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Reverb block

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Iii-reverb.jpg Reverb block.png

Available on which products

  • Axe-Fx III: 2 blocks
  • FM9: 2 blocks
  • FM3: 1 block
  • Axe-Fx II: 2 blocks
  • AX8: 1 block
  • FX8: 2 blocks
  • FAS-FX Reverb plugin

Channels or X/Y switching

  • Axe-Fx III and FM9 and FM3: 4 channels
  • Axe-Fx II: X/Y
  • AX8: X/Y
  • FX8: X/Y

FAS-FX Reverb plugin

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

Reverb in the Axe-Fx III, FM9 and FM3

Reverb has been improved in firmware Ares and later. Also, lots of types have been added, including various "cloud" types and a Deluxe Spring Reverb.

(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." [1]

Firmware Ares 12.xx:

"Added ULTRA-HIGH quality mode to Reverb block. This mode increases the modulation at the expense of a slight increase in CPU usage."

"Improved Spring Reverb algorithm. Also added “Boiiinnng!” parameter which controls the “springiness” of the reverb."

"Improved Reverb algorithms."

"Added ECONOMY mode to Reverb block. This can be used to reduce CPU usage when building elaborate presets."

"Improved CPU usage for Reverb block in high-quality modes."

Firmware Cygnus 16.xx:

"All the reverb types have been updated in addition to updated algorithms. You should find the reverbs to be more natural and spacious sounding now." [2]

Camilo Velandia demonstrates Reverb types in the Axe-Fx III


The Reverb block can create shimmer reverb effects on its own through built-in pitch shifting (firmware 17.01 and later). Several reverb types are included to demonstrate the capability: Andromeda - Aquarius - Capricorn - Centaurus - Gemini - Pegasus - Sagitarius - Ursu Major.

The Pitch Direction parameter controls the direction of the pitch shifters. FORWARD runs both shifters forward. REVERSE runs both in reverse. FOR/REV runs Voice 1 forward and Voice 2 in reverse. REV/FOR runs Voice 1 in reverse and Voice 2 forward.

The Pitch Position parameter controls the location of the pitch shifters. INPUT locates the shifters at the input of the block. MATRIX locates the shifters inside the reverb matrix. FEEDBACK locates the shifters in the matrix feedback loop. MATRIX and FEEDBACK can yield more intense effects and also helps the reverb “stick” to the notes better. FEEDBACK builds slightly slower than MATRIX. CAUTION: The MATRIX and FEEDBACK positions can cause instability at high values of Pitch Mix and Pitch Feedback. In general Pitch Feedback should be low or zero when using MATRIX or FEEDBACK.

The shimmer effect can also be created with the Plex Delay effect, and with separate Reverb and Pitch blocks.

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 in Sun Studio
  • North and South Church — inspired by the Bricasti [3]
  • "cloud" types: probably inspired by the Big Sky

"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." [4]

"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." [5]

"An "environment simulator" is reverb. There are dozens of different room shapes/types in the Reverb block. Turn down the late level and increase the early reflections level to hear the "room" shape." [6]

"The algorithm is not intended to be a binaural simulator." [7]

Factory presets

Many factory presets contain some kind of reverb. Search the Factory presets page for "reverb".

Position of the Reverb block on the grid

In the studio, in DAWs and 19" rigs reverb usually is placed somewhere at the end of the chain.

"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."

Put the Spring Reverb type before the Amp block for authenticity.

"Our spring reverb is great. You just need to put it before the amp block as this is equivalent to how it would be in the amp. If you put it after the amp block it won't sound the same." [8]

"The spring reverb in a fender amp is "effectively" in front because the preamp is essentially linear. Any distortion in a Fender comes from the power amp. Since the preamp is linear putting the reverb before the preamp is the same as putting it after preamp since linear systems are commutative, i.e. x + y = y + x. Also, our spring reverb algorithm has been updated recently and sounds better than before. I used our Fender '63 Reverb unit as the reference." [9]

"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." [10]

"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." [11]

"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." [12]

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." [13]

"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." [14]

Read this: PRE and POST routing


Spillover refers to the functionality to keeping the trails of the reverb audible after switching the effect off or switching presets, scenes, channels or X/Y. Read this: Spillover

Reverb and CPU usage

Reverb requires a lot of CPU power.

There are four Quality modes in firmware Ares and later: Economy, Normal, High, Ultra-High. Higher quality means higher CPU usage.

"Reverb uses a ton of CPU. On Axe-Fx 3 the reverbs have a "Ultra" mode that makes them sound better at the cost of more CPU usage." [15]

If CPU usage on the AX8 and FX8 gets above 90%, the processor will start disabling blocks. Usually Reverb is first, because it tends to suck up the most CPU power.

On the FM3, amp modeling and reverb do NOT run on the same processor; this was true for an early beta version but not in the final release version.

On the FM9, the two Reverb blocks run in a dedicated DSP core, without much of an impact on the the overall CPU usage.

Common methods to reduce CPU usage in the Reverb block: switch to lower quality, reduce Density, or use Spring Reverb which 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 or stereo reverb

The dry signal that passes through the Reverb block stays stereo (if it's stereo). The signal which gets reverberated is first summed to mono. The resulting reverberated output of the Reverb block ("wet signal") is stereo, except for the Spring reverb type.

The stereo reverb output can be made 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."

"The non-spring types create stereo." [16]

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

Room ambience

Room ambience is a type of reverb that recreates the ambience of a room. The Cab block in most of Fractal Audio's hardware processors provides this feature.

Firmware Ares and later adds floor reflection modeling to room ambience.

"The intensity of the floor reflections can be adjusted with the new FLOOR REFLECTIONS parameter. Floor reflections play a large role in “amp in the room” sound. If the amp is on a carpet the floor reflections are minimal. If the amp is on a wood or other hard surface the floor reflections are significant. Existing presets will initialize this value to 0% so as to not change the sound. The default value is 50%. Note that negative values, while not realistic, are supported which inverts the reflection. MIC SPACING sets the stereo width of early reflections by simulating mic separation in the virtual space."

You can also use the regular Reverb block for simulating room ambience.

"The room stuff in the Cab block is a stripped-down, simplified version of the early reflections generator in the Reverb." [17]

"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." [18]

"If you just want the "room" stuff turn the reverb level down and the early reflections level up." [19]

"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." [20]

(firmware Ares 2.05) "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." [21]

Firmware 17 and later add support for FullRes IRs to the Cab block and IR Player block. This allows using captures of stereo room mics for ambience ("amp+cab in the room"), as well as convolution reverb of max. 1.33 seconds.


Parameters table

Parameter Axe-Fx III / FM9 / FM3
Input Select yes
Input Gain yes
Type yes
Time yes
Pre-Delay yes
Size yes
Crossover Freq yes
Low Freq Time yes
High Freq Time yes
Early Level yes
Late Level yes
EQ parameters: Freq, Q, Gain, Slope yes
HF Decay Time, LF Decay Time yes
LF Crossover yes
Echo Density yes
Early Diffusion yes
Early Diffusion Time yes
Early Decay yes
Late Input Mix yes
Quality yes
Stack/Hold (firmware Ares 12 and later) yes
Wall Diffusion yes
Input Diffusion yes
Input Diffusion Time yes
Mic Spacing yes
Mod: Depth, Rate yes
LFO Phase yes
Pitch Mix yes
Pitch Shift 1,2 yes
Splice Time yes
Pitch Feedback yes
Pitch Direction yes
Pitch Position yes
Voice Balance yes
Stereo Spread yes
Ducker: Attenuation, Threshold, Threshold yes
Spring: Number, Tone, Drive, Boiiinnng! yes


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


There are four Quality modes in firmware Ares and later: Economy, Normal, High, Ultra-High. High quality uses significantly more CPU but provides world-class reverberation algorithms. Ultra-High increases modulation at the expense of a slight increase in CPU usage. In most situations, especially live, Normal or Economy provide the desired results and most listeners won't notice any difference. There's no Quality parameter in the Spring Reverb type.

The Axe-Fx II, FX8 and AX8 default to Normal Quality, where the Axe-Fx III and FM9 default to High Quality. The FM3 defaults to Normal Quality.

"Reverb uses a ton of CPU. On Axe-FX3 the reverbs have a "Hi Quality" mode that makes them sound better at the cost of more CPU usage." [22]

”In the Reverb block it selects a more advanced algorithm.“ [23]


Maximum Reverb Time is 100 seconds in firmware Ares.


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." [24]

"When activating reverb hold the time is increased to infinity. This causes a bunch of stuff to be recalculated which can cause a click." [25]

Firmware Ares 12 added the "Stack" feature:

"Added Stack/Hold parameter to Plex Delay and Plex Verb types in the Plex block. When set to STACK incoming audio is stacked on existing audio and held. When set to HOLD existing audio is held."

When attaching a pedal to Off/Stack/Hold, Heel is Off, Stack is middle and Toe is Hold.

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 firmware Ares.

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." [26]

"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." [27]

"Another way to EQ the Reverb is to adjust the Low/High Freq Times. The reverb is essentially three bands. Time sets the RT60 (decay time) of the mid band. Low/High Freq Times adjust the decay times of the Low and High bands, respectively, relative to the mid band." [28]

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.” [29]

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

Effects Mix is another Global parameter that boosts or cuts the Mix value of all blocks for which the GLOBAL MIX parameter is set to “ON”.

Tips and tricks

Alternative reverb effects

Turn up Diffusion in the Delay block to smear the delays trails and turn it into reverb.

Select Plex Verb in the Plex Delay block for ambient reverb.

Gated reverb

Try factory preset: Gated Reverb or ADSR Reverb.

More information on the user forum:

More information