Class 9 Science Notes Chapter 12 (Chapter 12) – Examplar Problem (English) Book
Alright class, let's focus on Chapter 12: Sound, from your NCERT Exemplar. This is a crucial chapter, not just for your school exams but also for various government exams where basic science concepts are tested. Pay close attention to the definitions, characteristics, and applications.
Chapter 12: Sound - Detailed Notes for Exam Preparation
1. Production of Sound:
- Definition: Sound is a form of energy that produces the sensation of hearing in our ears.
- Source: Sound is produced by vibrating objects. Vibration is the rapid to and fro or back and forth motion of an object.
- Examples: Vibrating strings of a guitar, vibrating air column in a flute, vibrating drum membrane, vibrating vocal cords in humans.
2. Propagation of Sound:
- Medium Required: Sound needs a material medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum. This is demonstrated by the bell jar experiment.
- Mechanism:
- The vibrating object disturbs the particles of the medium immediately around it.
- These particles displace adjacent particles, transferring the disturbance (energy) without the particles themselves travelling far from their mean positions.
- This disturbance travels through the medium as a wave.
- Nature of Sound Waves: Sound waves are longitudinal waves.
- Longitudinal Wave: A wave in which the particles of the medium vibrate parallel to the direction of propagation of the wave.
- Sound travels as a series of compressions (C) and rarefactions (R).
- Compression: A region of high pressure and density where particles are crowded together.
- Rarefaction: A region of low pressure and density where particles are spread apart.
- The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength (λ).
3. Characteristics of a Sound Wave:
- Frequency (ν or f):
- Definition: The number of complete oscillations (or vibrations or cycles) per second. It is also the number of compressions or rarefactions passing a point per second.
- Unit: Hertz (Hz). 1 Hz = 1 oscillation per second.
- Determines Pitch: Higher frequency means higher pitch (shriller sound, like a whistle); lower frequency means lower pitch (flatter sound, like a drum).
- Amplitude (A):
- Definition: The maximum displacement of the particles of the medium from their mean (undisturbed) position.
- Unit: Metre (m) or units of density/pressure.
- Determines Loudness: Larger amplitude means louder sound; smaller amplitude means softer sound. Loudness is also related to the energy of the sound wave (Loudness ∝ Amplitude²).
- Loudness is measured in decibels (dB).
- Time Period (T):
- Definition: The time taken for one complete oscillation (or vibration or cycle). It is also the time taken for two consecutive compressions or rarefactions to pass a point.
- Unit: Second (s).
- Relationship with Frequency: T = 1/ν or ν = 1/T.
- Wavelength (λ):
- Definition: The distance between two consecutive compressions (C) or two consecutive rarefactions (R). It is the distance travelled by the wave during one time period (T).
- Unit: Metre (m).
- Speed (v):
- Definition: The distance travelled by the wave per unit time.
- Formula: Speed = Distance / Time = Wavelength / Time Period = λ / T
- Since ν = 1/T, the fundamental wave equation is: v = νλ (Speed = Frequency × Wavelength).
- Speed of sound depends on:
- Medium: Generally, v(solid) > v(liquid) > v(gas). (e.g., Speed in air ≈ 344 m/s at 22°C, Speed in water ≈ 1500 m/s, Speed in steel ≈ 5000 m/s).
- Temperature: Speed increases with an increase in temperature (e.g., speed in air at 0°C is ≈ 331 m/s).
- Humidity: Speed increases slightly with an increase in humidity in the air.
- Speed of sound is independent of pressure (for gases, under normal conditions), frequency, and amplitude.
- Timbre (or Quality):
- Definition: The characteristic of a sound that distinguishes it from another sound of the same pitch and loudness.
- Reason: Most sounds are a mixture of several frequencies. Timbre depends on the waveform, i.e., the combination of frequencies (fundamental and overtones/harmonics) present in the sound. Allows us to distinguish between a violin and a flute playing the same note at the same loudness.
4. Reflection of Sound:
- Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection:
- The angle of incidence equals the angle of reflection.
- The incident wave, the reflected wave, and the normal at the point of incidence all lie in the same plane.
- Echo:
- Definition: The repetition of sound caused by the reflection of sound waves from a hard surface (like a cliff, wall, or building).
- Condition for hearing a distinct echo: The time interval between the original sound and the reflected sound (echo) must be at least 0.1 seconds. This is due to the persistence of hearing in the human ear.
- Minimum distance for echo:
- Distance = Speed × Time
- The sound travels to the reflector and back, so the distance is 2d.
- 2d = v × t
- d = (v × t) / 2
- Taking v ≈ 344 m/s (speed of sound in air) and t = 0.1 s,
- d = (344 × 0.1) / 2 = 34.4 / 2 = 17.2 metres.
- So, the minimum distance between the source/observer and the reflecting surface should be 17.2 m to hear a distinct echo. This distance changes with the temperature of the air.
- Reverberation:
- Definition: The persistence of sound in a large hall due to repeated reflections from the walls, ceiling, and floor. If it's too long, the sound becomes blurred and distorted.
- Reduction: Reverberation is reduced by using sound-absorbing materials like compressed fibreboard, rough plaster, draperies, and carpets in auditoriums and concert halls.
5. Uses of Multiple Reflection of Sound:
- Megaphones, Horns, Trumpets, Shehnais: Designed to send sound in a particular direction without spreading, by confining the sound waves through multiple reflections within the tube.
- Stethoscope: A medical instrument used to listen to sounds produced within the body (heart, lungs). Sound reaches the doctor's ears by multiple reflections along the tube.
- Soundboards: Curved surfaces placed behind the stage in concert halls or auditoriums so that sound, after reflection, spreads evenly across the hall. Ceilings of halls are also often curved for the same purpose.
6. Range of Hearing:
- Human Audible Range: The range of frequencies that humans can hear is typically from 20 Hz to 20,000 Hz (or 20 kHz).
- Infrasound (or Infrasonic): Sounds of frequencies below 20 Hz.
- Produced by: Rhinoceroses, whales, elephants, earthquakes, volcanoes.
- Ultrasound (or Ultrasonic): Sounds of frequencies above 20,000 Hz (20 kHz).
- Produced and heard by: Dolphins, bats, porpoises, rats. Moths have highly sensitive hearing for ultrasound to evade bats.
7. Applications of Ultrasound:
Ultrasound waves have high frequency, allowing them to travel along well-defined paths even in the presence of obstacles, and they carry high energy.
- Industrial Uses:
- Cleaning parts in hard-to-reach places (spiral tubes, electronic components).
- Detecting cracks and flaws in metal blocks without damaging them.
- Medical Uses:
- Echocardiography (ECG - related but distinct, ECG is electrical): Ultrasound waves are used to create images of the human heart. (Correction: Echocardiography uses ultrasound for heart imaging. ECG measures electrical activity).
- Ultrasonography: Used to get images of internal organs (liver, gall bladder, uterus, kidney). Used to monitor fetal development during pregnancy.
- Breaking kidney stones into fine grains (lithotripsy).
- SONAR (SOund Navigation And Ranging):
- Principle: Uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.
- Working: A transmitter produces and transmits ultrasonic waves. These waves travel through water, reflect off objects (seabed, submarine, shoal of fish), and are detected by a receiver.
- Calculation: The distance (d) to the object is calculated using d = (v × t) / 2, where v is the speed of sound in seawater and t is the time interval between transmission and reception (echo time). This technique is called echo-ranging.
- Uses: Determine the depth of the sea, locate underwater hills, valleys, submarines, icebergs, sunken ships. Bats use a similar technique (echolocation) to navigate and find prey in the dark.
8. Structure of the Human Ear:
- Function: Converts sound waves (pressure variations in the air) into electrical signals that travel to the brain via the auditory nerve.
- Parts:
- Outer Ear (Pinna): Collects sound from the surroundings. Directs sound into the auditory canal.
- Auditory Canal: Sound travels down this canal to the eardrum.
- Middle Ear:
- Eardrum (Tympanic Membrane): Vibrates when sound waves strike it.
- Three Bones (Ossicles): Hammer (Malleus), Anvil (Incus), Stirrup (Stapes): Amplify the vibrations received from the eardrum. The stirrup transmits the amplified vibrations to the oval window of the inner ear.
- Inner Ear:
- Cochlea: A spiral-shaped cavity filled with fluid. Converts pressure variations (amplified vibrations) into electrical signals. Contains nerve cells sensitive to sound.
- Auditory Nerve: Transmits the electrical signals from the cochlea to the brain, where they are interpreted as sound.
Multiple Choice Questions (MCQs)
-
Sound cannot travel through:
(a) Solids
(b) Liquids
(c) Gases
(d) Vacuum -
The characteristic of sound that distinguishes a shrill sound from a flat sound is:
(a) Amplitude
(b) Frequency
(c) Wavelength
(d) Loudness -
The minimum distance required between the source of sound and the reflector to hear a distinct echo (at approx. 22°C) is:
(a) 10.0 m
(b) 17.2 m
(c) 34.4 m
(d) 0.1 m -
Which part of the human ear converts sound vibrations into electrical signals?
(a) Eardrum
(b) Hammer, Anvil, Stirrup
(c) Cochlea
(d) Auditory Nerve -
SONAR technology uses:
(a) Infrasonic waves
(b) Audible sound waves
(c) Ultrasonic waves
(d) Radio waves -
A sound wave has a frequency of 2 kHz and a wavelength of 35 cm. How long will it take to travel 1.4 km?
(a) 2.0 s
(b) 1.0 s
(c) 0.5 s
(d) 4.0 s -
Loudness of sound primarily depends on the wave's:
(a) Frequency
(b) Speed
(c) Wavelength
(d) Amplitude -
Which of the following frequency ranges represents infrasound?
(a) 0 Hz - 20 Hz
(b) 20 Hz - 200 Hz
(c) 20 Hz - 20,000 Hz
(d) Above 20,000 Hz -
Reverberation in a large hall can be reduced by using:
(a) Polished surfaces
(b) Hard walls
(c) Sound-absorbing materials like carpets and curtains
(d) Metal sheets on the ceiling -
Sound waves in air are:
(a) Transverse waves
(b) Longitudinal waves
(c) Electromagnetic waves
(d) A mix of transverse and longitudinal waves
Answers to MCQs:
- (d) Vacuum
- (b) Frequency (determines pitch)
- (b) 17.2 m
- (c) Cochlea
- (c) Ultrasonic waves
- (a) 2.0 s (Explanation: ν = 2 kHz = 2000 Hz; λ = 35 cm = 0.35 m. Speed v = νλ = 2000 Hz × 0.35 m = 700 m/s. Distance = 1.4 km = 1400 m. Time = Distance / Speed = 1400 m / 700 m/s = 2 s)
- (d) Amplitude
- (a) 0 Hz - 20 Hz (Frequencies below 20 Hz)
- (c) Sound-absorbing materials like carpets and curtains
- (b) Longitudinal waves
Make sure you understand the concepts behind each point and MCQ answer. Revise these notes regularly. Good luck with your preparation!