Chapter 21 – The Nature of Sound Section 2 – The Properties of Sound pp. 606 - 611

TERMS TO LEARN Pitch – how high or low a sound is Infrasonic – a term that describes sound with frequency lower than 20 Hz Ultrasonic – term that describes sounds with frequencies higher than 20, 000 Hz

TERMS TO LEARN 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- how loud or soft a sound is Decibel – the most common unit used to express loudness.

You are the commander of a space station located about halfway between Earth and the moon. You are in the command center, and your chief of security tells you that sensors have just detected an explosion 61,054 km from the station. How long will it be before you hear the sound of the explosion?

SPEED OF SOUND Depends on the medium through which it is traveling. Changes when the medium changes. Sound travels faster in steel than in water. Speed of sound depends on temperature. The cooler the temperature, the slower the speed of sound. Complete Math Practice on p. 607.

PITCH The pitch of a sound is determined by the frequency of the sound wave. High pitch = high frequency Low pitch = low frequency

PITCH The average human ear can hear sounds between 20 Hz and 20,000 Hz. The lowest sound a pipe organ can make has a frequency of about 40 Hz. The screech a bat makes is 10,000 Hz or higher.

FREQUENCIES YOU CANNOT HEAR Sounds with frequencies lower than 20 Hz are called infrasonic. “Infra” means “below”. Sounds with frequencies higher than 20,000 Hz are called ultrasonic. “Ultra” means “beyond” Ultrasonic waves are used to clean jewelry and to remove ice from metal.

THE DOPPLER EFFECT For sound waves, the Doppler effect is the apparent change in frequency of a sound caused by the motion of either the listener or the source of the sound.

LOUDNESS Loudness is related to amplitude. The higher the amplitude of the wave, the louder the sound is. The lower the amplitude of the wave, the softer the sound is. Loudness is measured in decibels (dB).

SOME COMMON DECIBEL LEVELS Whisper = 20 decibels Normal conversation = 60 dB Automobile horn = 115 dB Rock concert = 115 dB Threshold of pain = 120 dB Jet engine 30 m away = 140 dB

SOME COMMON DECIBEL LEVELS

SEEING SOUNDS An oscilloscope is used to graph representations of sound waves.

HOW IT WORKS A microphone converts the sound waves into an electric current. The oscilloscope converts the electric current into graphs. The highest point (crests) represent compressions of the sound wave; the lowest point (trough) represents rarefactions.