A sound module is an electronic musical instrument without a human-playable interface such as a piano-style musical keyboard. Sound modules have to be operated using an externally connected device, which is often a MIDI controller, of which the most common type is the musical keyboard (although wind controllers, guitar controllers and electronic drum pads are also used). Controllers are devices that provide the human-playable interface and which may or may not produce sounds of its own. Another common way of controlling a sound module is through a sequencer, which is computer hardware or software designed to record and play back control information for sound-generating hardware (e.g., a DJ may program a bassline and use the sound module to produce the sound). Connections between sound modules, controllers, and sequencers are generally made with MIDI (Musical Instrument Digital Interface), which is a standardized protocol designed for this purpose, which includes special ports (jacks) and cables.
Sound modules may use any number of technologies to produce their sounds. A sound module may be an analog or digital synthesizer, a sampler, or a rompler. Electronic drum modules are sound modules which specialize in drumkit and percussion sounds. Drum modules may be triggered by external trigger pads or pickups attached to an acoustic drum as well as through MIDI controller pads. Drum modules are distinguished from drum machines through their lack of dedicated on-board triggers and lack of an integrated sequencer. Sound modules are often rack-mountable, but might also have a table-top form factor, particularly when the intended user is a DJ or record producer. The height of a sound module is often described in rack units ("U") or unit. Small sound modules are mostly 1U in height, the larger models a multiplication e.g. 2U or 3U. Despite the name, "sound module", most sound modules do not produce any audible sound until their output is plugged into a keyboard amplifier and loudspeaker or a PA system.
Because most electronic instruments are designed in a modularized way, manufacturers often release a sound module version of their fully integrated instruments. For example, the 1980s-era DX-7 synthesizer/keyboard was also sold as a standalone "sound module", the TX-7. A sound module may have all the other features of the controller-equipped version, but it often has a smaller display or limited programming controls. In this case, instrument and other sounds can be loaded through MIDI or external media. In some cases, sound modules have expanded capacity for sounds in comparison to the controller-equipped version. In the 2010s, hardware sound modules have largely been replaced with software equivalents ("software synths"), due to the increased speed and processing power of computers and their decrease in price. Nevertheless, some DJs, EDM musicians, and record producers continue to use vintage 1980s sound modules for their unique, retro sound.
A sound module may also be referred to as "tone generator", "synth module", "MIDI sound generator" or "rack-mount synthesizer". With electronic drums, the sound module is sometimes colloquially called the "brain".
A sound module has the same advantages over a fully integrated instrument as does any system with a modularized design:
- Cost — a sound module is cheaper than a comparable instrument equipped with a controller
- Space — a sound module takes up less room than an instrument equipped with a controller
- Portability - a performer going on tour can bring only her/his sound modules, so long as a MIDI controller is provided as "backline" gear (e.g., a MIDI keyboard). Many professional studios have a MIDI stage piano on hand.
- Expandability — many sound modules can be expanded with sounds and memory.
- Troubleshooting – if a sound module develops problems, just this one unit can be removed for repair or replacement, leaving the rest of a keyboard player's rig the same (e.g., other rack-mounted sound modules, power amps, etc.).
- Obsolescence cycles — when it becomes obsolete, a sound module can be replaced without changing a favorite controller, or vice versa.