Public Addressing System
A public address system (PA system) is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, and distributing the sound throughout a venue or building.
Simple PA systems are often used in small venues such as school auditoriums, mosque , PA systems with a larger number of speakers are widely used in institutional and commercial buildings, to read announcements or declare states of emergency.
Intercom systems, which are often used in schools, also have microphones in each room so that the occupants can reply to the central office.

There is disagreement over when to call these audio systems sound reinforcement systems or PA systems. Some audio engineers distinguish between the two by technology and capability, while others distinguish by intended use, e.g., sound reinforcement systems are for live music, whereas PA systems are for reproduction of speech and recorded music in buildings and institutions. This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable.

Small systems
The simplest PA systems consist of a microphone, a modestly-powered mixer amplifier and one or more loudspeakers. Simple PA systems of this type, often providing 50 to 200 watts of power, are often used in small venues such as school auditoriums, churches, and small bars. A sound source such as radio may be connected to a PA system so that music can be played through the system.
Control equipment monitors the amplifiers and speaker lines for faults before it reaches the loudspeakers. This control equipment is also used for separating zones in a PA system. The loudspeaker is used to transduce electrical signals into analog sound signals.

Large systems
Some PA systems have speakers that cover an entire campus of a college or industrial site, or an entire outdoor complex (e.g., an athletic stadium). More than often this PA system will be used as voice alarm system that make announcement during emergency to evacuate the occupants in a building.

Acoustic feedback
All PA systems have a potential for audio feedback, which occurs when sound from the speakers returns to the microphone and is then re-amplified and sent through the speakers again. Sound engineers take several steps to prevent feedback, including ensuring that directional microphones are not pointed towards speakers, keeping the onstage volume levels down, and lowering gain levels at frequencies where the feedback is occurring, using a graphic equalizer, a parametric equalizer, or a notch filter


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 sequence of waves of pressure that propagates through compressible media such as air or water. Sound can propagatethrough solids as well, but there are additional modes of propagation. During propagation, waves can be reflected, refracted, or attenuated by the medium.
The behavior of sound propagation is generally affected by three things:
 A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.
 The propagation is also affected by the motion of the medium itself. For example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.
 The viscosityof the medium also affects the motion of sound waves. It determines the rate at which sound is attenuatedFor many media, such as air or water, attenuation due to viscosity is negligible.
When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused)


Physics of sound

The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.
Longitudinal and transverse waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves (in solids) are waves of alternating shear stress at right angle to the direction of propagation.
Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium.Sound wave properties and characteristics
Sound waves are often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:
 Frequency, or its inverse, the period
 Wavelength
 Wavenumber
 Amplitude
 Sound pressure
 Sound intensity
 Speed of sound


Speed of sound

The speed of sound depends on the medium the waves pass through, and is 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 to its density. Those physical properties and the speed of sound change with ambient conditions.



Acoustics
Acoustics is the interdisciplinary (mortabet be baghiye reshteha )science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical or audio engineer. The application of acoustics can be seen in almost all aspects of modern society with the most obvious being the audio and noise control industries.

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

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. Since the human ear does not have a flat spectral response, sound pressures are often frequency weighted so that the measured level matches 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.




Equipment for dealing with sound

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