Physics Chapter 12 Study Guide
Sound

The Production of Sound Waves

Compression–the region of a longitudinal wave in which the density and pressure are greater than normal.

Rarefaction–the region of a longitudinal wave in which the density and pressure are less than normal.

Characteristics of Sound Waves

Pitch–how high or low we perceive a sound to be, depending on the frequency of the sound wave.

Speed of Sound in different media

 Medium Speed (m/s) Gases air (0˚C) 331 air (25˚C) 346 air (100˚C) 366 helium (0˚C) 972 hydrogen (0˚C) 1290 oxygen (0˚C) 317 Liquids at 25˚C methyl alcohol 1140 sea water 1530 water 1490 Solids aluminum 5100 copper 3560 iron 5130 lead 1320 vulcanized rubber 54

How to calculate the speed of sound in air.

The speed of sound increases as the temperature increases.

Increases 0.6 m/s for every degree C˚, starting from 330 m/s at 0˚C

Propagation of Waves

The Doppler Effect

The Doppler Effect–frequency shift that is the result of relative motion between the source of waves and an observer.

Java Applet

Doppler Effect Equations

Sound Source Moving

f is the frequency of the source

v is the speed of sound

is the speed of the source

Use the – for moving toward and + for moving away.

Observer Moving

f is the frequency of the source

v is the speed of sound

is the speed of the observer

Use the + for moving toward and – for moving away

Both Source and Observer Moving

Use the top signs for moving towards and the bottom sign for moving away.

Example 1

A high speed train is traveling at a speed of 44.7 m/s when the engineer sounds the 415 Hz warning horn. The speed of sound is 343 m/s. What are the frequency and wavelength of the sound, as perceived by a person standing at a crossing when the train is (a) approaching and (b) leaving the crossing?

Example 2

A bicyclist and a car have the same speed and are moving toward each other. The car emits a sound of frequency 612.0 Hz, and the bicyclist hears the frequency as 635.0 Hz. The temperature at the time was 20˚C. How fast is each going?

Sound Intensity

Intensity-rate at which energy flows through a unit area perpendicular to the direction of motion.

Decibels - The relative intensity of a sound is called the intensity level or decibel level ß.

Decibel Level Table

Decibel Levels

Example 3

A speaker at an open-air concert emits 600 W of sound power, radiated equally in all directions.
a. What is the intensity of the sound 5.00 m from the speaker?

b. What sound intensity level would you experience there if you did not have any protection for your ears?

c. Earplugs you can buy in the drugstore have a noise reduction rating of 23 decibels. If you are wearing those earplugs but your friend Fred is not, how far from the speaker should Fred stand to experience the same loudness as you?

Shock Waves and Sonic Booms

Some Sonic Boom Videos

Resonance and Forced Vibration

Resonance–a condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system.