Sound: CBSE Class 9 Science Chapter 12 Guide

How is Sound Produced and How Does it Travel?

Sound originates from vibrating objects. Think about a school bell. When you strike it, it vibrates, and you hear a sound. If you touch the vibrating bell, you can feel the vibrations, and the sound stops. This is the fundamental principle: vibration is the source of all sound.

But how does this vibration reach your ears? Sound needs a material medium to travel. It cannot travel through a vacuum. This is why sound is called a mechanical wave. When an object vibrates, it pushes and pulls on the particles of the medium around it (like air). This creates a chain reaction. The vibrating object pushes the air particles in front of it, creating a region of high pressure called a compression (C). Then, as the object vibrates back, it creates a region of low pressure called a rarefaction (R). This series of compressions and rarefactions travels outwards from the source, carrying the sound energy. Your eardrum detects these pressure changes, and your brain interprets them as sound. The famous bell jar experiment, where a ringing bell inside a vacuum jar becomes inaudible as the air is pumped out, proves that sound requires a medium for propagation.

Key Characteristics of a Sound Wave

Wavelength (λ)

The distance between two consecutive compressions (C) or two consecutive rarefactions (R). Its SI unit is the metre (m).

Frequency (ν)

The number of complete oscillations or cycles per second. It determines the pitch of the sound (high frequency = high pitch). Its SI unit is hertz (Hz).

Time Period (T)

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

Amplitude (A)

The maximum displacement of the particles of the medium from their mean position. It determines the loudness of the sound (large amplitude = loud sound). It has the same unit as density or pressure.

Speed of Sound (v)

The distance which a point on a wave, such as a compression, travels per unit time. The speed of sound is related to frequency and wavelength by the equation: Speed (v) = Wavelength (λ) × Frequency (ν) or v = λν.

Worked Examples: Calculating Wave Properties

• Example 1: Find the wavelength of a sound wave. A sound wave has a frequency of 2 kHz and a speed of 400 m/s in a certain medium. Calculate its wavelength. Step 1: Identify the given values and convert units. Frequency (ν) = 2 kHz = 2 × 1000 Hz = 2000 Hz Speed (v) = 400 m/s Step 2: State the formula. The relationship between speed, frequency, and wavelength is: v = νλ Step 3: Rearrange the formula and solve for wavelength (λ). λ = v / ν λ = 400 m/s / 2000 Hz λ = 0.2 m Final Answer: The wavelength of the sound wave is 0.2 metres.
• Example 2: Find the frequency of a sound source. A sound source produces 500 compressions and 500 rarefactions in 2.5 seconds. What is the frequency of the sound wave? Step 1: Understand what one oscillation means. One compression and one rarefaction together constitute one complete oscillation or cycle. So, the source produces 500 complete oscillations. Step 2: Identify the given values. Number of oscillations = 500 Total time taken = 2.5 s Step 3: Use the definition of frequency. Frequency is the number of oscillations per second. Frequency (ν) = Total number of oscillations / Total time taken ν = 500 / 2.5 s ν = 200 Hz Final Answer: The frequency of the sound wave is 200 Hz.

Exam Tips & Common Mistakes

Pay close attention to these points to avoid losing marks in your exams:

1. Echo Calculation Error: When solving problems involving echoes, remember that the sound travels from the source to the reflecting surface and back to the listener. Therefore, the total distance travelled by the sound is 2d, where 'd' is the distance between the source and the reflector. A common mistake is using just 'd' in the formula speed = distance / time. The correct formula is v = 2d / t.

1. Unit Conversion: Always check the units! Frequency is often given in kilohertz (kHz) but needs to be converted to hertz (Hz) for calculations (1 kHz = 1000 Hz). Similarly, ensure distance is in metres (m) and time is in seconds (s) to get the speed in m/s.

1. Confusing Pitch and Loudness: Don't mix up these two concepts. Pitch is determined by frequency (high frequency = high pitch, like a whistle). Loudness is determined by amplitude (high amplitude = loud sound, like a drum beat hard).

Practice Questions with Solutions

• Q: A person claps his hands near a cliff and hears the echo after 4 seconds. What is the distance of the cliff from the person if the speed of sound in air is 346 m/s? A: Step 1: Identify the given values. Time for echo (t) = 4 s Speed of sound (v) = 346 m/s Step 2: Understand the distance travelled by the sound. The sound travels from the person to the cliff and back. Let the distance to the cliff be 'd'. So, the total distance travelled is 2d. Step 3: Use the speed-distance-time formula, modified for an echo. Speed = Total Distance / Time v = 2d / t Step 4: Rearrange the formula and solve for 'd'. 2d = v × t d = (v × t) / 2 d = (346 m/s × 4 s) / 2 d = 1384 / 2 d = 692 m Final answer: The distance of the cliff from the person is 692 metres.
• Q: A submarine emits a SONAR pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff? A: Step 1: Identify the given information. Time taken for the pulse to return (t) = 1.02 s Speed of sound in salt water (v) = 1531 m/s Step 2: Recognize this is an echo problem. The sound travels to the cliff and back. The total distance is 2d, where d is the distance to the cliff. Step 3: Apply the formula v = 2d / t. Step 4: Solve for d. d = (v × t) / 2 d = (1531 m/s × 1.02 s) / 2 d = 1561.62 / 2 d = 780.81 m Final answer: The cliff is 780.81 metres away from the submarine.
• Q: A sound wave has a frequency of 50 Hz and a wavelength of 4 m. How long will it take to travel 800 m? A: Step 1: Calculate the speed of the sound wave. The formula relating speed (v), frequency (ν), and wavelength (λ) is v = νλ. v = 50 Hz × 4 m v = 200 m/s Step 2: Now that we have the speed, we can calculate the time taken to travel a specific distance. Distance (d) = 800 m Speed (v) = 200 m/s Step 3: Use the basic formula: time = distance / speed. t = d / v t = 800 m / 200 m/s t = 4 s Final answer: It will take 4 seconds for the sound wave to travel 800 m.
• Q: Why can't astronauts on the Moon talk to each other directly like they do on Earth, even if they take off their helmets? A: Step 1: Recall the requirement for sound propagation. Sound is a mechanical wave, which means it requires a material medium (like a solid, liquid, or gas) to travel. It travels by causing vibrations in the particles of the medium. Step 2: Consider the environment on the Moon. The Moon has no atmosphere; it is a vacuum. A vacuum is a space devoid of matter, meaning there are no particles. Step 3: Connect the two points. Since there are no particles in the vacuum of space to vibrate and pass the sound energy along, sound waves cannot travel from one astronaut to another. They must use radios in their helmets, which use electromagnetic waves that can travel through a vacuum. Final answer: Astronauts cannot talk directly on the Moon because it is a vacuum, and sound waves need a medium to propagate.

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