Chapter 21 – The Nature Chapter 21 – The Nature of Soundof Sound

Section 2 – The Properties of Section 2 – The Properties of SoundSound

pp. 606 - 611pp. 606 - 611

TERMS TO LEARNTERMS TO LEARN

•Pitch – Pitch – how high or low a sound is

• Infrasonic – Infrasonic – a term that describes sound with frequency lower than 20 Hz

•Ultrasonic – Ultrasonic – term that describes sounds with frequencies higher than 20, 000 Hz

TERMS TO LEARNTERMS TO LEARN

•Doppler effect – Doppler effect – the apparent change in frequency of a sound caused by the motion of either the listener or the source of the sound

•Loudness- Loudness- how loud or soft a sound is

•Decibel – Decibel – the most common unit used to express loudness..

You are the commander of a space You are the commander of a space station located about halfway station located about halfway

between Earth and the moon. You between Earth and the moon. You are in the command center, and are in the command center, and

your chief of security tells you that your chief of security tells you that sensors have just detected an sensors have just detected an explosion 61,054 km from the explosion 61,054 km from the

station. How long will it be before station. How long will it be before you hear the sound of the you hear the sound of the

explosion?explosion?

SPEED OF SOUNDSPEED OF SOUND

• Depends on the Depends on the medium through which it is traveling.

• Changes when the Changes when the medium changes..• Sound travels faster in steel than in Sound travels faster in steel than in

water.water.• Speed of sound Speed of sound depends on temperature..

The cooler the temperature, the slower The cooler the temperature, the slower the speed of sound.the speed of sound.

• Complete Math Practice on p. 607.Complete Math Practice on p. 607.

PITCHPITCH

• The pitch of a The pitch of a sound is sound is determined by the frequency of the sound wave.

• High pitch = High pitch = high frequency

• Low pitch = Low pitch = low frequency

PITCHPITCH• The average human The average human

ear can hear sounds ear can hear sounds between between 20 Hz and 20,000 Hz..

• The lowest sound a The lowest sound a pipe organ can make pipe organ can make has a frequency of has a frequency of about about 40 Hz..

• The screech a bat The screech a bat makes is makes is 10,000 Hz or higher.

FREQUENCIES YOU CANNOT FREQUENCIES YOU CANNOT HEARHEAR

• Sounds with frequencies lower than 20 Sounds with frequencies lower than 20 Hz are called Hz are called infrasonic..

• ““Infra” means “Infra” means “below”.”.• Sounds with frequencies higher than Sounds with frequencies higher than

20,000 Hz are called 20,000 Hz are called ultrasonic..• ““Ultra” means “Ultra” means “beyond””• Ultrasonic waves are used to clean Ultrasonic waves are used to clean

jewelry and to remove ice from metal.jewelry and to remove ice from metal.

THE DOPPLER EFFECTTHE DOPPLER EFFECT• For sound waves, the Doppler effect is the For sound waves, the Doppler effect is the

apparent change in frequency of a sound apparent change in frequency of a sound caused by the motion of either the listener caused by the motion of either the listener or the source of the sound.or the source of the sound.

LOUDNESSLOUDNESS

• Loudness is related to Loudness is related to amplitude..

• The higher the amplitude of the The higher the amplitude of the wave, the wave, the louder the sound is..

• The lower the amplitude of the wave, The lower the amplitude of the wave, the the softer the sound is..

• Loudness is measured in Loudness is measured in decibels (dB).

SOME COMMON DECIBEL SOME COMMON DECIBEL LEVELSLEVELS

• Whisper = 20 decibelsWhisper = 20 decibels

• Normal conversation = Normal conversation = 60 dB

• Automobile horn = 115 dBAutomobile horn = 115 dB

• Rock concert = Rock concert = 115 dB

• Threshold of pain = Threshold of pain = 120 dB

• Jet engine 30 m away = 140 dBJet engine 30 m away = 140 dB

SOME COMMON DECIBEL SOME COMMON DECIBEL LEVELSLEVELS

SEEING SOUNDSSEEING SOUNDS

•An An oscilloscope is used to is used to graph graph representatiorepresentations of sound ns of sound waves.waves.

HOW IT WORKSHOW IT WORKS

• A microphone converts the sound A microphone converts the sound waves into an electric current.waves into an electric current.

• The oscilloscope converts the electric The oscilloscope converts the electric current into graphs. The highest current into graphs. The highest point (crests) represent point (crests) represent compressions of the sound wave; the of the sound wave; the lowest point (trough) represents lowest point (trough) represents rarefactions.