Sound is a mechanical form of energy that produces the sensation of hearing in our ears. It is produced by vibrating objects and travels as a wave through a material medium, carrying energy from one point to another.
11.1 Production of Sound
Sound is produced whenever an object is set into vibration. Vibration is a rapid back-and-forth motion of an object.
- Examples of Production: We can produce sound by striking, plucking, scratching, rubbing, blowing, or shaking objects.
- Human Voice: In humans, sound is produced due to the vibrations of the vocal cords.
- Stretched Strings: A plucked rubber band or the strings of a guitar vibrate to create sound.
11.2 Propagation of Sound
The matter or substance through which sound is transmitted is called a medium. It can be a solid, liquid, or gas.
- The Process: When an object vibrates, it displaces the particles of the medium in contact with it. These particles then exert a force on adjacent particles, passing the motion along. The particles themselves do not travel from the source to the ear; only the disturbance moves forward.
- Wave Nature: Because sound is characterised by the motion of particles in a medium, sound waves are called mechanical waves.
- Compressions and Rarefactions: * Compression (C): A region of high pressure and high density created when a vibrating object moves forward.
- Rarefaction (R): A region of low pressure and low density created when the object moves backward.
11.2.1 SOUND WAVES ARE LONGITUDINAL WAVES
Sound waves are longitudinal waves because the individual particles of the medium move in a direction parallel to the direction of the propagation of the disturbance. The particles simply oscillate back and forth about their position of rest.
- Transverse Waves: In these waves, particles oscillate up and down, perpendicular to the direction of wave travel (e.g., light waves or ripples in water).
11.2.2 CHARACTERISTICS OF A SOUND WAVE
We describe a sound wave using its frequency, amplitude, and speed.
| Characteristic | Definition | SI Unit |
| Wavelength ($\lambda$) | The distance between two consecutive compressions or rarefactions. | Metre ($m$) |
| Frequency ($\nu$) | The number of complete oscillations per unit time. | Hertz ($Hz$) |
| Time Period ($T$) | The time taken for one complete oscillation. | Second ($s$) |
| Amplitude ($A$) | The maximum disturbance in the medium on either side of the mean value. | Same as density/pressure |
Important Relationships:
- Frequency and Time Period: $\nu = \frac{1}{T}$
- Pitch: How the brain interprets the frequency of an emitted sound. High frequency means high pitch.
- Loudness: Determined by the amplitude. A larger amplitude results in a louder sound.
- Quality (Timbre): Allows us to distinguish between two sounds of the same pitch and loudness.
- Speed ($v$): Speed = Wavelength $\times$ Frequency ($v = \lambda \nu$).
11.2.3 SPEED OF SOUND IN DIFFERENT MEDIA
Sound travels at a finite speed which depends on the properties and temperature of the medium.
- Temperature Effect: In any medium, as we increase the temperature, the speed of sound increases.
- State of Matter: Sound travels fastest in solids, slower in liquids, and slowest in gases.
Questions and Answers
- Question: How does the sound produced by a vibrating object in a medium reach your ear?
- Answer: 1. The vibrating object pushes and pulls the surrounding air.2. This creates a series of compressions (high pressure) and rarefactions (low pressure).3. This disturbance travels through the air as a wave by setting neighbouring particles into motion.4. When the disturbance reaches the ear, it causes the eardrum to vibrate, allowing us to hear.
- Question: Why are sound waves called mechanical waves?
- Answer: They require a material medium (solid, liquid, or gas) for their propagation and involve the physical motion of particles.
- Question: Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s.
- Answer: 1. Speed ($v$) = $440~m/s$2. Frequency ($\nu$) = $220~Hz$3. Wavelength ($\lambda$) = $v / \nu = 440 / 220 = 2~m$.
11.3 Reflection of Sound
Sound bounces off solid or liquid surfaces just like light, following the laws of reflection.
- Laws of Reflection: The angle of incidence equals the angle of reflection, and the incident, reflected, and normal lines all lie in the same plane.
11.3.1 ECHO
An echo is a distinct reflected sound heard after the original sound has ceased.
- Persistence of Hearing: The sensation of sound stays in our brain for about $0.1~s$.
- Conditions for Echo: To hear a clear echo, the time interval between the original and reflected sound must be at least $0.1~s$.
- Minimum Distance: At $22^{\circ}C$, the minimum distance of the reflecting surface from the source must be $17.2~m$.
11.3.2 REVERBERATION
The persistence of sound due to repeated reflections in a big hall or auditorium is called reverberation.
- Problem: Excessive reverberation makes sound blurred and difficult to understand.
- Reduction: Halls use sound-absorbent materials like compressed fibreboard, heavy curtains, or rough plaster on walls and ceilings.
11.3.3 USES OF MULTIPLE REFLECTION OF SOUND
- Instruments: Megaphones, horns, trumpets, and shehanais use multiple reflections to guide sound towards the audience.
- Stethoscope: Heartbeats reach the doctor’s ears through multiple reflections within the tube.
- Sound Boards: Curved ceilings in concert halls or boards behind the stage ensure sound reflects evenly to all corners.
Questions and Answers
- Question: Why are the ceilings of concert halls curved?
- Answer: So that sound, after reflecting from the ceiling, reaches all corners of the hall evenly.
- Question: An echo is heard in 3 s. What is the distance of the reflecting surface if the speed of sound is 342 m/s?
- Answer: 1. Total distance = Speed $\times$ Time = $342 \times 3 = 1026~m$.2. Distance of reflecting surface = Total distance / $2 = 513~m$.
11.4 Range of Hearing
Human hearing is limited to a specific range of frequencies.
- Audible Range: Approximately $20~Hz$ to $20,000~Hz$ ($20~kHz$).
- Infrasound: Frequencies below $20~Hz$. Animals like rhinoceroses, whales, and elephants use this.
- Ultrasound: Frequencies above $20~kHz$. Produced by dolphins, bats, and rats.
11.5 Applications of Ultrasound
Ultrasound waves are high-frequency waves that can travel along well-defined paths and reflect even from small obstacles.
- Cleaning: Objects are placed in a cleaning solution and subjected to ultrasonic waves to remove dirt from hard-to-reach places.
- Flaw Detection: Used to detect cracks or holes inside metal blocks for big structures like buildings and bridges.
- Medical Imaging: * Echocardiography: To create images of the heart.
- Ultrasonography: To image internal organs (liver, kidneys, etc.) and check foetal growth during pregnancy.
- Kidney Stones: Breaking kidney stones into fine grains to be flushed out with urine.
Questions and Answers
- Question: What is the audible range of the average human ear?
- Answer: $20~Hz$ to $20,000~Hz$.
- Question: What are the typical wavelengths of sound waves in air corresponding to 20 Hz and 20 kHz? (Speed = 344 m/s)
- Answer: 1. For $20~Hz$: $\lambda = 344 / 20 = 17.2~m$.2. For $20~kHz$: $\lambda = 344 / 20,000 = 0.0172~m$.
- Question: Explain how defects in a metal block can be detected using ultrasound.
- Answer: 1. Ultrasonic waves are passed through the metal block.2. Detectors are placed on the other side.3. If there is a crack, the waves reflect back and do not reach the detector, indicating a flaw.
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