specific frequency ranges. The small volume of an isolation cabinet does not produce audible room reverberation, so the sound generally has to be enhanced with an electronic reverb. 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. 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. A speaker isolation cabinet can be combined with a Direct Inject signal. A DI signal is run from the guitar amplifier or from a guitar amp power attenuator to one channel of the mixing console. A miked guitar speaker in an isolation cabinet is run into another channel of the mixing console. The DI signal and miked guitar speaker can then be selectively blended, with the DI providing a more immediate, present, bright sound, and the microphone and guitar speaker providing a colored, distant, darker sound. Over stressing components Blowing a speaker is a significant possibility when using an isolation cabinet. 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