Sound - ICSE Class 10 Physics Study Notes

Sound is a mechanical wave that needs a material medium; it is produced by vibrating sources and travels as compressions and rarefactions. For ICSE Class 10, you need to connect the physics of wave motion to everyday phenomena like echo, pitch, and ultrasound. This note builds each idea from intuition first, then adds the exam-relevant definitions and formulas.

What is Sound? Production and Propagation

Sound is produced when an object vibrates and makes the particles of the surrounding medium oscillate. The disturbance travels outward as a wave, but the particles themselves only vibrate about their mean positions; they do not move with the wave.

• Vibration source: A tuning fork, vocal cord, or loudspeaker diaphragm vibrates to create sound.
• Medium needed: Sound cannot travel through vacuum. The bell jar experiment shows that when air is removed, the ringing bell is no longer heard.
• Compressions and rarefactions: The vibrating source creates alternate regions of high pressure and low pressure that move through the medium.

In air and other fluids, sound is a longitudinal wave because the particles vibrate parallel to the direction of wave travel. This is a key difference from light, which is a transverse wave.

Wave Parameters and the Wave Equation v = fλ

Every sound wave has measurable quantities. Frequency (f) is the number of complete oscillations per second, measured in hertz (Hz). Wavelength (λ) is the distance between two successive compressions or rarefactions. Amplitude is the maximum displacement of a particle from its mean position.

The speed of a wave is related to its frequency and wavelength by v = f × λ. This equation is universal and is frequently used in numerical problems.

• Worked example: A sound wave has frequency 512 Hz and wavelength 0.67 m. Then v = 512 × 0.67 ≈ 343 m/s. If the frequency doubles to 1024 Hz, the wavelength becomes about 0.335 m because the speed in the same medium is nearly constant.
• Time period: T = 1/f. For 512 Hz, T = 1/512 ≈ 0.00195 s.

Characteristics of Sound: Pitch, Loudness, and Quality

Pitch is the sensation of how high or low a sound appears. It is determined by frequency: a whistle has a high pitch, while a drum has a low pitch. Pitch is independent of loudness.

Loudness is the sensation of the strength of a sound. It depends mainly on the amplitude of vibration and the distance from the source. The physical intensity of sound is measured in decibels (dB), and loudness is the human perception of that intensity.

• Loudness and amplitude: Intensity is proportional to the square of amplitude, so doubling the amplitude makes the sound about four times more intense.
• Quality (timbre): Two sounds with the same pitch and loudness can still be distinguished by their quality, which depends on the waveform and the presence of overtones.
• Exam point: Pitch depends on frequency, loudness on amplitude, and quality on waveform.

Speed of Sound and Factors Affecting It

Sound travels at different speeds in different media because speed depends on how quickly particles can pass on the disturbance. In general, the order is solids > liquids > gases: steel ≈ 5000 m/s, water ≈ 1500 m/s, air ≈ 340 m/s at room temperature.

In air, the speed of sound increases with temperature. At 0°C it is about 331 m/s; at 20°C it is about 343 m/s. Humid air is slightly less dense than dry air, so sound travels faster in humid air.

• Worked example (thunder): Light reaches you almost instantly, but sound takes time. If thunder is heard 5 s after the lightning flash, distance = speed × time = 343 × 5 = 1715 m, about 1.7 km.
• Important: In a given medium, the speed of sound is independent of frequency and amplitude. That is why sounds of different pitches from the same distance reach you at the same time.

Reflection of Sound: Echo and Reverberation

When sound strikes a hard, large surface, it reflects. The reflected sound heard after the original sound is called an echo. For a distinct echo, the reflected sound must reach the ear at least 0.1 s after the original sound, because the human ear cannot separate two sounds closer than that.

This 0.1 s limit gives a minimum distance for an echo in air. The sound travels to the reflecting surface and back, so the total distance is 2d. Using v = 2d/t with t = 0.1 s and v ≈ 343 m/s, d = (343 × 0.1)/2 ≈ 17.2 m. Therefore, the reflecting surface must be at least about 17 m away.

• Worked example: If a cliff is 68.6 m away, the echo time is 2 × 68.6 / 343 = 0.4 s, which is clearly distinguishable.
• Reverberation: When many reflected sounds overlap and persist, the sound seems prolonged. This is reverberation, common in empty halls.
• Control: Curtains, carpets, and acoustic panels absorb sound and reduce reverberation.

Ultrasound, Infrasound, and Applications

Human ears normally detect sound frequencies from about 20 Hz to 20,000 Hz. Sounds below this range are called infrasound; sounds above it are called ultrasound. Bats and dolphins use ultrasound for navigation and hunting.

Ultrasound is useful because its short wavelength allows it to travel as a narrow beam and reflect from small objects. In medicine, ultrasonography uses reflected ultrasonic waves to form images of internal organs, and focused ultrasound can break kidney stones. In industry, ultrasound can detect cracks in metal blocks.

• SONAR: A ship sends an ultrasonic pulse toward the sea floor. If the echo returns after time t, depth = (v × t)/2. For example, in water with v = 1500 m/s and t = 2 s, depth = (1500 × 2)/2 = 1500 m.
• Infrasound sources: Earthquakes, volcanoes, and some large animals produce infrasound.
• Audible range: 20 Hz to 20,000 Hz; this range varies slightly with age.

Key takeaways

• Sound is a longitudinal mechanical wave that requires a medium; it cannot travel in vacuum.
• The wave equation v = fλ connects speed, frequency, and wavelength; in a given medium, higher frequency means shorter wavelength.
• Pitch depends on frequency, loudness depends on amplitude, and quality depends on waveform.
• Speed of sound is greatest in solids, then liquids, then gases; in air it increases with temperature.
• A distinct echo requires a minimum distance of about 17.2 m because the human ear needs at least 0.1 s between two sounds.
• The human audible range is 20 Hz to 20,000 Hz; ultrasound above this range is used in SONAR and medicine.

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