Audio engineers and other sound industry professionals disagree over whether these audio systems should be called Sound Reinforcement (SR) systems or Public Address (PA) systems. Some audio engineers distinguish between the two terms by technology and capability, while others distinguish by intended use (e.g., SR systems are for live music whereas PA systems are usually for reproduction of speech and recorded music in buildings and institutions). In some regions or markets, the distinction between the two terms is important. The terms are also considered interchangeable in some areas.
A typical sound reinforcement system consists of three parts: input transducers such as microphones, which convert sound energy into an audio signal; signal processors such as equalizers and amplifiers, which alter the audio signal characteristics; and output transducers such as loudspeakers, which convert the audio signal into sound for an audience.
Sound is taken and converted into electronic signal by an input transducer (such as a microphone or a pickup on an electric guitar or electric bass). A signal processor (such as a mixing console, amplifier, a reverb effect, or other devices) then alters the signal's equalization, balance, effects and amplitude. Finally, an output transducer such as a loudspeaker (or, for audio engineers, a set of headphones) converts the electronic signal back into sound, so that the listener can hear the end product. This basic concept of sound reinforcement systems encompasses anything from a simple system with only one microphone, amplifier and loudspeaker, to the complex systems in professional applications including multiple mixing boards, monitors and a vast selection of effects. There is debate on the classification of sound systems as "sound reinforcement systems" and "public address systems" depending on size and application.
For some sound engineers, a "Sound Reinforcement System" is a system where the audience could hear the speaker or the singer at the microphone without electronic assistance, such as in the case of a Minister addressing a congregation in a church. The components of the SR system amplify or "reinforce" the sound one hears to make it more intelligible. The "PA", or "Public Address System", also known as "commercial paging systems", tend more to be classified as a system where one could not at all hear the person at the microphone without amplification, as in the case of a school principal addressing all of the classrooms in a school through a PA system. In these PA systems, or distributed sound systems, there are multiple speakers located throughout a business, institution, or large complex.
Sound reinforcement in a large format system typically uses a signal path which starts with the directly-connected instrument or a microphone (transducer) which is plugged into the multicore cable (often called a "snake"). The snake routes the signals of all of the inputs to the Front of the House mixer and to the monitor consoles. Once the signal is in a channel on the console, this signal can be equalized, panned and routed to various mix buses. The signal may also be patched into an external effect processor present in the channel (e.g. gate, compressor, reverb).
The signal can be routed internally to a summation bus (also known as a "bus", mix group, subgroup or simply 'group'), to allow the engineer to control the levels of several related signals at once. For example, all of the different microphones for a drum set might be grouped together, so that the volume of the entire drum set sound can be controlled with a single fader. If an engineer was trying to record the different instruments of drumset with ten microphones and mix the sound into a rock song, and they did not have a bus or group function on their mixer, they would have to lower each individual mic fader to reduce the overall volume of the drums; with a mixer that has a "mix group" function, all of the faders in the group can be controlled by a single master fader. From here each signal is routed through the stereo masters on a console (left and right, or balance, pan, etc.). Additionally, each signal can be sent to separate outputs from the main console, typically referred to as Auxiliary sends, or "Auxes."
The next step in the signal path generally depends on the size of the system in place. In smaller systems, the main outputs would be sent to an additional equalizer, or directly to a power amplifier, with one or more loudspeakers (typically two) then connected to that amplifier. In large-format systems, the signal is first routed through an additional equalizer then to a crossover. A crossover splits the signal into multiple frequency bands, with each band being sent to separate amplifiers and speaker enclosures for low-, middle-, and high-frequency sounds. Low-frequency sounds are usually sent to subwoofers, and middle- and high-frequency sounds are usually sent to full-range speaker cabinets.
Microphones used for sound reinforcement can be positioned and mounted in many ways, including base-weighted upright stands, podium mounts, tie-clips, instrument-mounted and headset mounts. Headset mounted and tie-clip mounted microphones are often used with wireless transmission to allow performers or speakers to move freely. Early adopters of headset mounted microphones technology included country singer Garth Brooks, Kate Bush, and Madonna.
There are many other types of input transducers which may be used occasionally, including magnetic pickups used in electric guitars and electric basses, contact microphones used on stringed instruments, and piano and phonograph pickups (cartridges) used in record players .
Large music productions often use a separate stage monitor mixing console, which is dedicated to creating mixes for the performers' on-stage monitors. These consoles are often placed at the side of the stage so that the operator can communicate with the performers on stage. In cases where performers have to play at a venue that does not have a "Front of House" monitor engineer near the stage, the monitor mixing is done by the main sound engineer from from the main console, which is in the audience or in the back of the hall. This arrangement can be problematic, because the performers end up having to request changes to the monitor mixes with "...hand signals and clever cryptic phrases", and because the sound engineer cannot hear the changes that result in the monitors onstage.
Graphic equalizers have faders which resemble a frequency response curve plotted on a graph. Sound reinforcement systems normally use graphic equalizers designed on one-third octave centers. End-cut filters restrict a given channels bandwidth extremes, which can prevent subsonic disturbances and RF or lighting control disturbances from interfering with the audio system. End-cut filter sections are often included with graphic equalizers to give full control of the frequency range. If their response is steep enough, then high-pass filters (low-cut) and low-pass filters (high-cut) can function as end-cut filters. A feedback suppressor is a specialized type of filter which automatically detects and suppresses feedback by cutting a deep notch on the frequency which is feeding back.
Portions of the signal that function below the threshold are theoretically unaffected by the process. A theoretically perfect compressor would instantaneously transition from 1:1 to x:1 when the signal rises to any degree above the threshold. Due to design factors analog compressors cannot theoretically accomplish this perfectly. The area of transition between uncompression and full compression is referred to as the "knee" of a compressor. It is in some cases desirable for a compressor to affect a smooth transition from uncompression to full ratio implementation. One example of this principle in application is the "Overeasy" feature provided by dbx. This function provides a subjectively more pleasing sonic character for some signals acting at or near a threshold setting by gradually transitioning the ratio from 1:1 to x:1 rather than affecting an immediate change from 1:1 to x:1 at the threshold point.
Most presently available compressors also allow the user to increase the overall output level to account for losses due to gain reduction in order to ensure optimal signal-to-noise performance for devices receiving the compressed signal. This is typically referred to as "make-up gain". An input signal, compressed and properly "made-up" yields a signal with a higher average level at the compressors output.
Many compressors include user-adjustable "attack" and "release" times. These enable the user to alter the speed at which the compressor reduces (attacks) and restores (releases) gain. These settings are most often ennumerated using milliseconds, although many compressors allow release times in excess of 2 seconds. Techniques for determining ideal time-dynamic criteria are largely subjective, varying with application and operator preference. It is important to note that altering attack and release times alters the compressive process, creating variances in amounts of gain reduction for identical ratio and threshold settings. i.e. - Attack times that are slow when compared with signal composition may not affect compression for all signal transients.
Compressor applications vary widely, from objective system design criterion, to subjective applications determined by variances in program material and operator preference. Typical system design criterion specify limiters for component protection and gain structure control. Artistic signal manipulation is a subjective technique widely utilized by mix engineers in both live production and studio environments.
Noise gates are also used with the microphones placed near the different drums and cymbals in the drum kits in many hard rock and metal bands. Without a noise gate, the microphone for a specific instrument, such as the floor tom, will also pick up sounds from nearby drums or cymbals, which bleed into its microphone. With a noise gate, the threshold of sensitivity for each microphone for each instrument from the drum kit can be set so that only the direct striking of the instrument will be picked up by the microphone, and not the nearby sounds.
While the noise gate is sometimes misunderstood as a type of filter, which removes the unwanted sounds, it should be noted that the device does not remove any sounds from the music or speech. Instead, it opens and closes an electronic "gate" when there is no, or very little signal present. In this fashion, when a speaker pauses in a conference and looks through their notes, the microphones will not pick up the much quieter sounds of papers rustling. In a musical context, if there is a guitar amp which is humming, the most noticable point for the hum is usually in between songs, or during the rests when the guitarist is not playing. With the noise gate set to the proper threshold, the hum will not be picked up during the points where the guitarist is not playing.
A wide range of accessories are used in sound reinforcement systems. High-end audio cables are shielded to prevent hum and interference. Rack-mount cases, such as the industry standard 19-inch racks are used to store and transport effects units and amplifiers. Some racks have cushioned shock-mounting to protect equipment from impacts.
Like most sound reinforcement equipment, professional amplifiers are designed to be mounted on 19-inch racks. Many power amplifiers have internal fans to draw air across the heat sinks. Heat dissipation is an important factor for operators to consider when mounting amplifiers onto equipment racks, since they can generate a significant amount of heat
Since transistor power amplifiers used for large venues need to produce a high output, this usually means that most powerful amplifiers are very heavy. Most powerful amplifiers are Class AB amplifiers, which need bulky transformers made of copper wiring and large metal heat sinks for cooling. However, Class D amplifiers, which are much more efficient, weigh much less than Class-AB amplifiers producing an equivalent power output.
Digital loudspeaker system controllers (DLSC), also known as digital crossover networks, are most commonly used to process the final mix being sent from the mixing console to the amplifiers, and in turn to the loudspeakers. Multiple loudspeakers with a more narrow-band response, tailored to specific frequency bands, can be used together (i.e. lows, mids and highs) to obtain a more accurate reproduction of the input signal. This also makes more efficient use of amplifier power by sending each amplifier only the frequency band that its respective loudspeaker can reproduce. The crossover function of a DLSC does this splitting of the input signal into separate outputs for each speaker. Most DLSCs have calibration and testing functions, such as a tone generator, a pink noise generator coupled with a real-time analyzer and automated room optimization.
In the 1990s and 2000s, professional PA speakers were often "actively controlled" with electronic processors that automatically adjust crossover and equalization settings and protect the speakers. In the 2000s, PA speaker cabinets were increasingly built with protection circuitry that protects high and mid frequency drivers from accidental exposure to low frequency sound material. Some PA speakers also protect the high frequency drivers with "current-to-light" conversion circuitry, which takes excess current which would otherwise damage a horn, and uses it to light a small light bulb. Another circuit protection technique used to protect horns is current sensing / self-resetting breakers, which protects the horn in cases of high-volume feedback (e.g., a microphone being accidentally dropped into a monitor).
As well, many PA speaker companies have begun providing Neutrik Speakon multi-pin connectors, instead of the decades-old 1/4" jacks. The Speakon connectors provide a much larger metal contact area for high current PA applications.
The four different types of transducers (woofers, compression drivers, and tweeters) all use the same components: a voicecoil, magnet, cone or diaphragm, and a frame or structure. PA speakers have a power rating (in watts) which indicates their maximum power capacity, to help users avoid overloading them with excessively powerful amplifiers. However, even an amplifier with less power output than a speaker's power rating can destroy the speakers if the amplifier signal becomes heavily distorted, especially if the distortion is from low-range sounds. The power rating of speakers is expressed either as RMS (Root Mean Square) or PGM (Program). The PGM rating became more widely used in the 2000s; it means "do not use more than the indicated power with this speaker." Trapezoidal or "wedge-shaped" speakers have a shape which allows the speakers to be grouped into arrays so that they can be mounted on rigging.
Professional PA speakers are usually designed with internal brace "flying" hardware (e.g., steel eyebolts) for safe suspension of speakers from rigging or ceilings. Many speakers are designed with interlocking corners so that they can be vertically or horizontally stacked for large concerts. Large, heavy PA speakers are often equipped with wheeled dollys, to facilitate on-stage placement.
Some mixing consoles designed for houses of worship have automatic mixers, which turn down unused channels to reduce noise, and automatic feedback elimination circuits which detect and notch out frequencies that are feeding back. These features may also be available in multi-function consoles used in convention facilities and multi-purpose venues.
Weekend band PA systems are a niche market for touring SR gear. Weekend bands need systems that are small enough to fit into a minivan or a car trunk, and yet powerful enough to give adequate and even sound dispersion and vocal intelligibility in a noisy club or bar. As well, the systems need to be easy and quick to set up. Sound reinforcement companies have responded to this demand by offering equipment that fulfills multiple roles, such as "amp-mixers" (a mixer with an integrated power amplifier and effects) and powered subwoofers (a subwoofer with an integrated power amplifier and crossover). These products minimize the amount of wiring connections that bands have to make to set up the system. Some subwoofers have speaker mounts built into the top, so that they can double as a base for the stand-mounted full-range PA speaker cabinets.
In live theater and drama, performers move around onstage, which means that wireless microphones may have to be used. Wireless microphones need to be set up and maintained properly, to avoid interference and reception problems.
Kai Harada's article Opera's Dirty Little Secret states that opera houses have begun using electronic acoustic enhancement systems "...to compensate for flaws in a venue's acoustical architecture." Despite the uproar that has arisen amongst operagoers, Harada points out that note of the opera houses using acoustic enhancement systems "...use traditional, Broadway-style sound reinforcement, in which most if not all singers are equipped with radio microphones mixed to a series of unsightly loudspeakers scattered throughout the theatre." Instead, most opera houses use the sound reinforcement system for acoustic enhancement, and for subtle boosting of offstage voices, onstage dialogue, and sound effects (e.g., church bells in Tosca or thunder in Wagnerian operas).
Acoustic enhancement systems include LARES (Lexicon Acoustic Reinforcement and Enhancement System) and SIAP, the System for Improved Acoustic Performance. These systems use microphones, computer processing "with delay, phase, and frequency-response changes", and then send the signal "... to a large number of loudspeakers placed in extremities of the performance venue." Another acoustic enhancement system, VRAS (Variable Room Acoustics System) uses "...different algorithms based on microphones placed around the room." The Deutsche Staatsoper in Berlin and the Hummingbird Centre in Toronto use a LARES system. The Ahmanson Theatre in Los Angeles, the Royal National Theatre in London, and the Vivian Beaumont Theatre in New York City use the SIAP system.
As well, rental systems need to be easy to use and set up, and they must be easy to repair and maintain for the renting company. From this perspective, speaker cabinets need to have easy-to-access horns, speakers, and crossover circuitry, so that repairs or replacements can be made. Since rental gear is often used by fairly inexperienced users, rental companies often rent powered amplifier-mixers, mixers with onboard effects, and powered subwoofers, which are easier to set up and use.
Another problem with designing sound systems for live music clubs is that the sound system may need to be used for both prerecorded music played by DJs and live music. If the sound system is optimized for prerecorded DJ music, then it will not provide the appropriate sound qualities needed for live music, and vice versa. Lastly, live music clubs can be a hostile environment for sound gear, in that the air may be hot, humid, and smoky; in some clubs, keeping amplifiers cool may be a challenge.
The oldest method of system calibration involves a set of healthy ears, test program material (i.e. music or speech), a graphic equalizer, and last but certainly not least, a familiarity with the proper (or desired) frequency response. One must then listen to the program material through the system, take note of any noticeable frequency changes or resonances, and subtly correct them using the equalizer. However, simply relying on a prior knowledge of how the program material "should" sound can be very subjective, and should be avoided if at all possible.
A more objective method of manual calibration requires a pair of high-quality headphones patched into the input signal before any processing (such as the pre-fade-listen of the test program input channel of the mixing console, or the headphone output of the CD player or tape deck). One can then use this direct signal as a near-perfect reference with which to find any differences in frequency response. This method may not be perfect, but it can be very helpful with limited resources or time, such as using pre-show music to correct for the changes in response caused by the arrival of a crowd.
Because this is still a very subjective method of calibration, and because the human ear is so dynamic in its own response, the program material used for testing should be as similar as possible to that for which the system is being used.
Since the development of digital signal processing (DSP), there have been many pieces of equipment and computer software designed to shift the bulk of the work of system calibration from human auditory interpretation to software algorithms that run on microprocessors.
One modern tool for calibrating a sound system using either DSP or Analog Signal Processing is a Real Time Analyzer (RTA). This tool is usually used by piping pink noise into the system and measuring the result with a special calibrated microphone connected to the RTA. Using this information the system can be adjusted to help achieve the desired response.