Physical Science 92B

Unit I - Waves

 
TOPIC: Waves and Sound
 
National Standards - Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter.
 
Georgia Standards - compare and contrast transverse and compressional waves
 
Compressions in Springs and air
** Sound is produced by the vibration (rapid back-and-forth motion) of some matter.
** As matter vibrates, it pushes again and again on the surrounding air.
** Air waves are made of compressions (more dense air) and rarefactions (thinner air).
 
Compressional waves
** When a musical instrument is played, something in it vibrates, or moves back and forth rapidly.
** Many compressions, one after the other, travel through the surrounding air.
* Traveling in between each two compressions is a rarefaction.
*** Air acts like a spring.
** In air, compressions travel outward in all directions as opposed to moving in one direction with a spring.
*** The compressions and rarefactions sent out through the air by a musical instrument form what is called a compressional wave.
** The air particles move back and forth (together and apart), but they do not travel with the wave.
** The number of compressions arriving per second is called the frequency.
*** The faster the source vibrates, the higher is the frequency.
** The SI unit for frequency is Hertz (Hz).
** Energy travels along with the compressions.
*** In a compressional wave, particles move back and forth along the direction of the wave motion.
Text pp. 505 - 508
Transverse Waves
*** In a transverse wave, particles move perpendicular, or crosswise, the the direction of the wave motion.
** Water waves that move across the surface of water show how transverse waves behave.
** A pebble thrown into water produces a ripple in the form of a circle that gets larger and larger.
** A leaf floating in the water will bob up and down as the ripples pass.
** Matter does not move along with a transverse wave.
* Water waves may be studied in a ripple tank.
** A transverse wave is started by something that vibrates.
Text pp. 499 - 504
 
Georgia Standards - Draw and label parts of a wave
identify properties of waves and wave motion
 
Parts of a Wave
** crest - high point of a wave
** trough - low point of a wave
** wavelength - distance between two neighboring crests
** amplitude - distance from the midpoint of a wave to the crest
* All transverse waves have crests and troughs.
** A compressional wave also has a wavelength and a trough.
** The wavelength is the distance between two neighboring compressions or rarefactions.
** The amplitude is the maximum distance a particle moves from its normal position as the wave passes by.
Text pp. 500
 
Georgia Standard - analyze wave interactions with matter including reflection, refraction, diffraction and interference
 
Reflection and Refraction
*** reflection - the change in direction of a wave caused by bouncing off a barrier.
*** refraction - the change in direction of a wave caused by crossing some kind of boundary.
 
Diffraction and Interference
*** diffraction - any spreading out of a wave after it passes through an opening in a barrier.
*** Interference - reinforcement or cancellation of two waves as they pass through each other.
** Interference has no effect on the waves themselves - they continue to move with the same frequency, wavelength, amplitude, and speed.
 
Sound Production
** The vibrations in your throat are produced by your vocal cords.
** People lose their voice when the vocal cords are swollen and cannot vibrate.
** When a guitar string is plucked, it vibrates.
*** All sounds are caused by vibrations.
Text pp. 507
 
Georgia Standard - relate the efficiency of sound transfer through a medium to density (temperature) and medium type
 
Sound Carriers
** Sound travels easily through air.
** Sound travels through gases, solids, and liquids.
** Some solids are not good carriers of sound and are called sound insulators.
ex. special tiles, carpet
** All sound insulators trap air.
** Matter must be present for sound to travel from one place to another.
** Sound does not travel through empty space.
Text pp. 507
 
The Speed of Sound
** An echo is proof that it does take some time for sound to get someplace.
*** Sound does not travel at the same speed in all kinds of matter.
*** In general, sound travels faster in solids than in liquids, and faster in liquids than in gases.
** Sound depends on temperature as well as on the kind of matter.
*** The warmer the matter, the faster it carries sound.
Text pp. 507
 
Georgia Standard - recognize the relationships between amplitude and loudness and frequency and pitch
 
Pitch
** Pitch refers to how high or low a sound is.
** The pitch may be changed by changing the length of the vibrating object.
** When the length of a vibrating string is shortened, the pitch rises.
** When a thick string vibrates, it produces a lower sound than a thinner string of the same length.
** The tightness of a string also affects the pitch.
** The tighter the string, the higher the sound.
Text pp. 508
 
Loudness and noise level
** The amount of energy used to produce a sound is one factor that affects the loudness.
** The more energy that is used to make the sound, the louder is the sound.
** The greater the distance from the source of sound, the quieter is the sound.
*** Intensity - amount of sound energy received per second.
** The human ear is more sensitive to certain pitches than others.
** Noise level compares the level of intensity of a sound with that of the quietest sound the ear can hear.
*** Noise level is measured in units called decibels.
** A sound of 0 decibels is the quietest sound that can be detected by the human ear.
** A sound of 120 decibels is the loudest the ear can hear without pain.
Text pp. 509 - 10
 
Georgia Standard - cite examples of Doppler shift in sound
Doppler Effect
** The change in wave frequency due to a moving wave source is called the Doppler effect.
** As a source of waves moves toward you, the waves are pushed together and the pitch is made higher.
** As the source of waves moves away from you, the waves are pulled apart and the pitch is lowered.
*** Ex. fire engine with siren moving past
automobile with horn blaring as it moves past
Text pp. 511
 
Georgia Standard - use the relationship between frequency, wavelength and wavespeed in simple calculations
 
Frequency, wave speed, and wavelength
** The frequency of a transverse wave is the number of crests passing through one place per second.
** The frequency of a wave is the same as the frequency of whatever produced the wave by vibrating.
** The wave speed is the speed at which the crests or compressions travel.(m/sec)
*** The higher the frequency is, the shorter the wavelength must be.
** The waves with lower frequencies have longer wavelengths.
** Compressional waves travel faster through warm air than through cold air.
*** wavelength = wave speed/frequency
Text pp. 513
 
Amplifying Sound
** Megaphones may seem to amplify sound.
** Sounds coming through a megaphone are best heard directly in line with the megaphone.
** Megaphone-shaped objects are used in receiving as well as sending sound.
** The ear is somewhat like a megaphone - it gathers sound and directs it inside the skull.
** "Hearing aids" used to be shaped like megaphones.
** Now we use something to convert sound to electrical signals before changing it back to sound.
 
Controlling Noises and Sound
*** Noise is any sound that is unwanted.
** Too much sound can be harmful.
** Loud noises overwork the ear.
* As the noise level increases, people are more likely to suffer hearing loss.
** Sounds with deicbel rating between 60 and 100 can be annoying.
** Sounds above 100 decibels that last a few minutes can cause temporary or permanent loss of hearing.
** Limits are now set on loudness of many noises.
** People who must work near loud noises are required to wear ear protectors.
* Echo - reflected sound.
** auditorium - "hear" "place for"
 
Quality
** Sounds which have the same loudness and pitch may sound quite different.
** The sounds of a piano and a violin are said to differ in quality.
** Objects made from different materials produce sounds of different quality.
** The shape of the vibrating object also affects the quality of sound.
Text pp. 513 - 21
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