Lesson 12: Study of Sound

In the figure : In the figure compressions and rarefactions form a sound wave.

Rule 1: Sound waves — Sound waves form a chain of compressions (higher density) and rarefactions (lower density). The particles of the medium oscillate parallel to the direction of propagation, so these are longitudinal waves (अनुदैर्ध्य). When a stone is dropped in still water, the particles move up and down, perpendicular to the propagation, so these are transverse waves (अनुप्रस्थ).

• Easy : Sound in air is longitudinal; water ripples are transverse.
• Medium : Where air is squeezed we get a compression; where it is spread out we get a rarefaction. A sound wave is a series of both.
• Exam-type : Sound cannot travel in a vacuum because there are no particles to compress and rarefy.

Rule 2: Wave terms — Wavelength λ is the distance between two successive compressions. Frequency ν is the number of oscillations per second. Amplitude A is the maximum pressure or density change. Time period T is the time for one oscillation. Frequency decides the pitch; amplitude decides the loudness.

• Easy : A high-pitched whistle has a high frequency; a drum has a low frequency.
• Medium : Turning up the volume increases the amplitude, so the sound is louder but the pitch stays the same.
• Exam-type : If 50 oscillations occur in 1 second, ν = 50 Hz and T = 1 ÷ 50 = 0.02 s.

Spot the difference

In the figure : In the figure speed comes from wavelength and frequency.

Rule 1: Speed and the medium — Sound is heard through an iron pipe before it is heard through air, so it travels faster in iron. Velocity = wavelength × frequency, v = νλ. At fixed conditions the velocity of sounds of different frequencies is nearly the same. It is highest in solids, least in gases and increases with temperature. Borelli and Viviani first calculated the speed of sound in the 1660s as 350 m/s; today it is 346 m/s in air.

• Easy : ν = 500 Hz and λ = 0.68 m give v = 500 × 0.68 = 340 m/s.
• Medium : λ = 2 m and v = 340 m/s give ν = 340 ÷ 2 = 170 Hz.
• Exam-type : Thunder is heard 3 s after the lightning. At 340 m/s the distance is 340 × 3 = 1020 m.

Rule 2: Speed in a gas — In a gas, v is directly proportional to √T (absolute temperature) and inversely proportional to √density and √molecular weight. Four times the temperature doubles the velocity; four times the density or molecular weight halves it. At a fixed temperature it does not depend on pressure.

• Easy : Nine times the temperature triples the speed, since √9 = 3.
• Medium : Sixteen times the density makes the speed one-fourth, since √16 = 4.
• Exam-type : Speed is 300 m/s. If the absolute temperature becomes 4 times, the new speed is 300 × 2 = 600 m/s.

Spot the difference

In the figure : In the figure the three frequency ranges of sound.

Rule 1: Three ranges — The human ear hears 20 Hz to 20,000 Hz, which is audible sound. Below 20 Hz is infrasound, for example a pendulum, or the earth's crust before an earthquake. Above 20 kHz is ultrasound. Children under 5 can hear up to 25 kHz. Bats, mice and dolphins produce ultrasound. Spallanzani found that bats fly in the dark by using their ears: ultrasound reflected from obstacles is received by their ears.

• Easy : 50 Hz is audible; 10 Hz is infrasound; 30 kHz is ultrasound.
• Medium : A 22 kHz sound is ultrasound for most adults, but a child under 5 can still hear it, as the limit for them is 25 kHz.
• Exam-type : Classify 15 Hz, 8000 Hz, 40 kHz: 15 Hz infrasound; 8000 Hz audible; 40 kHz ultrasound.

Rule 2: Uses of ultrasound — Communication between ships, joining plastic surfaces, sterilizing milk by killing bacteria, echocardiography and images of internal organs, cleaning intricate machine parts and locating cracks in metal blocks.

• Easy : Ultrasound cleans small parts of watches and machines.
• Medium : Ultrasound reflected from a crack inside a metal block shows where the crack is, without cutting the block.
• Exam-type : A heart doctor uses ultrasound to see the heart's working: this is called echocardiography.

Spot the difference

In the figure : In the figure an echo needs a wall at least 17.2 m away.

Rule 1: Reflection of sound and echo — Sound reflects from solid and liquid surfaces and follows the laws of reflection. A hard, flat surface is a good reflector; clothes, paper, curtains, carpets and furniture absorb sound and are bad reflectors. The brain retains a sound for 0.1 s. At 22 °C the speed of sound is 344 m/s, so the minimum distance to the reflecting surface is 17.2 m. Multiple reflections cause reverberation (अनुरणन), a continuous, louder, unclear sound. Golghumat at Vijapur gives multiple echoes.

• Easy : In a hall full of curtains and carpets the echo is weak, because they absorb sound.
• Medium : Minimum distance for an echo = v × t ÷ 2 = 344 × 0.1 ÷ 2 = 17.2 m.
• Exam-type : You clap and hear the echo after 0.5 s. At 344 m/s, distance = 344 × 0.5 ÷ 2 = 86 m.

Rule 2: SONAR and sonography — SONAR (Sound Navigation and Ranging) uses ultrasound to find the direction, distance and speed of underwater objects and the depth of the sea. Sonography uses ultrasound with a probe and gel to image internal organs. Its misuse for female foeticide is an offence under the PNDT Act.

In the figure : In the figure SONAR finds the depth of the sea.

• Easy : A 4 s echo in sea water at 1550 m/s gives depth = 1550 × 4 ÷ 2 = 3100 m.
• Medium : A ship gets an echo after 2 s. Depth = 1550 × 2 ÷ 2 = 1550 m.
• Exam-type : Why divide by 2? The sound goes down and comes back up, so the total path is twice the depth.

Spot the difference

In the figure : In the figure how sound travels through the ear.

Rule 1: Parts of the ear — The outer ear (pinna) collects sound with its funnel shape. In the middle ear, the eardrum is pushed inwards by a compression and pulled outwards by a rarefaction, so it vibrates. The inner ear has the snail-shaped cochlea (शंखिका), which converts vibrations into electrical signals that the auditory nerve carries to the brain.

• Easy : The pinna collects sound like a funnel.
• Medium : Order: pinna → eardrum → cochlea → auditory nerve → brain.
• Exam-type : A compression pushes the eardrum in; the next rarefaction pulls it out. So the drum vibrates at the frequency of the sound.

Rule 2: Care of the ears — Never insert sticks or pointed objects into the ear, and avoid very loud music with earphones.

• Easy : Cleaning the ear with a pin can pierce the eardrum.
• Medium : Loud music for long hours through earphones can damage hearing.
• Exam-type : Keep the volume low, take breaks, and see a doctor if there is pain or reduced hearing.

Spot the difference