Sound Revision Notes

Sound begins when an object vibrates, meaning it moves rapidly back and forth from its resting position. For example, when a guitar string is plucked, it vibrates, moving side to side repeatedly. This vibration disturbs the surrounding air particles.

The vibrating object pushes nearby air particles, causing them to move in a similar back-and-forth motion. This sets up a chain reaction where particles bump into neighbouring particles, transmitting the disturbance through the air.

• Vibration is a rapid back-and-forth movement.
• Objects like strings, drum skins, or vocal cords can vibrate to produce sound.
• The initial vibration disturbs nearby particles in the medium.

The movement of air particles caused by the vibrating object creates regions where particles are compressed together (compressions) and regions where particles are spread apart (rarefactions). These alternating compressions and rarefactions form longitudinal waves that travel through the air.

These waves carry energy away from the source without the net movement of air particles over long distances. Instead, particles oscillate about their equilibrium positions, passing the disturbance along.

• Sound waves are longitudinal pressure waves.
• Compressions are areas of higher pressure where particles are close together.
• Rarefactions are areas of lower pressure where particles are spread out.
• The wave propagates by particles oscillating parallel to the wave direction.

Sound requires a medium such as air, water, or solids to travel because it depends on particle interactions. Without particles to vibrate, such as in a vacuum, sound cannot propagate.

The speed of sound varies with the medium because particle spacing and bonding affect how quickly vibrations transfer. For example, sound travels faster in solids than in gases due to closer particle spacing.

• Sound cannot travel through a vacuum.
• Medium particles transmit sound by vibrating and colliding.
• Speed of sound depends on the medium's properties.

The frequency of the vibrating object determines the pitch of the sound we hear. Higher frequency vibrations produce higher-pitched sounds, while lower frequencies produce lower-pitched sounds.

The amplitude of the vibration affects the loudness of the sound. Larger amplitude vibrations cause greater displacement of particles, resulting in louder sounds.

For example, plucking a guitar string harder increases amplitude and loudness, while tightening the string increases frequency and pitch.

• Frequency of vibration controls pitch.
• Amplitude of vibration controls loudness.
• Examples: tighter strings produce higher pitch; harder plucks produce louder sound.

Sound is a wave characterised by its frequency, which is the number of vibrations or oscillations per second, measured in hertz (Hz). The human ear detects these vibrations as sound when they fall within a certain frequency range. This range determines which sounds we can hear and which we cannot.

• Frequency is measured in hertz (Hz).
• Sound frequency determines pitch: low frequency sounds have low pitch, high frequency sounds have high pitch.

Humans can hear sounds with frequencies from about 20 Hz to 20,000 Hz (20 kHz). Frequencies below 20 Hz are too low for the ear to detect and are called infrasound. Frequencies above 20 kHz are too high and are called ultrasound. This audible range allows humans to perceive a wide variety of sounds, from deep bass notes to high-pitched treble notes.

For example, the rumble of thunder is often below 20 Hz and is felt more than heard, while a dog whistle emits sounds above 20 kHz, inaudible to humans but heard by dogs.

• Lower limit of hearing: approximately 20 Hz.
• Upper limit of hearing: approximately 20 kHz.
• Sounds below 20 Hz = infrasound (inaudible).
• Sounds above 20 kHz = ultrasound (inaudible).

The human hearing range is adapted to detect sounds important for communication and environmental awareness. Speech frequencies typically lie between 300 Hz and 3,400 Hz, well within the audible range. Sounds outside this range are less relevant for everyday human interaction but can be important in other contexts, such as animal communication or medical imaging.

Understanding this range helps in designing audio equipment, hearing aids, and in studying how hearing loss affects perception.

• Speech frequencies lie within the audible range.
• Audio devices are designed to cover 20 Hz to 20 kHz.
• Hearing loss often affects higher frequencies first.

The upper limit of hearing tends to decrease with age, a condition called presbycusis. Exposure to loud noises can also damage the ear, reducing the range of audible frequencies. Individual variation means some people may hear slightly beyond the typical limits.

Testing hearing range involves playing sounds of different frequencies and noting the lowest and highest sounds a person can detect.

• Age reduces upper hearing limit.
• Loud noise exposure damages hearing.
• Individual hearing ranges vary.

Humans hear sounds approximately between 20 Hz and 20 kHz. Sounds below or above this range are inaudible and classified as infrasound or ultrasound respectively. This knowledge is essential for understanding human communication, designing audio technology, and protecting hearing health.

For example, when setting volume levels on audio devices, it is important to consider that sounds outside the audible range will not be heard but may still affect equipment or animals.

• Human hearing range: 20 Hz to 20 kHz.
• Infrasound and ultrasound lie outside this range.
• Applications include communication, technology, and health.

Sound is produced by vibrating objects, which create waves that travel through a medium by causing particles in that medium to vibrate. These waves are mechanical longitudinal waves, meaning the particle vibrations occur parallel to the direction of wave travel. Because they rely on particle interactions, sound waves cannot exist without a medium composed of matter.

• Sound waves are mechanical, requiring particle interaction.
• They consist of compressions and rarefactions in the medium.
• No particles mean no sound transmission.

Unlike electromagnetic waves, sound waves cannot travel through empty space (a vacuum) because there are no particles to transmit the vibrations. For example, in outer space, where there is a near vacuum, sound cannot be heard. On Earth, sound travels through air, water, and solids because these media have particles that can vibrate and pass on the sound energy.

• Sound cannot travel through a vacuum.
• Air, water, and solids provide particles for sound transmission.
• This is why astronauts cannot hear sounds in space without radio communication.

The speed of sound depends on the medium's density and elasticity. Sound travels fastest in solids because particles are closely packed and can transmit vibrations quickly. It travels slower in liquids and slowest in gases, where particles are more spread out. For example, sound travels at approximately 343 m/s in air, about 1480 m/s in water, and around 5000 m/s in steel.

• Sound speed increases with particle density and elasticity.
• Typical speeds: air ~343 m/s, water ~1480 m/s, steel ~5000 m/s.
• Closer particles transmit vibrations faster.

A simple experiment involves placing a ringing alarm clock inside a bell jar connected to a vacuum pump. Initially, the sound is clearly heard through the air inside the jar. As air is gradually removed, creating a partial vacuum, the sound becomes fainter until it is almost inaudible, demonstrating that sound needs air (a medium) to travel.

• Bell jar and vacuum pump apparatus.
• Sound fades as air is removed.
• Shows sound cannot travel through vacuum.

Understanding that sound requires a medium explains why sound cannot travel through space and why the medium affects how fast sound travels. This knowledge is essential in fields such as acoustics, engineering, and communication technologies. It also helps explain everyday phenomena like why sound is clearer underwater or why knocking on a door sounds different than knocking on a metal surface.

• Sound needs a medium to propagate.
• Medium type affects sound speed and quality.
• Important for practical applications and understanding natural phenomena.

Speed is defined as the distance travelled divided by the time taken. For sound, this means measuring how far sound waves travel through air and how long they take to do so. Since sound is a mechanical wave, it requires a medium such as air to propagate, and its speed depends on the properties of that medium. In air at room temperature, sound travels roughly at 340 metres per second, but this value can be experimentally determined by measuring distance and time accurately.

• Speed formula: speed = distance / time
• Sound requires a medium to travel
• Speed depends on medium properties

To measure the speed of sound, you need a known distance and a way to measure the time sound takes to travel that distance. A common method involves two people or two devices placed a measured distance apart, for example 100 metres. One person creates a sharp sound (like clapping or firing a starter pistol), and the other uses a stopwatch to measure the time from seeing the action to hearing the sound. Because light travels much faster than sound, the time delay corresponds to the sound travel time.

• Measure a straight-line distance between two points (e.g., 100 m)
• One person produces a sharp sound
• The other person measures time from seeing the sound source to hearing the sound
• Use a stopwatch or electronic timer for accuracy

Start the stopwatch the moment the sound source is seen (e.g., seeing a flash or movement) and stop it when the sound is heard. This time interval is the time taken for sound to travel the known distance. It is important to repeat the measurement several times to reduce random errors and calculate an average time. Record all times carefully and note environmental conditions such as temperature, which affect sound speed.

• Start timing on visual cue, stop timing on hearing sound
• Repeat measurements multiple times (at least 3)
• Record all times and calculate the average
• Note environmental factors like temperature

Use the formula speed = distance / time to calculate the speed of sound. For example, if the distance is 100 metres and the average time measured is 0.29 seconds, then speed = 100 m / 0.29 s ≈ 344.8 m/s. Always include units in your calculation and final answer. Check that the units are consistent: distance in metres (m) and time in seconds (s) give speed in metres per second (m/s).

• Apply speed = distance / time
• Example: distance = 100 m, time = 0.29 s
• Speed = 100 m ÷ 0.29 s = 344.8 m/s
• Include correct units in final answer

Several factors can affect the accuracy of the measurement: human reaction time when starting/stopping the stopwatch, environmental noise, and precise distance measurement. Using electronic timers and microphones can improve precision. Conducting the experiment in a quiet, open area reduces interference. Repeating trials and calculating an average reduces random errors. Temperature affects sound speed; warmer air increases speed, so record temperature to compare with theoretical values.

• Human reaction time can cause timing errors
• Use electronic timing devices for better accuracy
• Conduct experiment in quiet, open space
• Repeat trials and average results
• Record temperature to understand variations

Sound waves are mechanical waves that travel through a medium by vibrating particles. Two fundamental properties of sound waves are amplitude and frequency, which determine how we perceive sound in terms of loudness and pitch respectively. Understanding these properties helps explain why sounds differ in volume and tone.

• Sound waves have amplitude and frequency.
• Amplitude relates to energy carried by the wave.
• Frequency relates to how often the wave cycles per second.

Amplitude is the maximum displacement of particles in the medium from their rest position as the sound wave passes. It is directly related to the energy of the wave: larger amplitude means more energy. Loudness is the human perception of sound intensity, which depends on amplitude.

When the amplitude of a sound wave increases, the sound is perceived as louder. Conversely, a smaller amplitude results in a quieter sound. For example, when you turn up the volume on a speaker, the amplitude of the sound waves increases, making the sound louder.

Loudness is measured in decibels (dB), which is a logarithmic scale. A small increase in amplitude can cause a noticeable increase in loudness.

• Amplitude = maximum particle displacement in wave.
• Higher amplitude → louder sound.
• Lower amplitude → quieter sound.
• Loudness measured in decibels (dB).

Frequency is the number of complete wave cycles that pass a point each second, measured in hertz (Hz). It determines the pitch of the sound, which is how high or low a sound seems to a listener.

Higher frequency sound waves produce higher pitched sounds. For example, a whistle produces sound waves with a high frequency, resulting in a high pitch. Lower frequency waves produce lower pitched sounds, like the deep rumble of thunder.

Humans typically hear frequencies from about 20 Hz to 20,000 Hz. Sounds above this range are ultrasonic and below are infrasonic, both inaudible to humans.

• Frequency = number of wave cycles per second (Hz).
• Higher frequency → higher pitch.
• Lower frequency → lower pitch.
• Human hearing range approximately 20 Hz to 20 kHz.

Consider a guitar string: plucking it harder increases the amplitude of the vibrations, producing a louder sound without changing the pitch. Pressing the string at different lengths changes the frequency of vibration, altering the pitch but not the loudness.

Another example is a siren: as it approaches, the frequency (and thus pitch) appears higher due to the Doppler effect, but the amplitude controls how loud the siren sounds.

These examples show that amplitude and frequency independently affect loudness and pitch respectively.

• Hard pluck → higher amplitude → louder sound.
• Changing string length → changes frequency → changes pitch.
• Amplitude and frequency influence sound independently.

In summary, amplitude and frequency are key properties of sound waves that determine loudness and pitch. Increasing amplitude makes sounds louder, while increasing frequency raises the pitch.

This knowledge is important in many fields such as music, audio engineering, and acoustics. For example, sound technicians adjust amplitude to control volume and frequency to tune instruments.

Understanding these relationships also helps in designing hearing aids, speakers, and ultrasound devices.

• Amplitude controls loudness; frequency controls pitch.
• Adjusting amplitude changes volume without affecting pitch.
• Adjusting frequency changes pitch without affecting volume.
• Applications in music, technology, and medicine.

An echo is a sound that is heard after the original sound has been reflected from a surface and returned to the listener. When a sound is produced, it travels as waves through a medium such as air. If these sound waves encounter a surface, they can bounce back, similar to how light reflects from a mirror. The reflected sound wave that reaches the ear after a short delay is called an echo.

For example, if you shout towards a large wall or cliff, you may hear your voice repeated a moment later. This repeated sound is the echo.

• Echo is a reflected sound wave returning to the listener.
• It occurs after the original sound wave hits a surface and bounces back.
• The echo is heard as a distinct repetition of the original sound.

Sound waves travel through the air as vibrations of air particles. When these waves meet a hard, smooth surface, they reflect according to the law of reflection: the angle of incidence equals the angle of reflection. The reflected sound waves then travel back towards the source or another location where they can be heard.

The time delay between the original sound and the echo depends on the distance between the source and the reflecting surface. The greater the distance, the longer the delay before the echo is heard.

• Sound waves reflect off surfaces following the law of reflection.
• The echo’s delay depends on the distance sound travels to the surface and back.
• If the reflecting surface is too close, the echo merges with the original sound and is not heard separately.

Several factors influence whether an echo can be heard clearly:

Distance: The reflecting surface must be far enough away so that the reflected sound arrives at least 0.1 seconds after the original sound. Since sound travels at approximately 340 m/s in air, the surface should be at least 17 metres away (because the sound travels to the surface and back, total distance is 2 × 17 m = 34 m, and 34 m ÷ 340 m/s = 0.1 s).

Surface type: Hard, smooth surfaces reflect sound better than soft or irregular surfaces, which absorb or scatter sound waves, reducing echo clarity. Examples of good reflectors include walls, cliffs, and large buildings. Soft materials like curtains or carpets absorb sound and prevent echoes.

• Minimum distance for distinct echo is about 17 metres from the reflecting surface.
• Hard, smooth surfaces produce clearer echoes.
• Soft or irregular surfaces absorb sound, reducing echo strength.

Echoes are commonly experienced in large open spaces such as valleys, canyons, empty halls, or near cliffs. For instance, shouting in a canyon often produces a clear echo because the sound reflects off distant rock faces.

Echoes have practical applications too. Bats and dolphins use echoes to navigate and locate prey through echolocation, emitting sounds and interpreting the returning echoes. In human technology, sonar systems use echoes of sound waves underwater to detect objects or measure distances.

• Echoes occur naturally in large open spaces with reflective surfaces.
• Animals like bats use echoes for navigation (echolocation).
• Sonar technology relies on sound echoes to detect underwater objects.

An echo is the reflected sound wave heard after the original sound bounces off a surface and returns to the listener. The clarity and delay of an echo depend on the distance to the reflecting surface and the nature of that surface. Understanding echoes helps explain sound behaviour and has practical applications in nature and technology.

• Echo = reflected sound returning after a delay.
• Echo delay depends on distance and speed of sound.
• Hard surfaces reflect sound well; soft surfaces absorb it.
• Echoes are used by animals and in sonar technology.

Sound waves are mechanical waves that travel through a medium by vibrating particles. Humans can hear sounds with frequencies roughly between 20 Hz and 20 kHz. Ultrasound refers to sound waves with frequencies above 20 kHz, which are too high-pitched for human ears to detect.

For example, a dog whistle emits sound waves at frequencies above 20 kHz, which dogs can hear but humans cannot. This distinguishes ultrasound from audible sound.

• Ultrasound frequency > 20,000 Hz (20 kHz).
• Above human hearing range.
• Produced by vibrating objects at very high frequencies.

Ultrasound waves have shorter wavelengths than audible sound because wavelength (λ) is inversely proportional to frequency (f) when the speed of sound (v) in a medium is constant, according to the formula:

λ = v / f. For example, in air at 20 °C, sound travels at approximately 343 m/s. At 20 kHz, the wavelength is about 343 m/s ÷ 20,000 Hz = 0.01715 m (17.15 mm), which is much shorter than wavelengths of audible sounds.

Shorter wavelengths allow ultrasound waves to resolve smaller details when used in imaging or detection.

• Wavelength decreases as frequency increases.
• In air, ultrasound wavelengths are millimetres or less.
• Short wavelengths enable detailed imaging and detection.

The human ear detects sound by sensing vibrations in the ear drum within a certain frequency range. Frequencies above 20 kHz do not stimulate the ear drum effectively, making ultrasound inaudible to humans.

This inaudibility is why ultrasound can be used in applications without causing noise disturbance to people.

• Human hearing range: ~20 Hz to 20 kHz.
• Ultrasound frequencies exceed this range.
• Ultrasound is silent to human ears.

Ultrasound is widely used in medical imaging, such as prenatal scans, because it can penetrate soft tissues and reflect off boundaries between different tissues, creating images without harmful radiation.

In industry, ultrasound detects flaws inside materials by sending waves into objects and analysing reflected signals, a process called ultrasonic testing.

Ultrasound is also used for cleaning delicate items by producing high-frequency vibrations in liquids that remove dirt.

• Medical imaging (e.g., fetal scans).
• Industrial flaw detection (ultrasonic testing).
• Cleaning delicate objects using ultrasonic baths.

Ultrasound waves are sound waves with frequencies above 20 kHz, inaudible to humans due to their high frequency and short wavelength. Their ability to penetrate materials and provide detailed information makes them valuable in medical and industrial fields.

Understanding the frequency threshold and properties of ultrasound helps explain its unique uses compared to audible sound.

• Frequency > 20 kHz defines ultrasound.
• Inaudible to humans.
• Short wavelength enables detailed imaging and detection.
• Used in medicine, industry, and cleaning.

Sound waves are mechanical waves that require a medium, such as air, to travel. In air, sound waves are longitudinal waves, meaning the particles of the medium vibrate back and forth in the same direction as the wave travels. This contrasts with transverse waves, where particle vibration is perpendicular to wave direction.

When an object vibrates, it causes nearby air particles to oscillate, transmitting energy through the air as a wave of pressure variations. These pressure variations propagate away from the source, carrying the sound to our ears.

• Sound waves need a medium (air) to travel.
• They are longitudinal waves in air.
• Particles oscillate parallel to wave direction.

In a longitudinal wave, each air particle moves forward and backward along the line of wave travel. This oscillation does not transport the particles themselves over long distances but transfers energy through successive collisions between particles.

For example, when a speaker cone moves outward, it pushes air particles closer together, creating a region of high pressure. When it moves inward, it pulls particles apart, creating a region of low pressure. This cycle repeats rapidly, producing a wave of alternating high and low pressure regions.

• Particles oscillate parallel to wave direction.
• Energy is transferred by particle collisions.
• Particles do not travel with the wave.

Compressions are areas where air particles are pushed closer together than normal, resulting in increased pressure and density. These occur when the vibrating source pushes air particles forward, compressing them.

In a sound wave, compressions correspond to the peaks of the pressure wave. For example, when a drum skin strikes air, it creates compressions that travel outward as sound.

• Compressions have higher pressure and particle density.
• They form when particles are pushed together.
• Represent pressure peaks in the wave.

Rarefactions are areas where air particles are spread further apart than normal, causing a decrease in pressure and density. These occur when the vibrating source moves back, creating a partial vacuum that pulls particles apart.

Rarefactions correspond to the troughs of the pressure wave. For instance, after the drum skin moves inward, it creates a rarefaction that follows the compression in the sound wave.

• Rarefactions have lower pressure and particle density.
• They form when particles are pulled apart.
• Represent pressure troughs in the wave.

A sound wave consists of a repeating pattern of compressions and rarefactions moving through the air. As the source vibrates, it continuously creates these alternating regions, which propagate away from the source at the speed of sound.

This pattern can be visualised using a slinky toy: compressing coils together and then stretching them apart along the length of the slinky simulates compressions and rarefactions, demonstrating longitudinal wave motion.

Understanding this pattern is essential for explaining how sound travels and how changes in pressure correspond to the audible sound we perceive.

• Sound waves are alternating compressions and rarefactions.
• They propagate as a pressure wave through air.
• Slinky toy models longitudinal waves effectively.

Sound waves are longitudinal waves that travel through a medium such as air, water, or solids. Unlike transverse waves, where particles move perpendicular to the wave direction, in longitudinal waves particles oscillate parallel to the direction the wave travels.

This oscillation causes regions where particles are pushed together and regions where they are pulled apart, creating variations in pressure within the medium. These variations are essential to understanding how sound propagates.

• Sound waves are longitudinal waves.
• Particles oscillate parallel to wave direction.
• Pressure variations occur due to particle movement.

Compressions are areas within a sound wave where particles are pushed closer together than their normal spacing. This increased particle density results in a region of higher pressure compared to the surrounding medium.

For example, when a vibrating object pushes air molecules forward, it compresses them into a smaller volume, increasing the local pressure. This high-pressure region moves through the medium as the wave travels.

Because particles are closer together, the pressure in compressions is above the atmospheric pressure level.

• Particles are closer together in compressions.
• Compressions correspond to high-pressure regions.
• They form when particles are pushed forward by a vibrating source.
• Pressure in compressions is greater than atmospheric pressure.

Rarefactions occur where particles are spread further apart than normal, creating a region of lower pressure within the medium.

Following a compression, the particles move apart as the vibrating source pulls back, reducing the local particle density and pressure below atmospheric levels.

This low-pressure region travels through the medium alternating with compressions, forming the characteristic pattern of a longitudinal wave.

• Particles are more spread out in rarefactions.
• Rarefactions correspond to low-pressure regions.
• They form when particles move apart after a compression.
• Pressure in rarefactions is less than atmospheric pressure.

When an object vibrates, it causes the particles in the surrounding medium to oscillate back and forth along the direction of wave travel.

During the forward movement of the vibration, particles are pushed together forming a compression. During the backward movement, particles are pulled apart forming a rarefaction.

This alternating sequence of compressions and rarefactions moves through the medium as a sound wave, carrying energy without net movement of particles over long distances.

• Vibrating objects cause particle oscillations.
• Forward motion creates compressions; backward motion creates rarefactions.
• Compressions and rarefactions propagate the sound wave.
• Particles oscillate around fixed positions, not traveling with the wave.

The pressure variations in compressions and rarefactions are crucial for the detection of sound by our ears and by instruments.

Our eardrums respond to these pressure changes, converting them into nerve signals interpreted as sound.

Understanding these pressure differences helps explain how sound intensity and loudness relate to the amplitude of compressions and rarefactions, though this objective focuses on qualitative description only.

• Pressure variations enable sound detection.
• Eardrums respond to alternating high and low pressure.
• Amplitude relates to the magnitude of pressure changes.
• Qualitative understanding aids comprehension of sound wave behaviour.

Sound is a mechanical wave that requires a medium to travel through. It moves by causing particles in the medium to vibrate and pass the vibration on to neighbouring particles. The speed at which sound travels depends on how quickly these vibrations can be transferred from particle to particle.

• Sound needs particles to vibrate and transfer energy.
• The closer and more tightly bound the particles, the faster the vibration passes through.

In solids, particles are packed very closely together in a fixed, orderly arrangement. This close packing means particles can quickly pass vibrations to their neighbours.

In liquids, particles are still close but not fixed in place; they can move around each other. This makes vibration transfer slower than in solids.

In gases, particles are far apart and move freely, so vibrations take longer to pass from one particle to another.

• Solids: particles tightly packed and fixed.
• Liquids: particles close but mobile.
• Gases: particles far apart and free-moving.

Elasticity is a measure of how well a material returns to its original shape after being disturbed. Materials with high elasticity transmit vibrations more efficiently, increasing sound speed.

Solids generally have higher elasticity than liquids and gases, which helps sound travel faster through them.

• Higher elasticity means faster vibration transfer.
• Solids have higher elasticity than liquids and gases.

Combining particle arrangement and elasticity explains why sound travels fastest in solids, slower in liquids, and slowest in gases.

For example, sound travels through steel (a solid) at about 5000 m/s, through water (a liquid) at about 1500 m/s, and through air (a gas) at about 340 m/s at room temperature.

• Sound speed in solids > liquids > gases.
• Example speeds: steel ~5000 m/s, water ~1500 m/s, air ~340 m/s.

If you clap your hands underwater and in air, the sound reaches your ears faster underwater because water is denser and more elastic than air, allowing vibrations to travel more quickly.

This difference can be demonstrated by measuring the time taken for a sound to travel a fixed distance in air and in water using sensors and a timer.

• Sound travels faster underwater than in air.
• Measurement involves timing sound travel over a set distance in each medium.