Chapter 12: Study of Sound
Comprehensive chapter summary with detailed explanations and examples.
Grade 9 Learning: Chapter 12: Study of Sound
Introduction to Sound
Sound is a form of energy that produces the sensation of hearing. It is produced by vibrations and travels through a medium in the form of waves. Understanding sound involves exploring how it is produced, how it travels, its characteristics, and its various applications.
Sound Production
Sound is produced by vibrating objects. When an object vibrates, it sets the particles of the surrounding medium (like air) into vibration, creating a disturbance that travels as a wave.
Examples of Sound Production:
- Vibrating vocal cords produce human voice.
- Vibrating strings of a guitar produce musical notes.
- Vibrating diaphragm of a loudspeaker produces sound.
- Vibrating prongs of a tuning fork produce sound.
Sound Propagation
Sound needs a medium to travel. It cannot travel through a vacuum. Sound travels as a **longitudinal wave**, meaning the particles of the medium vibrate parallel to the direction of wave propagation. This creates regions of compression (high pressure) and rarefaction (low pressure).
- Medium: Sound can travel through solids, liquids, and gases.
- Speed of Sound: The speed of sound depends on the properties of the medium. It is generally fastest in solids, slower in liquids, and slowest in gases.
- Speed of sound in air (at 22°C) $\approx 344$ m/s.
- Speed of sound in water $\approx 1500$ m/s.
- Speed of sound in steel $\approx 5100$ m/s.
Characteristics of Sound Waves
Sound waves have several characteristics that determine how we perceive sound.
1. Wavelength ($\lambda$)
The distance between two consecutive compressions or two consecutive rarefactions of a longitudinal wave. Unit: meter (m).
2. Amplitude (A)
The maximum displacement of the particles of the medium from their mean position. It determines the loudness of the sound. Unit: meter (m).
3. Frequency ($\nu$ or f)
The number of oscillations or vibrations per unit time. It determines the pitch of the sound. Unit: Hertz (Hz).
Relationship between speed, wavelength, and frequency:
$v = \nu \lambda$ (Speed = Frequency × Wavelength)
4. Time Period (T)
The time taken for one complete oscillation. It is the reciprocal of frequency: $T = 1/\nu$. Unit: second (s).
Perception of Sound
We perceive sound based on its pitch, loudness, and quality.
1. Pitch
- Determined by the frequency of the sound wave.
- Higher frequency = Higher pitch (shrill sound).
- Lower frequency = Lower pitch (flat sound).
- Example: A child's voice typically has a higher pitch than an adult's voice.
2. Loudness
- Determined by the amplitude of the sound wave.
- Larger amplitude = Louder sound.
- Smaller amplitude = Softer sound.
- Unit: Decibel (dB).
- Example: Shouting produces a louder sound (larger amplitude) than whispering.
3. Quality (Timbre)
- Determined by the shape of the sound wave, which depends on the mixture of different frequencies (overtones) present in the sound.
- Allows us to distinguish between two sounds of the same pitch and loudness produced by different sources.
- Example: The sound of a piano and a flute playing the same note (same pitch and loudness) can be distinguished by their quality.
Reflection of Sound
Like light, sound also obeys the laws of reflection. When sound waves strike a hard surface, they bounce back.
1. Echo
- An echo is the repetition of sound caused by the reflection of sound waves from a distant obstacle (like a wall or mountain).
- For a distinct echo to be heard, the minimum distance between the source of sound and the reflecting surface should be approximately 17.2 meters (at 22°C). This is because the human ear can distinguish two sounds if they reach the ear with a time interval of at least 0.1 seconds.
Calculation for Echo Distance:
Speed of sound in air ($v$) = 344 m/s
Time interval for distinct echo ($t$) = 0.1 s
Total distance traveled by sound ($2d$) = $v \times t$
$2d = 344 \times 0.1 = 34.4$ m
Minimum distance to obstacle ($d$) = $34.4 / 2 = 17.2$ m
2. Reverberation
- Reverberation is the persistence of sound in a large hall or auditorium due to repeated reflections from walls, ceiling, and floor, even after the sound source has stopped.
- Excessive reverberation can make speech unclear.
- To reduce reverberation, sound-absorbing materials (e.g., curtains, carpets, acoustic panels) are used on walls and ceilings of concert halls and auditoriums.
Range of Hearing
The human ear can only perceive sounds within a certain frequency range.
- Audible Range: The range of frequencies that the average human ear can hear is approximately 20 Hz to 20,000 Hz (20 kHz).
- Infrasound: Sounds with frequencies below 20 Hz. These are inaudible to humans.
- Examples: Sounds produced by elephants, whales, earthquakes, some vibrations.
- Ultrasound: Sounds with frequencies above 20,000 Hz (20 kHz). These are also inaudible to humans.
- Examples: Sounds produced by bats, dolphins, dogs (can hear up to 50 kHz).
Uses of Ultrasound
Ultrasound has numerous practical applications due to its high frequency and ability to travel in straight lines without much diffraction.
1. Medical Applications
- Ultrasound Imaging (Sonography): Used to image internal organs (e.g., heart, liver, kidney, fetus during pregnancy). It's non-invasive and safe.
- Echocardiography: Used to image the heart.
- Breaking Kidney Stones: High-intensity ultrasound waves can break kidney stones into fine grains that can be flushed out with urine.
- Therapeutic uses: For muscle pain relief.
2. Industrial Applications
- Cleaning: Used to clean hard-to-reach parts of objects (e.g., electronic components, intricate machinery parts) by creating high-frequency vibrations in cleaning solutions.
- Detecting Flaws in Metals: Used to detect cracks and flaws in metal blocks and structures without damaging them.
- Sonar (Sound Navigation And Ranging): Used to measure the depth of the sea, locate underwater objects (submarines, shipwrecks, fish schools). It works by sending ultrasonic waves and measuring the time taken for the echo to return.
Sonar Principle:
A sonar device sends ultrasonic waves into the water. These waves travel through the water, hit an object, and are reflected back to the detector. The time taken for the echo to return is measured. Knowing the speed of sound in water, the distance to the object can be calculated using the formula: Distance = (Speed × Time) / 2.
Structure of the Human Ear
The human ear is a complex organ responsible for hearing. It can be divided into three main parts:
- Outer Ear: Consists of the pinna (outer visible part) and the ear canal (auditory canal). It collects sound waves and directs them to the eardrum.
- Middle Ear: Separated from the outer ear by the eardrum (tympanic membrane). It contains three tiny bones (ossicles): malleus (hammer), incus (anvil), and stapes (stirrup). These bones amplify and transmit vibrations from the eardrum to the inner ear. The Eustachian tube connects the middle ear to the throat, equalizing pressure.
- Inner Ear: Contains the cochlea (snail-shaped, responsible for hearing) and the vestibular system (responsible for balance). Vibrations from the middle ear are converted into electrical signals in the cochlea, which are then sent to the brain via the auditory nerve.
How We Hear:
Sound waves enter the ear canal, causing the eardrum to vibrate. These vibrations are amplified by the three ossicles in the middle ear and then transmitted to the fluid in the cochlea of the inner ear. The fluid vibrations create electrical signals that are sent to the brain, where they are interpreted as sound.
Conclusion
Sound is a fascinating phenomenon that allows us to perceive the world through vibrations. From its production and propagation as longitudinal waves to its characteristics like pitch and loudness, sound plays a vital role in our daily lives. The principles of sound reflection are utilized in various technologies, and the intricate structure of the human ear enables us to experience this auditory world.
References
- Maharashtra State Board Science and Technology Standard Nine Textbook (Latest Edition) - Chapter 12: Study of Sound.
- Maharashtra State Board 9th Standard Science Syllabus.
- Balbharati Science and Technology Textbook Part 1.
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