Chapter 20 Sound Notes

Kenneth W. Ford is a physicist, author, and physics educator.
He has been a University of California professor, a college president, and executive director and CEO of the American Institute of Physics.
He taught grades 9 through 12 at Germantown Academy in Pennsylvania.
He is an expert and author on quantum physics and is passionate about teaching and flying small airplanes and gliders.
He is a close friend who refines the author's writing before it is published.
His book Basic Physics inspired the author when writing Conceptual Physics, as did The Feynman Lectures on Physics and Physics for the Inquiring Mind.
He was one of the first reviewers of Conceptual Physics, praising and adopting it for his classes, and alerting the author to errors and suggesting improvements.
His books include The Quantum World: Quantum Physics for Everyone, 101 Quantum Questions: What You Need to Know About the World You Can't See, Basic Physics, and Building the H Bomb.
Building the H Bomb recounts his personal history with his late mentor, John Wheeler, as part of the team that developed the first hydrogen bomb.
The author dedicated the 8th edition of Conceptual Physics to Kenneth W. Ford, calling him an eminent physicist and great human being.

The question of whether a tree falling in a forest with no listener produces sound depends on the definition of sound.
Some argue that sound is subjective and requires a listener, while others argue that sound is an objective thing.
Scientists take the objective position and define sound as a form of energy that exists whether or not it is heard.

Most sounds are waves produced by the vibrations of matter.
Examples include vibrating strings in a piano, violin, and guitar; a vibrating reed in a saxophone; a fluttering column of air in a flute; and vibrating vocal cords.
The original vibration stimulates the vibration of something larger or more massive, such as the sounding board of a stringed instrument or the air in the throat and mouth of a singer.
This vibrating material sends a disturbance through the surrounding medium, usually air, in the form of longitudinal waves.
Under ordinary conditions, the frequencies of the vibrating source and the sound waves are the same.
Pitch is the subjective impression about the frequency of sound, with high-pitched sounds having a high frequency of vibration and low-pitched sounds having a low frequency of vibration.
The ear of a young person can normally hear pitches corresponding to the range of frequencies between about 20 to 20,000 hertz.
Sound waves with frequencies below 20 hertz are infrasonic, and those with frequencies above 20,000 hertz are called ultrasonic.
Infrasonic: frequency too low for human hearing.
Ultrasonic: frequency too high for human hearing.

Most sounds are transmitted through the air, but any elastic substance can transmit sound.
Elasticity is the property of a material that has changed shape in response to an applied force and then resumes its initial shape once the distorting force is removed.
Steel is an elastic substance, while putty is inelastic.
In elastic liquids and solids, the atoms are relatively close together, respond quickly to one another's motions, and transmit energy with little loss.
Sound travels about 4 times faster in water than in air and about 15 times faster in steel than in air.
Air is a poor conductor of sound relative to solids and liquids.
Liquids and crystalline solids are generally excellent conductors of sound, with the speed of sound generally greater in solids than in liquids, and greater in liquids than in gases.
Sound won't travel in a vacuum because there is nothing to compress and expand.

When we clap our hands, the sound produced is nonperiodic and consists of a wave pulse that travels outward in all directions.
When a door is opened, it pushes the molecules next to it away from their initial positions and into their neighbors, creating a pulse of compressed air called a compression.
When a door is closed, it pushes some air molecules out of the room, producing an area of low pressure behind the door, creating a zone of lower-pressure air is rarefied, called a rarefaction.
With all wave motion, it is not the medium itself that travels across the room but the energy-carrying pulse.
If you continually swing the door open and closed in periodic fashion, you can set up a wave of periodic compressions and rarefactions.
When the prong of a tuning fork moves toward the tube, a compression enters the tube. When the prong swings away in the opposite direction, a rarefaction follows the compression.
A radio loudspeaker is a paper cone that vibrates in rhythm with an electrical signal.
Air molecules next to the vibrating cone of the speaker are themselves set into vibration, which vibrates against neighboring particles, creating rhythmic patterns of compressed and rarefied air.
The resulting vibrating air sets your eardrum into vibration, which sends cascades of rhythmic electrical impulses along the cochlea nerve canal and into your brain, allowing you to listen to music.

The loudspeaker of a radio or other sound-producing system changes electrical signals into sound waves.
The electrical signals pass through a coil wound around the neck of a paper cone, which acts as an electromagnet, located near a permanent magnet.
When current flows one way, magnetic force pushes the electromagnet toward the permanent magnet, pulling the cone inward.
When current flows in the opposite direction, the cone is pushed outward.
Vibrations in the electrical signal cause the cone to vibrate, producing sound waves in the air.

Light travels faster than sound.
The speed of sound depends on wind conditions, temperature, and humidity.
It does not depend on the loudness or the frequency of the sound; all sounds in the same medium travel at the same speed.
The speed of sound in dry air at 0°C is about 330 meters per second.
Water vapor in the air increases this speed slightly.
Sound travels faster through warm air than cold air because the faster-moving molecules in warm air bump into one another more often and therefore can transmit a pulse in less time.
For each degree rise in temperature above 0°C, the speed of sound in air increases by 0.6 meter per second.
In air at a normal room temperature of about 20°C, sound travels at about 340 meters per second.

Wave motion of all kinds possesses energy of varying degrees.
Sound has an extremely small amount of energy, requiring only a small amount of energy to produce.
For example, 10 million people talking at the same time would produce sound energy equal only to the energy needed to operate a common flashlight.
Sound energy dissipates to thermal energy while sound travels in air.
For waves of higher frequency, the sound energy is transformed into internal energy more rapidly than for waves of lower frequencies.
As a result, sounds of low frequencies travel farther through air than sounds of higher frequencies. That's why the foghorns of ships have a low frequency.