Chapter 24 –Sound 24.1 –Properties of Sound pp

TERMS TO LEARN Pitch – how high or low a sound is Pitch – how high or low a sound is Infrasonic – a term that describes sound with frequency lower than 20 Hz Infrasonic – a term that describes sound with frequency lower than 20 Hz Ultrasonic – term that describes sounds with frequencies higher than 20, 000 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 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 Loudness- how loud or soft a sound is Decibel – the most common unit used to express loudness. 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. Depends on the medium through which it is traveling. Changes when the medium changes. Changes when the medium changes. Sound travels faster in steel than in water. Sound travels faster in steel than in water. Speed of sound depends on temperature. The cooler the temperature, the slower the speed of sound. Speed of sound depends on temperature. The cooler the temperature, the slower the speed of sound.

24.1 The speed of sound The speed of sound in normal air is 343 meters per second (660 miles per hour). The speed of sound in normal air is 343 meters per second (660 miles per hour). Sound travels through most liquids and solids faster than through air. Sound travels through most liquids and solids faster than through air. Sound travels about five times faster in water, and about 18 times faster in steel. Sound travels about five times faster in water, and about 18 times faster in steel.

24.1 The speed of sound Objects that move faster than sound are called supersonic. Objects that move faster than sound are called supersonic. If you were on the ground watching a supersonic plane fly toward you, there would be silence. If you were on the ground watching a supersonic plane fly toward you, there would be silence.  The sound would be behind the plane, racing to catch up.

24.1 The speed of sound Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr. Passenger jets are subsonic because they travel at speeds from 400 to 500 mi/hr.

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

24.1 The frequency of sound Almost all the sounds you hear contain many frequencies at the same time. Almost all the sounds you hear contain many frequencies at the same time. Humans can generally hear frequencies between 20 Hz and 20,000 Hz. Humans can generally hear frequencies between 20 Hz and 20,000 Hz.

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

24.1 The frequency of sound Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. Sounds near 2,000 Hz seem louder than sounds of other frequencies, even at the same decibel level. According to this curve, a 25 dB sound at 1,000 Hz sounds just as loud as an 40 dB sound at 100 Hz. According to this curve, a 25 dB sound at 1,000 Hz sounds just as loud as an 40 dB sound at 100 Hz.

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

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

SOME COMMON DECIBEL LEVELS

24.1 Recording sound One second of compact-disc-quality sound is a list of 44,100 numbers which represents the amplitudes converted sounds.

24.1 Recording sound To play the sound back, the string of numbers is read by a laser and converted into electrical signals again by a second circuit which reverses the process of the previous circuit.

24.1 Recording sound The playback circuit converts the string of numbers back into an electrical signal. The electrical signal is amplified to move the coil in a speaker and reproduce the sound.

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

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