Sound Waves: Characteristics and Applications — Notes

Production of Sound

Sound is produced by vibrating objects. A vibration is defined as the rapid, back-and-forth motion of an object. When an object vibrates, it disturbs the particles of the medium around it, creating a sound wave that travels to our ears.

How Vibrations Produce Sound

• Striking an object: When you strike a drum, a bell, or even a metal object, it causes the object to vibrate. These vibrations are passed on to the surrounding air, producing sound.
• Plucking a string: Musical instruments like guitars and violins produce sound when their strings are plucked. The vibrating strings cause the air around them to vibrate, creating sound.
• Blowing air: When you blow through a flute or a pipe, the air column inside vibrates. This vibration of air produces the sound you hear.
• Vocal cords: In humans and many animals, sound is produced by the vibration of vocal cords. These are two muscular flaps located in the larynx (voice box) in the throat. When we speak or sing, air expelled from the lungs causes these vocal cords to vibrate. The tongue, lips, and nasal cavity then modify these vibrations to form speech or music.

Example: The activity with a rubber band stretched across a cardboard box demonstrates this. When plucked, the rubber band vibrates and produces sound. When the vibration stops, the sound ceases. Changing the tension of the rubber band alters the speed of vibration, which in turn changes the pitch of the sound.

The Tuning Fork

A tuning fork is a scientific instrument commonly used to study sound.

• It is a U-shaped metal bar with a stem.
• The two arms of the 'U' are called prongs or tines.
• When the prongs are struck gently against a soft pad (like rubber), they vibrate, producing a pure, specific tone.
• If you touch a vibrating tuning fork to the surface of water, you can observe the water rippling, which visually confirms that the prongs are vibrating.

Remember

Sound is always produced by a vibrating source. If an object is not vibrating, it cannot produce sound.

Exam-smart

You might be given a scenario describing an object producing sound (e.g., a plucked guitar string, a vibrating tuning fork, a human speaking). You will need to identify that the underlying cause of the sound is vibration and potentially relate it to the specific object's method of vibration.

Propagation of Sound

Sound is produced by vibrating objects. These vibrations create disturbances in the medium surrounding the source, which then travel outwards. This movement of a disturbance through a medium is called propagation of sound.

Sound can travel through different types of materials, known as mediums. These mediums can be solids, liquids, or gases.

Investigating Sound Travel Through Different Mediums

Activity 1: Sound Through Solids

• Stand on opposite sides of a desk.
• Have a friend gently knock or scratch the desk.
• Listen to the sound with your ear in the air.
• Now, place your ear against the desk and close your other ear.
• Listen again. You will notice that the sound is heard more clearly through the desk.
• Conclusion: This demonstrates that sound can travel through solids.

Activity 2: Sound Through Liquids

• Fill a large tub or bucket with water.
• Take two metal spoons.
• Tap the spoons together in the air and listen to the sound.
• Now, submerge the spoons in water (without touching the sides or bottom) and tap them together again.
• You will hear the sound of the submerged spoons, which has traveled through the water to your ears.
• Conclusion: This shows that sound can also travel through liquids.

Medium for Sound Propagation

The material through which sound propagates is called a medium. Sound travels from its source to your ear by propagating through a medium.

A space where there is no medium, i.e., no matter, is called a vacuum. Sound cannot travel through a vacuum.

The Vacuum Bell Jar Experiment

This experiment is a common demonstration to prove that sound needs a medium to propagate.

1. An electric bell is placed inside a bell jar.
2. The bell is switched on, and the initial loudness of the sound is noted.
3. As air is gradually pumped out of the bell jar using a vacuum pump, the sound of the bell becomes fainter.
4. When a near vacuum is created inside the jar, very little or no sound is heard, even though the bell is visibly ringing.
5. When air is allowed back into the bell jar, the sound becomes audible again and its loudness increases.

Conclusion: This experiment clearly shows that sound cannot propagate in a vacuum because there is no medium (air) for the sound vibrations to travel through.

Remember

Sound requires a medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum.

Exam-smart

A scenario might describe a sound heard from a distance, and you'd need to identify the medium through which it traveled. For example, hearing a train's horn through the tracks (solid) or the sound of a ship's horn through water (liquid). You might also be asked to explain why astronauts in space cannot hear each other's voices directly without special equipment.

Sound Needs a Medium to Propagate

Sound is a form of energy that travels as waves. For these waves to travel from one point to another, they require a substance, known as a medium. This medium can be a solid, a liquid, or a gas. Without a medium, sound cannot travel.

Why Sound Needs a Medium: Sound is produced by the vibration of objects. These vibrations disturb the particles of the medium surrounding the vibrating object. These disturbed particles then collide with their neighbours, passing on the disturbance. This chain reaction of collisions and vibrations allows the sound to propagate. If there are no particles (like in a vacuum), there is nothing to disturb and nothing to carry the vibrations.

Analogy: The Slinky Imagine a slinky spring laid out horizontally. If you push and pull one end, a disturbance travels along the slinky. The individual coils of the slinky move back and forth, but they don't travel to the other end. Instead, they pass the motion to the next coil. This is similar to how sound travels through a medium: the particles of the medium vibrate about their fixed positions, but the disturbance (the sound wave) moves forward.

Types of Waves: Sound waves are a type of mechanical wave, meaning they require a medium to propagate. Specifically, sound waves are longitudinal waves. In longitudinal waves, the particles of the medium vibrate back and forth parallel to the direction in which the wave is travelling. This creates regions of compression (where particles are closer together and density is higher) and rarefaction (where particles are farther apart and density is lower).

Example: Space Travel Astronauts in outer space cannot hear each other directly because space is a near vacuum. They must use special communication devices within their spacesuits that convert sound into radio waves, which can travel through vacuum.

Remember

Sound cannot travel in a vacuum. This is why you can see a lightning strike but hear the thunder later, as light travels much faster than sound.

Exam-smart

A question might describe an experiment with a bell in a vacuum jar, where the sound fades as air is pumped out. You would need to explain this using the concept that sound requires a medium.

Energy of Sound Waves

Sound waves carry energy. When a source vibrates, it transfers energy to the surrounding medium, causing its particles to vibrate. This energy is then transmitted through the medium by the propagation of the sound wave.

Demonstration: Grains on a Drumhead If you stretch a sheet over a container and sprinkle grains on it, and then produce a loud sound near it, the grains will jump. This happens because the sound energy causes the sheet to vibrate, transferring energy to the grains and making them move. The louder the sound, the more energy it carries, and the higher the grains will jump.

Microphones and Speakers:

• Microphones convert sound energy into electrical energy. The sound waves cause a diaphragm to vibrate, and these vibrations are converted into an electrical signal.
• Speakers do the opposite: they convert electrical energy back into sound energy. An electrical signal makes a cone or diaphragm vibrate, producing sound waves.

Remember

Sound is a form of energy transfer, not matter transfer. The particles of the medium vibrate but do not travel with the wave.

Exam-smart

You might be asked to explain why sound energy is transferred when a wave propagates, or how a microphone works, linking it to the energy carried by sound waves.

Graphical Representation of a Sound Wave

A sound wave can be represented graphically by plotting the variations in the density of the medium against distance or time.

• Density vs. Distance: When a sound wave travels through a medium, the density of the medium changes. In a compression, the density is higher than average. In a rarefaction, the density is lower than average. A graph showing density versus distance will have peaks (crests) corresponding to compressions (maximum density) and troughs corresponding to rarefactions (minimum density).
• Density vs. Time: At a fixed point in the medium, the density also varies with time as compressions and rarefactions pass by. A graph of density versus time will also show periodic variations, with peaks and troughs.

Remember

Crest represents maximum density (compression), and Trough represents minimum density (rarefaction).

Exam-smart

You may be given a graph of density vs. distance and asked to identify compressions, rarefactions, wavelength, or amplitude.

Wavelength, Frequency and Time Period

These terms describe the characteristics of a sound wave:

• Wavelength ($\lambda$): The distance between two consecutive compressions or two consecutive rarefactions. It is measured in metres (m).
• Frequency ($\nu$): The number of complete oscillations (compressions and rarefactions) that pass a fixed point per unit time. It is measured in hertz (Hz) or per second ($s^{-1}$).
• Time Period (T): The time taken for one complete oscillation to pass a fixed point. It is measured in seconds (s).

Relationship: Frequency and time period are inversely related:

A wave with a higher frequency completes more oscillations in a given time.

Activity: Using a mobile app to analyse sounds can help you hear how different musical notes correspond to different frequencies. Higher notes have higher frequencies.

Example: If 10 density oscillations occur in 2 seconds, the frequency is $10\mathrm{/}2=5$ Hz, and the time period is $1\mathrm{/}5=0.2$ seconds.

Remember

Higher frequency means higher pitch (shriller sound). Lower frequency means lower pitch (deeper sound).

Exam-smart

You might be given the number of oscillations and time, and asked to calculate frequency and time period, or vice versa.

Amplitude and Intensity of Sound Waves

• Amplitude: The maximum change in density of the medium from its average density. A larger amplitude means a greater difference between the highest and lowest density in the compressions and rarefactions. Amplitude is directly related to the energy carried by the sound wave. A wave with larger amplitude carries more energy.
• Intensity: The amount of sound energy passing through a unit area perpendicular to the direction of wave propagation in unit time. Intensity is proportional to the square of the amplitude.

Distance and Intensity: As a sound wave travels away from its source, it spreads out over a larger area. Consequently, the energy is distributed over a greater area, and the intensity of the sound decreases with distance from the source.

Remember

Larger amplitude means louder sound and more energy. Intensity decreases with distance.

Exam-smart

Questions might ask how loudness relates to amplitude and intensity, or why sound seems quieter when you are further from the source.

Speed of Sound

The speed of sound is how fast a sound wave travels through a medium.

Formula: The speed of sound ($v$) is related to its wavelength ($\lambda$) and frequency ($\nu$) by the equation:

Since $\nu =1\mathrm{/}T$, we can also write:

Factors Affecting Speed:

• Medium: Sound travels fastest in solids, slower in liquids, and slowest in gases.
• Temperature and Humidity: In gases like air, the speed of sound increases with temperature and humidity.

Examples:

• The speed of sound in air is approximately 344 m/s at 22°C.
• The speed of sound in steel is much higher, around 5000 m/s.

Example Calculation: If a sound wave has a frequency of 20 Hz and a wavelength of 17.2 m, its speed is $20\text{ Hz}\times 17.2\text{ m}=344\text{ m/s}$.

Remember

Speed of sound depends on the medium, not its frequency or wavelength (these adjust to maintain the speed).

Exam-smart

You might be given the speed of sound and frequency, and asked to find the wavelength, or vice versa. Problems involving lightning and thunder rely on the difference in speed between light and sound.

Human Perception of Sound

While physical properties like frequency and amplitude are measurable, how humans perceive sound is subjective and described by terms like loudness and pitch.

• Pitch: Our perception of how high or low a sound is. It is primarily determined by the frequency of the sound wave. Higher frequencies are perceived as higher-pitched (shrill) sounds, while lower frequencies are perceived as lower-pitched (deep) sounds.
• Loudness: Our perception of the intensity of a sound. It is related to the amplitude of the sound wave. Larger amplitudes generally lead to louder sounds.

Audible Range: Humans can typically hear sounds with frequencies ranging from about 20 Hz to 20,000 Hz (20 kHz). Sounds with frequencies below 20 Hz are called infrasonic, and those above 20 kHz are called ultrasonic. This range can vary with age and individual differences.

Example: A whistle has a high pitch because it produces a sound with a high frequency. Thunder has a low pitch because it is produced by a low-frequency sound wave.

Remember

Pitch relates to frequency. Loudness relates to amplitude/intensity.

Exam-smart

You could be asked to differentiate between pitch and loudness, or to relate them to the physical properties of frequency and amplitude. You might also be asked about the human audible range.

Sound Waves

Audible Range of Hearing

• Audible range refers to the spectrum of sound frequencies that humans can perceive.
• The typical human hearing range is from 20 Hz to 20,000 Hz (or 20 kHz).
• This range can vary among individuals and tends to decrease with age.
• Frequencies outside this range are not heard by humans.

Infrasonic and Ultrasonic Waves

• Infrasonic waves are sound waves with frequencies below 20 Hz.
• Ultrasonic waves are sound waves with frequencies above 20 kHz (20,000 Hz).
• Humans cannot hear these waves, but some animals can detect them.
• Animals that detect ultrasound: Dogs, cats, bats, dolphins.
• Animals that detect infrasound: Elephants.

Loudness and Intensity

• Loudness is how humans perceive the amplitude of a sound wave.
• Larger amplitude sounds are perceived as louder.
• Smaller amplitude sounds are perceived as softer.
• Intensity is a measurable quantity related to loudness.
• Loudness generally decreases as the distance from the sound source increases.
• While often used interchangeably in everyday language, intensity is objective and measurable, while loudness is subjective and depends on the listener.
• Loudness is commonly measured in decibels (dB).
• Rustling leaves: few dB.
• Normal conversation: ~60 dB.
• Firecrackers: > 100 dB.
• A small increase in dB level signifies a large increase in sound intensity.

Noise Pollution

• Unwanted or harmful sound is called noise.
• Noise pollution is a significant environmental and health issue.
• Exposure to sound levels above recommended limits, especially for extended periods, can negatively impact:
• Health
• Sleep patterns
• Hearing ability
• Prolonged exposure to loud sounds can lead to hearing loss, which can be diagnosed using audiograms.
• Hearing aids assist individuals with hearing loss by amplifying sounds using a microphone, amplifier, and speaker.

The Human Ear and Sound Perception

• The eardrum, a thin membrane in the ear, vibrates when sound waves enter.
• Tiny bones in the ear amplify these vibrations.
• The cochlea converts amplified vibrations into electrical signals.
• The brain interprets these electrical signals as sound.
• Having two ears allows for sound localization: the brain uses the tiny time difference (often less than 0.001 seconds) between when a sound reaches each ear to determine its origin's direction.

Animal Hearing

• Different animals have unique ways of sensing sound and vibrations:
• Snakes and fish: Sense vibrations through their bodies.
• Some insects: Possess ear-like organs on various body parts.

Tone, Musical Notes, and Timbre

• A tone is a sound characterized by a single frequency, often produced by instruments like tuning forks or whistling.
• A musical note is more complex, consisting of a fundamental frequency (the lowest) and overtones (higher frequencies). This combination creates a rich sound.
• Indian string instruments (sarangi, sitar, veena) may use extra strings to enhance this complexity.
• Timbre is the unique quality of sound that distinguishes instruments even when they play the same note at the same loudness. It arises from the instrument's shape, material, and construction, which determine the pattern and intensity of overtones.
• An octave is the interval between two notes where one's fundamental frequency is double the other's (e.g., 200 Hz and 400 Hz).

Sir C.V. Raman and Acoustics

• Sir C.V. Raman, an Indian Nobel laureate, studied acoustics.
• His work included investigating Indian percussion instruments like the tabla and mridangam to understand their sound production.

Indian Percussion Instruments and the Syaahi

• Indian drums like the tabla feature a black patch called the syaahi on the center of the drumhead.
• The syaahi modifies the membrane's vibrations, enabling the production of a rich variety of sounds and offering significant tonal control.

Remember

The human audible range is limited, and frequencies outside 20 Hz to 20 kHz are classified as infrasonic or ultrasonic. Loudness is a perception based on amplitude, while intensity is a measurable property.

Exam-smart

A question might present a scenario involving sound levels (e.g., dB readings near a construction site) and ask about potential noise pollution effects or how the loudness would change if the listener moved further away. You'd apply concepts of dB, intensity, and the inverse square law (implicitly) to explain these effects. It could also provide sound frequencies and ask to identify if they are audible, infrasonic, or ultrasonic.

Reflection of Sound

Reflection of sound is the phenomenon where sound waves bounce off obstacles like solids or liquids. These reflected sound waves behave similarly to light waves, following the laws of reflection.

• The angle of incidence (the angle between the incoming sound wave and the normal to the reflecting surface) is equal to the angle of reflection (the angle between the reflected sound wave and the normal).
• The incident sound wave, the reflected sound wave, and the normal all lie in the same plane at the point of incidence.

This principle is the basis for understanding phenomena like echoes and reverberation, and is crucial for acoustic design in spaces.

Echo

An echo is the phenomenon where a sound is heard again after it reflects off a distant surface. This occurs when you shout near a mountain, cliff, or in a long corridor and hear your voice return after a delay.

• How it happens: Sound waves travel from the source, hit a hard surface (like a mountain or wall), and bounce back towards the listener.
• Conditions for hearing an echo: For the human ear to distinguish an echo as a separate sound from the original sound, there must be a minimum time gap of 0.1 seconds between them. If the time gap is less than 0.1 seconds, the reflected sound blends with the original sound and is not perceived as distinct.

Calculating the distance to the reflecting surface: If the speed of sound in air is approximately $340\text{ m/s}$, and the minimum time to distinguish an echo is 0.1 s, then the sound travels a total distance of:

This 34 m is the distance the sound travels to the reflecting surface and back. Therefore, the minimum distance from the reflecting surface to the listener is half of this total distance:

• Factors affecting echoes: Echoes are heard best from hard and smooth surfaces (like rock walls or large buildings) because they reflect sound efficiently. Soft surfaces (like curtains) tend to absorb sound, and rough surfaces scatter sound in many directions, making echoes difficult to hear.

Example: If you clap and hear an echo after 0.5 seconds, and the speed of sound is 340 m/s, the distance to the wall is:

Remember

An echo is heard when the time delay between the original sound and the reflected sound is at least 0.1 seconds. This implies a minimum distance to the reflecting surface.

Exam-smart

A question might provide the time delay of an echo and the speed of sound. You need to calculate the distance to the reflecting surface by first finding the total distance travelled by the sound and then dividing it by two.

Reverberation

Reverberation is the persistence of sound in a large space (like a hall or auditorium) after the original sound source has stopped. It is caused by multiple reflections of sound waves from the various surfaces within the space.

• How it happens: In large enclosed spaces, sound waves bounce repeatedly off walls, ceiling, and floor. If these reflections arrive at the listener's ear with very short time intervals (less than 0.05 seconds between successive reflections), they merge and prolong the sound, rather than being perceived as separate echoes.

• Impact of reverberation:

• Desirable reverberation: In modern auditoriums and concert halls, architects design for controlled reverberation to enhance the clarity and richness of speech and music. This makes the sound pleasant and ensures it reaches all parts of the audience.

• Undesirable reverberation: Excessive reverberation can lead to a garbled or muddled sound, making speech unintelligible and music distorted.

• Controlling reverberation: Reverberation is managed by using sound-absorbing materials.

• Materials that reduce reverberation: Soft furnishings like upholstered chairs, curtains, carpets, and porous surfaces absorb sound energy.

• Materials that promote reflection: Hard, smooth surfaces reflect sound, contributing to reverberation.

Remember

Reverberation is caused by multiple, closely spaced sound reflections, unlike echoes which are single reflections heard after a distinct time delay.

Exam-smart

A question might describe a large hall or auditorium and ask how to improve its acoustics for clear speech. The answer would involve reducing reverberation using sound-absorbing materials. Conversely, to create a "live" sound in a music venue, one might want to allow for controlled reverberation.

Ultrasonic and Infrasonic Waves, and their Applications

Sound waves whose frequencies fall outside the range of human hearing (20 Hz to 20 kHz) are broadly classified into two categories: infrasonic waves and ultrasonic waves. These waves, though inaudible to us, have significant applications in various fields due to their unique properties.

Infrasonic Waves

Infrasonic waves are sound waves with frequencies below 20 Hz. While humans cannot hear these frequencies, they are produced by various natural phenomena and some artificial sources.

Sources of Infrasonic Waves:

• Natural phenomena: Earthquakes, volcanic eruptions, avalanches, thunderstorms, and even the movement of large ocean waves generate infrasonic waves.
• Biological sources: Some large animals like elephants and rhinoceroses can produce and detect infrasonic sounds for communication over long distances.
• Artificial sources: Large machinery, wind turbines, and even traffic can produce infrasonic noise.

Applications of Infrasonic Waves:

• Monitoring natural disasters: Scientists use infrasonic detectors to monitor and even predict events like earthquakes and volcanic eruptions by analyzing the infrasonic waves they produce.
• Studying severe weather: Infrasonic waves can travel long distances through the Earth and air, making them useful for detecting and tracking severe storms.
• Detection of aircraft and submarines: In some military applications, infrasonic sensors are used to detect the presence of large objects like aircraft or submarines.

Ultrasonic Waves

Ultrasonic waves are sound waves with frequencies above 20 kHz. These waves are commonly used in a wide range of technological and medical applications.

Characteristics of Ultrasonic Waves:

• High frequency: Their frequency is greater than the upper limit of human hearing.
• Short wavelength: Due to their high frequency, they have a short wavelength, which allows for better resolution in imaging and detection.
• Directional propagation: They can be transmitted in a narrow beam, making them useful for targeted applications.

Applications of Ultrasonic Waves:

Example Calculation (Sonar): A naval sonar signal is sent into seawater and returns after 0.90 seconds. The speed of sound in seawater is 1530 m/s. To find the distance to the object, we first find the time taken to reach the object (half the total time) and then multiply by the speed. Time to object = $0.90\text{ s}\mathrm{/}2=0.45\text{ s}$ Distance = Speed $\times$ Time = $1530\text{ m/s}\times 0.45\text{ s}=688.5\text{ m}$

Remember

The human audible range is typically from 20 Hz to 20 kHz. Frequencies below this are infrasonic, and above are ultrasonic.

Exam-smart

Questions might involve calculating distances using sonar/echolocation data, or identifying applications of infrasonic and ultrasonic waves based on their characteristics. You will need to apply the formula: distance = speed × time. For echoes, remember the time given is for the round trip.

Echolocation

Echolocation is a biological sonar system used by certain animals, most famously bats and dolphins, to perceive their environment. It is also the principle behind human-made sonar technology.

How Echolocation Works:

1. Emission of sound: The animal or device emits a sound pulse, typically in the ultrasonic range.
2. Reflection: These sound waves travel outwards and reflect off objects in the environment (prey, obstacles, the seabed, etc.).
3. Reception of echoes: The animal or device detects the returning echoes.
4. Interpretation: The brain or processing unit analyzes the time delay between emission and reception, the direction of the returning echo, and changes in frequency (Doppler effect) to determine the location, size, shape, texture, and movement of objects.

Animals that Use Echolocation:

• Bats: Use echolocation to navigate in darkness and hunt insects. They emit high-frequency clicks.
• Dolphins and Whales: Use clicks and whistles to navigate, find food, and communicate in murky waters.
• Some birds: Such as the oilbird and some swiftlets, use echolocation to navigate in dark caves.

Human Applications of Echolocation:

• Sonar (Sound Navigation and Ranging): Used by ships and submarines to map the seabed, locate shipwrecks, schools of fish, and detect other vessels.
• Ultrasonic distance sensors: Used in vehicles for parking assistance and in robotics for obstacle detection.
• Medical ultrasonography: While often referred to as ultrasonography, it is a form of echolocation used to visualize internal organs.

Remember

Echolocation relies on the principle of sound reflection and the time it takes for echoes to return.

Exam-smart

A question might describe an animal (like a bat) or a device (like sonar) and ask you to explain how it navigates or finds objects, referencing the concept of echolocation and sound wave reflection. You might be asked to compare biological echolocation with human-made sonar.