Applications of Mechanical Waves

Mechanical waves transfer energy through matter. Unlike electromagnetic waves, mechanical waves require a medium such as a solid, liquid, or gas through which to travel.

Mechanical waves are involved in many technologies and everyday experiences, including:

• speech and hearing
• music
• medical ultrasound
• sonar
• earthquake detection
• engineering inspections
• noise control
• musical instruments
• ocean waves

Understanding properties such as frequency, wavelength, amplitude, speed, reflection, refraction, diffraction, and interference allows scientists and engineers to use mechanical waves to solve practical problems.

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Mechanical Waves Need a Medium

A mechanical wave cannot travel without matter.

The medium might be:

• gas — sound travelling through air
• liquid — sonar travelling through seawater
• solid — seismic waves travelling through rock

Particles in the medium oscillate and transfer energy to neighbouring particles.

The particles do not normally travel all the way from the source to the receiver.

Instead:

matter oscillates locally while energy travels through the medium.

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Mechanical Waves in Everyday Life

Mechanical waves are constantly around us.

Examples include:

• sound from a conversation
• music from speakers
• vibrations in musical instruments
• ripples across water
• waves at a beach
• vibrations from machinery
• seismic waves from earthquakes
• ultrasound used in hospitals
• sonar used by ships

Many of these applications involve sound waves, which are mechanical waves.

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Sound Waves

Sound is produced by a vibrating source.

Examples include:

• vocal cords
• speaker cones
• guitar strings
• drum skins
• tuning forks
• engines

In air, sound travels mainly as a longitudinal wave consisting of alternating:

compressions and rarefactions

Air particles oscillate parallel to the direction of wave travel.

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Hearing

The human ear detects sound waves.

A simplified sequence is:

1. A source vibrates.
2. Sound waves travel through the air.
3. Sound enters the ear canal.
4. The sound causes the eardrum to vibrate.
5. Vibrations are transferred through tiny bones in the middle ear.
6. Vibrations reach the inner ear.
7. Sensory cells convert the vibrations into electrical signals.
8. Signals travel to the brain.

The brain interprets these signals as sound.

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Frequency and Pitch

The frequency of a sound wave is the number of complete vibrations occurring each second.

Frequency is measured in:

hertz (Hz)

Frequency is strongly related to pitch.

In general:

higher frequency → higher pitch

lower frequency → lower pitch

For example, a high musical note has a greater frequency than a low musical note.

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Amplitude and Sound

The amplitude of a sound wave is related to the size of the pressure disturbance.

Greater-amplitude sound waves generally have:

• greater intensity
• more energy passing through a given area
• greater perceived loudness, although human loudness perception is more complex than amplitude alone

Therefore:

amplitude and frequency describe different properties.

Frequency is mainly associated with pitch.

Amplitude is associated with the strength of the disturbance.

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Human Hearing

Young people with healthy hearing can typically detect frequencies roughly between:

20 Hz and 20 000 Hz

although the exact range varies among individuals and generally changes with age.

Sound below the normal human hearing range is called:

infrasound

Sound above it is called:

ultrasound

Humans cannot normally hear ultrasound, but it has many important applications.

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Ultrasound

Ultrasound is sound with a frequency above the upper limit of normal human hearing.

In many applications:

f > 20 000 Hz

Ultrasound is still a mechanical wave.

It therefore requires matter through which to travel.

Ultrasound can travel through:

• body tissues
• liquids
• solids

It cannot travel through a perfect vacuum.

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Medical Ultrasound

One of the best-known applications of ultrasound is medical imaging.

A device called a transducer sends high-frequency sound pulses into the body.

When these waves encounter boundaries between different tissues, some of the sound is reflected.

The returning waves are detected by the transducer.

A computer processes these echoes to produce an image.

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How an Ultrasound Image Is Produced

A simplified process is:

1. The transducer produces ultrasound pulses.
2. The waves travel into the body.
3. Some waves encounter boundaries between different tissues.
4. Part of the wave energy is reflected.
5. Reflected waves return to the transducer.
6. The equipment measures the return time and strength of the echoes.
7. A computer uses this information to construct an image.

The system therefore relies heavily on:

reflection of mechanical waves.

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Measuring Depth with Ultrasound

The time taken for an ultrasound pulse to return can be used to estimate the depth of a reflecting boundary.

Use:

distance = speed × time

But the measured time includes the journey:

transducer → object → transducer

Therefore:

depth = vt/2

where:

v = speed of sound in the tissue

t = total return time

The division by 2 is essential.

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Worked Ultrasound Example

Suppose an ultrasound pulse travels through tissue at:

1540 m/s

The echo returns after:

0.00010 s

Calculate the depth of the reflecting structure.

Use:

depth = vt/2

depth = (1540 × 0.00010)/2

depth = 0.154/2

depth = 0.077 m

or:

7.7 cm

The reflecting structure is approximately 7.7 cm beneath the transducer.

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Why Ultrasound Gel Is Used

During an ultrasound examination, gel is usually placed between the transducer and the skin.

Air gaps would cause strong reflection at the boundary and interfere with transmission of the ultrasound into the body.

The gel:

• removes most air gaps
• improves contact
• allows more ultrasound energy to enter the body

This is an example of wave physics directly influencing medical practice.

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Uses of Medical Ultrasound

Ultrasound can be used to investigate:

• pregnancy and fetal development
• the heart
• blood vessels
• kidneys
• liver
• muscles and tendons
• other soft tissues

Different ultrasound techniques are designed for different medical purposes.

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Doppler Ultrasound

The Doppler effect is the change in observed frequency caused by relative motion between a wave source and an observer or reflecting object.

Medical Doppler ultrasound can use frequency changes in waves reflected from moving blood cells.

This can provide information about:

• blood-flow direction
• blood-flow speed
• circulation
• heart and blood-vessel function

The technology combines reflection with the Doppler effect.

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Advantages and Limitations of Medical Ultrasound

Some advantages include:

• real-time imaging
• no ionizing radiation
• portable equipment is possible
• useful for many soft tissues
• relatively rapid examinations

Some limitations include:

• air can strongly interfere with transmission
• bone can block or strongly reflect ultrasound
• image quality depends on the tissues being examined
• interpretation requires skill

No imaging technique is ideal for every situation.

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Sonar

Sonar stands for Sound Navigation and Ranging.

Sonar uses sound waves, usually in water, to:

• detect objects
• measure distances
• determine water depth
• map the seafloor
• locate underwater structures

Sonar works using the reflection of sound waves.

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How Sonar Works

A basic sonar system works like this:

1. A transmitter produces a sound pulse.
2. The pulse travels through water.
3. The wave reaches an object or the seabed.
4. Some sound is reflected.
5. The reflected wave returns.
6. A receiver detects the echo.
7. The travel time is measured.
8. Distance is calculated.

Again:

distance to object = vt/2

because the sound makes a round trip.

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Worked Sonar Example

A ship sends a sound pulse toward the seabed.

The echo returns after:

2.0 s

Suppose the speed of sound in seawater is:

1500 m/s

Total distance travelled:

distance = vt

distance = 1500 × 2.0

distance = 3000 m

This includes the downward and upward journeys.

Therefore:

depth = 3000/2

depth = 1500 m

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Sonar Applications

Sonar can be used for:

• measuring ocean depth
• mapping the seafloor
• locating underwater objects
• navigation
• fish finding
• scientific research
• studying underwater geology

Some marine animals also use sound in ways similar to sonar.

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Echolocation

Animals such as bats and dolphins use echolocation.

They produce sounds and detect returning echoes.

The timing and properties of the echoes can provide information about:

• distance
• direction
• size
• movement
• sometimes surface characteristics

Echolocation is a natural example of applying sound-wave reflection.

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Sound Reflection and Echoes

An echo occurs when sound reflects from a surface and returns to the listener or detector.

Reflection is useful in:

• sonar
• ultrasound
• echolocation
• acoustic measurements

But reflection can also create problems.

For example, excessive reflections inside a room can make speech difficult to understand.

Engineers therefore sometimes want sound to reflect and sometimes want it to be absorbed.

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Acoustics

Acoustics is the study of sound and its behaviour.

Engineers and architects apply acoustics when designing:

• classrooms
• theatres
• concert halls
• recording studios
• cinemas
• offices
• transportation systems

They consider how sound:

• reflects
• absorbs
• diffracts
• interferes
• travels through different materials

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Designing a Concert Hall

A concert hall should allow sound to reach the audience clearly.

Too much absorption can make music sound weak.

Too much reflection can create confusing echoes and excessive reverberation.

Engineers therefore select:

• room shape
• wall materials
• ceiling structures
• acoustic panels
• seating materials

to control how sound behaves.

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Reverberation

Reverberation occurs when reflected sound continues to be heard for a short time after the original sound is produced.

Some reverberation can improve music.

Too much reverberation can make:

• speech unclear
• recordings muddy
• classrooms difficult to understand

Acoustic engineers therefore try to control reverberation time.

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Sound Absorption

Soft or porous materials can absorb some sound energy.

Examples include:

• carpets
• curtains
• acoustic foam
• upholstered seats
• specialized wall panels

When sound is absorbed, some of its energy is converted into other forms, mainly thermal energy within the material.

Sound-absorbing materials can reduce unwanted reflections.

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Noise Control

Mechanical-wave principles are also used to reduce unwanted noise.

Possible strategies include:

• absorbing sound
• blocking transmission
• isolating vibrations
• changing surface shapes
• reducing vibration at the source
• using interference

Applications include:

• buildings
• vehicles
• factories
• aircraft
• machinery
• headphones

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Active Noise Cancellation

Some headphones use active noise cancellation.

A microphone detects incoming noise.

The electronics produce another sound wave designed to interfere destructively with the unwanted sound.

When the waves combine appropriately:

positive pressure disturbance + negative pressure disturbance → reduced resulting disturbance

This is an application of wave interference.

It works particularly well for some steady, low-frequency sounds.

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Resonance

Resonance occurs when a system is driven at or near one of its natural frequencies, producing a large response.

Examples include:

• musical instruments
• vibrating strings
• air columns
• bridges
• buildings
• machinery

Resonance can be useful or dangerous.

Musical instruments use resonance to strengthen desired sounds.

Engineers often design structures to avoid dangerous resonant vibrations.

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Mechanical Waves in Musical Instruments

Musical instruments depend heavily on mechanical waves.

In a guitar:

• strings vibrate
• waves travel along the strings
• the body of the guitar vibrates
• surrounding air vibrates
• sound reaches the listener

In wind instruments:

• air columns vibrate

In drums:

• stretched membranes vibrate

The instrument's shape and materials influence which frequencies are strengthened.

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Standing Waves

Reflections can combine with incoming waves to produce standing waves.

Standing waves contain positions called:

nodes — little or no displacement

antinodes — maximum displacement

Standing waves are important in:

• strings
• pipes
• musical instruments
• acoustic systems
• mechanical structures

Engineers must sometimes encourage standing waves and sometimes prevent them.

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Mechanical Waves in Engineering

Engineers use mechanical waves to:

• inspect materials
• detect cracks
• measure thickness
• locate structural defects
• study vibrations
• reduce noise
• monitor machinery
• investigate earthquakes
• design buildings and bridges

Mechanical waves can provide information about structures without requiring them to be cut open or dismantled.

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Ultrasonic Testing

Ultrasonic testing is a type of non-destructive testing.

High-frequency sound waves are sent into a material.

If the waves encounter:

• cracks
• voids
• boundaries
• defects

some energy may be reflected.

The returning signals can reveal the location of hidden defects.

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Why Non-Destructive Testing Is Useful

Imagine checking a metal component inside an aircraft.

Cutting the component open to look for cracks would destroy it.

Ultrasonic testing can help inspect the component while leaving it intact.

This can be useful for:

• aircraft parts
• pipelines
• railway components
• bridges
• pressure vessels
• welded joints

Wave reflection provides information about structures that cannot easily be seen.

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Vibration Monitoring

Machines contain moving parts that create mechanical vibrations.

Engineers can measure:

• vibration frequency
• amplitude
• changes over time

An unusual vibration pattern may indicate:

• imbalance
• loose components
• bearing wear
• misalignment
• structural damage

Monitoring waves and vibrations can therefore help identify problems before equipment fails.

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Seismic Engineering

Buildings and bridges can be affected by seismic waves during earthquakes.

Engineers study:

• ground-motion frequency
• amplitude
• resonance
• damping
• wave transmission through soil

Structures can then be designed to reduce dangerous motion.

Techniques may include:

• flexible structural systems
• dampers
• reinforced frames
• base isolation

Wave science therefore contributes directly to earthquake-resistant design.

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Damping

Damping reduces the amplitude of oscillations by removing energy from a vibrating system.

Damping is important in:

• vehicles
• buildings
• bridges
• machinery
• suspension systems

For example, shock absorbers reduce repeated vehicle oscillations after a wheel encounters a bump.

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Waves in Vehicles

Mechanical waves and vibrations occur throughout vehicles.

Examples include:

• engine vibration
• tyre vibrations
• suspension oscillations
• sound in the passenger compartment
• vibration through the vehicle frame

Engineers try to control these waves to improve:

• comfort
• safety
• durability
• noise levels

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Water Waves in Engineering

Water waves are also mechanical waves.

Engineers study them when designing:

• harbours
• breakwaters
• offshore platforms
• coastal defences
• ships
• wave-energy devices

Important wave behaviours include:

• reflection
• refraction
• diffraction
• interference

Understanding these processes helps engineers predict how waves will interact with structures.

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Seismic Waves in Earth Science

Seismic waves are another major application of mechanical-wave principles.

P-waves and S-waves travel through Earth after earthquakes.

Their behaviour helps scientists investigate:

• Earth's mantle
• the liquid outer core
• the solid inner core
• internal boundaries
• earthquake locations

S-waves cannot travel through Earth's liquid outer core, providing important evidence about Earth's internal structure.

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Comparing Important Applications

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Using Travel Time to Measure Distance

Ultrasound and sonar use the same basic principle.

A pulse travels to an object and returns.

If:

v = wave speed

t = round-trip time

then:

distance to object = vt/2

This equation works because:

total distance = 2 × object distance

Therefore:

object distance = total distance/2

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Worked Engineering Example

An ultrasonic pulse travels through a metal component at:

5900 m/s

An echo from a defect returns after:

0.000020 s

Calculate the depth of the defect.

Use:

depth = vt/2

depth = (5900 × 0.000020)/2

depth = 0.118/2

depth = 0.059 m

or:

5.9 cm

The defect is approximately 5.9 cm below the surface.

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Reflection

Reflection occurs when a wave encounters a boundary and some of its energy returns.

Applications include:

• sonar
• ultrasound imaging
• echolocation
• ultrasonic testing

The time and strength of the reflected wave can reveal information about the reflecting object.

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Refraction

Refraction occurs when a wave changes speed and direction as it enters a different medium or region.

Applications include:

• seismic studies
• medical ultrasound
• underwater acoustics
• engineering measurements

Engineers must account for refraction when waves pass through materials with different properties.

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Diffraction

Diffraction is the spreading of waves around obstacles or through openings.

Sound diffraction explains why you may hear someone even when you cannot see them directly around a doorway or corner.

Diffraction becomes particularly noticeable when the wavelength is similar to the size of the opening or obstacle.

Lower-frequency sounds have longer wavelengths and can often diffract around large obstacles more noticeably than higher-frequency sounds.

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Interference

Interference occurs when two or more waves overlap.

Their displacements combine.

Constructive interference produces a larger resulting disturbance.

Destructive interference produces a smaller resulting disturbance.

Applications include:

• active noise cancellation
• acoustic engineering
• vibration control

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The Doppler Effect

The Doppler effect occurs when there is relative motion between a wave source and an observer.

A familiar example is an approaching ambulance.

As it approaches:

observed frequency is higher

After it passes and moves away:

observed frequency is lower

This causes the familiar change in perceived pitch.

The Doppler effect is also used in medical and engineering applications.

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Everyday Mechanical Waves

Mechanical waves appear in many ordinary situations.

Talking

Vocal cords vibrate and produce sound waves.

Listening to music

Speakers vibrate and produce pressure waves in air.

Playing guitar

Waves form on strings and vibrations produce sound.

Swimming

Water waves move across the pool.

Driving

Mechanical vibrations travel through the vehicle.

Using headphones

Speaker components create sound waves near the ear.

Hearing thunder

Pressure waves from rapidly expanding air travel through the atmosphere.

Feeling an earthquake

Seismic waves produce ground motion.

Recognizing these examples helps connect wave theory with everyday experience.

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Choosing the Right Wave Principle

When analyzing an application, identify the physical principle being used.

Ask:

Does the wave bounce back?

→ reflection

Does it bend because its speed changes?

→ refraction

Does it spread around an obstacle?

→ diffraction

Do waves combine?

→ interference

Does a system respond strongly at a particular frequency?

→ resonance

Does observed frequency change because of motion?

→ Doppler effect

This approach is more useful than simply memorizing applications.

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A Complete Application Example

Suppose a research boat uses sonar.

The speed of sound in the water is:

1500 m/s

An echo returns from an underwater object after:

0.80 s

Calculate the distance to the object.

Use:

distance = vt/2

distance = (1500 × 0.80)/2

distance = 1200/2

distance = 600 m

The object is approximately:

600 m away

The physics involved includes:

• production of a mechanical wave
• transmission through water
• reflection from the object
• detection of the echo
• measurement of travel time
• calculation of distance

A single technology can therefore involve several wave principles.

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Evaluating Wave Technologies

Mechanical-wave technologies have strengths and limitations.

When evaluating an application, consider:

• accuracy
• range
• resolution
• safety
• cost
• material being investigated
• environmental conditions
• interference from other waves
• absorption of wave energy
• speed of the wave in the medium

For example, sonar may work extremely well underwater but is not designed to operate through empty space because sound requires a medium.

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A Reliable Strategy for Analyzing Mechanical-Wave Applications

When given an unfamiliar application:

1. Identify the source of the wave.
2. Identify the type of mechanical wave.
3. Identify the medium.
4. Determine what is vibrating.
5. Identify the wave property being measured.
6. Look for reflection, refraction, diffraction, interference, resonance, or the Doppler effect.
7. Determine whether travel time is being measured.
8. Use v = fλ if frequency, wavelength, or speed is involved.
9. Use distance = vt/2 for a reflected pulse when appropriate.
10. Interpret the result in context.
11. Consider limitations of the technology.

This strategy allows wave concepts to be applied to unfamiliar situations.

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Common Mistakes

Thinking mechanical waves can travel through a vacuum

Mechanical waves require a material medium.

Thinking sound carries air from the source to the listener

Air particles oscillate locally while energy travels.

Thinking ultrasound is electromagnetic radiation

Ultrasound is high-frequency sound and is a mechanical wave.

Thinking ultrasound means extremely loud sound

"Ultra" refers to frequency above normal human hearing, not loudness.

Forgetting to divide by 2 in echo calculations

The measured time normally includes the trip to the object and back.

Confusing pitch and loudness

Frequency is associated mainly with pitch. Amplitude/intensity is associated with loudness.

Confusing reflection and refraction

Reflection is the return of wave energy from a boundary. Refraction involves a change in direction associated with changing wave speed.

Assuming all reflected waves are unwanted

Reflection is essential to sonar, ultrasound imaging, and ultrasonic testing.

Assuming all reflection is useful

Too much reflection can create unwanted echoes and reverberation.

Thinking resonance is always harmful

Resonance is deliberately used in musical instruments but can be dangerous in structures and machinery.

Thinking wave technologies rely on only one wave property

Many technologies combine several principles, such as reflection, timing, frequency, and interference.

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Key Terms

Mechanical wave — wave that requires a material medium through which to travel.

Medium — material through which a mechanical wave propagates.

Sound wave — mechanical pressure wave produced by a vibrating source.

Ultrasound — sound with a frequency above the normal upper limit of human hearing.

Transducer — device that converts energy from one form into another; ultrasound transducers produce and detect sound waves.

Echo — reflected sound detected after the original sound.

Sonar — technology that uses sound transmission and reflection to detect objects or determine distances underwater.

Echolocation — use of emitted sounds and returning echoes to obtain information about surroundings.

Reflection — return of wave energy from a boundary.

Refraction — change in wave direction associated with a change in wave speed.

Diffraction — spreading of waves around obstacles or through openings.

Interference — combination of overlapping waves.

Constructive interference — interference that produces a larger resulting disturbance.

Destructive interference — interference that produces a smaller resulting disturbance.

Resonance — large response produced when a system is driven near one of its natural frequencies.

Standing wave — wave pattern produced by interference that contains nodes and antinodes.

Node — position of minimum displacement in a standing wave.

Antinode — position of maximum displacement in a standing wave.

Doppler effect — change in observed frequency due to relative motion.

Acoustics — study of sound and its behaviour.

Reverberation — persistence of sound due to repeated reflections.

Damping — reduction of oscillation amplitude through energy loss.

Non-destructive testing — inspection of a material or structure without damaging it.

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Key Takeaways

• Mechanical waves require matter through which to travel.
• Mechanical waves transfer energy while particles mainly oscillate around equilibrium positions.
• Sound is one of the most important mechanical waves in everyday life and technology.
• Sound in air is primarily a longitudinal pressure wave.
• Frequency is associated mainly with pitch.
• Wave amplitude and intensity are related to the strength of a sound.
• Ultrasound is sound with frequency above normal human hearing.
• Medical ultrasound uses reflected sound waves to investigate structures inside the body.
• Ultrasound does not use ionizing radiation.
• Ultrasound gel improves transmission by reducing problematic air gaps between the transducer and skin.
• Doppler ultrasound can provide information about moving blood.
• Sonar uses sound and echoes to detect underwater objects and measure distances.
• For a reflected pulse, distance = vt/2.
• Animals such as bats and dolphins use echolocation.
• Acoustic engineers control reflection, absorption, diffraction, interference, and reverberation.
• Active noise cancellation uses destructive interference.
• Resonance can strengthen useful vibrations or create dangerous ones.
• Standing waves are important in musical instruments and other vibrating systems.
• Ultrasonic testing can detect hidden defects without destroying a component.
• Vibration monitoring can help engineers detect developing problems in machinery.
• Seismic-wave principles help scientists study Earth and engineers design safer structures.
• Water-wave principles are important in coastal and marine engineering.
• Reflection, refraction, diffraction, interference, resonance, and the Doppler effect have important practical applications.
• Mechanical waves occur throughout everyday life, from conversation and music to transportation, medicine, engineering, and natural hazards.
• Understanding wave behaviour allows scientists and engineers to use mechanical waves as tools for communication, measurement, imaging, detection, monitoring, and design.

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Check Your Understanding

1. What makes a wave a mechanical wave?
2. Explain how sound travels from a speaker to a listener without the same air particles travelling across the entire room.
3. Explain the difference between frequency and amplitude in a sound wave and how each affects what we hear.
4. What is ultrasound, and why is it still classified as a mechanical wave?
5. Describe how reflected ultrasound waves can be used to create a medical image.
6. An ultrasound pulse travels through tissue at 1540 m/s and returns after 0.000080 s. Calculate the depth of the reflecting structure.
7. Explain why ultrasound gel is placed between an ultrasound transducer and the patient's skin.
8. Describe how Doppler ultrasound can provide information about blood flow.
9. Explain how sonar uses reflection and travel time to determine the distance to an underwater object.
10. A sonar pulse travels through seawater at 1500 m/s and returns from the seabed after 1.6 s. Calculate the water depth.
11. Explain how bats or dolphins use echolocation and identify the wave principle involved.
12. Describe how reflection, absorption, and reverberation influence the design of a concert hall or classroom.
13. Explain how destructive interference is used in active noise-cancelling headphones.
14. An ultrasonic inspection pulse travels through steel at 5900 m/s. An echo from a crack returns after 0.000030 s. Calculate the depth of the crack and explain why this technique is useful to engineers.
15. Choose one application of mechanical waves from medicine, sonar, music, earthquake engineering, coastal engineering, noise control, or material testing. Explain the type of wave involved, the medium through which it travels, at least two wave principles used, and how understanding those principles makes the technology useful.