Study of Sound
Introduction to Sound
What is Sound? Sound is a form of energy that we hear with our ears. It travels as waves and needs a medium like air, water, or solids to move.
How Sound Travels? Sound waves create areas of compression (where particles are close together) and rarefaction (where particles are spread out). These waves make particles in the medium vibrate back and forth.
Types of Waves:
- Longitudinal Waves: Sound waves are longitudinal because particles move parallel to the wave’s direction (e.g., pushing and pulling air).
- Transverse Waves: In contrast, waves on water are transverse, where particles move up and down, perpendicular to the wave’s direction.
Representation of Sound Waves:
- Graphs show changes in density or pressure as sound moves, with alternating high and low areas (see Figure 12.1).
Key Terms
Wavelength (λ): The distance between two consecutive compressions or rarefactions, shown by the Greek letter lambda.
Frequency (υ): The number of vibrations per second, measured in Hertz (Hz), which decides the pitch (high or low sound).
Amplitude (A): The maximum change in pressure or density, which decides the loudness of the sound.
Time Period (T): The time taken for one complete vibration, related to frequency (T = 1/υ).
Velocity of Sound
Definition: The speed at which sound travels is called its velocity, calculated as distance divided by time (Velocity = Distance/Time).
Formula: Velocity (v) = Wavelength (λ) × Frequency (υ) or v = λ/T.
Factors Affecting Velocity:
Medium: Sound travels fastest in solids, slower in liquids, and slowest in gases because particles are closer in solids.
Temperature: Velocity increases with higher temperature (e.g., by 0.6 m/s per 1°C rise in air).
Density and Molecular Weight: In gases, velocity decreases if density or molecular weight increases. For example, sound is faster in hydrogen (molecular weight 2) than oxygen (molecular weight 32).
Pressure: Velocity doesn’t change with pressure if temperature is constant.
Example: Sound travels faster in iron pipes than in air, as seen in the “Try This” activity.
Velocity in Different Media (at 25°C):
- Air: ~346 m/s
- Water: ~1480 m/s
- Steel: ~5000 m/s
Audible, Infrasound, and Ultrasound
Audible Sound: Humans hear sounds between 20 Hz and 20,000 Hz (20 kHz).
Infrasound: Sounds below 20 Hz (e.g., earthquake vibrations) that humans can’t hear but animals like dogs can.
Ultrasound: Sounds above 20 kHz (e.g., bat sounds) used in technology like sonography.
Special Abilities:
- Bats and dolphins use ultrasound to navigate and find prey.
- Young children and some animals can hear up to 25 kHz.
Reflection of Sound
What is Reflection? Sound waves bounce off solid or liquid surfaces, following the laws of reflection (angle of incidence = angle of reflection).
Good and Bad Reflectors:
- Hard, flat surfaces (e.g., walls) are good reflectors.
- Soft materials (e.g., curtains, carpets) absorb sound and are bad reflectors.
Try This Activity: Use cardboard tubes to hear a clock’s ticking reflect off a wall, adjusting angles to match incidence and reflection.
Echo
Definition: An echo is the repeated sound you hear after it reflects off a surface.
Condition for Echo: The reflecting surface must be at least 17.2 m away at 22°C (based on sound speed of 344 m/s and brain retaining sound for 0.1 s).
Temperature Effect: The minimum distance changes with air temperature since sound speed varies.
Reverberation
What is It? Reverberation happens when sound reflects multiple times in a closed space, creating a continuous sound.
Example: In an empty room or auditorium, reflections mix, making speech unclear.
Solution: Use sound-absorbing materials like curtains or carpets to reduce it.
SONAR (Sound Navigation and Ranging)
Purpose: Used to find the direction, distance, and speed of underwater objects.
How It Works:
- A transmitter sends ultrasonic waves.
- Waves reflect off objects and are received back.
- Time difference and sound speed (e.g., 1550 m/s in water) calculate distance.
Uses: Measures sea depth, locates submarines, icebergs, or sunken ships.
Sonography
What is It? Sonography uses ultrasound to create images of internal organs for medical diagnosis.
Procedure: A probe with gel sends ultrasound into the body; reflected waves form images on a computer.
Uses: Checks heart conditions, baby growth, or swelling, and is painless.
Caution: Misuse (e.g., female foeticide) is illegal under the PNDT Act.
Human Ear
Parts and Functions:
- Outer Ear (Pinna): Collects sound waves and funnels them into the ear canal.
- Middle Ear: Contains the eardrum, which vibrates with sound waves, and three small bones (hammer, anvil, stirrup) that amplify the vibrations.
- Inner Ear: The cochlea converts vibrations into electrical signals, sent to the brain via the auditory nerve.
Care Tips:
- Avoid inserting objects into the ear.
- Don’t use earphones at high volume to protect the eardrum.
Solved Examples
Time for Sound to Travel 1.5 km:
- Given: λ = 0.25 m, υ = 1500 Hz, distance = 1.5 km = 1500 m.
- v = υλ = 1500 × 0.25 = 375 m/s.
- Time = Distance/Velocity = 1500/375 = 4 s.
Depth of Sea Using SONAR:
- Given: v = 1550 m/s, time = 4 s (2 s one way).
- Distance = v × t = 1550 × 2 = 3100 m (depth).
Frequency of Sound:
- Given: λ = 0.01 m, v = 340 m/s.
- υ = v/λ = 340/0.01 = 34,000 Hz (above human hearing range).
Key Points to Remember
- Sound needs a medium to travel and varies with temperature and material.
- Ultrasound and infrasound have special uses, while echoes and reverberation affect sound clarity.
- The ear is a sensitive organ that needs proper care to function well.
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