Cab Sims, IRs and FRFR: What Each Part of a Modeler Rig Does

A modern digital guitar rig can pack an amplifier, speaker cabinet, microphone chain, effects and routing into one pedalboard or plugin. That convenience also makes it easy to lose track of which part is doing what. The simplest way to keep track is to treat the signal chain as three separate jobs: amp stage → cabinet stage → playback stage.

The amp stage creates or reproduces the behaviour of the amplifier, whether through a pre-built amp model or a profile or capture of a real rig. (How those representations are made is the subject of Amp Modeling, Profiling and Neural Capture Explained.) The cabinet stage shapes that sound as a guitar speaker and microphone chain would. The playback stage is the physical system that finally turns the processed signal back into sound: an FRFR cabinet, a PA, studio monitors, headphones, or a conventional guitar cabinet driven by a power amp.

These stages can live inside one device, and some captures include more than one of them, but they are not interchangeable. An IR is not an FRFR speaker. A cab sim is not an amp capture. And an FRFR cabinet is not another cabinet simulation.

An impulse response (IR) is not an audio recording in the usual sense. In signal processing, it is a way of characterising a linear time-invariant system. ^S8 In guitar rigs, cabinet IRs are commonly used to reproduce the measured response of a speaker, cabinet and microphone setup. A typical one represents a particular cabinet and speaker captured with a particular microphone at a particular position; change the microphone or its position and you can get a different IR.

Exactly what an IR contains depends on how it was made. Line 6, for example, describes cab IRs as measurements of speaker-cabinet and microphone combinations, and notes that a capture may also include some post-processing such as EQ. ^S1

That is why an IR is fundamentally different from an amp model or a neural capture of an amplifier, which is intended to reproduce nonlinear behaviour such as distortion, compression and gain-dependent response. A safe working definition: a guitar cabinet IR is a measured linear response used to reproduce the filtering of a particular speaker, cabinet and microphone chain. It should not be described as a complete model of an amplifier.

An IR is data. A cab sim is the processing stage that recreates the cabinet part of the rig, and it may use one static IR, many IRs or a more elaborate IR-based system.

Line 6 says each Helix Stadium Cab is built from thousands of IRs, and lets the player move a virtual microphone by changing its position and distance from the speaker. Imported IR files, meanwhile, are processed internally as 2048-sample, 48 kHz, 32-bit floating-point mono files. ^S1 Fractal Audio’s AM4 manual describes DynaCab cabinets with visual microphone positioning, alongside a separate User Cab mode for loading conventional IR files. ^S3 Two Notes’ Captor X software has separate Virtual Cabinet and IR Loader modes: Virtual Cabinet uses its DynIR cabinets with selectable microphones and positions, while IR Loader loads third-party WAV IRs. ^S6

Saying “cab sim” and “IR” as though they mean the same thing therefore hides a real difference, and the details of IR length, format and resolution belong to each platform’s implementation, not to the idea of an IR itself.

A capture may or may not already include the cabinet stage. An amp-only capture can be followed by a separate cab sim or IR. A capture made from a miked amplifier and cabinet may already contain the speaker and microphone response, in which case adding another virtual cabinet is normally unnecessary. The source of a profile or capture matters as much as its name.

Look at what comes after the modeler. If the destination is a PA, an FRFR cabinet or studio monitoring, the signal normally needs its virtual cabinet stage. IK’s TONEX Board guide says to use a Tone Model with its cabinet, a VIR cabinet or a custom IR enabled in that case. ^S4

If the destination is a traditional guitar speaker cabinet, the logic changes. Line 6 says that with real power amps and wooden cabinets, the player should avoid adding a redundant Cab block ^S1; IK suggests bypassing any cabinet in a TONEX Board preset ^S4; and Fractal says to bypass speaker simulation when using a neutral power amp and guitar cab, adding that a traditional (non-neutral) power amp also needs Power Amp Modeling disabled. ^S3

A useful default: modeler → PA, FRFR or monitors, virtual cab normally on. Modeler → power amp → real guitar cabinet, virtual cab normally off. It is a default signal-flow rule, not a law against experimenting.

Players sometimes call the mistake “double cab”. IK’s TONEX Pedal manual uses the phrase “unwanted double cab filtering” ^S5: a signal that already carries virtual cabinet filtering is played through another physical guitar speaker, so two speaker responses are stacked.

FRFR stands for full-range, flat-response. In the guitar-modeling world it describes a powered playback system meant to reproduce the finished modeler signal over a much wider range than a conventional guitar speaker. Fender, for example, calls its Tone Master FR-12 a full-range, flat-response powered speaker designed for digital guitar amp modelers and profilers. ^S7

The key word is playback. An FRFR system does not create the amp model or the capture, and it does not replace the cab simulation inside the modeler; it reproduces the signal it is given.

“Flat response” should not be read as a promise of mathematical neutrality. Real speakers have drivers, enclosures, crossovers, dispersion patterns, sometimes built-in EQ, and they interact with the room. FRFR is best understood as a design goal and product category: a system intended to add less deliberate guitar-speaker coloration than a conventional guitar cabinet. Guitar Radar has covered examples such as HeadRush’s FRFR CAB 8 and 12.

A guitarist standing beside a conventional guitar cabinet hears the speaker directly in the room. A cab IR played through an FRFR monitor often represents something different: the sound of that cabinet through a microphone. Fractal’s AM4 manual notes that the close-miked sound of an IR differs from the “amp-in-the-room” experience. ^S3

Line 6 draws the same line in its Powercab guide: Flat mode is intended for full-range reproduction of a modeler signal, while Speaker Modeling mode is designed to give more of the experience of playing through a guitar speaker in the room. ^S2 Neither perspective is automatically better; they are different listening positions in the signal chain.

Live rigs do not have to choose. Fractal documents a split mode in the AM4 manual: one output keeps cab simulation for an FRFR speaker, mixer or interface, while the other bypasses it before feeding a power amp and a real guitar cabinet. ^S3 That gives front of house a direct, cabinet-simulated feed while the player stands in front of a physical cabinet.

The same logic explains why modeler-oriented power amplifiers exist: products such as Avantone Pro’s GTR-100 address the stage between a digital processor and the loudspeaker rather than the modeler itself.

A few neutral questions help locate most problems. Is a virtual cab or IR still active while the signal goes to a real guitar cabinet? Is an amp-only model or capture reaching a PA or FRFR system without any cabinet stage? Does the capture already include a cabinet, so that a second one is being added? And is the difference you notice between FRFR and a guitar cabinet a difference in perspective, miked cabinet versus speaker in the room, rather than a fault? Answering those usually shows which stage is responsible.