Cheatography
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sound notes for students of class 9
This is a draft cheat sheet. It is a work in progress and is not finished yet.
INTRODUCTORY TERMS
sound is a form of energy which produces a sensation of hearing in our ears. it is produced by vibrating objects.
vibration is the rapid to and fro motion of an object.
the matter or substance through which sound is transmitted is called a medium. it may be solid, liquid or gas.
a wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. |
COMPRESSIONS AND RAREFACTIONS
compression is the region of high pressure |
rarefaction is the region of low pressure |
pressure is related to the number of particles of a medium in a given volume. more density of the particles in the medium gives more pressure and vice versa.
LONGITUDINAL WAVES /\ TRANSVERSE WAVES
particles of the medium vibrate parallel to the direction of wave |
particles of the medium vibrate at right angles to the direction of wave |
waves consist of compressions and rarefactions |
waves consist of crests and troughs |
propagates through solid, liquid and gas |
propagates through solid and liquid |
waves cannot be polarized |
waves can be polarized |
ex. sound waves, ultrasound waves, seismic P-waves |
ex. light waves, radio waves, seismic S-waves |
CHARACTERISTICS OF A SOUND WAVE
the distance between two consecutive compressions and two consecutive rarefactions is called the wavelength. SI unit is m.
the magnitude of maximum displacement in the medium on either side of the mean value is the amplitude of a wave. SI unit is that of density or pressure.
frequency is the number of oscillations per unit time. SI unit is Hz.
time taken by two consecutive compressions/rarefactions to cross a fixed point is called the time-period. SI unit is s.
speed of sound is the distance travelled by a wave per unit time. |
CHARACTERISTICS OF SOUND
how the brain interprets the frequency of an emitted sound is called its pitch. pitch corresponds on frequency.
quality or timbre of sound is that characteristic which enables us to distinguish one sound from another having the same pitch and loudness.
a sound of a single frequency is called a tone.
sound produced due to a mixture of several frequencies is called a note.
noise is unpleasant to the ear. music is pleasant to hear and is of rich quality. |
LOUDNESS /\ INTENSITY
is a measure of response of the ear to the sound |
is the amount of sound passing each second through a unit area |
SI unit is dB |
SI unit is W/m2 |
THE LAW OF REFLECTION OF SOUND
states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane. |
ECHO
the repetition of a sound caused by reflection of sound waves is called an echo.
conditions for hearing an echo
time interval b/w source and reflected sound = more than 0.1s.
minimum distance b/w source and reflector = 17.2m.
intensity/loudness of sound should be sufficient for reaching the ear so as to be audible.
size of reflector must be large. |
REVERBERATION
repeated reflection that results in persistence of sound is called reverberation.
ex. megaphones, loudhailers, horns, stethoscopes, ceilings of concert, cinema and conference halls [curved soundboard]
examples of sound absorbers: compressed fibreboard, plaster, draperies |
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RANGE OF HEARING
INFRASOUND |
ULTRASOUND |
sounds of frequencies less than 20Hz |
sounds of frequencies higher than 20kHz |
rhinoceroses [5Hz], whales & elephants |
dolphins, bats, rats & porpoise |
humans: 20Hz-20kHz; children under 5 and dogs: 20Hz-25kHz
SOUND WAVES /\ LIGHT WAVES
longitudinal waves |
transverse waves |
cannot travel through vacuum |
can travel through vacuum |
speed of sound in air is 343m/s |
speed of light in air is 3.108m/s |
have low frequency and high wavelength |
have high frequency and low wavelength |
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