Physical Science


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.

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.

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.

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

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.

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.

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.
 
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.

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.

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.