Sound reinforcement system in the context of "Audio engineering"

Audio feedback (also known as acoustic feedback, howlround in the UK, or simply as feedback) is a positive feedback situation that may occur when an acoustic path exists between an audio output (for example, a loudspeaker) and its audio input (for example, a microphone or guitar pickup). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The frequency of the resulting howl is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen, hence it is also known as the Larsen effect.

Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a sound reinforcement system or PA system. Audio engineers typically use directional microphones with cardioid pickup patterns and various electronic devices, such as equalizers and, since the 1990s, automatic feedback suppressors, to prevent feedback, which detracts from the audience's enjoyment of the event and may damage equipment or hearing.

A loudspeaker enclosure or loudspeaker cabinet is an enclosure (often rectangular box-shaped) in which speaker drivers (e.g., woofers and tweeters) and associated electronic hardware, such as crossover circuits and, in some cases, power amplifiers, are mounted. Enclosures may range in design from simple, homemade DIY rectangular particleboard boxes to very complex, expensive computer-designed hi-fi cabinets that incorporate composite materials, internal baffles, horns, bass reflex ports and acoustic insulation. Loudspeaker enclosures range in size from small "bookshelf" speaker cabinets with 4-inch (10 cm) woofers and small tweeters designed for listening to music with a hi-fi system in a private home to huge, heavy subwoofer enclosures with multiple 18-inch (46 cm) or even 21-inch (53 cm) speakers in huge enclosures which are designed for use in stadium concert sound reinforcement systems for rock music concerts.

The primary role of an enclosure is to prevent sound waves generated by the rearward-facing surface of the diaphragm of an open speaker driver interacting with sound waves generated at the front of the speaker driver. Because the forward- and rearward-generated sounds are out of phase with each other, any interaction between the two in the listening space creates a distortion of the original signal as it was intended to be reproduced. As such, a loudspeaker cannot be used without installing it in a baffle of some type, such as a closed box, vented box, open baffle, or a wall or ceiling (infinite baffle).

Audio crossovers are a type of electronic filter circuitry that splits an audio signal into two or more frequency ranges, so that the signals can be sent to loudspeaker drivers that are designed to operate within different frequency ranges. The crossover filters can be either active or passive. They are often described as two-way or three-way, which indicate, respectively, that the crossover splits a given signal into two frequency ranges or three frequency ranges. Crossovers are used in loudspeaker cabinets, power amplifiers in consumer electronics (hi-fi, home cinema sound and car audio) and pro audio and musical instrument amplifier products. For the latter two markets, crossovers are used in bass amplifiers, keyboard amplifiers, bass and keyboard speaker enclosures and sound reinforcement system equipment (PA speakers, monitor speakers, subwoofer systems, etc.).

Crossovers are used because most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and absence of distortion. Most hi-fi speaker systems and sound reinforcement system speaker cabinets use a combination of multiple loudspeaker drivers, each catering to a different frequency band. A standard simple example is in hi-fi and PA system cabinets that contain a woofer for low and mid frequencies and a tweeter for high frequencies. Since a sound signal source, be it recorded music from a CD player or a live band's mix from an audio console, has all of the low, mid and high frequencies combined, a crossover circuit is used to split the audio signal into separate frequency bands that can be separately routed to loudspeakers, tweeters or horns optimized for those frequency bands.

A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or audiobooks. CD players may be part of home stereo systems, car audio systems, personal computers, or portable CD players such as CD boomboxes. Most CD players produce an output signal via a headphone jack or RCA jacks. To use a CD player in a home stereo system, the user connects an RCA cable from the RCA jacks to a hi-fi (or other amplifier) and loudspeakers for listening to music. To listen to music using a CD player with a headphone output jack, the user plugs headphones or earphones into the headphone jack.

Modern units can play audio formats other than the original CD PCM audio coding, such as MP3, AAC and WMA. DJs playing dance music at clubs often use specialized players with an adjustable playback speed to alter the pitch and tempo of the music. Audio engineers using CD players to play music for an event through a sound reinforcement system use professional audio-grade CD players. CD playback functionality is also available on CD-ROM/DVD-ROM drive-equipped computers as well as on DVD players and most optical disc-based home video game consoles.

Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or compressing an audio signal's dynamic range. Compression is commonly used in sound recording and reproduction, broadcasting, live sound reinforcement and some instrument amplifiers.

A dedicated electronic hardware unit or audio software that applies compression is called a compressor. In the 2000s, compressors became available as software plugins that run in digital audio workstation software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect. A limiter is a compressor with a high ratio and, generally, a short attack time.

A feedback suppressor is an audio signal processing device which is used in the signal path in a live sound reinforcement system to prevent or suppress audio feedback.

Digital feedback reduction is the application of digital techniques to sound reinforcement in order to reduce audio feedback and increase headroom.