Sound

This article is about acoustic waves. For other uses, see Sound (disambiguation).
Sound is a disturbance of mechanical energy that propagates through matter as a wave. Sound is characterized by the properties of waves, which are frequency, wavelength, period, amplitude, and speed.

Humans perceive sound by the sense of hearing. By sound, we commonly mean the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider definition of sound that includes low and high frequency vibrations in air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases, liquids, solids, and plasmas.

The matter that supports the sound is called the medium. Sound propagates as waves of alternating pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate. The scientific study of sound is called acoustics.

Noise is often used to refer to an unwanted sound. In science and engineering, noise is an undesirable component that obscures a wanted signal.


Perception of Sound

A schematic representation of hearing. (Blue: sound waves. Red: eardrum. Yellow: cochlea. Green: auditory receptor cells. Purple: frequency spectrum of hearing response. Orange: nerve impulse)Sound is perceived through the sense of hearing. Humans and many animals use their ears to hear sound, but loud sounds and low-frequency sounds can be perceived by other parts of the body through the sense of touch as vibrations. Sounds are used in several ways, notably for communication through speech and music. They can also be used to acquire information about properties of the surrounding environment such as spatial characteristics and presence of other animals or objects. For example, bats use echolocation, ships and submarines use sonar and humans can determine spatial information by the way in which they perceive sounds.

Humans can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range) although this range varies significantly with age, occupational hearing damage, and gender; the majority of people can no longer hear 20,000 Hz by the time they are teenagers, and progressively lose the ability to hear higher frequencies as they get older. Most human speech communication takes place between 200 and 8,000 Hz and the human ear is most sensitive to frequencies around 1000-3,500 Hz. Sound above the hearing range is known as ultrasound, and that below the hearing range as infrasound.

The amplitude of a sound wave is specified in terms of its pressure. The human ear can detect sounds with a very wide range of amplitudes and so a logarithmic decibel amplitude scale is used. The quietest sounds that humans can hear have an amplitude of approximately 20 µPa (micropascals) or a sound pressure level (SPL) of 0 dB re 20 µPa (often incorrectly abbreviated as 0 dB SPL). Prolonged exposure to a sound pressure level exceeding 85 dB can permanently damage the ear, resulting in tinnitus and hearing impairment. Sound levels in excess of 130 dB are more than the human ear can safely withstand and can result in serious pain and permanent damage. At very high amplitudes, sound waves exhibit nonlinear effects, including shock.

The way in which sound travels or propagates is difficult to imagine, as sound appears to humans as invisible. Imagine a long tube exposed to air whereby sound travels longitudinally through it. The air acts like a Slinky spring in this tube. As sound is generated at one end, the wave will begin to travel down through the air in the tube, (watching an earth worm move by pulsating its long body on the top of the ground helps to visualize this same phenomenon). The length of pulse cycle will determine the sound wave length. Low bass sounds will have large pulse lengths, in the order of 10-50 feet long, where high treble sounds will have pulse lengths as small as 1/2 an inch.


Speed of Sound

Main article: Speed of sound
The speed at which sound travels depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium and its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in air and other gases depends on temperature. In air, the speed of sound is approximately 344 m/s, in water 1500 m/s and in a bar of steel 5000 m/s. The speed of sound is also slightly sensitive (to second order) to the sound amplitude, resulting in nonlinear propagation effects, such as the weak production of harmonics and the mixing of tones. (see parametric array).


Sound Pressure

Main article: Sound pressure
Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure averaged over a given interval of time. In a soundwave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity in time.

The loudest sound ever historically reported was the 1883 volcanic eruption of Krakatoa whereby sound levels reached levels of 180 dBSPL 100 miles (160 km) away.

Sound Pressure Level

As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale.

The sound pressure level (SPL) or Lp is defined as


where p is the root-mean-square sound pressure and p0 is a reference sound pressure. (When using sound pressure levels, it may be important to quote the reference sound pressure used.) Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water.
Since the human ear does not have a flat spectral response, sound pressure levels are often frequency weighted so that the measured level will match perceived levels more closely. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to noise and A-weighted sound pressure levels are labeled dBA. C-weighting is used to measure peak levels.


Examples of sound pressure and sound pressure levels


Source of sound sound pressure sound pressure level
pascal dB re 20 µPa
threshold of pain 100 134
hearing damage during short-term effect 20 approx. 120
jet engine, 100 m distant 6–200 110–140
jack hammer, 1 m distant / discotheque 2 approx. 100
hearing damage during long-term effect 0.6 approx. 90
major road, 10 m distant 0.2–0.6 80–90
passenger car, 10 m distant 0.02–0.2 60–80
TV set at home level, 1 m distant 0.02 ca. 60
normal talking, 1 m distant 0.002–0.02 40–60
very calm room 0.0002–0.0006 20–30
leaves noise, calm breathing 0.00006 10
auditory threshold at 2 kHz 0.00002 0


Equipment for dealing with sound
Equipment for generating or using sound includes musical instruments, hearing aids, sonar systems and sound reproduction and broadcasting equipment. Many of these use electro-acoustic transducers such as microphones and loudspeakers.

Intensity

In physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light).

In physics, the word "intensity" is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match.

If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity decreases in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies. The consequence of this is that the net power coming from the source must be constant, thus:


where P is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. That P is a constant. If the source is radiating uniformly, i.e. the same in all directions, and we take A to be a sphere centered on the source (so that I will be constant on its surface), the equation becomes:


where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (4πr2 is the expression for the surface area of a sphere). Solving for I, we get:


If the medium is damped, then the intensity drops off more quickly than the above equation suggests.

Anything that can carry energy can have an intensity associated with it. For an electromagnetic wave, if E is the complex amplitude of the electric field, then the power carried by the wave is given by

,
and the intensity is obtained multiplying this expression by the velocity of the wave, c / n:

,
where n is the refractive index, c is the speed of light in vacuum and ε0 is the electric permittivity in vacuum.

Photometry and Radiometry

In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists.

Comment #83578347956485830

Hi! there, here I am again. Well as of now I am happy coz I passed the tests that we have took in our Physics Class but I am not acting as a person who makes some easy easy matters then I should take care of my scores as I am about to struggle different lessons in Physics. Maybe I should balance my scores, I mean I should not take them for granted.

Steps in Component Method

1. Draw each vector and show its components.
2. Determine the magnitude and direction of the components by using the trigonometric functions.
3. Find the sum of the x-components.
4. Find the sum of the y-components.
5. the answers obtained in steps 3 and 4 are the x and y components of the resultant vector. Use these components to find the magnitude and the direction of the resultant vector, using the Pythagorean Theorem and the Trigonometric Functions.
6. Check your answer by comparing it with the result obtained in Graphical Method (optional).
   

Steps in Graphical Method

1. First, choose an appropriate scale and coordinate system for the given vectors.
2. Draw the first vector starting from the origin of the coordinate system. Draw the second vector starting from the head of the first vector. Proceed to draw the remaining vectors starting from the head of the most recent vector drawn. All vectors must be connected in series, head-to-tail fashion.
3. Draw a new vector connecting the tail of the first to the head of the last vector drawn. the new vector is the resultant of the given vectors.
   

Resultant Vector

Scalar Quantities can be addedand subtracted like ordinary numbers provided the scalars have the same units. For vectors, the sum depends on the direction of the vectors. The sum of two or more vectors is represented by a single vector called the Resultant.

Common Prefixes of the Metric System


Prefix, Symbol, Factor

milli, m, 0.001
centi, c, 0.01
deci, d, 0.1
deka, da, 10
hecto, h, 100
kilo, k, 1000
mega, m, 1000000
giga, g, 1000000000