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Reverb block

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Reverb block.png

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

Camilo Velandia demonstrates Reverb types in the Axe-Fx III

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: 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.

"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."

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

"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
"The preamp of a Fender doesn't distort much, if at all. Therefore it can be considered linear. Linear systems are commutative. Therefore reverb before the amp is the same as reverb between the preamp and poweramp." source
"The reverb cannot be placed between the preamp and power amp. Doesn't matter though as putting the reverb before the amp block is the same thing for Fender amps because the preamp doesn't distort." source

About the order of Reverb and EQ:

"Reverb is linear time-invariant (LTI) which means it's commutative. IOW, you can put EQ before or after and it will sound the same. It doesn't add harmonics or overtones, by definition. Now our reverb algorithms aren't exactly LTI because they have modulation but they are "wide sense stationary" which means for all intents and purposes you can treat them as linear." source


Read this: Spillover.

Room ambience

"If you just want the "room" stuff turn the reverb level down and the early reflections level up." source
"The trick on the Axe-Fx to get room ambience is to do the following in the Reverb block:
  1. Set Predelay to 0
  2. Increase Early Level and/or decrease Late Level.
  3. Adjust Size and Mix to taste.
When Predelay is at zero the Reverb block is simulating a room. Most of the Reverb presets have a bit of predelay because that is typically done on recordings to get the reverb "out of the way". Likewise the Early Reflections are mixed low because most engineers/producers find that they clutter the mix. Playing solo that "in the room" thing is cool but it makes the sound difficult to mix." source
(firmware 2.05 for the Axe-Fx III) "The early reflections in both the cab and reverb block were tweaked. The cab block now assumes a dipole radiator so the reflections off the front wall are inverted." source

The Cab block also provides room ambience.

"The room stuff in the Cab block is a stripped-down, simplified version of the early reflections generator in the Reverb." source
"Frankly the best way to do it is to use the Early Reflections in the Reverb block to adjust your room level to taste. It's the same algorithm. The room stuff in the cab block is for those who add reverb in post-processing. The Early Reflections in the Reverb block are matched to the shape and size of the room so they are inherently better." source

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.

Reverb and CPU usage

When CPU usage on the AX8 and FX8 gets above 90%, they will start disabling blocks. Usually that’s Reverb first because it uses the most CPU.

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

Note: 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.

Mono and stereo reverb

The Reverb's output is stereo. You can make it mono by setting Stereo Width to 0%.

The signal at the input of the Reverb block (only the part for reverberation) is summed to mono.

"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 for reverberation 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 table

Parameter Axe-Fx III Axe-Fx II AX8, FX8
Type yes
Time yes
Size yes
Predelay yes
Early Level yes
Late Level yes
HF Decay Time yes
LF Decay Time yes
LF Crossover yes
Early Decay yes
Late Input Mix yes
Quality yes
Hold yes
Wall Diffusion yes
Input Diffusion yes
Diffusion Time yes
Crossover Frequency yes
Low Frequency Time yes
High Frequency Time yes
Echo Density yes
Early Diffusion yes
Early Diffusion Time yes
Mic Spacing yes
Stereo Spread yes
Low Cut yes
High Cut yes
Low Mid = Freq 1, Q 1, Gain 1 yes
High Mid = Freq 2, Q 2, Gain 2 yes
Mod Depth yes
Mod Rate yes
LFO Phase yes
Ducker Threshold yes
Ducker Attenuation yes
Ducker Release yes
Spring — Number of Springs yes
Spring — Tone yes
Spring — Drive yes

Reverb 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.

Reverb Time

Maximum Reverb Time is 100 seconds.


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

"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
"When activating reverb hold the time is increased to infinity. This causes a bunch of stuff to be recalculated which can cause a click. I'll have to look into it." source

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.


Values beyond +/-100% increase the apparent image beyond the stereo field.

Low Freq Time and High Freq Time

"These set the decay time relative to the midband time for the low frequencies and high frequencies. If you set the Time to, say, 10 seconds and High Freq Time to 0.1 the high frequencies will decay to -60 dB in 1 second." source

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 set the amount of 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

Gated reverb

Create a gated reverb

Shimmer reverb

There's no "shimmer" Reverb type. Use the Plex Delay block instead.

More information about Reverb