NATURE OF SOUND,HUMAN EAR AND Quantification of sound in terms of SPL and PWL

NATURE OF SOUND

Simply it is sound pollution. The unwanted sound during any purpose is noise pollution. The sound become unwanted when it interfere or disturb the normal human activities and wildlife.

The sound is one kind of longitudinal wave in which the particles oscillate simple harmonic motion in the same direction of the wave propagation. Sound waves can't be transmitted through the vacuum. The transmission of sound requires a medium that can be solid, liquid or gas etc. it is a variation in pressure. The region of increased pressure of sound wave is called a compression. The region of the decreased pressure sound wave is called a rarefaction.

The sources of sound are

• Solids Vibrating
• Rapid expansion or compression

Laminar air flow around blunt obstacles may the result in the formation of vortices (the plural of vortex) that snap off with a characteristic frequency. This process is called vortex shedding and is another means by which the sound waves are formed. This is how a whistle or flute produces sound.

The term involves in sound are amplitude, loudness, wavelength, frequency, pitch, intensity, tone. Humans are generally capable of hearing sounds between 20 Hz and 20 kHz. Sounds with frequencies above this range of hearing of human are called ultrasound. Sounds of frequencies below this range of human hearing are called infrasound.

HUMAN EAR

The human ear is very highly sensitive to sound receptor in which pressure fluctuations in the outer ear is transformed into the vibrations of small bones (the ossicles) in the middle ear that are ultimately communicated to the cochlea which is located in the inner ear where the vibrations are further transformed by stereocilia(hair cells) into neural impulses distributed by frequency. The human ear consists of three parts and they are,

Outer ear

The outer ear is an external portion of the ear and it includes the fleshy visible pinna (also called the auricle), the ear canal, and the outer layer of the eardrum.

The pinna consists of curving outer rim called the helix, the inner curved rim is called the antihelix and open into the ear canal.The tragus protrudes and partially obscures to the ear canal as does the facing to antitragus.The empty region in front of the ear canal is called the concha. The ear of canal stretched for about 2.5cm. The ist part of the canal is surrounded by cartilage while the 2nd part near the eardrum is surrounded by bone. The bony part is known as the auditory bulla and is formed by the most tympanic part of temporal bone. The skin which is surrounding the ear canal contains ceruminous and sebaceous glands that produce protective ear wax. The ear which canal ends at the external surface of the eardrum.

The two sets of muscles that associated with outer ear: In some mammals, these muscles can adjust the direction of the pinna. In humans, these muscles have little effect. The ear muscles are supplied by the most facial nerve which supplied sensation to the skin of ear itself as well as to external ear cavity. The greatest auricular nerve, auricular nerve, auriculotemporal nerve lesser and greater occipital nerves of cervical plexus of all supply sensation to the parts of the outer ear and surrounding skin.

The pinna consists of the single piece of the elastic cartilage with a complicated relief on an inner surface and a smooth configuration on its posterior surface. The earlobe consists of an areola and adipose tissue.The symmetrical arrangement of two ears allows for localization of sound. The brain accomplishes this comparing arrival times and intensities from each ear in circuits located in superior of olivary complex and the trapezoid bodies which are connected through pathways to both ears.

Middle ear

The middle ear is lies between the outer ear and inner ear. It consists of an air-filled cavity called tympanic cavity and it includes the three ossicles with their attaching ligaments.The auditory tube and round oval windows. The ossicles are three small bones that function is together to receive, amplify and transmit the sound from the eardrum to inner ear. The ossicles are the malleus, incus and the stapes (stirrup). The stapes is smallest named bone in the body. The middle ear connects to the upper throat at nasopharynx through the pharyngeal opening of the Eustachian tube.

Inner ear

The inner ear sits within the temporal bone in a complex cavity called bony labyrinth. A central area known as vestibule contains two small fluid-filled recesses, utricle, and saccule. These connect to semicircular canals and cochlea. There are the three semicircular canals angled at the right angled to each other which are responsible for dynamic balances. The cochlea is the spiral shell-shaped organ for the sense of hearing. These are the structures together create the membranous labyrinth.

FUNCTION

HEARING

Sound waves are travel through the outer ear are modulated by the middle ear and are transmitted to a vestibulocochlear nerve in the inner ear. This nerve transmits information to the temporal nerve of the brain, where it is registered as sound

BALANCE

The ear facilitates two types of balance: static balance which allows a person to feel effects of the gravity and the dynamic balance which allows a person to sense acceleration.

Quantification of sound in terms of SPL and PWL

Sound pressure level (SPL)

It is the pressure level of sound in decibel with respect to the reference pressure of sound. The range of sound pressures that can be heard by the human ear is very large. The minimum sound pressure for a healthy person is approximately 20 x 10-6 Pa. The minimum audible level range occurs at about 4,000 Hz and physical limit imposed by molecular motion. Lower sound pressure levels would be altered by thermal noise due to molecular motion in the air.

The level of sound pressure p is then said to be Lp decibels(dB) i.e.greater or less than reference sound pressure Pref according to the following equation given by,

Lp= 10log (P/Pref)

Sound power level (PWL)

Sound power level (PWL) or acoustic power level is a logarithmic measure of the power of sound relative to a reference value. Sound power level is denoted Lw and measured in dB, is defined by

Lw = 10 log [(sound power)/ (reference power)] (dB)

The internationally agreed reference power is 10-12 W. Again, the following convenient form which is obtained when reference sound power is introduced into the above equation,

Lw =10 logW + 120 (dB)

Thus SPL = SWL + 10 log (Q/4*pi*r2) where Q shape parameter

References:
1. Mackenzie L. Davis & David A. Cornwell, “Introduction to Environmental Engineering”, McGraw Hill.
2. Gilbert M. Masters, Standford University, “Introduction to Environmental Engineering and Science”, Printice Hall.
3. Stephan Konz, Kansas State University, “Work design”, Grid Publishing Inc., Colombus, Ohio
4. C. S. Rao, “Environmental Pollution Control Engineering”, New age International (P) Limited, Publishers, India.