle.

  • A sound wave of low intensity loses its energy more quickly, and travels a shorter distance than a sound wave of higher intensity.

  • Loudness: the human perception of sound intensity.

  • When you hear different sounds, you do not need special equipment to know which sounds have greater intensity.

  • When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do.

  • As the intensity of a sound wave increases, the loudness of the sound you hear increases.

  • Decibel: Each unit on the scale for sound intensity

  • Sounds with intensity levels above 120 dB may cause pain and permanent hearing loss.

• Pitch: how high or low a sound seems to be.

  • For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second.

  • When a sound wave with high frequency hits your ear, many compressions hit your eardrum each second.

  • Every note has a different frequency, which gives it a distinct pitch.

  • A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz.

    • The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz.

  • Ultrasonic: sound frequencies above 20,000 Hz

    • Ultrasonic waves are used in medical diagnosis and treatment.

  • Infrasonic, or subsonic, waves have frequencies below 20 Hz—too low for most people to hear.

• The Doppler Effect: The change in pitch or wave frequency due to a moving wave source

  • The Doppler effect occurs when the source of a sound wave is moving relative to a listener.

  • You also can observe the Doppler effect when you are moving past a sound source that is standing still.

  • The faster the change in position, the greater the change in frequency and pitch.

  • The Doppler effect also occurs for other waves besides sound waves.

  • Doppler radar can show the movement of winds in storms, and, in some cases, can detect the wind rotation that leads to the formation of tornadoes. This can help provide early warning and reduce the injuries and loss of life caused by tornadoes.

Section 3: Music

• What is music?

  • Music and noise are caused by vibrations— with some important differences.

  • Music: made of sounds that are deliberately used in a regular pattern.

  • Every material or object has a particular set of frequencies at which it vibrates.

  • Musical instruments contain strings, membranes, or columns of air that vibrate at their natural frequencies to produce notes with different pitches.

  • The sound produced by musical instruments is amplified by resonance.

• Sound Quality: describes the differences among sounds of the same pitch and loudness.

  • Objects can be made to vibrate at other frequencies besides their natural frequency.

  • The specific combination of frequencies produced by a musical instrument is what gives it a distinctive quality of sound.

  • Even though an instrument vibrates at many different frequencies at once, you still hear just one note.

  • The main tone that is played and heard is called the fundamental frequency.

  • In addition to vibrating at the fundamental frequency, the string also vibrates to produce overtones.

  • Overtone: a vibration whose frequency is a multiple of the fundamental frequency.

    • These overtones produce an instrument’s distinct sound quality.

  • The number and intensity of overtones vary with each instrument.

• Musical Instruments

  • A musical instrument is any device used to produce a musical sound.

  • Soft violins, screaming electric guitars, and elegant harps are types of string instruments.

    • In string instruments, sound is produced by plucking, striking, or drawing a bow across tightly stretched strings.

  • Resonator: a hollow chamber filled with air that amplifies sound when the air inside of it vibrates.

  • Brass and woodwind instruments rely on the vibration of air to make music.

  • In brass and wind instruments, the length of the vibrating tube of air determines the pitch of the sound produced.

  • Since ancient times, people have used drums and other percussion instruments to send signals, accompany important rituals, and entertain one another

    • Percussion instruments are struck, shaken, rubbed, or brushed to produce sound.

    • Other types of percussion instruments include cymbals, rattles, and even old-fashioned washboards.

• Beats

  • You may have heard a pulsing variation in loudness, called beats.

  • When two instruments play at the same time, the sound waves produced by each instrument interfere.

  • When compressions and rarefactions overlap each other, the loudness decreases.

  • The frequency of the beats that you hear decreases as the two waves become closer in frequency.

Section 4: Using Sound

• Acoustics: the study of sound.

  • The sounds and their reflections reach your ears at different times, so you hear echoes. This echoing effect produced by many reflections of sound is called reverberation.

• Echolocation: the process of locating objects by emitting sounds and interpreting the sound waves that are reflected back.

  • At night, bats swoop around in darkness without bumping into anything.

    • Their senses of sight and smell help them navigate. Many species of bats also depend on echolocation.

• Sonar: a system that uses the reflection of underwater sound waves to detect objects

  • Because the speed of sound in water is known, the distance to the object can be calculated by measuring how much time passes between emitting the sound pulse and receiving the reflected signal.

  • The idea of using sonar to detect underwater objects was first suggested as a way of avoiding icebergs, but many other uses have been developed for it.

• Ultrasound in Medicine

  • High-frequency sound waves are used in more than just echolocation and sonar.

    • Ultrasonic waves also are used to break up and remove dirt buildup from jewelry.

    • Chemists sometimes use ultrasonic waves to clean glassware.

    • One of the important uses of ultrasonic waves, though, is in medicine.

  • Reflected ultrasonic waves are used to detect and monitor conditions such as pregnancy, certain types of heart disease, and cancer.

  • Like X rays, ultrasound can be used to produce images of internal structures.

  • Medical professionals use ultrasound to examine many parts of the body, including the heart, liver, gallbladder, pancreas, spleen, kidneys, breast, and eye.

  • High-frequency sound waves can be used to treat certain medical problems.