What is a public address system
What is a Public Address system? A Public Address system is a specific type of sound system. When designing a commercial sound system, the process includes considering many nuanced design criteria; including, the size or volume of the space, noise levels, how many people will be listening, and where those listeners are located. The first and most fundamental design question is, will the sound system be used for speech, music, or both? By describing a sound system as a Public Address (PA) system, that fundamental design question is already answered. The primary purpose of a Public Address system is to provide clear and intelligible speech.
BASIC public address SYSTEM CONFIGURATIONS
While Public Address systems vary significantly in their construction and configuration, we can generally categorize them in a couple of different ways. One is by size. Small PA systems generate a few to hundreds of watts of audio power, are designed for one listener up to hundreds of listeners, and generally serve a single space or facility. Large PA systems generate thousands of watts and are designed for hundreds to thousands of listeners in auditoriums, stadiums, across campuses, or entire cities.
PA systems, like all sound systems, including those used exclusively for music, may also be categorized as “single point source” or “distributed.” In their highly regarded textbook “Sound System Engineering,” Don and Carolyn Davis point out: “from the standpoint of electro-acoustic engineering, a single source system is always preferred. Any other choice entails a sizeable compromise. That is not to say that other choices cannot be made to work well; it simply means that by a wide margin, the highest quality for the lowest cost lies in the design of a single point source system. Whenever possible.”
When contrasting a single point source with a distributed design, the most important distinction is where the energy is emanated. In a large single point source PA system, thousands of watts of power emanate from principally one location. In a large distributed PA system, speakers delivering very low power emanate from thousands of locations.
The simplest, smallest PA systems tend to be battery-powered portable systems like the iconic “megaphone” or “bull horn.” While these systems are highly effective, they are not the topic of this article. If you’re interested in portable hailing devices like the megaphone, see our article https://torrencesound.com/ahd-acoustic-hailing-device/
For this article, Small Public Address Systems consist of a microphone or a recorded sound playback device such as an mp3 player, an amplifier, and one or more loudspeakers. Small PA systems typically produce somewhere between 10 to 500 hundreds of watts of power and are used in small venues such as school auditoriums, churches, and meeting rooms – anywhere you need to speak to a group of people all at once and be heard. Small PA systems may extend through an entire building, such as restaurants, stores, elementary schools, or office buildings. Because the primary intended purpose of PA systems is to allow a presenter to address a group of listeners, microphones and telephone system handsets are the primary audio source of a PA system. A sound source such as a streaming device, mp3 player, compact disc player, or radio may also be connected to a PA system so that music can be played for listeners. However, the audio fidelity may be compromised because the system is optimized for speech.
In addition to the amplifier, Public Address systems of all sizes often include control and monitoring equipment, including; Mixers, Filters, Equalizers, Compressors, Expanders, Audio Routers, Delays, LED indicator lights, Volume (VU) Meters, and Headphones. Control and monitoring equipment may take the form of a programmable Digital Signal Processor (DSP) that is not only capable of providing all of these functions over multiple channels but can also store signal tones and prerecorded sounds, including prerecorded announcements and emergency messages. If you’re interested in systems that make automated announcements in hospitals for rapid response, see our page https://torrencesound.com/automated-code-announcements.
Sources like microphones and telephone system paging feeds are connected to the inputs of the PA system. A “preamplifier” boosts the low-level microphone signal. It then passes the audio onto the power amplifier(s) that boost the signal further so it can be heard through the loudspeakers. Most small PA system amplifiers will include a mixer, preamp, tone controls, and basic routing functions housed in a single shelf or rack-mounted package.
The loudspeaker is the device that converts the amplified electrical signals into sound. When a system calls for only one or two loudspeakers, a PA amplifier with an 8-ohm speaker output is appropriate. When the design calls for multiple speakers “distributed” throughout an office or restaurant, a “constant voltage” PA amplifier is selected. Depending on local practices, constant voltage amplifiers usually amplify the audio signals to a speaker line level of 25V, 70V, or 100V.
When a more sophisticated solution is desired, PA amplifiers can include control equipment that monitors the amplifier outputs and speaker lines for faults and alerts staff when there is a failure. This control equipment can also be used to create multiple separate speaker zones in a PA system.
The Rauland Telecenter U is a Public Address system with monitoring and control features specifically designed for educational environments. The Telecenter U features time tone signals played by completely customizable schedules to accommodate class change, prerecorded announcements, handsfree intercom calling into classrooms, and much more. To see more about the Telecenter products, visit https://torrencesound.com/school-security-systems.
Torrence is the exclusive distributor of Rauland Telecenter products in Northwest Ohio.
LARGE single point source PA SYSTEMS
The voice coil of a loudspeaker can have an impedance specification from 2 to 30 ohms. Impedance is the loudspeaker’s resistance to AC current, otherwise known as the “load.” PA system amplifiers are designed to drive into a specified load, usually 4, 8, or 16 ohms. Care must be taken to match the amplifier’s output to the load of the loudspeaker(s). Some amplifiers have several outputs to accommodate for different loudspeaker loads, but usually, the manufacturer states the amplifier performance based on a specified load impedance. For example, “Output 20 watts into 8 ohms”. This means that the amplifier will give its optimum performance of 20 watts with a speaker load of 8 ohms.
There are techniques to match various speaker loads to an amplifier output. For an amplifier designed for a 16 ohm load, two 8 ohm loudspeakers wired in series will make the total load 16 ohms. Some slight loss of quality will result, but for most purposes, this is acceptable. Alternatively, it’s possible to drive two 16 ohm loudspeakers from an amplifier designed for a load of 8 ohms by wiring the two 16 ohm loudspeakers in parallel. This is a better method of driving two loudspeakers from one amplifier. The same principles can be applied to a larger number of loudspeakers.
If an amplifier designed for 16 ohms is connected to a 4 ohm load, the result will be a louder and perhaps distorted sound, and the amplifier may suffer damage. Conversely, if an amplifier designed for a 4 ohm load is connected to a 16 ohm loudspeaker the result will be loss of output level. Although amplifiers can usually accept a load from half to double the one specified, the only sure way of getting good results is to follow the manufacturer’s instructions. Ideally, where cost is not the determining factor, each loudspeaker should have its own amplifier.
The Ohio State University Football Stadium “The Shoe” is one of the most recognizable landmarks in all of college athletics. With its present seating capacity of 102,780, Ohio Stadium is the fourth-largest on-campus facility in the nation it’s also one of the best examples of a large single point source, direct voice coil public address system. “The Shoe” is equipped with a Meyer Sound LEO linear large-scale sound system. The system consists of two hanging arrays of 28 Meyer LEO-M loudspeakers with bass augmented by ten Meyer 100LFC low-frequency control elements. The PA system is mounted inside the video screen scoreboard at the south end of the impressive oval. The single source PA system projects sound across the bowl, reaching over 900 feet to the top seats on the north end with crisp voice announcements and fan-pumping music over the loud, spirited crowd.
LARGE DISTRIBUTED PA SYSTEMS
Large distributed PA systems typically have amplifiers that output high power at high voltage through hundreds to thousands of daisy-chained small loudspeakers in various locations. Each loudspeaker has a transformer between its voice coil and the amplifier to lower the voltage on the speaker line. This configuration is called a Constant Voltage, Distributed Design loudspeaker system, or simply a Constant Voltage or High Voltage Distributed system.
In Ohio and North America, constant voltage amplifier output levels are standardized at 25V, 70V and 100V.The constant voltage system was devised to overcome the many problems that arise from feeding an amplifier directly to many loudspeakers over very long cable runs. The constant voltage design is most common in commercial PA systems, where very large areas need to be covered – like shopping malls, universities, airports, and convention centers.
The Rauland Telecenter brand of products are designed to monitor and control large distributed PA systems with features specifically designed for educational facilities. The Telecenter U features zone control so announcements can be delivered to specific areas, time tone signals played by completely customizable schedules, prerecorded announcements, handsfree intercom, and much more. To see more about large distributed PA systems for schools, visit https://torrencesound.com/school-security-systems
Constant voltage systems are powerful tools for delivering voice announcements to thousands of listeners and over enormous geographic areas instantly. By utilizing IP networking, a distributed Public Address system can cover the entire campus of a large university, industrial complex, a multi-building hospital network, even an entire city.
Commercial constant voltage PA systems are very flexible, but they tend to be optimized for speech, size, and scalability rather than full bandwidth sound reproduction. That said, if a constant voltage PA system is to be used for providing background, foreground environmental, or mood music, with a little extra investment and care on the part of the designer, a commercial constant voltage PA system can provide impressive results. For more information on background and foreground music, see our pages https://torrencesound.com/background-music https://torrencesound.com/foreground-music
CONSTANT VOLTAGE SYSTEMS SIMPLIFIED
Constant voltage PA systems may seem complicated, but their design and configuration is relatively simple to understand using standard household AC power circuits as an analogy. If you have a 120v- 15amp circuit feeding the outlets in your kitchen, you can plug in many appliances without concern until the total load exceeds 15amps and the breaker blows. Each appliance draws a defined amount of power from the circuit, so you can calculate the load on the kitchen circuit by simply adding up the current draw of the refrigerator, microwave, coffee pot, and so on. Because there is sufficient voltage on the line, you rarely notice the additional current draw when you run your toaster or microwave.
A constant voltage PA system amplifier is rated by its Output Voltage and total Watts of output power. A very common example is an amplifier that provides 35watts of power on a 70v speaker line. The PA system speaker circuit is built out using speakers (or paging horns) that have an integral transformer with a primary winding designed for a 70v line and secondary windings that provide a variety of “tap” options rated in the Watts of power the speaker will draw from the amplifier.
Common transformer tap values for commercial constant voltage loudspeakers are 1/2w, 1w, 2w, 5w, and 10w. Using the 35watt amplifier example, here are a few different ways the PA System can be configured using the taps on the speaker transformer:
- Three speakers tapped 10w and one speaker tapped at 5w
- Seven speakers tapped at 5w
- Thirty-five speakers tapped at 1w
- Ten speakers tapped at 2w, twenty speakers tapped at 1/2w, and one speaker tapped at 5w
How do you know what tap to use? Answer: It depends. A sound system designer considers dozens of variables when choosing loudspeakers and their power requirements for any given application. Ceiling height, the area that a single loudspeaker can cover (influenced by the speaker mounting height), ambient noise in the room, reflective or absorptive surfaces nearby, and so on.
A few examples:
- A small quiet office with one occupant – 1/2w
- A long hallway in an office building or hospital – 1w with speakers every 10 feet for coverage
- Noisy cafeteria (depending on ceiling height) – 5w with speakers spaced 15 feet apart
- Warehouse paging horn – 10w
Another unique advantage of a constant voltage PA system is you can place simple passive wall-mounted volume controls in multiple locations along the speaker line run to provide control of individual speakers or groups of speakers serving a common area. The speaker tap determines the maximum volume of the loudspeaker, and the in-line volume control allows the user to adjust the volume of the speaker(s) according to need.
It’s always wise to size the amplifier with more power than is needed for the current configuration. Adding additional speakers to a constant voltage system can be done by adding a new speaker line attached at the amplifier, extending an existing speaker line, or “T-tapping” a new branch off an existing speaker line.
A sound contractor can determine the total load of an existing constant voltage speaker system using a specialized electronic meter called an Impedance Bridge and little math using Ohms law. The two most common mistakes building owners make are installing too many speakers for the amplifier’s capacity and wiring a standard 8 ohm loudspeaker on the line, which acts as a short-circuit on a constant voltage system.
All PA systems have the potential for audio feedback, which occurs when a microphone picks up sound from the speakers, which is re-amplified and sent through the speakers again. It often sounds like a loud high-pitched squeal or screech, and can occur when the volume of the system is turned up too high. Feedback only occurs when the loop gain of the feedback loop is greater than one, so it can always be stopped by reducing the volume sufficiently.
Sound designers and engineers take several steps to maximize gain before feedback, including keeping microphones at a distance from speakers, 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, notch filter or a digital processor (DSP) that incorporates all of the above.
Feedback prevention devices detect the start of unwanted feedback and use a precise notch filter to lower the gain of the frequencies that are feeding back. Some automated feedback detectors require the user to “set” the feedback-prone frequencies by purposely increasing gain (during a sound check) until some feedback starts to occur. The device then retains these frequencies in its memory and it stands by ready to cut them. Some automated feedback prevention devices can detect and reduce new frequencies other than those found during the sound check, or when a commercial PA system is commissioned by the system designer.
TELEPHONE PAGING SYSTEMS
In most cases, the facility telephone system is used as the sole access point to the PA system. A paging announcement can conveniently be made from any telephone device connected to the phone system. A specialized circuit that integrates and isolates the phone system and PA system is required.
A typical private branch exchange (PBX) telephone system uses an analog paging adaptor between the telephone system and the PA system. A separate paging controller is connected to a trunk port of the telephone system and is accessed as either a designated directory number or central office line. Most small office telephone Key-Systems usually including a paging output port as part of the system. In either case, the telephone system is being used as an input source for the PA system voice announcements rather than a dedicated microphone. The telephone to PA system adapter or interface is the circuit that connects analog audio from the telephone system without interfering with the operation of, or being harmful to either the telephone system or the PA System.
VOIP PAGING GATEWAYS / SIP PAGING ADAPTERS
VOIP phone service and VOIP-based phone systems are rapidly replacing the analog PBX and Key Systems used in government, education, healthcare, industry, and business. The voice signal in an IP phone system is digital data until it is converted to analog voice at the endpoint phone device. A separate VOIP Paging Gateway or SIP Paging Adapter is required to convert the digital voice public address message encoded at the phone handset to an analog signal that can be patched into the PA system amplifier input.
SIP Paging Adapters connect to the IP phone system using a POE (power over ethernet) port and provide a 600 ohm analog output that can be patched to the PA system “paging” input. For small offices, there are SIP Paging Adapters include a low-power amplifier that can be used to power a handful of 8 ohm speakers in a small office environments. For larger facilities, the analog output jack is used to provide public address announcements through the in-house PA system.
PA OVER IP
PA over or IP refers to PA paging and intercom systems that use an Internet Protocol (IP) network, instead of a central amplifier, to distribute the audio signal to paging locations across a building or campus, or anywhere else in the reach of the IP network, including the Internet. Network-attached amplifiers and intercom units are used to provide the communication function. At the transmission end, a computer application transmits a digital audio stream via the local area network, using audio from the computer’s sound card inputs or from stored audio recordings. At the receiving end, specialized intercom modules integrated with a traditional loudspeaker (known as IP speakers) receive these network transmissions and reproduce the analog audio signal. These are small, specialized network appliances addressable by an IP address, just like any other computer on the network.
WMT PA SYSTEMS
Wireless Mobile Telephony (WMT) PA Systems refers to PA paging and [intercom] systems that use any form of Wireless mobile telephony system such as GSM networks instead of a centralized amplifier to distribute the audio signal to paging locations across a building or campus, or other location. The GSM mobile Networks are used to provide the communication function. At the transmission end, a PSTN Telephone, mobile phone, VOIP phone or any other communication device that can access and make audio calls to a GSM based mobile SIM card can communicate with it.
At the receiving end, a GSM transceiver receives these network transmissions and reproduces the analogue audio signal via a Power Amplifier and speaker (the PA system). This was pioneered by Stephen Robert Pearson of Lancashire, England who was granted patents for the systems, which also incorporate control functionality. Using a WMT (GSM) network means that live announcements can be made to anywhere in the world where there is WMT connectivity. The patents cover all forms of WMT i.e., 2G, 3G, 4G …..xxG. A UK company called Remvox Ltd (REMoteVOiceeXperience) has been appointed under license to develop and manufacture products based on the technology.
LONG LINE PA
A Long-Line Public Address (LLPA) system is any public address system with a distributed architecture, normally across a wide geographic area. Systems of this type are commonly found in the rail, light rail, and metro industries, and let announcements be triggered from one or several locations to the rest of the network over low bandwidth legacy copper, normally PSTN lines using DSL modems, or media such as optical fiber, or GSM-R, or IP-based networks.
Rail systems typically have an interface with a passenger information system (PIS) server, at each station. These are linked to train describers, which state the location of rolling stock on the network from sensors on trackside signaling equipment. The PIS invokes a stored message to play from a local or remote digital voice announcement system, or a series of message fragments to assemble in the correct order, for example: ” / the / 23.30 / First_Great_Western / Night_Riviera_sleeper_service / from / London_Paddington / to / Penzance / …. / will depart from platform / one / this train is formed of / 12_carriages /.” Messages are routed via an IP network and are played on local amplification equipment. Taken together, the PA, routing, DVA, passenger displays and PIS interface are referred to as the customer information system (CIS), a term often used interchangeably with passenger information system.
POTS LINE PAGING INTERFACE
A phone patch, or phone tap, is necessary to interface line-level analog audio to and from plain old telephone service or POTS. The phone patch allows connecting standard audio equipment to a phone line, while isolating the audio equipment from ring tone and line voltage. It operates in parallel with the telephone, with a circuit design that disturbs normal operation very little due to its high impedance input (if the hold resistor is not needed). The typical standard characteristics of a POTS audio output is, a two wire twisted pair cable, 600 ohms impedance, an average level of -9 dBm (275 mV) [0 dBm = 1 mW (0.775V) into 600 ohms], 48 VDC typical talk voltage, 20-26 mA DC current, 200-300 ohms DC resistance, ring voltage of 90 VRMS, 20Hz (2 secs on, 4 secs off), AC 45db signal to noise, and a bandwidth of 300 – 3.3 kHz (3 kHz BW), positive tip or green wire tied to ground so it measures 48 VDC relative to ring or red wire.
A single transformer and capacitor can be used as a telephone to audio amplifier interface in a pinch; however, adding a few more components greatly improve the performance. The transformer provides the necessary isolation, while the capacitor blocks the DC voltage from the transformer. The diagram shows an enhanced version:
The MOV (metal-oxide varistor), or any similar transient voltage suppressor, is required due to the extreme lighting-induced voltage spikes that can travel on telephone lines (thousands of volts). It needs a maximum operating voltage of at least 250 Vrms. This seems extreme for a 48 VDC powered line, but the telephone company tests their lines by adding as much as another 200 volts, so you must guard against the worst case.
It is not necessary to add a matching capacitor to the other leg of the transformer primary (to preserve the line balance) as will be seen shortly. Use a non-polar type since the polarity of the DC voltage cannot be guaranteed and oftentimes reverses with different operating modes. The value is not critical and depends on the reflected impedance seen by the series capacitor. Normal usage for this type of phone patch is either to drive a high impedance (>10k ohms) input of a recorder or an amplifier, or, if used in the opposite direction (i.e., to add audio to the phone line), driven from a low impedance (<300 ohms) output. The voltage rating must be high enough to withstand the usual DC voltage (and variations) plus the AC ring voltage; a value of 250 Vrms is recommended. Since a 1µF/250V non-polar capacitor can be quite large (and expensive), consider paralleling two or more small non-polar caps (e.g., two 0.47µf/250V, or three 0.33µF/250V rated, etc.).
The resistor, R1, is necessary if the circuit must hold the line, i.e., look-like a phone off- hook. It must be selected to draw enough DC current to drop about 6 volts. A big problem comes in predicting this value. The DC source is typically 48 volts, but can vary anywhere from 42-54 volts, and sometimes much more (24 60 volts).
Resistor R2 is a good idea to make the line driving impedance higher when using the patch to add audio (total equals R2 + line driver output impedance).
When designing a phone patch you do not have to worry much about what the telephone line looks like when the phone is on-hook. This is good because while described as a 600-ohm balanced line, the on-hook line (the off-hook line is quite different, and will be discussed next) never measures 600 ohms, nor is it very well balanced. Variations from 500-2500 ohms are reported for the ungrounded side of the line, simultaneous with the grounded side measuring 0-700 ohms hardly a balanced line. Plus the DC resistance of the telephone cabling is not trivial, easily amounting to as much as 1500 ohms for locations a few miles from the central office (26 AWG is common, measuring 440 ohms/mile).
Once the phone is answered, the system goes into off-hook mode and sends out the dial tone. Now the line is predictable with the balanced output impedance measuring about 400 ohms (±25%), split evenly between the two lines, with the voltage ranging from 42 volts to as high as 80 volts. Still not well balanced, but a lot closer than the on-hook values.
The back-to-back zener diodes in the secondary clamp any high voltage (including any ring voltage that may appear) that gets through the transformer and protects the downstream equipment. Their value is pretty arbitrary and is determined by what the interfaced unit can withstand. The 1N746s limit the output to 4 volts peak-to-peak, or 1.4 Vrms. Likewise the power rating need not be excessive; 1/2-watt is enough.
In other systems, paging equipment is not built into the telephone system. Instead the system includes a separate paging controller connected to a trunk port of the telephone system. The paging controller is accessed as either a designated directory number or central office line. In many modern systems, the paging function is integrated into the telephone system, so the system can send announcements to the phone speakers.
LIVE MUSIC PA SYSTEMS
As noted previously, Public Address systems are intentionally frequency bandwidth limited to provide the best possible speech intelligibility for critical announcements. A sound system used for full-range music reproduction will be configured differently than a Public Address system. The design and components used for a live music PA will largely be determined by how Small or Large the performance space is.
SMALL VENUE LIVE MUSIC SYSTEMS
Small clubs, bars, and coffeehouses use a fairly simple set-up, with the front-of-house (FOH) speaker cabinets (and subwoofers, in some cases) aimed at the audience and monitor speaker cabinets aimed back at the performers so they can hear their vocals and instruments. In many cases, front-of-house speakers are elevated, either by mounting them on poles or by “flying” them from anchors in the ceiling. The FOH speakers are elevated to prevent the sound from being absorbed by the first few rows of audience members. The subwoofers do not need to be elevated because deep bass is omnidirectional. In the smallest coffeehouses and bars, the audio mixer may be onstage so that the performers can mix their own sound levels. In larger venues, the audio mixer may be located in or behind the audience seating area so that an audio engineer can listen to the mix and adjust the sound levels.
The adjustments to the monitor speaker mix may be made by a single audio engineer using the main mixing board or by a second audio engineer who uses a separate monitor mixing board. With the advent of new technology, artists are now doing individual monitor mixes themselves using a personal monitor mix system and wireless earbuds instead of monitor speakers. An added benefit of removing loud monitor speakers from the stage is it also improves the FOH sound. The sound coming from rear-facing musician monitors bounces off the wall behind the performers and combines with the FOH mix leading to a muddy, “smeared” sound in smaller venues. Loud stage monitors can also force the mix engineer to mix louder than they might prefer to get FOH mix over the volume of the stage monitors.
LARGE VENUE LIVE MUSIC SYSTEMS
For popular music concerts, a more powerful and complicated PA System is used to provide live sound reproduction. In a concert setting, there are typically two complete PA systems: the “main” system and the “monitor” system. Each system consists of a mixing board, sound processing equipment, amplifiers, and speakers. The microphones that are used to pick up vocals and amplifier sounds are routed through both the main and monitor systems. Audio engineers can set different sound levels for each microphone on the main and monitor systems. For example, a backup vocalist whose voice has a low sound level in the main mix may ask for a much louder sound level through her monitor speaker, so she can hear herself singing.
The “main” system (also known as Front of House, commonly abbreviated FOH), which provides the amplified sound for the audience, typically uses several powerful amplifiers that drive a range of large, heavy-duty loudspeakers—including low-frequency speaker cabinets called subwoofers, full-range speaker cabinets, and high-range horns. A large club may have amplifiers that provide 3000 to 5000 watts of power to the “main” speakers. An outdoor concert may use 10,000 or more watts.
The monitor system reproduces the sounds of the performance and directs them towards the onstage performers (typically using wedge-shaped monitor speaker cabinets) to help them to hear the instruments and vocals. In British English, the monitor system is referred to as the “foldback.” The monitor system in a large club may provide 500 to 1000 watts of power to several foldback speakers; at an outdoor concert, there may be several thousand watts of power going to the monitor system.
At a concert using live sound reproduction, sound engineers and technicians control the mixing boards for the “main” and “monitor” systems, adjusting tone, levels, and overall volume. Touring productions travel with relocatable large line-array PA systems, sometimes rented from an audio equipment hire company. The sound equipment moves from venue to venue along with various other equipment such as lighting and projection.