The production of sound happens in different ways. Some of them are listed below:

• In simple terms, we can say that sound is produced whenever any object creates vibrations. 
• Vibration or sounds can be caused due to the motion of objects and materials.
• The rapid to-and-fro motion of any object is called oscillation, and this produces sound. Sound can be monotone or can consist of multiple frequencies.

For example, when a rubber band is plucked, it creates a vibration, resulting in the production of sound.

Also, when a tuning fork is struck to a rubber pad, it vibrates, creating a sonorous sound.

After understanding what sound is and how it is produced, the next concept we need to understand is the propagation of sound. 

• The phenomenon of sound travelling from one object to another is called the propagation of sound. 
• Sound always needs a medium to travel, due to which, whenever sound is created, it starts to vibrate the particles around it. 
• When sound leaves a source, it starts to vibrate the nearest particles in the medium and keeps following this process until it reaches a listener. 
• When any vibrating object produces a disturbance, it gets carried through the medium. However, in the entire process, none of the particles moves forward. They just create a chain of vibration by disturbing the particles in front of them.

Reflection of light occurs as the waves encounter a reflecting surface or its borders, not absorbing the radiation energy while the waves bounce back from the body. When light rays reflect a body, they travel from one transparent medium to another or move through a medium with a frequently altered composition. According to the law of reflection:

• The incident ray, the reflected ray and the normal to the surface at the point of incidence all lie in the same plane.
• The angle of incidence is equal to the angle of reflection.

Till now, you would have understood that sound is a wave formed due to a disturbance in the particles. Now, we need to get a better understanding of sound as a longitudinal wave and the types of waves.

Based on the direction of propagation, a wave can be categorised into two types—transverse and longitudinal waves. 

•  Transverse waves
  The transverse waves are those waves in which the particles of the medium resonate to-and-fro, perpendicular to the wave’s direction. The best example of transverse waves is light waves.
• Longitudinal waves
  This is the kind of wave in which the particles resonate to-and-fro in the same direction as that of the wave. Sound waves are an example of longitudinal waves.

Here are a few characteristics of sound waves:

• Amplitude
  When a wave disturbance is plotted on a graph, the amplitude is the height of the crest and trough. The amplitude of any sound wave is defined by how high or low the parabola is formed. Amplitude is denoted by ‘A’, and the SI unit of amplitude is metre. Amplitude is also directly proportional to the loudness of a wave. This means the greater the amplitude higher will be the loudness. Amplitude highly depends on the force by which the particles vibrate, and, therefore, softness, loudness and amplitude are related to one another.
• Wavelength
  Wavelength is defined as the distance between two consecutive crests or troughs. It is denoted by λ (lambda), and the SI unit of wavelength is metre. Wavelength is also considered as the distance at which the graph is repeated.
• Frequency
  The frequency of the sound wave is defined as the number of vibrations that are completed by any particle in one second.Frequency = Number of oscillations/Total TimeEvery sound wave has compressions and rarefactions, and the frequency of any sound wave can be determined by simply calculating the number of compressions or rarefactions in one second. The SI unit of frequency is Hertz (Hz).
• Time period
  The time taken by any particle to complete one oscillation is called the time period. It is represented by ‘T’, and its SI unit is seconds.Time period and frequency are related to each other because the time period is the number of oscillations in one second and frequency is the total number of compressions and rarefactions in one second.
• Pitch
  Pitch and frequency are related to each other. Pitch is defined as the sensation of the frequency of an emitted sound. The pitch of any sound is directly proportional to the frequency of vibrations. A high-pitch sound has a higher frequency of vibration and is known as a shrill sound, whereas a low-pitch sound is called a heavy sound. This is measured in decibels. The unit of loudness is also measured in decibels.

This is a crucial chapter for students because there are several concepts that students have to use even in their college. The chapter holds a good score in JEE and other competitive examinations. Students who wish to take science as their stream in Class 11th should concentrate well on this chapter and learn all the fundamentals.

1. Sound travels faster through which state of matter?
   Sound travels faster through solids as compared to other states of matter.
2. What is the SI unit of frequency?
   The SI unit of frequency is Hertz.
   
3. Can sound travel in a vacuum? If not, then why.
   Sound cannot travel in a vacuum because it always needs a medium for travelling.