13.1 Sound Wavespp 479 - 486

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