Sound Class 9 Notes: CBSE Science Chapter 12 Revision

Key Concepts: Must Remember

• Sound is a form of energy produced by vibrations.
• It is a mechanical wave and requires a material medium (solid, liquid, or gas) for its propagation.
• Sound waves are longitudinal waves, meaning particles of the medium oscillate parallel to the direction of wave propagation.
• A sound wave consists of alternating regions of compressions (high pressure, high density) and rarefactions (low pressure, low density).
• The speed of sound is maximum in solids, less in liquids, and least in gases. It increases with temperature.
• Key characteristics of a sound wave: Wavelength (λ), Frequency (ν), Time Period (T), Amplitude (A).
• Pitch depends on frequency; Loudness depends on amplitude.
• Echo is the phenomenon of reflection of sound, clearly heard after the original sound. Reverberation is the persistence of sound due to multiple reflections.
• The human ear can detect sounds between 20 Hz and 20,000 Hz (audible range).
• Ultrasound (frequencies > 20 kHz) has various applications like SONAR, medical imaging, and cleaning.

Essential Definitions for Exams

Sound

A form of energy that produces the sensation of hearing, caused by vibrations that travel as waves through a medium.

Vibration

The rapid to-and-fro or up-and-down motion of an object about its mean position.

Longitudinal Wave

A wave in which the particles of the medium oscillate parallel to the direction of wave propagation. Sound waves are longitudinal.

Wavelength (λ)

The distance between two consecutive compressions or two consecutive rarefactions. SI unit: meter (m).

Frequency (ν or f)

The number of oscillations or cycles per unit time. SI unit: Hertz (Hz).

Time Period (T)

The time taken for one complete oscillation. It is the reciprocal of frequency (T = 1/ν). SI unit: second (s).

Amplitude (A)

The maximum displacement of the particles of the medium from their mean position. Relates to the loudness of sound.

Pitch

The characteristic of sound that allows us to distinguish between a sharp (high-pitched) and a flat (low-pitched) sound. It depends on frequency.

Loudness

The characteristic of sound that determines how intense or faint a sound appears. It depends on the amplitude of the sound wave. Measured in decibels (dB).

Echo

The phenomenon of hearing the distinct reflected sound wave after the original sound, due to reflection from a distant obstacle.

Ultrasound

Sound waves with frequencies higher than the upper limit of human hearing (above 20,000 Hz).

How Sound is Produced and Propagates

Sound is fundamentally produced by vibrating objects. When an object vibrates, it sets the particles of the surrounding medium (like air) into motion. For instance, consider a vibrating tuning fork. As its prongs move forward, they push the air molecules in front of them, creating a region of high pressure and high density. This region is called a compression. These compressed molecules then transfer their energy to adjacent molecules, and so on, propagating the compression outwards.

When the prongs move backward, they create a space, pulling the air molecules apart. This results in a region of low pressure and low density, known as a rarefaction. As the prongs continue to vibrate back and forth, they generate a series of alternating compressions and rarefactions that travel through the medium. This propagation of compressions and rarefactions is what we perceive as a sound wave.

Crucially, sound waves are mechanical waves, meaning they require a physical medium (solid, liquid, or gas) to travel. They cannot travel through a vacuum because there are no particles to vibrate and transmit the energy. Furthermore, sound waves are longitudinal waves, which means the individual particles of the medium oscillate parallel to the direction in which the wave energy is moving. The energy is transferred from one particle to the next, but the particles themselves do not travel along with the wave; they simply oscillate around their equilibrium positions. The speed at which sound travels depends on the properties of the medium, such as its elasticity and density. Sound travels fastest in solids, slower in liquids, and slowest in gases.

Characteristics of a Sound Wave & Related Formulas

Worked Examples: Applying Formulas

• {"title":"Example 1: Wavelength Calculation","description":"Q: A sound wave has a frequency of 2 kHz and a wavelength of 35 cm. How long will it take to travel 1.5 km?\nA: Given: ν = 2 kHz = 2000 Hz, λ = 35 cm = 0.35 m.\nFirst, calculate speed (v): v = λ × ν = 0.35 m × 2000 Hz = 700 m/s.\nDistance (d) = 1.5 km = 1500 m.\nTime (t) = d / v = 1500 m / 700 m/s = 2.14 seconds (approx)."}
• {"title":"Example 2: Echo Distance","description":"Q: A person claps near a cliff and hears the echo after 4 seconds. If the speed of sound in air is 340 m/s, what is the distance between the person and the cliff?\nA: For an echo, the sound travels to the cliff and back. So, total distance = 2 × distance to cliff.\nTotal distance = speed × time = 340 m/s × 4 s = 1360 m.\nDistance to cliff = 1360 m / 2 = 680 meters."}

SONAR: Sound Navigation And Ranging

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Exam Tip: Avoiding Common Mistakes

1. Units are Crucial: Always convert all quantities to SI units (meters, seconds, Hertz) before performing calculations. For instance, cm to m, kHz to Hz, km to m.
2. Echo vs. Reverberation: Understand the key difference. An echo is a distinct reflected sound, while reverberation is the persistence of sound due to multiple, closely spaced reflections.
3. Speed of Sound: Remember that the speed of sound changes with the medium and temperature. For echo problems, use the given speed or standard speed (e.g., 343 m/s at 20°C).
4. Echo Calculation: For distance calculations involving echoes, remember to divide the total distance (speed × time) by 2, as sound travels to the object and back.
5. Pitch vs. Loudness: Differentiate clearly: Pitch is determined by frequency, Loudness by amplitude.

Quick Revision Check

• Q: Why are sound waves called mechanical waves? A: Sound waves are called mechanical waves because they require a material medium (solid, liquid, or gas) for their propagation and cannot travel through a vacuum.
• Q: What is the audible range of hearing for humans? A: The audible range for human hearing is from 20 Hz (Hertz) to 20,000 Hz.
• Q: How does the loudness of sound relate to its amplitude? A: The loudness of sound is directly proportional to the square of its amplitude. A larger amplitude means a louder sound.
• Q: Name two applications of ultrasound. A: Two applications of ultrasound are in SONAR (Sound Navigation And Ranging) for measuring ocean depth, and in medical imaging (ultrasonography) for viewing internal organs.

Frequently Asked Questions