The Nature of Sound What is Sound? Properties of Sound Interactions of Sound Waves

What is Sound?  Sound is created by vibrations.  A sound wave is a longitudinal wave caused by vibrations and carried through a substance or medium. The particles vibrate BACK AND FORTH!  Sound waves travel in ALL directions away from their source.  The air or MEDIUM sound travels through DOES NOT travel with the sound waves.

Properties of Sound  MEDIUM  Sound can travel through ALL four phases of matter: SOLID, LIQUID, GAS, AND PLASMA.  The type of medium that sound is traveling through determines HOW FAST it travels. Sound travels fastest through a SOLID because the molecules are TIGHTLY PACKED, CLOSE TOGETHER.  Sound travels slower through a liquid and even SLOWER through a gas or plasma.  TEMPERATURE  Temperature also affects the speed of sound.  Colder temperatures SLOW sound down.  Hotter temperatures SPEED sound up. Sound Travel

 A measure of how HIGH or LOW a sound is, depending on the FREQUENCY of the sound wave is called PITCH. Properties of Sound Pitch  Sounds that have a frequency too HIGH for people to hear are called ULTRASONIC. FREQUENCIES HEARD BY DIFFERENT ANIMALS ANIMALFREQUENCY RANGE (Hz) BAT2,000 to 110,000 PORPOISE75 to 150,000 CAT45 to 64,000 BELUGA WHALE1,000 to 123,000 ELEPHANT16 to 12,000 HUMAN20 to 20,000 DOG67 to 45,000

Have you ever noticed the sudden change in pitch as an ambulance siren goes past you?  The pitch you heard was HIGHER as the ambulance moved toward you than it was after it passed.  The HIGHER pitch is the result of the DOPPLER EFFECT.  Doppler Effect: an observed CHANGE in the FREQUENCY of a wave when the source or observer is MOVING. Properties of Sound Doppler Effect

Textbook p. 608

Textbook p. 610

 The bouncing back of sound wave after it strikes a barrier is a REFLECTION.  When a sound bounces back off of a flat surface it REFLECTS, which causes an ECHO.  To prevent echoes, auditoriums and recording studios often have material to ABSORB the sound.  ECHOLOCATION the process of using reflected sound waves to find objects; used by animals such as bats.  SONAR a type of electronic echolocation. ULTRASONIC waves are used because their short wavelengths give more details about the objects they reflect off.  (Textbook p. 613, Figure 3) Interactions of Sound Waves

 Sound waves interact through INTERFERENCE.  CONSTRUCTIVE INTERFERENCE: when several instruments of the same kind play the same note. The COMPRESSIONS of one wave OVERLAP the COMPRESSIONS of another wave, resulting in a louder sound.  DESTRUCTIVE INTERFERENCE: when waves reflect off of walls in a room and interfere with the sound. The COMPRESSIONS of one wave overlap the RAREFACTIONS of another wave, causing you to hear the sound well or even at all. Interference of Sound Waves

 STANDING WAVE: a pattern of vibration that simulates a wave that is standing still.  Waves and reflected waves of the same frequency are going through the string. Where you see the maximum amplitude, waves a interfering CONSTRUCTIVELY. Where the string seems to be standing still, waves are interfering DESTRUCTIVELY.  RESONANCE: a phenomenon that occurs when two objects naturally VIBRATE at the same FREQUENCY; the sound produced by one object causes the other object to VIBRATE.  Musical instruments use resonance to make sound. An acoustic guitar has a hollow body that amplifies the sounds. Interference of Sound Waves