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ICSE Class 8 Physics Question 1 of 17

Sound — Question 1

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Question 1

Two waves of the same pitch have amplitudes in the ratio 1:3. What will be the ratio of their:

(i) loudness

(ii) pitch?

Answer

(i) Loudness (L) ∝ [amplitude (a)]2

Ratio of loudness =

L1L2=(a1)2(a2)2=(1)2(3)2=19\dfrac{\text{L}_1}{\text{L}_2} = \dfrac{(\text{a}_1)^2}{(\text{a}_2)^2} = \dfrac{(1)^2}{(3)^2} = \dfrac{1}{9}

So L1 : L2 = 1 : 9.

(ii) Frequency remains the same if the pitch remains unchanged.

pitch of 1st wavepitch of 2nd wave=11=1:1\dfrac{\text{pitch of 1st wave}}{\text{pitch of 2nd wave}} = \dfrac{\text{1}}{\text{1}} = 1:1

So pitch will be 1 : 1.

Chapter Overview: Sound

Sound is produced by vibrating objects and travels as a mechanical wave through a medium (solid, liquid, or gas). It cannot travel through vacuum. The speed of sound is fastest in solids (~5100 m/s in steel), slower in liquids (~1500 m/s in water), and slowest in gases (~344 m/s in air at 20°C). Key characteristics include frequency (vibrations per second, measured in hertz), amplitude (maximum displacement from rest position), and wavelength (distance between consecutive compressions). Loudness depends on amplitude and is measured in decibels (dB). Pitch depends on frequency — higher frequency means higher pitch. The audible range for humans is 20 Hz to 20,000 Hz. Sounds below 20 Hz are infrasound; above 20,000 Hz are ultrasound. The human ear has three parts: outer ear (pinna, ear canal), middle ear (eardrum, three bones), and inner ear (cochlea, auditory nerve). Noise pollution above 80 dB is harmful and can be controlled using silencers, sound barriers, and planting trees.

Key Definitions & Concepts

Term Definition / Details
VibrationRapid back-and-forth motion of an object that produces sound
FrequencyNumber of vibrations per second. Unit: hertz (Hz). f = 1/T
AmplitudeMaximum displacement from rest position. Determines loudness.
PitchHow shrill or bass a sound is. Depends on frequency.
LoudnessVolume of sound. Depends on amplitude. Measured in decibels (dB).
Audible RangeFrequency range humans can hear: 20 Hz to 20,000 Hz
Noise PollutionHarmful level of unwanted sound. Above 80 dB is dangerous.

Must-Know Points for Exams

  • Sound is produced by vibrations and needs a medium to travel.
  • Speed: Solids > Liquids > Gases. Sound cannot travel in vacuum.
  • Loudness depends on amplitude; pitch depends on frequency.
  • Human audible range: 20 Hz to 20,000 Hz.
  • Infrasound (< 20 Hz): elephants, earthquakes. Ultrasound (> 20,000 Hz): bats, medical scans.
  • Ear: Pinna → Ear canal → Eardrum → 3 bones → Cochlea → Auditory nerve → Brain.

Quick Self-Test

  1. Define the main concept of this chapter in one sentence.
  2. List the key types or categories discussed in this chapter.
  3. Give three real-life examples related to the main concept.
  4. Draw and label the key diagram of this chapter from memory.
  5. State the main law or principle covered in this chapter.