Sound 9th
Introduction
Sound is a form of energy that produces the sensation of hearing in our ears. This energy travels from one point to another in the form of waves.
When we speak, clap, or play a musical instrument, we cause the air around us to vibrate. These vibrations travel through the air to reach our ears, where they are interpreted as sound.
Production of Sound
Sound is always produced by vibrating objects.
Examples of Sound Production:
- Vocal Cords: When you speak, the air from your lungs passes through your voice box (larynx), causing two small membranes (vocal cords) to vibrate.
- Musical Instruments:
- Drum: The stretched membrane (drumhead) vibrates when struck.
- Guitar/Sitar: The stretched strings vibrate when plucked.
- Flute: The air column inside the flute vibrates when blown into.
- Tuning Fork: When struck on a rubber pad, the prongs of the tuning fork vibrate rapidly, producing a distinct sound.
Conclusion: All objects that produce sound are in a state of vibration.
Propagation of Sound: The Need for a Medium
Sound needs a material medium to travel. It cannot travel through a vacuum.
The process by which sound travels is called propagation. When a vibrating object moves back and forth, it pushes the particles of the medium (like air) nearest to it. These particles, in turn, exert force on the next layer of particles, causing them to vibrate as well. This process continues until the disturbance reaches the ear.
Key Point:
It is the disturbance (energy) that travels, not the particles themselves. The particles of the medium only oscillate about their mean positions.
Experiment: Sound Needs a Medium (Bell Jar Experiment)
- Place an electric bell inside an airtight glass jar (a bell jar).
- Connect the bell to a battery and switch it on. We can clearly hear the sound of the bell.
- Now, connect a vacuum pump to the bell jar and start pumping the air out.
- As the air is removed, the loudness of the sound gradually decreases, even though the electric bell is still ringing.
- When most of the air is pumped out (creating a near-vacuum), the sound becomes extremely faint or vanishes completely.
Conclusion: The absence of air (a medium) stops the propagation of sound, proving that sound requires a material medium (solid, liquid, or gas) to travel.
Sound Waves: Longitudinal Waves
A wave is a disturbance that travels through a medium, transferring energy from one point to another without the transfer of matter.
There are two main types of mechanical waves:
1. Transverse Waves
In a transverse wave, the particles of the medium oscillate perpendicular (at right angles) to the direction of wave propagation.
- Crest (C): The highest point of the wave.
- Trough (T): The lowest point of the wave.
- Example: Waves created on the surface of water, light waves (electromagnetic waves).
2. Longitudinal Waves
In a longitudinal wave, the particles of the medium oscillate parallel to the direction of wave propagation.
Sound waves are longitudinal waves.
- Compression (C): Regions of high pressure and high density where the particles of the medium are crowded together.
- Rarefaction (R): Regions of low pressure and low density where the particles of the medium are spread apart.
Sound travels as a series of compressions and rarefactions that move through the medium.
Characteristics of a Sound Wave
1. Wavelength (𝜆)
- Definition: The distance between two consecutive compressions (C) or two consecutive rarefactions (R).
- Unit: Metre (m).
2. Frequency (𝜈)
- Definition: The number of complete oscillations (or the number of compressions and rarefactions) passing a fixed point per unit time.
- Formula: $$\nu = \frac{1}{T}$$ (where T is the Time Period).
- Unit: Hertz (Hz), where 1 Hz = 1 oscillation per second.
- Significance: Frequency determines the pitch of the sound (how shrill or flat the sound is). Higher frequency means higher pitch.
3. Time Period (T)
- Definition: The time taken for one complete oscillation of a particle in the medium.
- Formula: $$T = \frac{1}{\nu}$$.
- Unit: Second (s).
4. Amplitude (A)
- Definition: The maximum displacement of the particles of the medium from their original mean position.
- Unit: Metre (m) or Pascal (Pa) (when describing pressure variations).
- Significance: Amplitude determines the loudness of the sound. A larger amplitude means a louder sound.
5. Speed (or Velocity) of Sound (v)
- Definition: The distance a point on a wave travels per unit time.
- Fundamental Formula:
$$\text{Speed} = \frac{\text{Distance}}{\text{Time}}$$
- Wave Equation (Relationship between v, \(\nu \), and \(\lambda\):
The time taken for one wavelength \(\lambda\) to pass is the Time Period (T). Since \(v = \frac{\lambda}{T}\) and \(T = \frac{1}{\nu}\), we get:
\(v = \nu \times \lambda\) (Speed of Sound = Frequency \(\times\) Wavelength)
Speed of Sound in Different Media
The speed of sound depends on the properties of the medium through which it travels, specifically its temperature, density, and elasticity.
| Medium | State | Speed (m/s) (at \(25^{\circ}\text{C}\)) |
|---|---|---|
| Aluminium | Solid | 6420 |
| Steel | Solid | 5960 |
| Water (Distilled) | Liquid | 1498 |
| Water (Sea) | Liquid | 1531 |
| Air | Gas | 346 |
| Oxygen | Gas | 316 |
| Hydrogen | Gas | 1284 |
Important Observations:
- Speed in Solids > Speed in Liquids > Speed in Gases.
Sound travels fastest in solids and slowest in gases. - Effect of Temperature: The speed of sound in a gas increases with an increase in temperature.
Sonic Boom and Shock Waves
When an object travels at a speed greater than the speed of sound in air (which is about 343 m/s), it is said to be travelling at supersonic speed.
- Shock Waves: Objects travelling at supersonic speeds produce powerful shock waves in the air.
- Sonic Boom: The shock waves carry a large amount of energy, generating a very sharp, loud sound known as the sonic boom.
- Effect: The pressure waves from a sonic boom can even shatter glass and damage buildings.
Characteristics of Human-Perceived Sound
1. Loudness
- Definition: Loudness is a measure of the sound energy reaching the ear per second.
- Physical Basis: Loudness is proportional to the square of the amplitude \(A^2\) of the wave. High Amplitude \(\implies\) Loud Sound | Low Amplitude \(\implies\) Soft Sound
- Unit: Loudness is measured in decibels (dB).
2. Pitch
- Definition: Pitch allows us to distinguish between a "shrill" (high-pitched) sound and a "flat" (low-pitched) sound.
- Physical Basis: Pitch is directly determined by the frequency \(\nu\) of the wave. High Frequency ⇒ High Pitch | Low Frequency ⇒ Low Pitch
3. Quality (Timbre)
- Definition: Quality allows us to distinguish between two sounds having the same pitch and loudness.
- Physical Basis: Quality depends on the shape of the sound wave, which is a combination of the main frequency and its overtones.
Reflection of Sound
Like light, sound waves also follow the laws of reflection:
- The angle of incidence is equal to the angle of reflection \(\angle i = \angle r\).
- The incident sound wave, the reflected sound wave, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
1. Echo
An echo is the repetition of sound caused by the reflection of sound waves from a hard surface (like a wall, hill, or cliff).
Condition for Hearing a Distinct Echo:
The reflected sound must reach the ear at least 0.1seconds after the original sound was produced (persistence of hearing).
Minimum Distance Calculation:
Using v = 344 m/s and t = 0.1s, the total distance traveled is 2d = vt.
The minimum distance to hear a distinct echo is \(\mathbf{17.2\ \text{m}}\).
2. Reverberation
- Definition: Reverberation is the repeated reflection of sound that causes the persistence of sound in a large enclosed space.
- Effect: Excessive reverberation makes the sound blurred and confusing.
- Control: It is reduced by using sound-absorbing materials like compressed fibreboard, rough plaster, or acoustic panels on walls and ceilings.
Range of Hearing
1. Audible Range
The range of frequencies that the average human ear can detect is from \(\mathbf{20\ \text{Hz}}\) to \(\mathbf{20,000\ \text{Hz}} (\text{or } 20\ \text{kHz}\)).
2. Infrasound (Infrasonic)
Sound waves with frequencies below \(20\ \text{Hz}\).
Examples: Earthquakes, vibrations from large animals like rhinoceroses.
3. Ultrasound (Ultrasonic)
Sound waves with frequencies above \(20,000\ \text{Hz}\).
Examples: Used by bats and dolphins for echolocation.
Applications of Ultrasound
Ultrasound is useful because it travels in straight, well-defined paths.
- Cleaning: To clean hard-to-reach parts of machines and electronic components.
- Medical Imaging (Sonography): To create images of internal organs (safe and non-invasive).
- To Break Kidney Stones (Lithotripsy): High-energy waves break stones into fine grains.
- SONAR (Sound Navigation and Ranging): To measure distance and direction of underwater objects.
SONAR Working Principle
The distance to an underwater object is calculated by measuring the time ($t$) the ultrasound takes for the round trip (transmission and reflection).
The division by 2 accounts for the sound travelling to the object and back.
The Human Ear
The ear converts pressure variations into electrical signals for the brain.
[Image of the structure of the human ear, labeled parts]
1. Outer Ear
- Pinna: Collects sound waves.
- Auditory Canal: Channels waves to the eardrum.
- Eardrum (Tympanic Membrane): Vibrates in response to pressure changes (compressions push inward, rarefactions pull outward).
2. Middle Ear
- Three Bones (Ossicles): Hammer, anvil, and stirrup.
- Function: Amplify the vibrations from the eardrum and transmit them to the inner ear.
3. Inner Ear
- Cochlea: Coiled, fluid-filled organ where amplified vibrations generate electrical signals.
- Auditory Nerve: Carries the electrical signals to the brain for interpretation as sound.
Formulae Summary
| Quantity | Symbol | Relationship | Unit |
|---|---|---|---|
| Speed of Wave | v | \(v = \nu \times \lambda\) | m/s |
| Time Period | T | \(T = 1 / \nu\) | s |
| Wavelength | \(\lambda\) | \(v = \lambda / T\) | m |
| Distance (Echo/SONAR) | d | \(d = \frac{v \times t}{2}\) | m |