What Is The Auto-Wah/Envelope Filter Modulation Effect?

Auto-wah/envelope filters produce an instantly recognizable modulation effect that will surely bring the Funk to your music.

Whether the effect is achieved via an effect pedal, an audio plugin or another piece of hardware, the auto-wahs/envelope filters effect is a great choice for adding interest to our sound and is worth understanding.

In this article, we'll discuss the envelope filters/auto-wah filter effect in detail to learn how it works and how it's achieved. We'll also look at some examples of envelope filter effects units and touch on how they work.

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What Is An Envelope Filter?

An envelope filter can be basically understood by simply summing the two words together. It is an audio filter that is controlled via an envelope.

More specifically, an envelope filter will work with an envelope follower that generates an envelope based on the amplitude (over time) of the envelope filter's input signal. This envelope will then control the cutoff/centre frequency of the chosen filter type (high/low-pass, band-pass, band-stop, etc.) and sweep it across the frequency spectrum.

So, the input signal amplitude determines the cutoff frequency of the filter. Therefore, transients in the signal will cause a significant and fast sweep of the filter. This filter movement causes the “wah-wah” type of effect we've come to cherish in envelope filters (which is sometimes referred to, aptly, as “auto-wah”).

The way in which the envelope follower will create the controlling envelope is pretty straightforward. The filter option and shape can be rather versatile and different from envelope filter to envelope filter.

Here’s a simple illustration of a signal's amplitude over time and its resulting envelope:

This envelope is then used to control the filter's position (whether it’s a high-pass, low-pass or band-pass) across the frequency spectrum.

When the envelope reaches its peak, the filter will be moved to one extreme of its overall range. As the envelope comes to its minimum value, the filter will be at the other extreme of its overall range.

Let's consider a low-pass filter being controlled by an envelope in an upward-sweeping motion. This means that the envelope's minimum value coincides with the lowest cutoff frequency, and as the envelope reaches its high point, the filter's cutoff frequency is swept upward to its highest cutoff frequency.

This can be visualized in the image below:

The most basic envelope filter signal flow diagram would look something like this:

As we can see, the input signal is fed into the input of the audio filter and the envelope detector/follower.

The envelope generated by the detector is fed into the filter to control the cutoff frequency parameter (and perhaps other parameters).

The stronger the input signal, the stronger the envelope, and the more extreme the filter movement.

Some envelope filters have mix controls, allowing users to combine the dry and filtered signals at the output. That setup would resemble the following:

To further our understanding of envelope filters, let's consider both envelopes and filters separately.

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What Is An Envelope?

The first word of envelope filter is “envelope.” We've briefly discussed what an envelope is in the previous section. Let's define it a bit further in this section.

What is an audio envelope? An envelope is a description of how a sound or audio signal changes over time. It can describe the amplitude, pitch, timbre, filtering, or any other parameter of the sound/audio signal. Envelopes tend to trigger with each note and define the note/source throughout its duration.

An envelope, put simply, describes how a signal or sound changes over time. Though many parameters or aspects of a signal/sound could be defined by an envelope (such as the cutoff frequency value in an envelope filter), it is typically the amplitude of the signal.

Envelopes can describe the transient burst and subsequent quieting of acoustic sounds; they can be generated electronically (common in synthesizer designs) or detected from a signal (as is the case with the envelope followers/detectors used in envelope filters).

The time-varying value of an electrical envelope can be used to manipulate all sorts of parameters. This is the case with plenty of synth patches. Envelope filters, as discussed, use their detected envelope to modulate the cutoff frequency (and potentially other parameters like resonance) within the filter circuit.

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What Is A Filter?

The second word of envelope filter is “filter.” Let's talk about filters in a bit more detail here.

What is an audio filter? An audio filter is a frequency-dependent amplifier (or attenuator) circuit that works within (and beyond) the audio frequency range of 20 Hz to 20,000 Hz. There is a wide variety of filters, some of which amplify/boost, attenuate/cut and/or pass defined frequency ranges/bands.

The filters used in typical envelope filters will remove frequencies from the signal below and/or above-defined frequency points. By removing frequencies from the output signal, a filter can drastically change the perceived tone/timbre of the audio.

Remember that the “defined frequency points” are modulated by the envelope in an envelope filter!

Depending on the type of filter, these points will be considered either cutoff frequencies (as is the case with high and low-pass filters) or centre frequencies (as is the case with band-pass and band-stop filters).

The four filter types mentioned above will make up the vast majority of envelope filter filters. Even then, band-stop filters are rarely used, leaving us with three main filter types.

Note that, in order to get that “envelope filter sound,” many of the filters will have resonance peaks at their cutoff or centre frequency. This is a boost in frequency-dependent sensitivity directly at the centre or right above or below the cutoff frequency.

With that, let's quickly go over the four main filter types. I'll include visual representations with resonance peaks at the cutoff or centre frequencies.

Low-Pass Filter

What is a low-pass filter in audio? A low-pass filter (LPF) “passes” the low frequencies below their cutoff frequency while progressively attenuating frequencies above their cutoff. In other words, low-pass filters remove high-frequency content from an audio signal above a defined cutoff point.

A low-pass filter will cause the envelope filter sweep to quickly remove higher-end energy from the signal while sweeping a boosted narrow resonant peak. It will keep the bass intact throughout.

Band-Pass Filter

What is a band-pass filter in audio? A band-pass filter “passes” a band of frequencies (a defined range above a low cutoff frequency and below a high cutoff frequency) while progressively attenuating frequencies below the low cutoff and above the high cutoff.

A band-pass filter will cause the envelope filter to sweep a defined band of frequencies with a centre resonant peak while removing energy above and below the band.

High-Pass Filter

What is a high-pass filter in audio? A high-pass filter (HPF) “passes” the high frequencies above their cutoff frequency while progressively attenuating frequencies below the cutoff frequency. In other words, high-pass filters remove low-frequency content from an audio signal below a defined cutoff point.

A high-pass filter will cause the envelope filter sweep to quickly remove lower-end energy from the signal while sweeping a boosted narrow resonant peak. It will keep the high frequencies intact throughout.

Band-Reject Filter

What is a band-reject filter in audio? A band-stop filter (aka band-reject or notch filter) works by removing frequencies in a specified band within the overall frequency spectrum. It allows frequencies below the low cutoff frequency to pass along with frequencies above the high cutoff frequency.

A band-stop/notch filter will cause the envelope filter to sweep a notch filter across the frequency spectrum with the potential for resonance peaks at either side of the notch.

These filters also find themselves in EQ processors and other audio effects.

To get the “auto-wah” sound we love out of an envelope filter, the filter must have two characteristics:

• A resonant frequency where the filter EQ peaks, producing a boost of a certain narrow band of frequencies. This mimics the natural formants of speech and gives us that wah-like sound.
• A sweepable centre frequency for the filter that moves according to the signal dynamics. This sweeps the filter and the filter resonance to give us the movement required of a wah/auto-wah.

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Envelope Filter Parameters

Now that we understand the basics of envelope filters, let's get into where the real magic happens with this powerful effect: the adjustable parameters.

In this section, we'll really develop our comprehension of envelope filters and how they work.

Envelope filters are generally very flexible in their performance thanks to manually adjustable parameters that affect the automatic nature of the effect.

Envelope filter effects units/plugins will often have many (if not all) of the following controls:

• Filter Type
• Response/Attack
• Speed/Decay
• Sensitivity
• Range
• Q/Peak/Resonance
• Sweep Direction
• Mix

Filter Type

Envelope filters will generally perform with one filter at a time. However, most envelope filter effect units will have options for various filters.

As mentioned, envelope filters are generally designed to perform with three basic filter types: low-pass, band-pass and/or high-pass. Band-stop/notch filters are less common but also possible.

In envelope filters that offer multiple filter types, the choices will be selectable.

Once again, I'll offer a few illustrations of these filter types with resonance peaks included:

Response/Attack

An envelope filter's response or attack parameter will alter how the envelope detector produces the attack portion of the envelope in reaction to an incoming signal.

Generally speaking, these controls affect time. Increasing the attack or decreasing the response would actually make the detector slower, which produces a longer attack region in the resulting envelope.

Relating that to images, rather than having something like this (fast attack/response):

We’d have something like this (slow attack/response):

Read the manual or simply use your ears to understand how this parameter works. It could be the opposite of what was stated here, meaning that increasing the response/attack parameter would decrease the detected envelope's attack period.

Note that controls labelled as “response” may also affect the decay of the envelope, which brings us to our next point.

Speed/Decay

An envelope filter's speed/decay parameter will alter how the envelope detector produces the envelope's decay time in reaction to an incoming signal.

Increasing the decay or decreasing the speed would draw out the decay portion of the pedal’s envelope.

So instead of having this:

We’d have something like this:

Read the manual or simply use your ears to understand how this parameter works. It could be the opposite of what was stated here, meaning that increasing the speed/decay parameter would decrease the detected envelope's decay period.

As an aside, attack and release controls can also be seen on compressors.

Sensitivity

The sensitivity effectively refers to how well the envelope detector will react to the incoming signal and, therefore, how the envelope filter will filter the signal.

Lower sensitivity ratings require higher input signal levels to trigger the same envelope amplitude and level of filtering. Higher sensitivity ratings will produce the same amount of filtering with less signal strength at the input.

Range

The range parameter effectively narrows or widens the sweeping range of the envelope filter. A larger range typically yields a more noticeable effect as the resonant peak is swept over a wider range of frequencies (at a faster “frequency-shift per time”).

Q/Peak/Resonance

The Q, peak or resonance control on an envelop filter will affect the amplitude and/or width of the resonant peak at the filter's cutoff or centre frequency.

Note that high and low-pass filters are governed by cutoff frequencies, while band-pass and band-stop/notch filters are defined by centre frequencies.

Q is a dimensionless quantity that refers to the narrowness (or width) of an EQ boost/peak. In the case of an envelope filter, the Q/ determines how sharp or smooth the resonant frequency peak is, though it doesn’t alter the overall boost at the resonant frequency.

Peak controls will alter the relative amplitude spike at the resonance frequency.

The Q/peak controls can also be consolidated into a single control labelled as “resonance”.

Here’s an illustration of the Q factor that should help us to visualize what Q controls do:

Sweep Direction

The sweep direction parameter (sometimes referred to as drive) can be toggled between upward and downward settings. This control adjusts the direction in which the envelope will sweep the filter.

The upward setting will cause the envelope to sweep the filter upward, where the maximum amplitude of the envelope will coincide with the highest cutoff/centre frequency of the filter.

The downward setting will cause the envelope to sweep the filter downward, where the maximum amplitude of the envelope will coincide with the lowest cutoff/centre frequency of the filter.

Mix

The mix/blend control parameter of an envelope filter effectively mixes the direct/dry signal with the filtered/wet signal at the output. By mixing the two signals together, we can maintain the fullness of the direct signal while obtaining the character of a modulated envelope filter.

The MXR M82 Bass Envelope is one of the best for getting a funky auto-wah on the bass guitar. It features 4 of the 8 parameters we’ve mentioned (the dry and FX controls both have to do with the mix).

The Mojo Hand Wonder Filter covers the other 4 control parameters mentioned here (filter type, attack, range, sweep direction/drive).

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Envelope Filter Unit/Plugin Examples

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at 4 different envelope filter units to help solidify our understanding of this effect.

In this section, we’ll discuss:

• 500 series envelope filter unit: Moog The Ladder Envelope Filter
• 19″ rack mount envelope filter unit: Mutronics Mutator
• Envelope filter effect pedal: Mu-Tron Micro-Tron IV
• Envelope filter plugin: Kuassa Efektor WF3607

Moog The Ladder Envelope Filter

The Moog The Ladder Envelope Filter is a “dynamic transistor ladder filter” based on Robert Moog's original ladder filter design. Combine this fact with the built-in envelope follower, and we have ourselves a superb envelope filter.

First off, this 500 series envelope filter has the option of engaging either a high-pass filter or a low-pass filter. The slope of the selected filter can be set to 6 dB/octave or 12 dB/octave via the 2-pole or 4-pole switch.

Let’s have a look at other The Ladder Envelope Filter controls:

• Drive: adjusts the input signal level and can cause analog saturation and distortion.
• Output: adjusts the output level of the unit.
• Attack: determines how quickly the envelope follower will respond to the incoming signal (whether that's the input or a sidechain).
• Release: determines how quickly the envelope follower's release responds to the audio signal.
• Amount: determines how the envelope's control voltage affects the cutoff frequency. A negative amount will cause the envelope to decrease the cutoff frequency below its starting point. A positive amount will cause the envelope to increase the cutoff frequency below its starting point.
• Cutoff: determines the starting cutoff frequency point of the filter.
• Resonance: adjusts the feedback level within the filter circuit to boost the resonance at the cutoff frequency.

There are also easy modifications that can be made to the Moog Ladder Envelope Filter, including ground lifting and stereo linking.

Mutronics Mutator

The Mutronics Mutator looks like a powerful envelope filter because it is a powerful envelope filter. Unfortunately, this unit has been discontinued.

This unit is a stereo analog audio filter with an envelope follower and a wide range of controls.

Firstly, because it's stereo, it has two independent VCFs (voltage-controlled filters), one per channel. These VCFs can be controlled via their own LFO (low-frequency oscillator) or, as we'd expect from an envelope filter, via its own envelope follower section.

The envelope follower of each channel will effectively contour the input signal amplitude of that channel and produce an envelope to control the channel's voltage-controlled filter's cutoff frequency.

The envelopes can be outputted to control other units as well.

They can alternatively be used to control the gate of the Mutator unit when in gate mode. In gate mode, the signal will only be passed through to the output as the envelope surpasses a certain threshold.

The filter has the typical envelope sweep (upward or downward and by how much), cutoff (the original cutoff frequency point), and resonance (feedback level) controls.

Though the Mutornics Mutator hardware unit has been discontinued, Softube makes a plugin emulation of the Mutronics Mutator.

Mu-Tron Micro-Tron IV

The Mu-Tron Micro-Tron IV is the successor to the classic Mu-Tron III envelope filter and is perfectly sized for guitar pedalboards.

Let’s have a look at the Micro-Tron IV’s controls:

• Mode:
  • LP (low-pass filter): passes all the signals below the peak and attenuates signals above the peak.
  • BP (band-pass filter): passes signals only near the peak and attenuates signals below and above the peak.
  • HP (high-pass filter): passes all signals above the peak and attenuates signals below the peak.
• Range:
  • LO (low): sets the filter sweep to a low range for most rhythm guitar, bass, and lower fret uses.
  • HI (high): sets the filter sweep to a high range for accenting string harmonics or up-the-neck and solo work.
• Level: controls the output volume of the affected signal.
• Peak (Q): controls the filter audio response from very weak to very strong.
• Gain: controls the envelope and filter audio drive.
• On/Off Footswitch: engages or disengages the envelope filter effect
• Up/Down Footswitch: alters the sweep direction.

This pedal also has a CV input to accept an expression pedal for even more control over the envelope-sweeping parameters. On top of that, we can send the generated envelope from the Micro-Tron IV to other units to link up control of multiple units together.

Kuassa Efektor WF3607

The Kuassa Efektor WF3607 is a wah-wah/envelope filter combo plugin.

In addition to the envelope follower (with amount, attack and release controls), the WF3607 also has a mod wheel for “pedal position.” We can choose to either modulate and/or automate the pedal position or, alternatively, use the input signal to control the envelope, which will, in turn, control the filter.

Regardless of how we control the filter's cutoff, there are additional controls for the sweep range, sweep direction, Q (resonance) and output level. There is also an option for stereo linking.

There are three styles of wah-wah:

• CB: Dunlop Cry Baby
• AX: Vertex Axis Wah
• VG: Vox Grey Wah

And three styles of envelope filtering:

• MuLP: Mu-Tron Low Pass
• MuBP: Mu-Tron Band Pass
• MuHP: Mu-Tron High Pass

Note that the envelope filters are based on the aforementioned Mu-Tron unit.

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Related Questions

What is the audio wah effect? Wah (or Wah-Wah) is a filtering effect that is common on guitars and keyboard instruments. Wah is achieved by sweeping one or more boosts in EQ up and down in frequency, thereby mimicking the human vowel sound of “wah.”

What is an amplitude envelope? An amplitude envelope is a way of describing the amplitude of a sound or audio signal over time. With audio electronics, an amp envelope is defined largely by the typical attack, decay, sustain and release characteristics of regular envelopes.