so much so that it needs to be considered part of the instrument's tone. If the clean signal from a guitar amplifier or pre-amplifier is captured directly (i.e., before it is sent to a speaker cabinet) it will very often be rather brittle and thin, with no "resonant" depth, particularly if the guitar signal is from a string pickup (i.e., the type used in a solid-body electric guitar). Vibration pickups, or microphone pickups, as used in many hollow body electro-acoustic guitars, aren't as strongly affected. The result can sound excessively shrill, scratchy and fizzy, completely different from the smooth tones that listeners hear in recordings or live performances. When driven hard, guitar speakers produce complex behavior, which affects the sound of the instrument. There will be some power compression, several kinds of distortion, even mechanical limiting as one or more drivers approach their physical limits (e.g. cone excursion). A guitar speaker shows a nonlinear frequency response depending on the speaker's load, e.g. the frequency response, and various distortions, at small amplitudes is different from those at large amplitudes. Isolation cabinets and emulation devices A guitar speaker isolation cabinet contains a guitar speaker and one or two microphones in a single- or double-layer soundproofed box. These devices allow the capture of the sound of a guitar amplifier and speaker being played at high levels, while minimizing "bleed through" between tracks in the mix. These are almost exclusively used in recording studios and during live performances being recorded. As an alternative to the isolation cabinet, there are guitar speaker cabinet emulating circuits or signal processors (also known as direct boxes or preamplifier-DI boxes), allowing the sound of a guitar amplifier to be fed directly into a PA system or recording equipment without the need for a speaker cabinet and microphone. Direct boxes are used more often with electric bass than with electric guitar, because the tone of a guitar amplifier and speaker is often considered to be a key element of an electric guitarist's tone. While DI boxes are used to route an electric bass signal to a mixing board, the audio engineer also often uses a mic set up in front of the bassist's speaker enclosure, to capture the bass player's preamped, equalized signal from the speaker cabinet. The engineer can then use either the DI out signal or the miked cabinet in the live or recording mix, or use a