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Audio system information
1. Components of a typical system
2. Some important audio terminology
1. Components of a typical system
Most public address (PA)/audio/sound systems in live theatre, productions and events have the following components. Note that the components are listed in 'signal flow' order, that is, the order in which the devices interact with the signal (sound) in a typical system.
- Sound sources: equipment that generates sound as an analogue, electrical or digital signal. Examples include microphones, CD players, electric guitars, synthesisers and samplers.
- A mixing desk: a device with which to control, balance and treat the various sound sources. The mixer may have a series of inputs which allow most types of sound sources to be connected and routed to the next part of the signal chain.
- Signal processors: devices used to treat or manipulate the signal from the sound sources so as to make it conform to the desired effect. Examples include compressors, reverb units and equalisers. Some of these may be in the mixing desk and some may be external devices.
- Amplifiers: devices designed to increase the level of the mixed and treated signals to the point at which they can be transmitted to a loudspeaker to create sufficient volume.
- Loudspeakers (or speakers): these convert the electrical signal from an amplifier into sound waves and direct the resultant sound into the auditorium.
A simple signal flow diagram for a small PA system
2. Some important audio terminology
A few basic terms are used frequently when dealing with audio and sound reinforcement system:
- Amplitude: the 'height' of a waveform, heard by the ear as volume. Increasing the amplitude of a noise will make it sound louder.
- dB or decibels: these are figures used to express the volume or Sound Pressure Level (SPL) of a noise. Odb is at the threshold of hearing - only just audible. 40 dB is at normal conversation level. 100 dB is equivalent to the sound of a train passing nearby. 130 dB is the threshold of pain and 150 dB is a jet engine at one metre.
- Frequency: the 'speed' of a waveform, heard as pitch, that is, how high or low sounds are to the ear.
- Ohms: the unit of electrical resistance. For example, the higher the ohm rating of a speaker, the greater the level of signal required to create the same volume from a lower ohm rated speaker.
- SPL: Sound Pressure Level is literally loudness, often expressed in dB.
- Watts: the unit of electrical power derived from the current (or 'quantity' of electricity) multiplied by the voltage (or 'pressure' at which the current is delivered). A small practice amp for a guitar may only provide ten watts, whereas a loud amplifier for a small theatre may be 400 watts per side and a large amplifier for an arena rock concert may be many thousand watts.
Except for frequency (which is heard as pitch), all of the terms explained above are either directly or indirectly concerned with signal level. This is due partly to the fact that sound operators are constantly trying to achieve the perfect balance of signal levels, but also because different components are designed to work optimally at very different levels microphone level, line level or loudspeaker level.
- Microphone level: microphone (or mic) level is a very low level of voltage, measured in thousandths of a volt, which is present at the output of a microphone. Microphone signals must be amplified at a pre-amp or at the microphone input on a mixing desk.
- Line level: equipment such as CD players, tape recorders, MiniDisc, signal processors and mixing desks transmit (and receive) their audio signals at line level. This is still a relatively low level, with the standard set at 0.775 volts, usually referred to as 0dBu, which is equivalent to the softest sound audible to a young person with excellent hearing.
- Loudspeaker level: loudspeaker (or speaker) level is significantly higher than line level. Signals at line level would not produce enough power to make loudspeakers move and would therefore not produce sound. Line level signals are increased by power amplifiers which increase the voltage of the signal. This then becomes an increase in amplitude and thus SPL at the speaker.
3. Microphones
Microphones are the most frequently used audio device in live theatre, productions and events. They convert sound wave energy into electrical energy and are therefore the exact opposite of speakers. There are several different types of microphone:
- Dynamic microphone: a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil. Dynamic microphones are robust and relatively inexpensive and are used in a wide variety of applications.
Studio Condenser microphone |
XY Stereo Condenser microphone |
Dynamic Instrument microphone |
Dynamic Vocal microphone |
- Condenser microphone (also known as a capacitor microphone): the diaphragm acts as one plate of a capacitor, and vibrations produce changes in a voltage maintained across the capacitor plates. Condenser microphones are expensive and require an external power supply but give a high-quality sound signal and are used in laboratory and studio recording applications.
- Foil or back electret microphone: a relatively new type of condenser microphone which is now one of the most widely used. They are used in many applications, ranging from high-quality PA to built-in microphones in small sound recording devices. Like other condenser microphones they require an external power supply or a battery. They are frequently phantom-powered in sound reinforcement applications.
- Ribbon microphones: a thin, corrugated metal ribbon is suspended in a magnetic field. Vibration of the ribbon in the magnetic field generates a changing voltage. Ribbon microphones detect sound in a bi-directional pattern and this characteristic makes them useful in applications such as radio and television interviews, where it cuts out much extraneous sound.
- Boundary microphone: is mounted on a flat plate which acts as a reflective surface directing sound into the microphone capsule. They are used for general pick-up over a large area without as many reflected sound waves to interfere.
- PZM (Pressure Zone Microphone): an omnidirectional boundary microphone for picking up sound from all around.
- Piezo microphone: uses the phenomenon of piezoelectricity - (that is, the tendency of some materials to produce a voltage when subjected to pressure) to convert vibrations into an electrical signal. This type of microphone is often used to microphone acoustic instruments for live performance (most acoustic/electric guitars use piezo pick-ups) or to record sounds in unusual environments, for example underwater.
Directionality
This is the phenomenon by which a microphone is more sensitive to sounds coming from particular directions.
Omnidirectional |
Bi-directional |
Cardioid |
Hypercardioid |
Shotgun |
Depending on various aspects of a microphone's construction, it may be equally sensitive to sound coming in all directions (an omnidirectional microphone), or it may be more sensitive to sound coming from a particular direction (a unidirectional microphone). The most common of the unidirectional microphones is sometimes called a cardioid microphone, because the sensitivity pattern somewhat resembles the shape of a heart. Most vocal mikes are cardioid or hyper-cardioid (similar to cardioid but with a tighter area of front sensitivity and a tiny lobe of rear sensitivity). Some microphones have more complex sensitivity patterns. Most ribbon microphones are bi-directional, receiving sound from both in front of and behind the element. This type of response is also known as a figure-8 pattern, because of its shape. Shotgun microphones, the most directional form of studio microphone, reserve most of their sensitivity for sounds directly in front of, and to a lesser extent, the rear of the microphone. Shotgun microphones also have small lobes of sensitivity to the left and right.
4. Connectors
There is an extremely large number of different types of connectors used in the audio world. The most frequently encountered and therefore the most important types of connector are:
Three-pin XLR or Canon connectors
These are used for audio gear destined for the performance stage or recording studio. The three-pin XLR (eXchanging Line Resistor) connector utilises a 'balanced' circuit, which consists of two phase-opposite signals on discrete conductors and a third conductor that provides a ground reference. Noise (such as induced hum) will appear equally - but out of phase - on both conductors where it can then be easily cancelled. It is very common for microphones to connect to mixers with a male three-pin XLR to a female three-pin XLR cable.
Male and female XLR plugs |
RCA phono plug |
TRS quarter inch Jack (phone) plug |
RCA (phono) plug
This is the most common connector type on consumer video, as well as for both digital and analogue audio devices. It is not a very good connector but it is unfortunately what equipment manufacturers choose to use. It was originally designed to connect turntables and amplifiers inside phonographs made by RCA. It is an unbalanced connector and best used over only short distances. Most hi-fi gear and consumer/domestic audio gear uses pairs of RCA phono to RCA phono cables, with both devices having sockets.
1/4 inch 'phone' plug or jack
Not to be confused with the RCA phono plug, this was developed by AT&T for use in early manual telephone switchboards, hence its name 'phone plug'. It can interconnect three conductors, referred to as tip, ring, and sleeve (TRS), and is therefore still a balanced connector. ¼ inch jacks are more commonly associated with semi-professional and project studio gear although the smaller size of the connector compared to the three-pin XLR plug has meant they are becoming more popular on professional gear. Most signal processors use ¼ inch jacks to connect with mixers. Many small amplifiers connect to speakers with ¼ inch jack to ¼ inch jack cables, with both devices having sockets.
TOSLINK
| Short for Toshiba Link and called ADAT Optical by Alesis, TOSLINK is also a CD 'red book' standard that allows for digital audio (both left and right channels or multi-channel sound) to be transported between components using an optical conductor and light as the carrier. Contrary to popular belief, TOSLINK does not use laser light but instead relies on the output of a simple LED. The Achilles heel of TOSLINK is the optical quality of the interface. The plastic conductors used in cheap cables can damage data and compromise performance. TOSLINK connection performance is somewhat limited by the bend radius (bending a light conductor causes internal reflections and signal loss) of the conductor design. Additionally, the TOSLINK interface is length sensitive with maximum performance available only with runs of less than 20 feet. TOSLINK is used by Alesis and many other companies to connect digital multitrack recorders, digital mixers and other digital peripherals. |
TOSLINK |
IEEE 1394 or FireWire or MLan or i.Link
| IEEE 1394, also named Firewire by Apple, MLan by Yamaha and i.Link by Sony, is a standard I/O interface designed to service the demands of the emerging multimedia bus requirements. 1394 is unique in its ability to carry video and audio with outstanding quality, based on its high bandwidth capabilities. This simple to use, peer-to-peer connection system can carry a dense multimedia data stream between computers, video cameras, high-fidelity audio products and imaging devices at a maximum speed of 400Mbps. The hot plugging capabilities of 1394 make it ideal as a consumer interface as well as a professional protocol, allowing users to daisy chain devices together on the fly. |
Various FireWire connectors |
5. Mixing desks
A mixing desk, also called a mixing console, mixing board, sound board or simply mixer, is designed to manage the many and varied sound sources, sound effects, signal processors, amplifier and speaker configurations required for more complex productions, events or live theatre. The overall job of a mixer is to combine many input signals into fewer output signals. Despite the huge number of possible input channels (desks typically have 8, 12, 16, 24, 32 or up to 128 inputs), each channel is almost always identical, and the sound sources are usually mixed down to a few stereo pairs of output. The process of mixing can be simplified as follows: signals are sent to an input, levels are increased/altered, signals are treated, equalised and balanced then sent out to amplifiers and subsequently speakers. Mixers can be understood as plumbing for audio signals. Through a series of pipes and taps (circuits and pots/faders) the material is routed to where it is needed and sent on its way.
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