1.The Definition of PA System
Public Address (PA) is a system managed by user and used for serving the public in a limited domain, in which signals are transmitted by lines. The signals can also be transmitted by wireless, but these PA systems are not typical.

Normally, PA System are used in some public areas, such as community areas, government departments, enterprises, schools, commercial buildings, supermarkets and so on for releasing news, messages, background music and emergency announcements, etc. So PA system mainly covers emergency broadcast, business announcement and background broadcast.

In some emergency situations, such as ire Alarmerrorist attacking and so on, PA system will play an important role in saving lives, for example, the 11 Disaster in the World Trade Center.

The Use of PA Amplifiers (70V/100V)

1. The length of lines in PA system can be about thousands of meters. Signals are transmitted in the mode of high voltage & low current for reducing damping.
2. The PA amplifier outputs are rated with 70V/100V/150V/200V/250V.
3. The PA speakers are high impedance with 70V/100V/ 150V /200V/250V input terminals for adapting to the line voltage.
4. The microphone has the priority in sequence of inputs.
5. The lines are used by audio frequency lines.



Sound Production and Spread
Sound is produced by mechanical vibration.
Only the sound with vibrative frequency between 20~20000Hz can be heard.
Sound can spread in the media: air etc. with sound waves.


Avoiding Interference

Sound Speed, Frequency, Wavelength, Phase

Refraction, Reflection and Absorption of Sound Wave

Sound Mixing and Its Time
Sound Mixing Phenomena


Sound Mixing Radii

Sound Mixing Time

Background Yawp
The yawp in a hall with air-conditions, lighting, and electro acoustic equipments turned on, without signals added up


Several Representative Sound Pressure Levels (SPL)
SPL meter can tell __ (dB)
0 dB for sound only heard, 120 dB can make ears pains.
Background yawp in silence is about 50 dB.
Face to face talk is about 60~70 dB.
Cinema dialogues are about 80 dB.
Sound in a disco hall is about 110 dB.
Site sound for a jet plane taking off is about 140 dB.