Available on which Fractal Audio products

Axe-Fx III: 2 blocks
Axe-Fx II: 2 blocks
AX8: 1 block
FX8: 2 blocks
FAS-FX Reverb

X/Y and channels

Axe-Fx III: 4 channels
Axe-Fx II: X/Y
AX8: X/Y
FX8: X/Y
FAS-FX Reverb: n/a

FAS-FX Reverb plugin

Fractal Audio's reverb is available as a software plugin (AAX, AU, VST): FAS-FX Reverb.

Reverb in the Axe-Fx III

"The new reverb algorithms use more CPU than those from the Axe-Fx II would use but they sound better. Also the reverb defaults to high-quality mode whereas the II defaults to normal-quality mode." source

Demo of Reverb types in the Axe-Fx III, by Camilo Velandia

Reverb types

The reverbs (types) are based on several algorithms, such as:

SPRING: simulates the physical spring reverb in a guitar amp.
ROOM: simulates natural realistic short reverb in a room.
CHAMBER: simulates natural reverb in a boxy chamber (bright, resonant reverb).
HALL: simulates natural long reverb in a concert hall. Similar to Room but a little less smooth and with some response peaks. Use this when you want your sound to stand out.
PLATE: simulates a vibrating reverb plate with a smooth sound.
STUDIO: models classic digital studio reverb units.
TUNNEL: simulates natural reverb in a tunnel (long, narrow space, great for special effects).

Some of the types are modeled after famous reverb units:

London Plate: probably based on the EMT 140 plate reverb.
Sun Plate: probably based on the plate reverb used on Sun Studio records.
North and South Church: probably inspired by the Bricasti. source

"The reverb block in the Axe-Fx is very powerful. Turn the Early Level up and the Late Level down and you can hear the different "spaces". There's a variety of room shapes from almost square to long and narrow along with plates and springs. The Size parameter allows you to scale the dimensions." source

"David Griesinger probably knows more about reverb than everyone else combined. He's the father of the Lexicon reverbs. According to him, and I have no reason to doubt him, real reverb (i.e. reverb from a real space) is actually inferior to synthetic reverb. This is due to human perception. Real reverb (and by extension convolution reverb) actually reduces intelligibility and clarity due to the particular nature of the decay, the decay being exponential. Synthetic reverb allows one to craft the decay curve thereby rendering improved clarity. If the decay curve is flat for a period and then exponential it doesn't clutter the desired program material. The new reverb algorithms in the Axe-Fx are based on his theories." source

Position of the Reverb block

Reverb usually comes after amp/cab, in studio environments and rack rigs.

But try the Spring Reverb before the Amp block for authenticity.

"If there isn't distortion or modulation in the delay/reverb then the order is irrelevant since they are then Linear Time Invariant (or shift invariant in digital parlance). If there is a small amount of distortion or modulation then the order is probably still irrelevant. If there is a lot of distortion or modulation then the order may make a difference. However, typically the biggest difference, as noted above, is series vs. parallel since h1(t)*h2(t) is not the same as h1(t)+h2(t). If LTI h1*h2 = h2*h1. It may seem counter-intuitive that the order doesn't matter but try it and you'll be surprised." source

"Placing reverb after the Cab is the recommended routing. The reverb is stereo. The Cab block may be mono so you would lose the stereo field. Both blocks are linear so there is no advantage to placing reverb before the Cab."

"For authentic spring reverb sound you want the reverb in front of the amp block. A big reason spring reverbs sound the way they do is that they get colored by the amp." source

Spillover of reverb when switching presets, scenes or channels

Read this: Spillover.

Room ambience reverb

"If you just want the "room" stuff turn the reverb level down and the early reflections level up." source

The Cab block also provides a room ambience reverb.

Reverb "Mix Law"

When using Mix to control the volume level of the Reverb, the volume level of the dry signal is affected also. In other words, when increasing Reverb with Mix, the dry signal's level decreases. To deal with this: turn up Mix to 50% and set Level to +3dB and use Input Gain to set the desired amount of Reverb. Or put the Reverb in a parallel row with Mix at 100% and use Level or Input Gain to set the desired reverb level.

CPU usage

When CPU usage on the AX8 and FX8 gets above 90%, they will automatically disable blocks. Usually that’s Reverb.

Important: when an effect such as Reverb is automatically disabled because of CPU overload, audio passes through as if it were a shunt. If the Reverb is placed in a parallel row, this can cause a signal level issue.

Common methods to reduce CPU usage by the Reverb block is to switch to Normal Quality, to reduce Density or to switch to Spring Reverb as it uses the least amount of CPU.

Mono and stereo reverb

Signals at the input of the Reverb block are summed to mono. The Reverb's output is stereo. You can make it mono by setting Stereo Width to 0%.

"Stereo imaging is not effected. The L/R summation is only into the reverb engine itself. The dry signal is unaffected." source

The FAS-FX Reverb plugin does not sum the input signal to mono, it keeps and processes both sides in stereo.

EQ-ing reverb

"The reverb tail is divided into three bands. The low-frequency band is defined by LF Xover and LF Time. LF Xover sets the crossover frequency between the low and mid bands. LF Time controls the decay time of the low band relative to the mid band. So if LF Time is 2.0 the low-frequency decay time will be twice as long as the mid-band time. You may be able to use this to reduce low-frequency buildup. High-frequency decay time works a bit differently. There is a single HF Time parameter. It controls the high-frequency absorption of the virtual room. The lower the value the faster the high frequencies decay relative to mid-band. A value of 1.0 means no high-frequency absorption. The EQ page then controls equalization of the resulting tail. You can further shape the sound of the reverb using this, if desired. Real rooms tend to have a slightly longer low-frequency decay and a shorter high-frequency decay. However if you're using LOTS of simulated reverb within a real reverberant environment then you can get low-frequency buildup. The tips above should help you adjust to the environment." source

Parameters

Parameters table

Parameter	Axe-Fx III	Axe-Fx II	AX8, FX8
Type	yes
Time	yes
Size	yes
Predelay	yes
Early Level	yes
Late Level	yes
Crossover Frequency, Low Frequency Time, High Frequency Time	yes
Mod Depth, Mod Rate	yes
Low Cut, High Cut	yes
Low Mid Frequency, Low/High Mid Q, Low/High Mid Gain,	yes
Ducker Attenuation, Threshold, Release	yes
Echo Density	yes
Early Diffusion	yes
Early Diffusion Time	yes
Early Decay	yes
Mic Spacing	yes
Late Input Mix	yes
Quality	yes
Hold	yes
Wall Diffusion	yes
Input Diffusion, Diffusion Time	yes
Stereo Spread	yes
Number of Springs	yes
Spring Tone	yes
Spring Drive	yes
Mod Depth, Mod Rate	yes
LFO Phase	yes

Quality

There are two “Quality” options in the Reverb block: Normal and High. High quality uses significantly more CPU but provides world-class reverberation algorithms. In most situations, especially live, Normal Quality already provides desired results.

The Spring Reverb type doesn't differentiate between Normal and High Quality.

The Axe-Fx II, FX8 and AX8 default to Normal Quality, where the Axe-Fx III defaults to High Quality.

Time

Maximum Reverb Time is 100 seconds.

"It requires a lot of calculations to change the Reverb time which is what Hold does (it increases it to a couple hours or something). If CPU use is high the calculations won't finish during one block which results in a click." source

Ducking

The Reverb block provides "ducking" parameters.

Hold

When Hold is activated, the wet input to the block is muted and time is set to infinity. This can be used to achieve pad sounds and drone notes/chords.

By attaching an external controller to Hold, an external pedal or switch can be used to control this "freeze" feature.

Tutorial by Cooper Carter for G66.

Early Diffusion

This sets the amount of diffusion in the early reflections. Higher values result in fuzzier and less distinct echoes. Lower values result in sharp, distinct reflections.

Early Diff Time

This scales the delay time of the early reflections diffusers. Adjust this control to suit the size and character of the simulated environment.

Late Diffusion

This parameter has been renamed Input Diffusion in the Axe-Fx III.

Early Decay

This controls the decay rate of the early reflections. Higher values yield faster decay.

The reverb tail is automatically set to the appropriate delay. In High Quality mode an additional parameter is available: Late Input Mix. This parameter controls the mix between the (possibly diffused) input and the early reflections data input to the late reverb algorithm. Thus this parameter mixes the output of the diffuser and the early reflections prior to inputting that data to the late reverb generator. With the Late Input Mix at 0% the High Quality mode is identical to the Normal Quality mode. Values greater than 0% mix early reflections data into the late reverb using a proprietary decorrelation technique which eliminates any metallic qualities associated with the typical diffuser techniques used in other products.

Rev Mix (Global Menu)

The Rev Mix parameter in the Global menu lets you increase or decrease the Reverb Mix level across all presets at once. This lets you tailor the reverb, based on the environment.

"Global Reverb and Effects Mix are relative. If set to 0% the mix is set by the preset. If set to, say, -10% the mix would be 10% less than the preset mix.” source

Note that Rev Mix may not work well with reverb blocks in parallel rows which have Mix at 100%.

Special effects

Room ambience in the Cab block

Read this: Cab block and IRs.

Gated reverb

Create a gated reverb.

More information

Reverb in Wikipedia.
Legacy thread providing great info for tweaking the Pre-delay parameter and creating ambience, sitting in a mix. Note: "tail delay" is also discussed but this parameter doesn't exist anymore in current firmware.
Scott Peterson's tutorial.
About acoustics and reverberation.

Reverb block

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