NCERT Class 8 Science Chapter: Sound — Concepts and Practice Questions

Understanding How Sound is Produced and Propagated

Sound is a form of energy that is produced by the mechanical vibrations of particles in a medium. When an object vibrates, it sets the neighboring particles of the medium into motion. This mechanical motion propagates as a wave through solids, liquids, and gases. Crucially, sound requires a material medium to travel and cannot propagate in a vacuum. For humans, the organ responsible for detecting these vibrations is the human ear. The outer ear or pinna collects sound waves, sending them down the ear canal to the eardrum, which vibrates and transmits the signals through the middle ear bones to the cochlea and inner auditory nerves. In this guide, you will master the physics behind these oscillations, how amplitude and frequency shape what we hear, and how to solve numerical problems involving frequency and time period.

Key Terminologies of Sound Waves

Vibration

The rapid, back-and-forth motion of an object about its central position.

Amplitude

The maximum displacement of a vibrating particle from its mean position.

Frequency

The number of oscillations or vibrations made by an object in one second, measured in Hertz (Hz).

Time Period

The time taken by a vibrating particle to complete one full oscillation.

Audible Range

The range of sound frequencies that humans can hear, typically between 20 Hz and 20,000 Hz.

Step-by-Step Guide to Calculating Wave Properties

1. Identify the Given Parameters — Examine the problem to find the total number of oscillations (vibrations) and the time duration in seconds.
2. Calculate the Frequency — Use the formula: Frequency (f) = Number of Oscillations / Total Time in seconds. Express the unit in Hertz (Hz).
3. Determine the Time Period — Use the reciprocal relation: Time Period (T) = 1 / Frequency. Express the answer in seconds.
4. Relate Amplitude to Loudness — Recall that Loudness is proportional to the square of the amplitude of vibration. If amplitude doubles, loudness increases four times.

Avoid Common Mistakes with Pitch and Loudness

Do not confuse high pitch with high loudness! Students often mistake high frequency sounds for loud sounds. Remember, pitch depends strictly on frequency (vibrations per second), whereas loudness depends on amplitude. A high-pitched mosquito buzz has high frequency but low amplitude, making it shrill but quiet. A loud drum beat has high amplitude but low frequency, making it deep and loud. Always write units clearly: Hertz (Hz) for frequency, seconds (s) for time, and decibels (dB) for loudness.

Practice Questions with Solutions

• Q: A pendulum makes 40 oscillations in 4 seconds. Calculate its frequency and time period. A: Step 1: Identify the given quantities. Number of oscillations = 40, and Time taken = 4 seconds. Step 2: Apply the frequency formula: Frequency = Total Oscillations / Total Time. Therefore, Frequency = 40 / 4 = 10 Hz. Step 3: Apply the time period formula: Time Period = 1 / Frequency. Therefore, Time Period = 1 / 10 = 0.1 seconds. Final answer: The frequency is 10 Hz and the time period is 0.1 seconds.
• Q: Explain why sound travels faster in solid steel rods than in air. A: Step 1: Analyze how molecules are packed. In solids (like steel), particles are very tightly packed together compared to gases (like air). Step 2: Explain the propagation of vibration. Because the molecules are close, the vibrational kinetic energy is transferred much more rapidly from one particle to the next. Step 3: Conclude that tighter bonds and high density in solids allow waves to propagate quickly. Final answer: Sound travels faster in solid steel because the closely-packed particles transfer vibrational energy much quicker than gaseous air molecules.
• Q: An ultrasound scanning device operates at 35,000 Hz. Can human ears detect this sound? Explain with reference to the human range of audibility. A: Step 1: Recall the standard human range of audibility. The human ear can only perceive sounds between 20 Hz and 20,000 Hz. Step 2: Classify the given sound. A sound with a frequency above 20,000 Hz is categorized as ultrasonic or ultrasound. Step 3: Compare the device frequency (35,000 Hz) with the limit. Since 35,000 Hz is greater than 20,000 Hz, it falls into the inaudible ultrasonic range. Final answer: No, human ears cannot detect this sound because 35,000 Hz is above the human upper limit of audibility (20,000 Hz).
• Q: What is the relation between the amplitude of a sound and its loudness? What happens to the loudness if the amplitude is tripled? A: Step 1: State the physical relationship. Loudness is directly proportional to the square of the amplitude of vibration (Loudness ∝ Amplitude²). Step 2: Set up the calculation for tripling. If new amplitude = 3 × old amplitude, then new loudness will be proportional to (3 × Amplitude)². Step 3: Compute the multiplier. (3)² = 9 times the original loudness. Final answer: Loudness is proportional to the square of amplitude. If the amplitude is tripled, the loudness increases by 9 times.

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