Sound Waves: Characteristics and Applications · Lesson 14 of 14
Chapter Summary and Practice
“Production, propagation, characteristics, perception, reflection and applications come together in one connected review.”
• Summarise how sound is produced and propagated. • Relate compressions rarefactions and longitudinal waves. • Use wavelength frequency time period amplitude intensity and speed correctly. • Distinguish pitch loudness and timbre. • Solve echo and sonar problems. • Explain applications of infrasonic and ultrasonic waves.
Sound begins with a vibrating source. The vibration creates a disturbance in a material medium. In gases such as air, this disturbance travels as alternating compressions and rarefactions. The medium's particles oscillate locally while energy and the density disturbance propagate outward.
Detailed Chapter Summary
Production and Propagation
Vibrating strings, membranes, air columns, tuning forks and vocal cords can act as sound sources. Sound can propagate through solids, liquids and gases, but it cannot propagate through vacuum because there are no material particles to pass the mechanical disturbance along.
Sound Waves and Energy
Sound is treated as a longitudinal mechanical wave. In a longitudinal wave, particles vibrate parallel to the propagation direction. Compressions are high-density regions and rarefactions are low-density regions. The particles do not flow from source to listener; energy is transferred through their vibration and interactions.
Graphical Representation
A density-versus-distance graph represents compressions as density above average and rarefactions as density below average. Maximum-density points appear as crests and minimum-density points as troughs. A density-versus-time graph can instead describe one location as the wave passes.
Characteristics of a Sound Wave
Distance between consecutive crests or consecutive troughs.
Number of density oscillations at a fixed point per unit time.
Time required for one complete density oscillation.
Maximum density change from average in a compression or rarefaction.
Sound energy crossing unit area perpendicular to propagation per unit time.
The speed of sound depends strongly on the medium. The chapter compares approximately 340 m s⁻¹ in air, 1500 m s⁻¹ in water and 5000 m s⁻¹ in steel at the stated conditions. Air temperature and humidity also affect sound speed.
Human Perception
Frequency is perceived mainly as pitch, while amplitude is perceived mainly as loudness. Typical human hearing extends from 20 Hz to 20 kHz. Below this lies the infrasonic range and above it the ultrasonic range. Timbre distinguishes instruments and voices that can play the same note at the same loudness.
Reflection, Echo and Reverberation
Sound can reflect from surfaces. A distinct reflection heard after sufficient delay is an echo. The distance to the reflector is calculated from the round-trip time using d = vt/2. Multiple reflections arriving close together create reverberation.
Applications Beyond Human Hearing
Infrasound helps detect natural events such as earthquakes and severe storms. Ultrasound is used in imaging, kidney-stone treatment, cleaning, welding and defect detection. Animals such as bats use ultrasonic echolocation, while humans use the same reflection principle in sonar.
Important Concepts and Formulas
| Concept | Key relationship |
|---|---|
| Sound production | Vibrating source |
| Propagation | Requires a material medium |
| Sound wave | Longitudinal mechanical wave |
| Frequency-time period | ν = 1/T |
| Wave speed | v = λν |
| Echo or sonar distance | d = vt/2 |
| Human audible range | 20 Hz to 20 kHz |
| Infrasonic | Below 20 Hz |
| Ultrasonic | Above 20 kHz |
At a Glance
Sound is produced by vibrating objects and propagates through solids, liquids and gases. It is a longitudinal mechanical wave and therefore requires a material medium. The travelling disturbance consists of compressions and rarefactions; medium particles vibrate about their mean positions rather than moving continuously with the wave.
Wavelength measures spatial repetition, frequency measures oscillations per second, time period measures time per oscillation, amplitude measures maximum density variation, intensity measures energy flow through unit area, and speed describes wave propagation.
Echoes and reverberations arise from reflection. Frequencies below the human range are infrasonic, while frequencies above it are ultrasonic. Ultrasound has important medical, industrial and ranging applications.
Revise, Reflect, Refine
Problem
Twenty compressions pass a point in four seconds. Find the frequency and time period.
- 1.Twenty compressions in four seconds represent 20 repeating compression arrivals.
- 2.Frequency = 20 ÷ 4 = 5 Hz.
- 3.Time period T = 1/5 s.
- 4.T = 0.2 s.
Problem
A sound wave has wavelength 3.44 m and speed 344 m s⁻¹. Find frequency and time period.
- 1.Use ν = v/λ.
- 2.ν = 344/3.44 = 100 Hz.
- 3.Use T = 1/ν.
- 4.T = 1/100 = 0.01 s.
Problem
A sonar signal returns after 5 s. If ultrasound speed in seawater is 1525 m s⁻¹, find the approximate depth.
- 1.Round-trip time = 5 s.
- 2.One-way time = 2.5 s.
- 3.Distance = 1525 × 2.5.
- 4.Distance = 3812.5 m.
- 5.The wreckage is approximately 3.81 km away in the one-way direction.
Problem
An ultrasonic parking sensor detects an obstacle 1.2 m away. If speed is 345 m s⁻¹, find the round-trip travel time.
- 1.Round-trip distance = 2 × 1.2 = 2.4 m.
- 2.Use t = distance/speed.
- 3.t = 2.4/345 s.
- 4.t ≈ 0.00696 s.
- 5.The round-trip time is approximately 7.0 ms.
Quiz
Which description best matches Wavelength?
Which description best matches Frequency?
Which term matches this description: Distance between consecutive crests or consecutive troughs.
Which term matches this description: Number of density oscillations at a fixed point per unit time.
Which expression represents Frequency and Time Period?
Practice Problems
- Explain which observation most directly shows that sound is a mechanical wave.
- Compare two wave graphs to identify which has greater wavelength and which has smaller amplitude.
- Draw a density graph for a wave with a stated wavelength and amplitude.
- Explain the scientific error in showing an audible explosion travelling through empty space.
- Calculate the extra thunder travel time over 1720 m when air changes from 344 m s⁻¹ to 331 m s⁻¹.
- Use an 8 cm repeated density spacing and a speed of 340 m s⁻¹ to calculate frequency.
- Compare two sounds travelling the same path in air and water using their return times.
The Journey Beyond
The chapter suggests extending learning beyond standard problems by investigating real sound and real spaces. These projects connect hearing safety, acoustic design, experimental measurement and digital tools.
- Investigate the effect of excessive earphone use on hearing, how hearing is tested and how hearing-support devices are used.
- Make a cardboard cone around a phone speaker and compare sound loudness with and without the cone.
- Investigate how curved walls and ceilings in concert or conference halls affect sound distribution.
- Measure the speed of sound in a large open ground by timing the delay between seeing a balloon burst and hearing the pop.
- Explore how temperature and humidity affect sound speed using suitable simulations or sound-analysis tools.
Sound is more than something we hear. It is a tool for investigating distant objects, living systems, materials, natural hazards and environments that may be difficult to study directly.