notes of sound class 9
Answers
Answer:
Medium
The matter or substance through which sound is transmitted is called a medium.
A medium is necessary for the propagation of sound waves.
The medium can be solid, liquid or gas.
Sound cannot travel in vacuum.
Wave that requires medium to propagate is called Mechanical wave
.Wave
Wave is a phenomenon or disturbance in which energy is transferred from one point to another without any direct contact between them. For example: Heat, light and sound is considered as a wave.
Types of Waves
On the basis of direction of propagation, waves are categorized into two parts:
(i) Longitudinal waves: These are the waves in which the particles of the medium vibrate along the direction of propagation of the wave. For example: sound wave.
(ii) Transverse waves: In this type of wave the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave. For example: waves produced in a stretched string.
Another type of waves which do not require any medium for propagation are called electromagnetic waves. These waves can travel through vacuum also. For example, light waves, X-rays.
Compressions and Rarefactions:
Compression is the part of wave in which particles of the medium are closer to one another forming the region of high pressure and density.
Compressions are represented by the upper portion of the curve called crest.
Rarefaction is a part of the wave where particles spread out to form a region of low pressure and density.
Rarefactions are represented by the lower portion of the curve called trough.
Characteristics of a sound wave
Sound waves have following characteristics:
(1) Amplitude
(2) wavelength
(3) Frequency
(4) Time period
(5)Velocity
Amplitude
The maximum displacement of each particle from its mean position is called amplitude.
It is denoted by A.
Its SI unit is metre (m).
Wavelength
The distance between two nearest (adjacent) crests or troughs of a wave is called its wavelength.
It is denoted by the Greek letter lamda (λ).
Its SI unit is metre.
Frequency
The number of vibrations per second is called frequency.
The SI unit of frequency is hertz (Hz).
The symbol of frequency is ν (nu).
Time period
The time taken to complete one vibration is called time period.
It is denoted by T.
Its SI unit is second (s).
The frequency of a wave is the reciprocal of the time period.
i.e., v = 1/T
Velocity
The distance travelled by a wave in one second is called velocity of the wave or speed of the wave.
Its S.I. unit is metres per second (m/s).
Velocity = Distance travelled/Time taken
⟹ v = λ/T , where λ = wavelength of the wave travelled in one time time period T)
⟹ v = λv (As 1/T = v)
Thus, Velocity = Wavelength × Frequency
v = λv is called the wave equation.
Pitch and loudness of Sound
Pitch: It represents shrillness or flatness of sound.
It depends on the frequency of vibration. Higher the frequency of sound wave, the higher will be the pitch of sound and vice-versa.
Loudness: It is a measure of the sound energy reaching the ear per second.
It depends on the amplitude of the sound wave.
It is measured in decibel ‘dB’.