RRB Group D Science Sound & Wave

In RRB Group D and NTPC 2026 exams, Science remains a high-weightage section where conceptual clarity translates directly into marks. Among the most frequently tested topics is Sound and Waves, a chapter that bridges fundamental physics with real-world applications. Based on the session by Arti Mam, here are the basic definitions of energy transmission and all the complex properties that define how we perceive sound.

Sound and Wave For Railway Exams 2026 

Waves transmit energy, categorized as mechanical (requiring a medium) or non-mechanical (no medium needed). Sound waves are mechanical and longitudinal, meaning particles vibrate parallel to wave propagation. Key wave properties include amplitude, wavelength, and frequency, related to velocity by v = fλ. Sound characteristics like pitch (frequency-dependent) and loudness (amplitude-dependent) are crucial. The speed of sound varies by medium, being fastest in solids. Echoes are reflected sound needing a minimum distance and time. Sound is also classified by frequency into audible, infrasonic (below 20 Hz), and ultrasonic (above 20,000 Hz) waves.

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How Sound and Waves are Related? 

To understand sound, one must first understand waves. Waves are fundamentally a mechanism for the transmission of energy. When a wave forms, it signifies that energy is being transferred sequentially from one molecule to the next, making the study of waves essential for comprehending sound phenomena in competitive examinations.

Types of Waves

Waves are broadly classified into two main types:

1. Mechanical Waves:

• Require a medium (solid, liquid, or gas) to travel.

• Examples: Sound waves, water waves.

• Because they need a medium, mechanical waves cannot travel in a vacuum. For instance, if a volcano were to erupt on the Moon, one would only see the light but hear no sound, as there is no atmosphere (medium) for sound to travel through.

2. Non-Mechanical Waves (Electromagnetic Waves):

• Do not require a medium to travel and can pass through a vacuum.

• Examples: Light waves, radio waves.

Classification of Mechanical Waves

Mechanical waves are further divided into two categories based on the direction of particle movement relative to the wave's direction:

1. Longitudinal Waves (अनुदैर्ध्य तरंगें):

• The direction of the particles' movement (or energy transfer) is the same as the direction of the wave's propagation.

• Sound waves are mechanical, longitudinal waves.

• These waves travel through compressions (regions of high pressure) and rarefactions (regions of low pressure).

2. Transverse Waves (अनुप्रस्थ तरंगें):

• The movement of the particles is perpendicular to the direction of the wave's propagation.

• If a wave is moving forward, the particles oscillate up and down.

Properties of a Wave

Understanding these properties is fundamental to wave mechanics:

• Crest (श्रृंग): The highest point of a wave.

• Trough (गर्त): The lowest point of a wave.

• Amplitude (आयाम): The maximum displacement from the equilibrium position to the crest (or trough). It represents the height of the crest and is related to the energy of the wave.

• Wavelength (तरंगदैर्ध्य, λ):

• The distance between two consecutive crests.

• The distance between two consecutive troughs.

• Important Note: The distance between one crest and the next trough is λ/2 (half a wavelength).

• Frequency (आवृत्ति, f):

• The number of vibrations or waves produced per second.

• Its unit is Hertz (Hz). Another unit for frequency is seconds⁻¹ (derived from f = 1/T).

• (Memory Tip: The instructor explains frequency using the word "frequently." Just as a relative who visits "frequently" comes repeatedly, frequency in physics refers to how often a wave cycle repeats in a given time.)

• Time Period (T):

• The time taken to complete one full vibration or wave cycle.

• The relationship between time period and frequency is T = 1/f.

• Velocity (v):

• The speed of the wave.

• The relationship between velocity, frequency, and wavelength is given by the formula: v = fλ.

• This may also be written as v = nλ, where 'n' represents frequency.

Characteristics of Sound

The human ear perceives different aspects of sound based on its physical properties:

• Pitch (तारत्व):

• Determined by the frequency of the sound wave.

• A high pitch (e.g., a woman's sharp voice) corresponds to a high frequency.

• Loudness (प्रबलता):

• Determined by the amplitude of the sound wave.

• A loud sound has a high amplitude and carries high energy, allowing it to travel farther (e.g., sound from a DJ system).

• Quality or Timbre (गुणता):

• Determined by the waveform of the sound.

• This characteristic allows us to distinguish between different instruments or voices even if they have the same pitch and loudness. Singers are noted to have a very good waveform.

Speed of Sound

The speed at which sound travels is highly dependent on the medium it propagates through:

• The speed is highest in solids, followed by liquids, and is slowest in gases.

• Solids > Liquids > Gases

• Example: The speed of sound in steel is over 5500 m/s.

• Speed of Sound in Air: Different sources provide slightly varied values, including 330 m/s, 332 m/s, and 343 m/s.

Factors Affecting the Speed of Sound

Here are the factors affecting the speed of sound: 

• Temperature: The speed of sound is directly proportional to the square root of the temperature. If the temperature of the air increases, the speed of sound increases. (e.g., sound travels faster in summer than in winter).

• Humidity: Sound travels faster in humid air than in dry air. This is because humid air is less dense than dry air.

• Pressure: Changing the pressure of a gas has no effect on the speed of sound, provided the temperature remains constant.

The Phenomenon of Echo (प्रतिध्वनि)

An echo is the repetition of sound caused by the reflection of sound waves from a surface.

• Condition for Echo: For a clear echo to be heard, the minimum distance between the source of the sound and the reflecting surface must be 17.2 meters.

• The time interval between the original sound and the reflected sound (echo) must be at least 0.1 seconds (the lecture states 'one second', which might be a simplified example; academically, 0.1s is the persistence of hearing).

• Important Note: Echoes are not easily produced in a typical room because objects like beds, cupboards, and furniture act as sound absorbers, preventing significant reflection.

Echolocation and Applications of Ultrasound

Arti Mam touched upon how animals use these waves. 

• SONAR (Sound Navigation and Ranging): Uses ultrasonic waves to measure the depth of the sea or locate underwater objects.

• Medical Use: Echocardiography and Ultrasonography (Ultrasound scans) use these high-frequency waves to see internal organs.

• Cleaning: Used to clean spiral tubes and electronic components where the human hand cannot reach.

Classification of Sound Waves by Frequency

Sound waves are categorized based on their frequency range, influencing whether humans and other animals can detect them.

• Audible Ranges for Animals:

• Humans: 20 Hz – 20,000 Hz

• Dogs: 45 Hz – 65,000 Hz