Public Address System

Introduction

  • Public Address System (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeakers, used to allow a person to address a large public, for example for announcements of movements at large and noisy air and rail terminals.
  • The simplest PA system consist of a microphone, an amplifier, and one or more loudspeakers. A sound source such as compact disc player or radio may be connected to a PA system so that music can be played through the system.
  • The process begins with a sound source (such as a human voice), which creates waves of sound (acoustical energy). These waves are detected by a microphone, which converts them to electrical energy. This signal is amplified in an amplifier up to a required level.
  • The loudspeaker converts the electrical signal back into sound waves, which are heard by human ears.

Conventional PA System:

  • Microphone: Captures the speaker’s voice and converts it into an electrical signal.
  • Amplifier: Receives audio signals as low voltage and low power, then boosts the voltage up to 100 volts and amplifies the current. At each speaker, the high voltage signal is reduced using a step-down transformer (LMT).
  • This technology has been in use for over 100 years and is well-known, with many experienced in its operation. However, a major drawback is that PA system cables cannot be shared with other low-voltage conduits or cable trays, leading to increased electromagnetic interference (EMI) that can affect IT infrastructure.
  • For zonal control in large areas, the system is divided into separate zones. Each zone’s speakers are wired independently and connected near the amplifier. Mechanical or electromechanical relay switches with push button controls are used to manage the zones. This setup allows users to select specific zones for music or announcements, leaving other areas unaffected.

Digital PA System:

  • In today’s digital landscape, various audio technologies such as Wi-Fi, Bluetooth, IP-based systems, and addressable options offer numerous benefits through flexible integration.
  • Digital Microphone Captures the speaker’s voice and converts it into a digital signal.
  • This digital audio signal can be transmitted via various media. At the speaker end, only low-voltage DC power and media control are needed, making the electronic components compact. Each zone or speaker can have a unique ID, allowing targeted announcements from a master control station.
  • Major advantages include eliminating the need for exclusive PA system cables, as a minimal amount of cable is sufficient. Additionally, digital systems facilitate easier integration and control of large area speaker setups.

Types of Speaker

Ceiling Mount Speaker (Dynamic Loudspeaker)

  • The most common type of driver, commonly called a dynamic loudspeaker. It has a light weight diaphragm, or cone, connected to a rigid basket, or frame, via a flexible suspension, commonly called a spider, that constrains a coil of fine tensile wire to move axially through a cylindrical magnetic gap.
  • When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it a variable electromagnet. The coil and the driver’s magnetic system interact, generating a mechanical force that causes the coil (and thus, the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical signal coming from the amplifier.

Cabinet Loud Speaker

  • The cabinet improves the acoustic response of the cone type speakers. The basic design consists of an enclosure with the loudspeaker unit set in the centre of a large box, which is completely air tight except for a port and the loudspeaker hole in the front panel. The port is so proportioned to the interior volume of the enclosure and to the loudspeaker characteristics that it functions acoustically as a low frequency loudspeaker.
  • Thus, the low frequency response is increased, and distortion generally experienced with a no ported enclosure, is reduced. The resonant frequency of a loudspeaker enclosure is damped by completely lining the interior surfaces of the enclosure with a highly absorbent material such as, rock wool. The resonant frequency of the panels may be damped to the use of diagonal braces and by filling unused spaces with sand.

Line Source or Column Speaker

  • Column Speakers use multiple speaker cones create a slim line column offering excellent vertical sound dispersion with a long ‘throw’, but limited horizontal coverage. For this reason, several column speakers can be mounted in a cluster and are often used around pillars for sound reinforcement.
  • On the axis of the system the sound waves from all the units are in phase and will therefore reinforce each other. Off this axis the different path lengths from the units will tends to cause cancellation. However it will show that phase cancellation can only occur if the wavelengths are comparable with or less than, the length of loudspeaker column.

High Fidelity (Hi‐Fi) Speaker

These are used to reproduce the generally audible frequency range of 50 Hz to 12 KHz (out of the entire audio range of 20 Hz to 20 KHz). The frequency response of ordinary speakers is irregular, with a number of resonant peaks and valleys, and has a range of about 60 Hz to 8 KHz only. By using a fairly large (30cm to 38 cm diameter) and heavy cone, the low frequency response of speakers can be extended downward to 45 or even 30 Hz but at the cost of high frequency response. It is difficult to design a single speaker to cover the entire audio range. One can use separate speakers for different audio ranges or combine large and small speakers into a single unit, mounted in line or coaxially.

Woofer

  • Woofer is designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The most common design for a woofer is the electro dynamic driver, which typically uses a stiff paper cone, driven by a voice coil which is surrounded by a magnetic field. The voice coil is attached by adhesives to the back of the speaker cone. The voice coil and magnet form a linear electric motor.
  • When current flows through the voice coil, the coil moves in relation to the frame according to Fleming’s left hand rule, causing the coil to push or pull on the driver cone in a piston‐like way. The resulting motion of the cone creates sound waves as it moves in and out.

Tweeter

A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz (generally considered to be the upper limit of human hearing). Specialty tweeters can deliver high frequencies up to 100 kHz. Tweeter in a two speaker system re‐produces frequencies from 1KHz onwards and in a three speaker system from 5 KHz onwards. Also, there is a super tweeter, which covers the range from 8 KHz onwards. A tweeter may be a small cone permanent magnet speaker or an electrostatic type.

Crossover Network

Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi‐fi speaker systems use a combination of multiple loudspeakers drivers, each catering to a different frequency band. Crossovers split the audio signal into separate frequency bands that can be separately routed to loudspeakers optimized for those bands.

The specific purpose of crossover network is:

  • To extend the frequency range by the use of two or more speakers of different
  • To avoid inter modulation distortion which may occur in a single
  • To limit the input to the most useful frequency range in a given
  • To protect a delicate HF unit from LF
  • To facilitate suitable placing of bass and treble speakers for natural

Horn Loud Speaker

  • A horn loudspeaker is a loudspeaker or loudspeaker element which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such, but rather improves the coupling efficiency between the speaker driver and the air. The horn can be thought of as an “acoustic transformer” that provides impedance matching between the relatively dense diaphragm material and the air of low density. The result is greater acoustic output from a given driver.
  • Horns have been used to extend the low frequency limit of a speaker driver. When mated to a horn, a speaker driver is able to reproduce lower tones more strongly. The flare rate and the mouth size determine the low frequency limit. The throat size is more of a design choice. Horns have been known to extend the frequency range of a driver beyond five octaves.
  • A horn facilitates the transfer of electrical energy into acoustical energy and, if properly designed will be so with a minimum of distortion. The design of loudspeaker horn is complex and requires careful consideration to prevent reflection of the acoustical energy back into the horn bell.
  • The area of the throat determines the loading on the diaphragm. If the area of the throat is small compared to the area of the diaphragm, the efficiency is increased because of the heavier loading effect. However, small throats require a longer horn, which increases the frictional losses.
  • The reflex loudspeaker  or  bullhorn,  a  type  of folded  horn speaker  used  widely in public address systems. To reduce the size of the horn, the sound follows in zigzag path through exponentially‐expanding concentric ducts in the central projection (b, c), emerging from the outer horn (d).