Sound

What you'll learn

• How sound is produced and travels.
• Frequency and amplitude — what they control.
• Properties of sound waves: wavelength, time period, speed.
• Audible range; ultrasound and infrasound.
• Noise pollution — causes, effects, control.

Key concepts

Production and propagation of sound

• Sound: a form of energy produced by vibrating objects.
• Vibration → compressions and rarefactions in surrounding medium → longitudinal wave.
• Sound is a mechanical wave — requires a medium to travel (cannot travel in vacuum).
  • Proof: Bell in a vacuum jar — as air is removed, sound fades even though bell still vibrates.
• Sound travels through: solids > liquids > gases (fastest in solids — particles closer together).

Speed of sound:

• Speed increases with temperature (air particles move faster → transmit vibration faster).
• Sound is ~880,000 times slower than light → we see lightning before hearing thunder.

Wave terminology

Frequency — what it controls

• Frequency determines pitch (how high or low the sound seems).
• High frequency → high pitch (shriller sound, like a whistle).
• Low frequency → low pitch (deeper sound, like a drum).
• Human audible range: 20 Hz to 20,000 Hz.

Amplitude — what it controls

• Amplitude determines loudness.
• Larger amplitude → louder sound.
• Smaller amplitude → softer (quieter) sound.
• Loudness is measured in decibels (dB).

• Sounds above 85 dB can damage hearing over time.
• Sounds above 130 dB cause immediate pain/damage.

Quality (Timbre)

• Quality (timbre): property that distinguishes sounds of the same pitch and loudness from different sources.
• Explains why a guitar and a violin playing the same note sound different.
• Depends on the waveform (mixture of frequencies produced).

Infrasound and Ultrasound

Applications of ultrasound:

• SONAR (Sound Navigation and Ranging): submarines/ships → send ultrasound pulses → measure echo time → calculate depth/detect objects. d = v × t/2.
• Echolocation (bats/dolphins): same principle to navigate and hunt.
• Medical ultrasonography: imaging internal organs (liver, kidney, foetus during pregnancy) — safe (no radiation).
• Cleaning: ultrasound in liquid vibrates and dislodges dirt from jewellery, surgical instruments.
• Industrial flaw detection: find cracks inside metal parts without cutting them open.
• Measuring distances: ultrasonic sensors in cars, parking aids.

Reflection of sound — Echo

• Echo: reflected sound heard after some time delay.
• For a distinct echo: reflecting surface must be at least 17 m from the source (sound travels to and back in > 0.1 s — minimum time for human ear to distinguish two sounds).
• Reverberation: multiple echoes causing sound to persist (in a closed hall/room). Controlled with sound-absorbing materials (curtains, carpets, foam panels).
• Uses of reflection: SONAR, echolocation, ultrasound imaging.

Noise pollution

• Noise: unwanted, unpleasant, loud sound.
• Noise pollution: excessive, loud or disturbing noise that harms health.

Sources:

• Vehicles (traffic); aircraft; factories/industries; construction; loudspeakers/firecrackers; home appliances.

Effects of noise pollution:

• Temporary or permanent hearing loss/damage.
• Increased blood pressure and stress hormones.
• Sleep disturbance → fatigue, reduced productivity.
• Difficulty in concentration, reduced cognitive performance.
• In extreme cases: psychological disturbances, cardiovascular problems.
• Wildlife: disrupts animal communication, migration, reproduction (especially marine life — sonar from ships).

Control measures:

• Source control: silencers on vehicles/machines; reduce use of horns; sound-insulated machines.
• Path control: noise barriers/walls along highways; sound-proof buildings; tree plantations (trees absorb sound).
• Receiver protection: ear plugs/muffs for workers; acoustic tiles in schools and hospitals.
• Law: Noise Pollution (Regulation and Control) Rules 2000 — India.
  • Residential: 55 dB (day) / 45 dB (night).
  • Industrial: 75 dB (day) / 70 dB (night).

Quick check

• How is sound produced? Can sound travel in vacuum? How do you know?
• What is the difference between frequency and amplitude? What does each control?
• What is the audible range of the human ear? What is ultrasound?
• State two medical and two industrial applications of ultrasound.
• What are the health effects of noise pollution? Name two control measures.

Open the Practice tab for graded questions on Sound.