Sound Waves: Characteristics and Applications Class 9 Notes

These notes on Sound Waves: Characteristics and Applications for Class 9 follow the chapter in order: how sound is produced, how it travels, how we draw and measure a sound wave, how we hear it, and how echoes, ultrasound and SONAR are used. Formulas come with worked numericals, and the important questions at the end have full answers.

How sound is produced

Sound is a form of energy produced by vibration, the periodic to-and-fro motion (oscillation) of an object. A plucked rubber band or a struck metal taal vibrates and makes sound; when the vibration stops, the sound stops. The vibrating object is the source.

• In a bansuri (flute), the air column inside the pipe vibrates.
• In humans, the vocal cords, stretched muscular flaps in the larynx (voice box), vibrate as air passes. The tongue, lips and mouth shape this into speech.
• A tuning fork is a U-shaped metal bar with two prongs and a stem. Struck on a rubber pad, its prongs vibrate. Touch a prong to water and you see ripples, proof that it is vibrating.

How sound travels

Sound needs a material medium: a solid, liquid or gas. Put your ear on a desk and you hear a knock through the wood; tap spoons under water and you hear them through the liquid. Because it needs a medium, sound is a mechanical wave. It cannot travel through a vacuum, which is why astronauts in near-vacuum space use electronic devices to talk.

Bell jar experiment: a ringing electric bell in a jar is heard clearly. As a pump removes the air, the sound gets fainter and disappears, though the hammer is still seen moving. When air is let back in, the sound returns.

Longitudinal wave: a piston moving forward pushes air particles together, making a high-density compression (C). Moving back, it makes a low-density rarefaction (R). Particles only vibrate about their mean positions, parallel to the direction the wave travels. The particles do not travel from the speaker to your ear; the energy does, passed on by collisions. Light is different: it is an electromagnetic wave and can cross a vacuum.

Drawing a sound wave

On a graph with distance on the x-axis and density on the y-axis, a horizontal line shows average density. Compressions are peaks and rarefactions are valleys.

• Crest: highest point, maximum density (centre of a compression).
• Trough: lowest point, minimum density (centre of a rarefaction).

You can also plot density at one fixed place against time.

Wavelength, frequency, period, amplitude and speed

• ν = 1/T: high frequency means a short time period.
• v = λν: in one medium, speed is generally constant, so if frequency goes up, wavelength goes down.
• Intensity is the sound energy passing through unit area, perpendicular to the direction of travel, per unit time. Bigger amplitude means more energy.

Worked example (from the chapter): 10 oscillations in 2 s. ν = 10 ÷ 2 = 5 Hz; T = 1 ÷ 5 = 0.2 s.

Speed of sound: fastest in solids, slower in liquids, slowest in gases. At 15 °C, roughly 5000 m/s in steel, 1500 m/s in water and 340 m/s in air. In air, speed rises with temperature: about 331 m/s at 0 °C, 340 m/s at 15 °C and 344 m/s at 22 °C.

Pitch, loudness and quality

• Pitch is how the brain reads frequency. High frequency sounds shrill, like a whistle; low frequency sounds deep, like thunder. In adolescence, boys' vocal cords thicken and vibrate less often, so their voices deepen.
• Loudness is how we sense amplitude, measured in decibels (dB). Normal talk is about 60 dB; sounds above 100 dB, like firecrackers, can be harmful. Noise is unwanted or harmful sound.
• Timbre (quality) lets us tell instruments apart on the same note and loudness. Most sounds mix a fundamental frequency with higher overtones, and an instrument's shape and material set its pattern.
• A tone is a single frequency (like a tuning fork); a note is a fundamental plus overtones.
• Human audible range: 20 Hz to 20,000 Hz (20 kHz).

Reflection, echo and reverberation

Sound reflects like light: angle of incidence = angle of reflection, and the incident ray, reflected ray and normal lie in one plane. Hard, smooth surfaces reflect well; curtains absorb.

• Echo: a distinct reflected sound. You need a gap of at least 0.1 s. At 340 m/s the sound travels 340 × 0.1 = 34 m there and back, so the wall must be at least 17 m away.
• Reverberation: sound persisting because of many reflections in a large hall, arriving less than 0.05 s apart. A little makes music rich; too much makes speech unclear. Halls use sound-absorbing panels and upholstered chairs.

Distance to a reflector = (v × t) ÷ 2, because sound goes there and back.

Worked example: echo heard after 0.5 s in air at 340 m/s. Distance = (340 × 0.5) ÷ 2 = 170 ÷ 2 = 85 m.

Infrasound, ultrasound and SONAR

• Infrasonic: below 20 Hz; from earthquakes, volcanic eruptions and large objects. Elephants can detect them.
• Ultrasonic: above 20 kHz. Bats use them for echolocation; dolphins and whales use similar methods underwater.
• Uses of ultrasound: cleaning delicate parts, finding hidden cracks in metal blocks, ultrasonography to image organs, and breaking kidney stones into small pieces that pass out of the body.

SONAR (Sound Navigation and Ranging) sends ultrasonic pulses from a ship and times the echo from the seabed or objects such as submarines, to find distance, direction and speed underwater.

Worked example (from the chapter): echo returns after 0.90 s in seawater (1530 m/s). Depth = (1530 × 0.90) ÷ 2 = 1377 ÷ 2 = 688.5 m.

Remember this

• Sound = mechanical, longitudinal wave; no sound in a vacuum.
• ν = 1/T and v = λν.
• Pitch ↔ frequency; loudness ↔ amplitude.
• Echo needs at least 0.1 s, so at least 17 m in air.
• Distance = (v × t) ÷ 2 for echoes and SONAR.

Important questions with answers

1. Why is sound called a mechanical wave? It needs a material medium to travel and cannot pass through a vacuum.

2. Do air particles travel from a speaker to your ear? No. They vibrate about their mean positions; only the energy (the disturbance) travels.

3. A sound has frequency 50 Hz. Find its time period. T = 1/ν = 1/50 = 0.02 s.

4. A 170 Hz sound travels in air at 340 m/s. Find its wavelength. λ = v ÷ ν = 340 ÷ 170 = 2 m.

5. Why do we not hear a distinct echo in a small classroom? The reflection returns in less than 0.1 s, so the brain cannot separate it from the original sound.

6. A clap's echo is heard after 1 s in air (340 m/s). How far is the wall? (340 × 1) ÷ 2 = 170 m.

7. A SONAR pulse returns after 2 s in seawater (1530 m/s). Find the depth. (1530 × 2) ÷ 2 = 1530 m.

8. What is the difference between a tone and a note? A tone has a single frequency; a note is a fundamental frequency with overtones.

9. Give two industrial uses of ultrasound. Cleaning delicate machine parts and detecting cracks inside metal blocks.

Common mistakes to avoid

• Forgetting to halve the time (or distance) in echo and SONAR sums.
• Mixing up pitch (frequency) and loudness (amplitude).
• Calling sound a transverse wave because the graph looks up-and-down. The graph shows density; the particles move back and forth.
• Saying the particles of air travel with the sound.

For more practice on wave numericals with step-by-step hints, study this chapter with Joy.