Properties of Sound

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1Summary 重點摘要

Sound is a longitudinal wave carried by the compressions and rarefactions of a medium — no sound in vacuum, about 340 m s⁻¹ in air. Loudness depends on amplitude, pitch on frequency and quality on waveform; echo ranging and ultrasonic sonar turn the wave equation into a way of measuring distances.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: How sound is made and travelsEvery sound starts with a vibration: a tuning fork, vocal folds or a guitar string pushes and pulls the surrounding air, forming alternating compressions and rarefactions — a longitudinal wave that spreads outward. Sound needs a medium: vacuum carries none. For example, put a ringing electric bell inside a glass jar and pump the air out: the ringing fades to almost nothing while the hammer is still striking — proof that sound needs air to reach our ears, while light is unaffected and we can still see the bell moving.
2. Step 2: Speed of sound, medium and temperatureThe speed of sound depends on the medium: particles bound most tightly in solids pass the vibration on fastest, and gases are slowest. Typical values: about 340 m s⁻¹ in air, about 1500 m s⁻¹ in water, and over 5000 m s⁻¹ in steel. Raising the temperature makes air particles move faster and slightly increases the speed. For example, put your ear to a rail and you hear an approaching train earlier than through the air, because steel carries sound more than ten times faster; soldiers of old slept with quivers on the ground to hear riders coming through the earth.
3. Step 3: Audible range and ultrasoundThe human ear hears roughly 20 Hz to 20 kHz; the upper limit is higher in children and falls with age, so many elders cannot hear the high-pitched beeps of electronics. Above 20 kHz is ultrasound; below 20 Hz is infrasound — both inaudible. Bats and dolphins use ultrasound to navigate and communicate; earthquakes and storms produce infrasound. For example, anti-mosquito devices emit tones near 20 kHz that most adults cannot hear while some teenagers can — a neat demonstration that the audible range differs from person to person.
4. Step 4: Loudness, pitch and qualityThree features of a note map onto three physical quantities: loudness to amplitude, pitch to frequency, quality (timbre) to waveform. Pluck a string harder and the amplitude grows — the note is louder; shorten or tighten the string and the frequency rises — the pitch is higher; the same note on a piano and a trumpet has different waveforms, hence different timbres. For example, a piano tuner tells instruments apart by timbre, and a doctor distinguishes heartbeats from murmurs the same way; turning up a radio raises the amplitude and loudness only — it never turns a man's voice into a woman's, because frequency and waveform are unchanged.
5. Step 5: Echo ranging — remember to halveSound reflected from an obstacle is an echo. In echo ranging, distance = (speed of sound × time) ÷ 2, because the sound goes there and back. For example, shout at a cliff and hear the echo 1.5 s later: the one-way time is 0.75 s, so the cliff stands 340 × 1.5 ÷ 2 = 255 m away. The ear can only separate an echo from the original if the gap exceeds about 0.1 s, meaning the obstacle is at least some 17 m off; in an ordinary classroom the walls are too near, the echo blends into reverberation, and no distinct echo is heard.
6. Step 6: Applications of ultrasoundUltrasound has high frequency and short wavelength, so it forms narrow, directional beams — ideal for precise probing. Ship sonar pulses ultrasound at the seabed and times the echo to chart depth; fishing boats find shoals the same way; hospital B-scans image fetuses and organs with no radiation; industry tests metal castings for hidden cracks. For example, if sonar receives the seabed echo 2 s after the pulse, the water depth is 1500 × 2 ÷ 2 = 1500 m; substituting 340 m s⁻¹ from air would be badly wrong, because the speed of sound in water is entirely different.
7. Step 7: Noise, decibels and ear protectionNoise is unwanted or harmful sound, and its strength is given in decibels: 0 dB is roughly the threshold of hearing, 30 dB a whisper, 60 dB conversation; prolonged exposure above 80 dB damages hearing, and above 120 dB can injure immediately. Decibels measure loudness level, not frequency — a favourite exam distinction. For example, ear plugs cut the sound reaching your ears by 20–30 dB and should be worn by construction workers, musicians and frequent flyers; playing earphones at maximum volume likewise causes permanent hearing loss over time.

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5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Sound is a longitudinal wave of compressions and rarefactions; it needs a medium — a vacuum carries no sound
• The speed of sound is about 340 m s⁻¹ in air; fastest in solids and slowest in gases, rising slightly with temperature
• Loudness — amplitude; pitch — frequency; quality — waveform; the audible range is about 20 Hz to 20 kHz
• Echo ranging: distance = speed × time ÷ 2; the speed changes with the medium
• Ultrasound (above 20 kHz) powers sonar and B-scans; noise is measured in decibels, and exposure above 80 dB harms hearing