Sound

Sound

• Sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.
• Sound is a longitudinal, mechanical wave. It is caused by the back and forth vibration of the particles of the medium through which the sound wave is moving.
• The vibrations of the object set particles in the surrounding medium in vibrational motion, causing the auditory receptors to detect them. This is called sound.
• Sound can travel through any medium, but it cannot travel through a vacuum. There is no sound in outer space.
• Sounds travel at 340.29 m/second at sea level and 20 degrees celsius.
• Sounds that are higher in frequency than audible sounds are known as ultrasonic sounds. Bats, whales, dolphins, dogs use ultra sound for echo location (to find either their location or to find or locate food, prey, or enemies and danger) and navigation.
• A cat can hear high frequency sounds two octaves higher than humans.
• Elephant communicate in sound waves in lower frequencies that human ears cannot detect.
• The study of sound waves is called acoustics.
• Flies are unable to hear any sounds.

Types of sound?

There are two types of sound, Audible and Inaudible.

• Inaudible sounds are sounds that the human ear cannot detect. The human ear hears frequencies between 20 Hz and 20 KHz.
• Sounds that are below 20 Hz frequency are called Infrasonic Sounds. Elephants use Infrasonic sounds to communicate with herds 100s of kms away.
• Sounds that are above 20 KHz frequency are called Ultrasonic Sounds. Dogs, Bats can hear high frequency sounds.

Production Of Sound:

Sound is a mechanical wave phenomenon and is normally associated with our sense of hearing. Sound is a property of vibrating objects.

• Sound is produced by vibrating sources in a material medium. Medium can be any gas, liquid or solid.
• The vibrating sources set the particles of a medium in vibration in such a way that sound travels outwards in the form of longitudinal waves.
• Some of the energy of the vibrations are transmitted over a distance.

Examples of vibrating sources:

• Musical instruments, like drums, guitar
• Hitting a piece of iron with a hammer
• Loudspeakers (Consists of a cone which vibrates under the effects of electricity and magnetism)
• Explosion resulting from explosives.
• Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear. Do you hear any sound?
• Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends.

TUNING FORK

In the above activities we have produced sound by striking the tuning fork. We set the objects vibrating and produce sound. Vibration means a kind of rapid to and fro motion of an object. The sound of the human voice is produced due to vibrations in the vocal cords. When a bird flaps its wings, do you hear any sound? Think how the buzzing sound accompanying a bee is produced. A stretched rubber band when plucked vibrates and produces sound. If you have never done this, then do it and observe the vibration of the stretched rubber band.

Propagation of Sound:

Vibration in the tuning fork produces disturbances in the surrounding air. When the prongs’ movement is outwards, the prongs push the surrounding air molecules away, creating a local compression.This disturbance of air layers is then passed from molecule to molecule by collisions, causing the local compression to move outwardly.

When the prongs’ movement is inwards, a partial void, or rarefaction is created. Pressure differences causes the air molecules to rush back into the region again. This periodic to-and-fro movement of the prongs will create alternating regions of compressions and rarefactions. The sound waves span outwardly parallel to the direction of the wave propagation (longitudinal nature).

• In air, compressions are regions where the pressure is higher than surrounding air and rarefactions are regions where pressure is lower than the surrounding air.

Notes:

• The energy of the sound waves is propagated and carried by colliding particles of a material medium. Hence, a (material) medium is required in order to transmit these (energy) waves.
• The speed of energy propagation is dependent on the proximity of these particles in a medium. Hence, given that the proximity of particles in the air, liquids or solids is different, the speed of sound differs in air, liquids and solids. Sound travels faster in denser media. It travels faster in liquids than in gases and fastest in solids.

Characteristics of sound wave:

Wavelength, Amplitude, Time-Period, Frequency and Velocity or Speed.

1. Wavelength

• The minimum distance in which a sound wave repeats itself is called its wavelength.
• That is it is the length of one complete wave. It is denoted by a Greek letter λ (lambda).
• We know that in a sound wave, the combined length of a compression and an adjacent rarefaction is called its wavelength.
• Also, the distance between the centres of two consecutive compressions or two consecutive rarefactions is equal to its wavelength.

Note: The distance between the centres of a compression and an adjacent rarefaction is equal to half of its wavelength i.e. λ/2. The S.I unit for measuring wavelength is metre (m).

2. Amplitude

• When a wave passes through a medium, the particles of the medium get displaced temporarily from their original undisturbed positions.
• The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium is called amplitude of the wave.
• In fact the amplitude is used to describe the size of the wave.
• The S.I unit of measurement of amplitude is metre (m) though sometimes it is also measured in centimetres.  Do you know that the amplitude of a wave is the same as the amplitude of the vibrating body producing the wave?

3. Time-Period

• The time required to produce one complete wave or cycle is called time-period of the wave.
• Now, one complete wave is produced by one full vibration of the vibrating body.
• So, we can say that the time taken to complete one vibration is known as time-period.
• It is denoted by letter T. The unit of measurement of time-period is second (s).

4. Frequency

• The number of complete waves or cycles produced in one second is called frequency of the wave.
• Since one complete wave is produced by one full vibration of the vibrating body, so we can say that the number of vibrations per second is called frequency.
• For example: if 10 complete waves or vibrations are produced in one second then the frequency of the waves will be 10 hertz or 10 cycles per second.
• The S.I unit of frequency is hertz or Hz. A vibrating body emitting 1 wave per second is said to have a frequency of 1 hertz. That is 1 Hz is equal to 1 vibration per second. 
• Sometimes a bigger unit of frequency is known as kilohertz (kHz) that is 1 kHz = 1000 Hz. The frequency of a wave is denoted by the letter f.
• The frequency of a wave is the same as the frequency of the vibrating body which produces the wave.

What is the relation between time-period and frequency of a wave?

The time required to produce one complete wave is called time-period of the wave. Suppose the time-period of a wave is T seconds.
In T seconds number of waves produced = 1
So, in 1 second, number of waves produced will be = 1/T
But the number of waves produced in 1 second is called its frequency.
Therefore, F = 1/Time-period
f = 1/T
where f = frequency of the wave
T = time-period of the wave

5. Velocity of Wave (Speed of Wave)

The distance travelled by a wave in one second is called velocity of the wave or speed of the wave. It is represented by the letter v. The S.I unit for measuring the velocity is metres per second (m/s or ms-1).

What is the relationship between Velocity, Frequency and Wavelength of a Wave?

Velocity = Distance travelled/ Time taken
v = f X λ
This formula is known as wave equation.
Where v = velocity of the wave
f = frequency
λ = wavelength
Velocity of a wave = Frequency X Wavelength

This applies to all the waves like transverse waves like water waves, longitudinal waves like sound waves and  the electromagnetic waves like light waves and radio waves

Reflection of sound

In simple terms, the reflection of sound is actually similar to the reflection of light. How? It also abides different laws of reflection, in which the angle of incidence does prove equal to the angle of reflection. In addition, sound rebounds from the surface of either solid or liquid similar to a billiard ball. For successfully experiencing the reflection of sound, it is important that the surface should be polished or rough, and that too of a considerably large size.

The two laws concerned with sound reflection are as follows:

• The incidence angle will always be equal to the reflection angle.
• Moreover, the incident sound waves, the normal at incidence point and reflected wave, all rest in a common plane.

Laws of Reflection of Sound

• The angle of incidence is always equal to the angle of reflection.
• The incident sound wave, the reflected wave and the normal wave at the point of incidence are in the same plane.

Application of Sound Reflection

Echo

It is the sound heard when reflections occur from a firm surface, for example, a wall or cliff. Echo is the repetition of sound even after the source has stopped vibrating. This is used by bats as well as dolphins for the detection of obstacles or navigation. Interestingly, SONAR follows the same principle for sound navigation. In SONAR, ultrasonic sound waves are transmitted in all directions through the ship and the signals received are later examined.

Sound Board

These are basically curved surfaces which are placed in a manner so that the sound source stays at the focus. In a soundboard, the sound waves are uniformly reflected. It can happen in an auditorium or hall, thus improving their quality.

Hearing Aid

A hearing aid is useful for people who face difficulty in hearing. In this device, the sound waves are established and reflected into a slimmer area directed to the ear.

Megaphone

These are horn-shaped tubes which prevent the extension of sound waves through successive reflections. This is achieved through the confinement of sound waves that happen in the tube.

Stethoscope

You might have seen a stethoscope hanging around your doctor’s neck. This device is used to hear sounds generated from internal organs in the human body. A stethoscope functions on the laws of sound reflection. The sound is received through the chest piece and delivered to the earpieces via multiple reflections occurring through a long tube. By listening to the sound with the help of stethoscope, doctors analyze the situation of an organ.

Range Of Hearing

• Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the range of levels.
• The human range is commonly given as 20 to 20,000 Hz, though there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal.
• Sensitivity also varies with frequency, as shown by equal-loudness contours. Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a normal.
• Several animal species are able to hear frequencies well beyond the human hearing range. Some dolphins and bats, for example, can hear frequencies up to 100,000 Hz. Elephants can hear sounds at 14–16 Hz, while some whales can hear infrasonic sounds as low as 7 Hz (in water).

Hearing Ranges Of Different Animals:

Animal Names	Hearing Range in Hertz
Humans	20-20,000
Rat	200-76,000
Rabbit	96-49
Porpoise	75-150,000
Parakeet	200-8,500
Oppossum	500-64,000
Killer Whale	800-13,500
Horse	55-33,500
Harp Seal	950-65,000
Harbor Porpoise	550-105,000
Fur Seal	800-50,000
Elephant	16-12,000
Dog	67-45,000
Chicken	125-2,000
Cat	45-64,000
Bottlenose Dolphin	90-105,000
Beluga Whale	1000-123,000
Bats	2000-110,000

Applications Of Ultrasound

Sound waves with frequencies higher than the upper audible limit of human hearing are called ultrasound.The concept of ultrasound is used in many different fields such as navigation, medicine, imaging, cleaning, mixing, communication, testing etc.

Cleaning:

In objects with parts that are difficult to reach, for example spiral tubes and electronic components, the process of ultrasonic cleaning is used. Here, the object is dipped in a solution of suitable cleaning material and ultrasonic waves are passed into it. As a result of this, high frequency waves are generated that cause the dirt and grease to detach from the surface.

Detection of cracks:

Ultrasound is used to detect cracks in the metallic components that are used in the construction of high rise structures such as buildings and bridges. They generate and display an ultrasonic waveform that is interpreted by a trained operator, often with the aid of analysis software, to locate and categorize flaws in test pieces. High frequency sound waves reflect from flaws in predictable ways, producing distinctive echo patterns that can be displayed and recorded by portable instruments. A trained operator identifies specific echo patterns corresponding to the echo response from good parts and from representative flaws. The echo pattern from a test piece may then be compared to the patterns from these calibration standards to determine its condition.

Echocardiography:

In the process of echocardiography, the ultrasonic waves are used to form an image of the heart using reflection and detection of these waves from various parts.

Lithotripsy:

Ultrasonic waves are used to break stones in the kidney. High-energy sound waves are passed through the body without injuring it and break the stone into small pieces. These small pieces move through the urinary tract and out of the body more easily than a large stone.

Echolocation:

Echolocation is the process where sound waves and echoes are used to determine objects in space. Echolocation is used by bats to navigate and find their food in the dark. Bats send out sound waves from their mouth and nose, which then hit the objects in their vicinity producing echoes, which are then received by the bats. The nature of the echo helps them determine the size, the shape and the distance of the object.

Ultrasonography:

Medical ultrasound is a diagnostic imaging technique based on ultrasound. It is used for the imaging of internal body structures such as muscles, joints and internal organs. Ultrasonic images are known as sonograms. In this process, pulses of ultrasound are sent to the tissue using a probe. The sound echoes off the tissue, where different tissues reflect sound varying in degrees. These echoes are recorded and displayed an image

Structure Of Human Ear

External ear:

External ear is composed of auricle and external auditory canal (meatus).

1. Auricle (pinna)

• Auricle is composed of thin plate of elastic cartilage covered by layer of skin.
• The funnel like curves of auricle collects sound wave and direct them to middle ear.
• The deepest depression called concha which is partly covered by two small projection; tragus in front and antitragus behind.

2. External auditory meatus

• External auditory meatus is slightly curved canal of about 2.5 cm long extending from floor of concha to tympanic membrane (ear drum).
• The meatus is lined with skin continuous with auricle. It contains two glands; sebaceous gland and ceruminous gland.
• Ceruminous gland are modified sweat gland that secretes cerumen (wax).

3. Tympanic membrane:

• It is oval bluish grey membranous structure located on medial part of auditory meatus.
• It separates external and middle ear.
• It is a stretchable organ capable of vibrating.
• It receive sound wave and amplify into appropriate magnitude.

Middle ear:

Middle ear is small chamber between tympanic membrane and inner ear. It consists of tympanic cavity and contains ear ossicles.

1. Tympanic cavity:

• Tympanic cavity is a narrow irregular air filled space in temporal bone. It is separated from external ear by tympanic membrane and medially from inner ear by bony wall.
• It has two opening; oval window and round window.
• In the anterior wall of tympanic cavity is an auditory tube, commonly called as Eustachian tube.

2. Eustachian tube

• Eustachian tube leads downward from tympanic cavity to nasopharynx.
• It is about 4 cm long.
• The mucus membrane lining the nasopharynx is also continuous with membrane of tympanic cavity through eustachiantube. As a result of which infection from nose or throat may spread to middle ear causing Otitis media.
• The main purpose of Eustachian tube is to maintain equal air pressure on both side of tympanic membrane by permitting air to pass from nasal cavity to middle ear.

3. Ear ossicles:

• The three ear ossicles (malleus, incus and stapes) form a chain of lever extending from tympanic membrane to inner ear.
• The ear ossicles transmit sound wave from ear drum to inner ear.
• Ear ossicles communicate the ear drum with internal ear through fenestra ovalis ( oval window).
• The ear ossicles are:

Malleus: it is hammer shaped bone whose handle is in contact with tympanic membrane and the head form movable joint with incus

Incus: it is middle anvil shaped bone.

Stapes: it is medial stirrup shaped bone. It head articulate with incus and foot plate fits into oval window. Stapes is the smallest bone in human body.

Inner ear:

The inner ear is also called as labyrinth because of its intricate structure of interconnecting chamber and passage.

It consists of two main structural parts one inside the other.

1. Bony labyrinth
2. Membranous labyrinth

1. Bony labyrinth:

• It is a series of hollow channels. It is filled with perilymph.
• Bony labyrinth consists of vestibule, three semicircular canal and spirally coiled cochlea.

2. Membranous labyrinth:

• It is surrounded by bony labyrinth. It is filled with endolymph and also contains the sensory receptors for hearing and equilibrium.
• Membranous labyrinth consists of three semi-circular ducts as well as utricle, saccule and cochlear duct, all are filled with endolymph enclosed by bony labyrinth.
• It also contains sensory receptors (cristae, ampullaris maculae and organ of corti).
• Semi-circular dusts are located within semicircular canal of bony labyrinth.
• Perilymph is located in space between duct and bony wall of semicircular canal.

Vestibule:

• Vestibule is the expanded part nearest the middle ear.
• It has two sacs; larger upper utriculus and smaller lower sacculus.
• Utriculus and sacculus is connected by utriculosaccular duct.
• The sensory sport (macula) is present in both utriculus and sacculus.
• The macula consists of otolith membrane having otolith (small crystal of CaCO3) which concerned with balancing of body.

Semicircular canal:

• It is associated with equilibrium or balancing not for hearing.
• There are three semicircular canals arises from utriculus; anterior, posterior and lateral canals
• The anterior and posterior canals opens at one end to form common duct called crus commune. One end of each semicircular canal is swollen to form ampulla.

Cochlea:

• It is spiral shaped resembling snail’s shell, wounded 2 ¾ times.
• It is the main hearing organ.
• It is connected with cerebrum by vestibulo-cochlear nerve.
• Cochlea is divided into 3 spiral fluid filled chamber.

i. Scala vestibuli:

• It communicate with vestibule.
• It contains perilymph

ii. Scala tympani:

• It ends at round window of tympanic cavity.
• It contains perilymph.

iii. Scala media or cochlear duct:

• It lies between scala vestibuli and scala tympani.
• It contains endolymph.
• Scala media or cochlear duct is separated from scala vestibuli by vestibular membrane and from scala tympani by basilar membrane.
• The basilar membrane has organ of corti formed about 24000 receptor auditory cells.

Organ of corti:

• It is the organ for hearing which is rested on basilar membrane.
• The organ of corti present within scala media of cochlea that receive and conduct sound stimulus.
• Organ of corti is an organized structure consisting of hair cells and supporting cells.
• Hair cells are arranged in rows along the length. The outer hair cells are arranged in three rows and inner hair cells are arranged in single row.
• Each outer and inner hair cell have sensory hair which are specialized microvilli.

Questions And Answers:

1. What type of waves are Sound Waves?

A. Latitudinal waves
B. Longitudinal waves
C. Latitudinal mechanical waves
D. Longitudinal waves
Ans. D
Sound Waves are longitudinal mechanical waves.

2. Which of the following is/ are not applications of Ultrasonic Waves?

(a) For measuring the depth of Sea.
(b) In sterilizing of a liquid.
(c) In Ultrasonography
(d) In sterilizing a needle.
Options are:
A. Both (a) and (b) Only (b)
B. Only (d)
C. Both (c) and (d)
D. Only (b)
Ans. B
Applications of Ultrasonic Waves are: sending signals, for measuring the depth of sea, for cleaning cloths, aeroplanes, machinery parts of clocks, for removing lamp-shoot from the chimney of factories, in sterilizing of liquid and in Ultrasonography.

3. What is the speed of sound in air?

A. 330 m/s
B. 332 m/s
C. 334 m/s
D. 336 m/s
Ans. B
The speed of Sound in Air (0°C) is 332 m/s and in Air (20°C) is 343 m/s.

4. What will be the effect of temperature on speed of sound?

A. The speed of sound decreases with the increases of temperature of the medium.
B. The speed of sound decreases with the decrease of temperature of the medium.
C. The speed of sound increases with the decrease of temperature of the medium.
D. The speed of sound increases with the increase of temperature of the medium.
Ans. D

5. Due to which phenomena sound is heard at longer distances in nights than in day?

A. Reflection
B. Refraction
C. Interference of sound
D. Diffraction of sound
Ans. B
Due to refraction, sound is heard at longer distances in nights than in day.