Sound is a form of energy that travels as a mechanical wave through a medium (solid, liquid, or gas). It cannot travel through vacuum. Sound is produced by vibrating objects and detected by our ears because the vibrations set up compressions and rarefactions in the medium.
Nature of Sound Waves
Sound travels as a longitudinal wave — the particles of the medium vibrate parallel to the direction of wave propagation. As a vibrating object moves forward it creates a region of high pressure called compression, and as it moves back it creates a region of low pressure called rarefaction. These compressions and rarefactions travel through the medium as the sound wave.
Characteristics of Sound
- Frequency (f) – Number of complete vibrations (cycles) per second. Unit: Hertz (Hz). Higher frequency = higher pitch.
- Amplitude (A) – Maximum displacement of particles from their mean position. Determines loudness — larger amplitude = louder sound.
- Wavelength (lambda) – Distance between two consecutive compressions (or rarefactions). Unit: metre (m).
- Time period (T) – Time for one complete vibration. T = 1/f.
- Speed (v) – Distance travelled by the sound wave per second. v = f x lambda.
Speed of Sound
- Sound speed depends on the medium and temperature:
- In air at 0°C: ~332 m/s; at 25°C: ~346 m/s.
- Sound travels faster in solids and liquids than in gases because particles are closer together.
- Approximate speeds: steel ~5000 m/s, water ~1500 m/s, air ~340 m/s.
Range of Hearing
- Audible range for humans: 20 Hz to 20,000 Hz (20 kHz).
- Infrasound: below 20 Hz. Produced by earthquakes, used by elephants and whales for communication.
- Ultrasound: above 20 kHz. Used by bats (echolocation) and dolphins. Humans cannot hear it.
Reflection of Sound and Echo
Sound reflects off hard surfaces, following the law of reflection (angle of incidence = angle of reflection). An echo is the reflection of sound heard after the original sound. For a distinct echo, the reflecting surface must be at least 17 m away from the source (because the ear can distinguish two sounds separated by at least 0.1 s, and sound travels ~340 m/s, so minimum distance = 340 x 0.1 / 2 = 17 m).
Reverberation is the persistence of sound due to multiple reflections, even after the source has stopped. Concert halls use sound-absorbing materials to reduce unwanted reverberation.
Applications of Ultrasound
- SONAR (Sound Navigation And Ranging): uses ultrasound pulses to detect objects underwater; the time taken for the echo to return is measured to calculate depth or distance.
- Medical imaging (sonography): ultrasound waves image internal organs safely (no radiation).
- Cleaning delicate parts: ultrasound vibrations dislodge dirt from jewellery, electronic components.
- Detecting cracks in metal blocks non-destructively.
The Human Ear
The outer ear (pinna) collects sound. Vibrations travel down the ear canal to the eardrum (tympanic membrane), which vibrates and transmits signals through three tiny bones (malleus, incus, stapes) to the cochlea in the inner ear, where they are converted to nerve signals sent to the brain.
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A sound wave has a frequency of 440 Hz and travels at 340 m/s. Find its wavelength.
v = f x lambda → lambda = v / f = 340 / 440 = 0.77 m (approx.).
A person claps near a hill and hears the echo after 0.6 s. If the speed of sound is 340 m/s, how far is the hill?
Total distance = speed x time = 340 x 0.6 = 204 m. Distance to hill = 204 / 2 = 102 m.
A sound wave has time period 0.002 s. What is its frequency?
f = 1/T = 1 / 0.002 = 500 Hz. This is in the audible range.
A ship uses SONAR. An ultrasound pulse is sent and the echo returns in 4 s. If speed of sound in water is 1500 m/s, what is the depth of the ocean floor?
Distance = speed x time / 2 = 1500 x 4 / 2 = 3000 m.
Explain why sound is louder when heard through a solid wall than through air over the same distance. Sound travels faster and with less energy loss through solids because particles are closer together, so the vibrations are transmitted more efficiently. Hence, intensity (loudness) is greater.
A bat emits ultrasound at 50,000 Hz. If the speed of sound in air is 340 m/s, what is the wavelength of this ultrasound?
lambda = v / f = 340 / 50,000 = 0.0068 m = 6.8 mm.
Why must the minimum distance for an echo be 17 m?
The human ear needs at least 0.1 s to distinguish between two sounds. In 0.1 s, sound covers 340 x 0.1 = 34 m. This distance is travelled to the wall and back, so the wall must be at least 34/2 = 17 m away.
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Key Formulas
Key formulas
Common mistakes
- Thinking sound travels faster in air than in solids or liquids — it is the opposite; sound is fastest in solids.
- Confusing pitch (related to frequency) with loudness (related to amplitude) — they are independent properties.
- Using full echo time (not halved) to calculate distance — remember sound travels to the object AND back, so divide by 2.
Summary
Sound is a longitudinal mechanical wave that requires a medium to travel. Its key characteristics are frequency (pitch), amplitude (loudness), wavelength, and speed. Ultrasound (above 20 kHz) has important medical and industrial applications. Echoes result from reflection of sound, requiring a minimum distance of 17 m for a distinct echo in air. The human ear detects vibrations in the range 20 Hz to 20,000 Hz.