Sound 13-1 A “physical phenomenon that stimulates the sense of hearing.”

What do you think? What is sound? What is sound? What do all of the sounds that you hear have in common? What do all of the sounds that you hear have in common? How do they differ? How do they differ? Can sounds travel through solids? Liquids? Gases? Can sounds travel through solids? Liquids? Gases? Is one type of material better for transmitting sound waves? Is one type of material better for transmitting sound waves? When race cars or emergency vehicles pass you, the sound changes. In what way, and why? When race cars or emergency vehicles pass you, the sound changes. In what way, and why?

What is Sound? Sound is a wave that is produced by vibrating material objects. Tuning forks, guitar strings, drum head, vocal cords, speakers The vibrating object pushes the air molecules together, forming a compression. It then spreads them apart, forming a rarefaction.

Graphing Sound Waves The diagram shows compressions (dark) and rarefactions(white). If you measured the pressure or density of the air and plotted these against position, how would the graph appear?

Sound is a mechanical wave Sound is a mechanical wave requires a medium. requires a medium. Cannot travel through a vacuum Cannot travel through a vacuum “Nobody can hear you scream in space” “Nobody can hear you scream in space” In air, sound is a longitudinal (pressure) wave In air, sound is a longitudinal (pressure) wave Sound Transmission

Demo: Sound in a vacuum

Click below to watch the Visual Concept. Visual Concept Sound Waves

Characteristics of Sound Frequency is the number of waves per second. Frequency is the number of waves per second. You have heard of ultrasound. What is it? You have heard of ultrasound. What is it? Frequencies audible to humans are between 20 Hz and Hz. Frequencies audible to humans are between 20 Hz and Hz. Middle C on a piano is 262 Hz. Middle C on a piano is 262 Hz. The emergency broadcast signal is Hz. The emergency broadcast signal is Hz. Infrasound frequencies are lower than 20 Hz. Infrasound frequencies are lower than 20 Hz. Ultrasound frequencies are greater than Hz. Ultrasound frequencies are greater than Hz.

Click below to watch the Visual Concept. Visual Concept Comparing Infrasonic and Ultrasonic Sounds

Pitch Pitch is the human perception of how high or low a sound appears to be. Pitch is the human perception of how high or low a sound appears to be. Pitch is primarily determined by frequency. Pitch is primarily determined by frequency. Pitch also depends slightly on other factors. Pitch also depends slightly on other factors. Higher frequencies appear to have a higher pitch when played loudly, even though the frequency does not change. Higher frequencies appear to have a higher pitch when played loudly, even though the frequency does not change.

Speed of Sound Sound waves travel though solids, liquids and gases. Sound waves travel though solids, liquids and gases. In which would the speed generally be greatest? Why? In which would the speed generally be greatest? Why? Solids. Because the molecules are more closely packed, the particles respond more rapidly to compressions. Solids. Because the molecules are more closely packed, the particles respond more rapidly to compressions.

Demo: Sound Propagation Sound travels better through elastic liquids and solids, such as water and rocks, than through air. Sound travels better through elastic liquids and solids, such as water and rocks, than through air. This is due to the close proximity of the atoms as they vibrate. This is due to the close proximity of the atoms as they vibrate.

How might the temperature of air affect the speed of sound waves? Why? How might the temperature of air affect the speed of sound waves? Why? Higher temperature increases the speed of the waves because the particles are moving faster and colliding more often. Higher temperature increases the speed of the waves because the particles are moving faster and colliding more often. Where v is the speed of sound in air measured in m/s and T is the temperature of the air measured in degrees Celsius ( o C). Where v is the speed of sound in air measured in m/s and T is the temperature of the air measured in degrees Celsius ( o C). Speed of Sound in Air

Check Yourself Find the speed of sound in air at room temperature (70 o F). Find the speed of sound in air at room temperature (70 o F).

Speed of Sound

Spherical Waves In which directions does sound propagate? The diagram shows: Crests or wave fronts (blue circles) Wavelength ( ) Rays (red arrows) Rays indicate the direction of propagation. How would these wave fronts appear different if they were much farther from the source?

Spherical Waves Wave fronts and rays become more nearly parallel at great distances. Plane waves are simply very small segments of a spherical wave a long distance from the source.

Reflection of Sound Waves Sound waves can reflect off of surfaces. Sound waves can reflect off of surfaces. An Echo is a reflection of sound emitted from a source. An Echo is a reflection of sound emitted from a source. The human ear can only distinguish sounds separated by about 1/10 of a second (0.1s). The human ear can only distinguish sounds separated by about 1/10 of a second (0.1s). i.e. For you to hear an echo, the reflected sound must arrive at least 0.1s after it was emitted. i.e. For you to hear an echo, the reflected sound must arrive at least 0.1s after it was emitted.

Sample Problem: What is the minimum distance away a reflective surface must be in order for you to hear an echo on a day when it is 20 o C. What is the minimum distance away a reflective surface must be in order for you to hear an echo on a day when it is 20 o C.

What do you think? On a blank sheet of paper reflect on the following: On a blank sheet of paper reflect on the following: What is sound? What is sound? What do all of the sounds that you hear have in common? What do all of the sounds that you hear have in common? How do they differ? How do they differ? Can sounds travel through solids? Liquids? Gases? Can sounds travel through solids? Liquids? Gases? Is one type of material better for transmitting sound waves? Is one type of material better for transmitting sound waves? When race cars or emergency vehicles pass you, the sound changes. In what way, and why? When race cars or emergency vehicles pass you, the sound changes. In what way, and why?