Isolation cabinet (guitar)

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The characteristic sound of a tube guitar amplifier as heard on the majority of professional recordings is achieved by playing the amplifier at high volumes, and using one or more microphones to capture the sound. Turning the volume up causes the pre-amplifier to drive the power amplifier into distortion and the loudspeaker to "break up", adding crunch and grit to the amplified tone. The speaker cone also generates more sound pressure, delivering more dynamic range and detail to the microphone.

However, an amplifier at full volume is extremely loud, posing a risk to hearing and an annoyance to neighbors, and will often drown out other instruments in a mix. A guitar speaker isolation cabinet is a sound-proof enclosure that surrounds, at minimum, the speaker and microphone and prevents sound leakage into the outside environment, enabling the amplifier to be turned up without excessive listening volume.

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[edit] Sizes and types

A guitar speaker isolation cabinet has a built-in mounting baffle for a guitar speaker and a permanently mounted microphone clip. A compact isolation cabinet contains a small guitar speaker such as 6 1/2" diameter and sometimes an attached power attenuator to prevent blowing the speaker.

A guitar speaker isolation box is large enough to contain a standard guitar speaker cabinet such as a 1x12" or 2x12" cabinet and a couple of compact microphone stands. Inexpensive but less effective implementations of this approach are to bury a guitar speaker and microphone in a closet, place gobo partitions around a speaker cabinet to somewhat deflect the sound, or form a tent with multiple layers of heavy blankets over a guitar speaker cabinet and microphone.

A isolation booth is a small room large enough to contain a single performer and his rig, enabling the instrument to interact with the amplifier while isolating the rest of the band from the extreme volume.

Finally, the live room of a recording studio provides sound isolation between an entire band and the control room, allowing the studio engineers to work at manageable volumes and prevent ear fatigue.

The frequency response of an isolation system depends on the number of microphones, the type of microphones, microphone positioning, cabinet dimensions, speaker size, speaker model, and the amount of sound-absorption material inside the speaker cabinet. To control the resulting response, a dedicated equalizer can be used to enhance or reduce specific frequency ranges.

[edit] Degree of sound isolation

A single-layer isolation cabinet or isolation box reduces the sound but does not make the speaker silent; significant bass leaks out. A double-layer box with dead space between the layers still leaks audible bass, if typical plywood thickness is used. Getting closer to silencing would require two very massive layers of plywood, MDF board, or soundproofing board such as Homasote or Wonderboard. An additional layer may be needed, such as for a 100-watt guitar amp with multiple efficient guitar speakers inside the box.

[edit] Combined approaches

To reduce the volume leakage or to prevent blowing the speaker or microphone, a power attenuator is sometimes used between the tube power amp and the guitar speaker in the isolation box. This reduces the power delivered to the speaker and thus the volume, but has some effect on speaker and microphone response.

To reduce volume on stage while staying near to a traditional guitar amp setup, a guitar amp can drive two parallel loads: a power attenuator driving a conventional guitar speaker cabinet (with no microphone), and a speaker isolation cabinet providing the signal for the mixer board and sound reinforcement system.

[edit] Overstressing components

Blowing a speaker is a significant possibility when using an isolation cabinet.[citation needed] A blown speaker usually has a broken wire in the coil and would need to be reconed. A blown speaker appears as an open or infinite resistance to the tube power amplifier and can "fry" expensive components in the amp, such as the output transformer or power tubes, which would then need to be replaced.

"Cranking an amp" means turning up a guitar power amplifier well into the region at which power-tube distortion is produced, generating as much as twice the amplifier's rated non-distorting wattage. Pushing a guitar amp to such an extent can destroy components of an amplifier whether using an isolation cabinet, dummy load, power attenuator, or conventional guitar speaker cabinet. In particular, tubes wear more quickly when they are consistently pushed into saturation.

[edit] See also

[edit] External links

  • Amptone - list of isolation cabinet products, and do-it-yourself isolation cabinet projects
  • Grendel Sound - Manufacturers of the Dead Room speaker isolation cabinet.