13.1 Sound Waves pp

Essential Questions  How do we perceive sound?  What conditions change the way in which we perceive sound?

Objective(s): Students will be able to…  Explain how sound waves are produced.  Relate frequency to pitch.  Compare the speed of sound in various media.  Relate plane waves to spherical waves.  Recognize the Doppler effect, and determine the direction of a frequency shift when there is relative motion between a source and an observer.

Agenda:  Introduction to Sound!  Notes:  The production of sound waves  Frequency and pitch  Ultrasonic waves  Speed and direction of sound  The Doppler effect  Pass back and discuss:  Pendulum lab reports  Chapter 12 Tests

How Sound is Made  The Basics:  A vibrating object compresses air (or another fluid) and transmits waves at certain frequencies.  Large hollow spaces (like piano bodies or your chest cavity) allow those vibrations to resonate.

Production of Sound Waves  A tuning fork vibrates in super-slow motion, moving the air particles around it.  When the fork forces air molecules closer together in high density, this is compression.  When the fork moves back and leaves a void for the air to return to fill, this is called rarefaction.

Sound Waves are Longitudinal  The air around a vibrating object is compressed and relaxed repeatedly, so sound waves are periodic and longitudinal.  Compression forms areas of high density, and rarefaction forms areas of low density.

Frequency of Sound Waves  Humans can hear between approximately 20 Hz and 20,000 Hz. These are called audible sound waves.  Frequency determines pitch, which is how high or low we perceive a sound to be.  Frequency is objective.  Pitch is subjective, based on our perception.

Audible Frequencies  Outside of 20 Hz–20,000 Hz, sound waves are still produced, but humans cannot hear them.  Think dog whistles.

Ultrasonic Waves  Sound waves with very high frequencies, about 10MHz (10,000,000 Hz) can be used to see inside of solid objects.  By bouncing the waves off objects and time how long it takes them to come back, images can be formed.

How Sound Waves Move  The speed of sound depends on the medium it travels through.  The closer the particles are packed, the faster the wave can transmit through the material.  See the table on p. 482.

How Sound Waves Move  Sound waves propagate in three dimensions (but we can look at them in 2D).  Circles represent areas of highest compression, called wave fronts.  The distance between each front is a wavelength.

How Sound Waves Move  As sound waves travel, if we look far enough away from the source…  The wave fronts can be approximated as parallel planes.  These are called plane waves.

The Doppler Effect  Relative motion creates a change in frequency.

The Doppler Effect  The frequency of the horn remains the same.  The perceived pitch of person A (left) is higher, because more wave fronts arrive per second.  The perceived pitch of person B (right) is lower, because less wave fronts arrive per second.

Recap  Sound waves are produced by vibrating objects compressing air into longitudinal waves.  The greater the frequency, the higher the perceived pitch.  Sound waves propagate spherically, but the “sides” of a sphere appear as planes from far enough away.  The Doppler effect describes the change in pitch due to the relative motion of an object.

Homework  -p486 #1-4, 6, 7