SOUND 24.1

Chapter Twenty-Four: Sound 24.1 Properties of Sound 24.2 Sound Waves 24.3 Sound Perception and Music

Chapter 24.1 Learning Goals Describe properties of sound. Explain how sound waves are created and recorded. Discuss examples of the Doppler effect.

Properties of Sound Investigation 24A Key Question: Does sound behave like other waves?

24.1 The frequency of sound The pitch of a sound is how you hear and interpret its frequency. A low-frequency sound has a low pitch. A high-frequency sound has a high pitch. Each person is saying “Hello”.

24.1 The frequency of sound 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.

24.1 The loudness of sound The loudness of a sound is measured in decibels (dB). The decibel is a unit used to express relative differences in the loudness of sounds.

24.1 The loudness of sound Most sounds fall between 0 and 100 on the decibel scale, making it a very convenient number to understand and use.

24.1 The frequency of sound 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.

24.1 Amplitude and decibels The amplitude of a sound increases ten times every 20-decibels.

24.1 The speed of sound 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 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. 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 A supersonic jet “squishes” the sound waves so that a cone-shaped shock wave forms where the waves “pile up” ahead of the plane. In front of the shock wave there is total silence.

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

24.1 The Doppler effect When the object is moving, the frequency will not be the same to all listeners. The shift in frequency caused by motion is called the Doppler effect. You hear the Doppler effect when you hear a police or fire siren coming toward you, then going away from you.

24.1 Recording sound To record a sound you must store the pattern of vibrations in a way that can be replayed and be true to the original sound. A microphone transforms a sound wave into an electrical signal with the same pattern of vibration.

24.1 Recording sound An “analog to digital converter” converts the electrical signal to digital values between 0 and 65,536.

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.

24.1 Recording sound Most of the music you listen to has been recorded in stereo. The slight differences in how sound reaches your ears lets you know where sound is coming from.