Reflection and Applications of Sound

Reflection Of Sound

When sound waves strike a surface, they can be reflected, just like light waves. The phenomenon of sound reflection follows the same laws as the reflection of light: the angle of incidence is equal to the angle of reflection, and the incident wave, the reflected wave, and the normal to the surface at the point of incidence all lie in the same plane.

Hard, smooth surfaces are good reflectors of sound, while soft, rough surfaces are poor reflectors and tend to absorb sound.

Reflection of sound gives rise to several interesting phenomena and applications.

Echo

An echo is the repetition of sound caused by the reflection of sound waves from a hard surface, such as a wall, cliff, or building. When a sound is produced, it travels outwards. If it encounters a reflecting surface at a sufficient distance, the reflected sound wave returns to the listener after the original sound has stopped.

Conditions for Hearing a Distinct Echo

For a distinct echo to be heard, the reflected sound wave must reach the listener at least 0.1 seconds after the original sound is heard. This is because the human ear's sensation of sound persists for about 0.1 seconds (persistence of hearing). If the reflected sound arrives within this time, it merges with the original sound. If it arrives after 0.1 seconds, it is perceived as a separate sound.

Let $v$ be the speed of sound in air, and $d$ be the distance between the source of sound (or listener, if at the same point) and the reflecting surface. The sound travels to the reflector and back, covering a total distance of $2d$. The time taken is $\Delta t = 2d/v$.

For a distinct echo, $\Delta t \ge 0.1$ seconds.

$ \frac{2d}{v} \ge 0.1 \implies d \ge \frac{0.1 \times v}{2} = \frac{v}{20} $

Taking the speed of sound in air at 20°C as approximately 343 m/s:

$ d \ge \frac{343 \text{ m/s}}{20} \approx 17.15 \text{ m} $

So, the minimum distance from the sound source to the reflector required to hear a distinct echo is approximately 17.15 metres.

In some cases, multiple echoes can be heard if there are several reflecting surfaces or if the sound reflects back and forth between two parallel surfaces. This is often observed in large empty halls or canyons.

Reverberation

Reverberation is the phenomenon of persistence of sound in an enclosed space due to multiple reflections from the walls, ceiling, floor, and other surfaces. Unlike a distinct echo where the reflected sound is heard after the original sound has stopped, in reverberation, the reflected sounds arrive quickly and mix with the original sound, prolonging the sound sensation.

Reverberation makes sound appear to linger in a room. In a large hall with hard, reflective surfaces, reverberation can be excessive, making speech unclear (muddling successive syllables) and music indistinct. In smaller rooms or rooms with sound-absorbing materials, reverberation is minimal.

The time it takes for the sound intensity to decay by a certain amount after the source stops is called the reverberation time. Controlling reverberation time is crucial for good acoustics in concert halls, auditoriums, and recording studios. Too much reverberation degrades clarity; too little can make the space sound 'dead'. Acousticians use sound-absorbing materials (like curtains, carpets, acoustic panels) and careful architectural design to control reverberation.

Uses Of Multiple Reflection Of Sound

While excessive reverberation can be problematic, controlled multiple reflections of sound are deliberately used in several applications:

1. Megaphones and Loudhailers: These devices are designed as conical tubes that reflect sound waves from the source (mouthpiece) outwards, concentrating the sound energy and directing it in a specific direction. This makes the sound travel farther and be heard more clearly.
2. Stethoscopes: Used by doctors to listen to internal body sounds (heartbeat, breathing). The sound from the chest piece travels through the tube to the ear pieces via multiple reflections off the walls of the tube. The narrow tube prevents the sound waves from spreading out, preserving their intensity.
3. Soundboards: Used in musical instruments like pianos, violins, and guitars. The vibrations of the strings are transferred to the soundboard, which vibrates and causes the surrounding air to vibrate. The shape of the soundboard reflects and amplifies the sound, radiating it effectively into the air.
4. Concert Halls and Auditoriums: Architectural design of these spaces, including the shape of walls, ceilings, and the placement of reflective and absorbing materials, is carefully planned to control reflections. This helps to distribute sound evenly throughout the hall and achieve an optimal reverberation time for clear listening (speech) or rich sound (music). Curved ceilings and walls, or angled surfaces, are used to direct sound towards the audience.

These examples demonstrate how the principle of sound reflection, especially multiple reflections, can be applied to amplify, direct, or distribute sound in useful ways.

Range Of Hearing

The human ear can perceive sound waves within a certain range of frequencies. This range is called the audible range.

Audible Range for Humans

The average range of hearing for healthy young adults is approximately from 20 Hertz (Hz) to 20,000 Hertz (Hz) or 20 kHz. Sounds with frequencies within this range are called audible sound or sonic sound.

• Sounds with frequencies below 20 Hz are called infrasound. Humans cannot hear infrasound, but some animals (like elephants, rhinoceroses, whales) can produce and perceive infrasound and use it for communication over long distances. Sources of infrasound include earthquakes, volcanoes, and large storms.
• Sounds with frequencies above 20 kHz are called ultrasound. Humans cannot hear ultrasound. However, many animals (like bats, dolphins, dogs, cats, mosquitoes) can produce and perceive ultrasound and use it for communication, navigation (echolocation), or hunting.

The upper limit of human hearing tends to decrease with age. Children can often hear frequencies up to 25 kHz or higher, while older adults may have difficulty hearing frequencies above 10-12 kHz.

The sensitivity of the human ear also varies with frequency; the ear is most sensitive to frequencies in the range of 2 kHz to 5 kHz, which is important for understanding human speech.

Applications Of Ultrasound

Although humans cannot hear ultrasound, these high-frequency sound waves have properties that make them extremely useful in various fields, including medicine, industry, and navigation. Ultrasound waves have shorter wavelengths than audible sound, which allows them to penetrate materials and resolve smaller details.

Properties of Ultrasound

• High frequency ($> 20$ kHz).
• Short wavelength (for a given speed, $\lambda = v/\nu$).
• Can travel as a directed beam (less diffraction compared to low frequencies).
• Can penetrate certain materials.
• Can be reflected from surfaces and boundaries.
• Carries high energy.

Uses of Ultrasound

1. Medical Imaging (Ultrasonography): Ultrasound is widely used in medical diagnosis. Ultrasound waves are sent into the body, and they reflect off different tissues and organs. The reflected waves (echoes) are detected and processed by a computer to create images of internal structures. This technique is safe (non-ionizing radiation, unlike X-rays) and commonly used for imaging fetuses during pregnancy, abdominal organs, heart, etc.
2. Medical Therapy: High-intensity focused ultrasound (HIFU) is used to destroy tissues (like tumours or kidney stones) without invasive surgery. Ultrasound can also be used for therapeutic heating of tissues.
3. Industrial Applications:
   • Flaw Detection: Ultrasound is used to detect cracks, flaws, or cavities in metal blocks or components. Ultrasound waves are sent into the metal, and if there is a flaw, the waves reflect back, indicating the defect's presence and location. This is crucial for quality control in manufacturing.
   • Cleaning: Ultrasound is used for cleaning intricate objects (like electronic components, jewellery, surgical instruments). The high-frequency vibrations in the cleaning fluid dislodge dirt and grease effectively.
   • Welding: Ultrasonic welding uses high-frequency vibrations to create friction and heat, joining plastic or metal parts.
4. Animal Communication and Navigation: Bats and dolphins use echolocation (sonar using ultrasound) for navigation and finding prey in darkness or murky water.

Sonar

SONAR stands for Sound Navigation And Ranging. It is a technique that uses sound waves (often ultrasound) to navigate, communicate with, or detect objects underwater. It works on the principle of echo reflection.

Working Principle of SONAR

(Image Placeholder: A ship on the water surface. Show a SONAR device below the ship. An outgoing sound pulse is emitted downwards. Show this pulse travelling through water. Show the pulse hitting an object (like a submerged submarine or the seabed). Show the reflected sound pulse (echo) travelling back upwards to the receiver on the ship. Indicate the time taken for the sound to travel down and back.)

A SONAR device consists of a transmitter and a receiver, usually mounted on a ship or submarine. The transmitter emits short pulses of sound waves (often ultrasonic waves due to their directionality and ability to travel well in water). These pulses travel through the water. If they encounter an object underwater (like a submarine, shipwreck, fish school, or the seabed), they are reflected back as echoes.

The receiver detects these echoes. By measuring the time interval ($\Delta t$) between the transmission of the pulse and the reception of the echo, and knowing the speed of sound in water ($v_{water}$), the distance ($d$) to the object can be calculated:

Distance travelled by sound = $2d$ (down and back)

$ 2d = v_{water} \times \Delta t $

$ d = \frac{v_{water} \times \Delta t}{2} $

The direction of the echo can also be used to determine the direction of the object. By scanning in different directions, a "sound map" of the underwater environment can be created.

Applications of SONAR

• Underwater Navigation: Determining depth (echo sounders), mapping the seabed (bathymetry).
• Object Detection: Locating submarines, shipwrecks, icebergs, fish schools.
• Communication: Used for underwater communication between vessels.
• Scientific Research: Studying marine life and the ocean floor.

The speed of sound in water is much higher than in air (approx. 1500 m/s). Factors like temperature, salinity, and pressure affect the speed of sound in water, and these variations must be accounted for in accurate SONAR measurements.

Structure Of Human Ear

The human ear is a remarkable and complex organ that detects sound waves from the environment and converts them into electrical signals that are sent to the brain, where they are interpreted as sound. The ear is divided into three main parts: the outer ear, the middle ear, and the inner ear.

(Image Placeholder: A diagram showing the cross-section of a human ear, labelling the main parts and key components: Pinna, Ear canal, Eardrum (tympanic membrane), Malleus, Incus, Stapes (ossicles), Oval window, Cochlea, Semicircular canals, Auditory nerve, Eustachian tube.)

Outer Ear

The outer ear consists of the pinna (the visible outer part of the ear) and the ear canal (auditory canal).

• Pinna: Funnel-shaped structure made of cartilage. It collects sound waves from the surroundings and directs them into the ear canal. Its shape also helps in locating the direction of sound.
• Ear Canal: A tube about 2.5 cm long that leads from the pinna to the eardrum. It channels the sound waves inwards and also protects the eardrum. It produces earwax, which traps dust and microorganisms.

Middle Ear

The middle ear is a small, air-filled cavity separated from the outer ear by the eardrum. It contains three tiny bones (ossicles) and is connected to the nasal cavity by the Eustachian tube.

• Eardrum (Tympanic Membrane): A thin, stretched membrane that vibrates when sound waves strike it. These vibrations mimic the pressure variations of the sound waves. It forms the boundary between the outer and middle ear.
• Ossicles: Three small bones that form a chain connecting the eardrum to the oval window (opening to the inner ear). They are the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus is attached to the eardrum, the incus is in the middle, and the stapes is attached to the oval window. These bones amplify and transmit the vibrations from the larger area of the eardrum to the smaller area of the oval window, increasing the pressure amplitude.
• Eustachian Tube: Connects the middle ear to the nasopharynx (upper part of the throat and nasal cavity). It helps to equalize the pressure on both sides of the eardrum, which is important for optimal hearing and preventing discomfort during changes in altitude or pressure.

Inner Ear

The inner ear (also called the labyrinth) is a complex structure embedded within the temporal bone of the skull. It contains the cochlea (responsible for hearing) and the semicircular canals (responsible for balance).

• Cochlea: A spiral-shaped, fluid-filled structure resembling a snail shell. It is the primary organ of hearing. Vibrations from the stapes are transmitted to the fluid inside the cochlea via the oval window. These vibrations create waves in the fluid, which stimulate thousands of tiny hair cells (sensory receptors) along the basilar membrane within the cochlea. Different frequencies of sound stimulate hair cells at different locations along the membrane.
• Auditory Nerve: The hair cells convert the mechanical vibrations into electrical signals (nerve impulses). These impulses are transmitted to the brain via the auditory nerve, where they are interpreted as sound of a specific pitch, loudness, and quality.
• Semicircular Canals: Three fluid-filled loops oriented in different planes. They are part of the vestibular system and are responsible for sensing head rotation and maintaining balance. (They are not involved in hearing).
• Oval Window: A membrane-covered opening connecting the stapes of the middle ear to the cochlea of the inner ear. It transmits vibrations from the ossicles to the inner ear fluid.
• Round Window: Another membrane-covered opening below the oval window, which allows the fluid in the cochlea to move in response to the vibrations transmitted through the oval window.

The process of hearing involves the conversion of sound waves in air into mechanical vibrations in the middle ear, and then into fluid waves and electrical signals in the inner ear, which are finally processed by the brain.