Sound

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SOUND

What is sound?Sound is a  mechanical wave that is

an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.

Sound is a physical entity, unlike noise which is the perception of sound. Sound is an high pressure wave front through a medium (such as air, or a liquid such as water, or a solid), wherein the wave front is followed by a reciprocal pressure differential behind it. Sound is produced when something disturbs the medium and sets in motion the molecules in the medium.

How sound is produced in humans ?

In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the voice box. It is at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air.

When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.

What is vibration?Fro or back and forth motion of an object is called vibration. Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic  such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

Vibration is occasionally "desirable". For example the motion of a tuning fork, the reed in a woodwind instrument or harmonica, or the cone of a loudspeaker is desirable vibration, necessary for the correct functioning of the various devices.

WHAT IS HERTZ(HZ)? The hertz is equivalent to cycles per second. In

defining the second the CIPM declared that "the standard to be employed is the transition between the hyperfine levels F = 4, M =  0 and F = 3, M = 0 of the ground state 2S1/2 of the cesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9 192 631 770 hertz" thereby effectively defining the hertz and the second simultaneously.

In English, hertz is used as a plural. As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz, 106 Hz), GHz (gigahertz, 109 Hz) and THz (terahertz, 1012 Hz). One hertz simply means "one cycle per second" (typically that which is being counted is a complete cycle); 100 Hz means "one hundred cycles per second", and so on.

The unit may be applied to any periodic event—for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz. The "frequency" (activity) of a periodic stochastic events, such as radioactive decay, is expressed in Becquerel. 

Even though angular velocity, angular frequency and hertz all have the dimensions of 1/s, angular velocity and angular frequency are not expressed in hertz,[5] but rather in an appropriate angular unit such as radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π rad/s or1 Hz, where the former measures the angular velocity and latter reflects the number of complete revolutions per second.

When an SI unit is spelled out in English, it should always begin with a lower case letter (hertz), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

Sound pressure level

Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. A square of this difference (i.e., a square of the deviation from the equilibrium pressure) is usually averaged over time and/or space, and a square root of this average provides a root mean square (RMS) value.

For example, 1 Pa RMS sound pressure (94 dBSPL) in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm  Pa) and (1 atm  Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny (relative to atmospheric) variation in air pressure at an audio frequency is perceived as a deafening sound, and can cause hearing damage, according to the table below.

As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or Lpis defined as

where p is the root-mean-square sound pressure and pref is a reference sound pressure. Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20µPa in air and 1 µPa in water. Without a specified reference sound pressure, a value expressed in decibels cannot represent a sound pressure level.

HOW DOES SOUND WORK?

Anything that moves back and forth makes sound. Moving back and forth is called vibrating. Pluck a guitar string and watch it vibrate back and forth. The vibrations make sound waves.

Sound waves are a lot like water waves. If you throw a pebble in a lake or pond, you can see the waves move out in circles. Sound waves move out in circles from whatever is vibrating.

Most sound waves you hear travel through air. Sound waves can also travel through water and even through solid things. You can hear your own voice because your solid skull bones vibrate.

Your ears pick up the sound waves. Your ear has three main parts that let you hear. The parts are called the outer ear, middle ear, and inner ear.

How the Larynx Works

When a person inhales and exhales, air quietly goes through the larynx. The larynx does not make noise unless the vocal cords vibrate. When a person inhales and exhales, air quietly goes through the larynx. The larynx does not make noise unless the vocal cords vibrate.

Vocal cords are muscles that are located inside the larynx.

The larynx has two vocal cords.

When a person speaks or makes a noise, the vocal cords vibrate as the person exhales and thus makes a noise.

The longer the vocal stretch, the more different the noise sounds. A person can control the pitch of their voice, by making the vocal cords stretch at different lengths.

A person does not conciously think about stretching his vocal cords when he speaks. Controlling vocal cords and changing the pitch of one's voice is learned as an infant. The noise made by the larynx combined with the shape of a persons mouth gives each person a unique sounding voice. The way a person moves her mouth, tongue and lips, changes the way the noise from her larynx will sound.

EAR

THE OUTER EAR

Your outer ear is the part that sticks out on the side of your head. The outer ear collects sound waves. The sound waves travel down a tube to your eardrum. Your eardrum is a thin layer that separates the outer ear from the middle ear. The sound waves make your eardrum vibrate.

THE MIDDLE EAR

Your middle ear is inside your head on the other side of your eardrum. Your middle ear has three tiny bones. Your vibrating eardrum makes the bones move. The bones carry the vibrations to your inner ear.

Your middle ear is filled with air. A tube connects your middle ear with your nose and throat. This tube helps keep too much air from building up inside your ear. The tube makes your ears pop in an elevator or airplane.

THE INNER EARYour inner ear has many parts and tubes that twist and turn. A part called the cochlea is very important for hearing. The cochlea looks like a snail shell. It is filled with fluid and tiny hairs. The moving bones in your middle ear make the fluid and tiny hairs in the cochlea move.The tiny hairs link to nerves. The nerves carry signals to your brain. Your brain tells you what the sound is. It tells whether you are hearing a train whistle or a bird singing.

VOICE BOX IN HUMANS.

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