Sound - Production and propagation of sound, amplitude, frequency, pitch, and audible sound range

Hello, bright young minds! Welcome to today’s physics session. Have you ever wondered how you hear your alarm clock ticking, the chirp of birds outside your window, or your teacher talking in class? All these experiences are brought to you by one wonderful phenomenon: Sound.

In this detailed tutorial designed according to your NCERT Class 8 Science curriculum, we will uncover what sound actually is, how it travels through space, and why some sounds are shrill while others are deep and loud. Let's begin!

1. How is Sound Produced? (The Magic of Vibrations)

To put it simply: Sound is produced by vibrating objects.

What is a Vibration?

A vibration (or oscillation) is a rapid, repeated to-and-fro or back-and-forth motion of an object about its central mean position.

Real-World Examples:

• The Rubber Band Experiment: Stretch a rubber band tightly between two pins or your fingers and pluck it in the middle. You will notice two things:
  1. The rubber band moves back and forth rapidly (vibrates).
  2. A humming sound is heard. If you hold the band to stop it from vibrating, the sound immediately stops!
• School Bell: Touch the school bell gently when it is ringing. You will feel a strong tingling sensation in your fingers. That sensation is vibration!
• Musical Instruments:
  • Sitar / Guitar: Sound is produced by vibrating strings.
  • Tabla / Drum: Sound is produced by a vibrating stretched membrane.
  • Flute: Sound is produced by vibrating air columns.

How do Humans Produce Sound?

In human beings, sound is produced by the voice box, also known as the Larynx.

• The larynx is located at the upper end of the windpipe.
• Two vocal cords are stretched across the larynx leaving a narrow slit between them for the passage of air.
• When your lungs force air through this slit, the vocal cords vibrate, producing sound!

2. How Does Sound Travel? (Propagation of Sound)

Sound cannot teleport from one place to another; it needs a pathway to travel. The movement of sound from the source to the listener is called propagation.

Sound Needs a Medium

A medium is any substance (solid, liquid, or gas) through which sound waves travel.

1. Gases: Sound travels through air. This is how we talk to each other every day.
2. Liquids: Aquatic animals like whales and dolphins communicate underwater because sound travels through water.
3. Solids: Place your ear flat on one end of a long wooden or metal table, and ask your friend to scratch the other end gently. You will hear the sound clearly! Sound actually travels fastest in solids.

Crucial Rule: Sound cannot travel through a vacuum!
A vacuum is an empty space completely devoid of matter/air. If there are no particles to vibrate, sound cannot travel. Astronauts on the Moon use radio communication because there is no air on the Moon for sound waves to travel.

3. Characteristics of a Sound Wave

To understand why a whistle sounds different from a bass drum, we need to look at three important physical quantities: Time Period, Amplitude, and Frequency.

A. Frequency and Time Period

When an object moves to and fro repeatedly, it performs oscillatory motion.

• Oscillation: One complete back-and-forth movement of a vibrating body about its mean position.
• Time Period ($T$): The time taken by a vibrating body to complete one single oscillation. It is measured in seconds ($s$).
• Frequency ($f$): The number of complete oscillations per second. $\text{Frequency } (f) = \frac{\text{Number of Oscillations}}{\text{Total Time Taken (in seconds)}}$
  • SI Unit of Frequency: Hertz (Hz).
  • $1\text{ Hz} = 1\text{ oscillation per second}$.

B. Amplitude and Loudness

• Amplitude ($A$): The maximum distance a vibrating body moves from its central mean (rest) position.
• Loudness: Loudness indicates how loud or soft a sound appears to our ears.

Relationship Between Loudness and Amplitude:

$\text{Loudness} \propto (\text{Amplitude})^2$

• If the amplitude of vibration is doubled, the loudness increases by a factor of $2^2 = 4$ times!
• Soft sound: Small amplitude (gentle tap on a drum).
• Loud sound: Large amplitude (hard hit on a drum).
• Unit of Loudness: Measured in Decibels (dB).
  • Normal breathing: $\approx 10\text{ dB}$
  • Soft whisper: $\approx 30\text{ dB}$
  • Normal conversation: $\approx 60\text{ dB}$
  • Heavy traffic: $\approx 80\text{ dB}$ (Sounds above $80\text{ dB}$ become painful and cause noise pollution).

C. Pitch (Shrillness) and Frequency

• Pitch: The characteristic of sound that allows us to distinguish between a sharp (shrill) sound and a deep (flat) sound.
• Pitch depends directly on Frequency:
  • High Frequency $\rightarrow$ High Pitch $\rightarrow$ Shrill Sound
    • Examples: A baby's cry, a whistle, a woman's voice, chirping of birds.
  • Low Frequency $\rightarrow$ Low Pitch $\rightarrow$ Grave / Deep Sound
    • Examples: A bass drum, a lion's roar, a man's voice.

4. Audible and Inaudible Sound Range

Human ears are incredible organs, but they cannot hear all possible sound frequencies!

Infrasonic (< 20 Hz)  |==== HUMAN AUDIBLE RANGE (20 Hz - 20,000 Hz) ====|  Ultrasonic (> 20,000 Hz)

1. Audible Range: The range of sound frequencies that a normal human ear can detect. $\text{Human Range of Hearing} = \mathbf{20\text{ Hz to } 20,000\text{ Hz}} \text{ (or } 20\text{ kHz)}$

2. Inaudible Sounds: Sounds that fall outside the human audible limits.

   • Infrasonic Sound (Infrasound): Frequencies below $20\text{ Hz}$.
     • Humans cannot hear this. Animals like elephants, rhinoceroses, and whales use infrasound to communicate over long distances. Earthquakes generate infrasound before the main shocks begin.
   • Ultrasonic Sound (Ultrasound): Frequencies above $20,000\text{ Hz}$ ($20\text{ kHz}$).
     • Humans cannot hear this. Dogs, bats, and dolphins can produce and hear ultrasound. Bats navigate and hunt insects in total darkness using ultrasound (Echolocation).
     • Medical Application: Ultrasound imaging equipment operates at frequencies far higher than $20,000\text{ Hz}$.

Quick Summary Checklist

• Production: Sound is produced by vibrating objects.
• Propagation: Needs a medium (solids, liquids, gases). Cannot travel in a vacuum.
• Loudness: Depends on Amplitude ($\text{Loudness} \propto \text{Amplitude}^2$). Unit: dB.
• Pitch: Depends on Frequency ($\text{Hz}$). High frequency = High pitch.
• Audible Range for Humans: $20\text{ Hz}$ to $20,000\text{ Hz}$.

5. Practice Questions with Solutions

Let’s test your understanding with three classic NCERT-pattern numericals and conceptual questions!

Question 1 (Numerical)

A pendulum oscillates 60 times in 3 seconds. Calculate its:

1. Frequency ($f$)
2. Time Period ($T$)

Solution:

1. Given:

   • Number of oscillations = $60$
   • Total time taken = $3\text{ seconds}$

2. Calculate Frequency ($f$): $\text{Frequency } (f) = \frac{\text{Total Oscillations}}{\text{Total Time}}$ $f = \frac{60}{3} = \mathbf{20\text{ Hz}}$

3. Calculate Time Period ($T$): $\text{Time Period } (T) = \frac{\text{Total Time}}{\text{Total Oscillations}} = \frac{1}{\text{Frequency}}$ $T = \frac{3}{60} = \frac{1}{20} = \mathbf{0.05\text{ seconds}}$

• Final Answer: The frequency is $20\text{ Hz}$ and the time period is $0.05\text{ seconds}$.

Question 2 (Conceptual)

Two musical instruments A and B produce sounds as described below:

• Instrument A vibrates $500$ times in one second with an amplitude of $1\text{ cm}$.
• Instrument B vibrates $150$ times in one second with an amplitude of $3\text{ cm}$.

Compare:

1. Which instrument produces a higher pitch? Explain why.
2. Which instrument produces a louder sound? Explain why.

Solution:

1. Pitch Comparison:

   • Frequency of Instrument A = $500\text{ Hz}$
   • Frequency of Instrument B = $150\text{ Hz}$
   • Answer: Instrument A produces a sound of higher pitch because pitch depends directly on frequency ($500\text{ Hz} > 150\text{ Hz}$).

2. Loudness Comparison:

   • Amplitude of Instrument A = $1\text{ cm}$
   • Amplitude of Instrument B = $3\text{ cm}$
   • Answer: Instrument B produces a louder sound because loudness depends on the square of the amplitude ($3\text{ cm} > 1\text{ cm}$).

Question 3 (Application-Based)

An astronaut standing on the surface of the Moon calls out to her teammate who is standing 5 meters away. Can the teammate hear her voice directly? Explain your answer with scientific logic.

Solution:

• Answer: No, the teammate cannot hear her voice directly.
• Reason:
  1. Sound is a mechanical wave that requires a material medium (solid, liquid, or gas) to propagate.
  2. The Moon has no atmosphere; there is a total vacuum on its surface.
  3. Because there are no air particles to vibrate and transfer sound energy, sound cannot travel through space on the Moon. Astronauts must use radio equipment to communicate.

Keep revising and exploring the world around you with scientific curiosity! Happy learning!