What Are Envelope Filter Effects Pedals & How Do They Work?


No effect brings the funk quite as much as the envelope filter. The wah-wah comes close, but its “automatic” cousin reigns supreme. The magic of the envelope filter, for both guitar and bass, can easily be found inside a small stompbox effects pedals.

What are envelope filters and how do they work? An envelope filter pedal (also known as auto-wah) is a stompbox unit that triggers a dynamic filter. This filter can be a low-pass, band-pass, or high-pass and acts according to the amplitude of the input signal. These pedals are typically designed for guitar or bass guitar.

In this article, we’ll develop our knowledge of envelope filters pedals, discussing the inner workings of these pedals and how they affect guitar and bass guitar signals. I’ll share a few envelope filter pedals throughout the article and offer some tips on how to get the most out of your auto-wah pedal(s).

Related article: Top 13 Best Envelope Filter Pedals For Guitar & Bass.


Table Of Contents


What Is An Envelope Filter?

An envelope filter can be understood by separating the two words.

The first word is envelope. An envelope, put simply, describes a signal or sound’s change over time.

Envelope generators are common in synthesizers and samplers. These are the circuits that produce the framework of a signal’s attack, decay, sustain and release.

Envelope generators are often used to define the amplitude of a key when pressed, held and released. They can also be used to affect many other parameters including, of course, a filters.

Acoustic sounds are also defined by envelopes. The initial transient and subsequent sustain of a sound is due to its envelope.

Of course, the envelopes of acoustic sounds are not programmed synthetically. However, with subtractive synthesis, we can use envelope generators to emulate the natural envelope of a sound in order to produce a synth patch that sounds like an instrument.

The second term is filter. A filter effectively alters the frequency response of a signal by either reducing or removing frequency above/below certain points along the frequency spectrum.

A filter, then, can alter the sound quite dramatically by changing its harmonic character. Filters are a major part of EQ.

To learn more about EQ pedals, check out my article What Are EQ Pedals (Guitar/Bass) & How Do They Work?

An envelope filter is a sort of dynamic EQ. It is a filter controlled by an envelope.

The envelope filter will read its input signal and use it to produce a corresponding envelope. This envelope with then act upon a filter to alter its cutoff frequency/frequencies in order to sweep it across the audible frequency spectrum.

That’s the basic definition of an envelope filter: a filter controlled by an envelope.

Let’s deepen our understanding by discussing envelopes, filters and, of course, envelope filters, in greater detail.


What Are Envelopes?

No, we’re not talking about the envelopes at the post office.

In terms of sound and audio, an envelope is a decription of how a sound changes over time. It is typically related to amplitude, though envelopes can also apply to filters, pitch and other effects and parameters of a sound/signal.

This article will focus more on how envelopes can describe/affect the filter of an audio signal.

Envelopes happen naturally in the world and can also be generated via electronics.

Sound sources produce transients: short-duration, high amplitude peaks at the beginning of their waveforms.

Here’s an illustration of a sound/audio wave transient:

Notice how that amplitude peaks and quickly comes down. This peak represents the picking, slapping, and even just the touching of a string. The more percussive we play or the harder we hit the strings, the more pronounced the transient.

These peaks typically contain a great deal of harmonic content that defines the timbre and tone of the sound.

The higher harmonic content loses energy faster than the lower content (the fundamental frequency of the note is generally the last to fade).

Combine this with the reduction of overall amplitude as the string vibrations get weaker and weaker and we have ourselves an envelope.

In fact, if we were to really get into it, we could develop an envelope model for each frequency of a sound. This is how additive synethesis aims to recreate real instruments.

Speaking of synthesis, we can also produce envelopes electronically. These envelope generators, again, are often related to amplitude, though envelopes can also apply to filters, pitch and other effects and parameters of the signal.

These envelope generators are commonplace in synthesizer and sampler designs. They are also used in envelope filter pedals to control an audio filter.

These envelope generators, like many audio effects, can be either analog or digital and can be built into hardware (analog and digital) or software (digital only).

The typical envelope generator has four parameters: attack, decay, sustain and release. Let’s have a look at a simple illustration of ADSR and describe each of the variables.

  • Attack: the time the audio takes to go from zero amplitude to peak amplitude as the note is played. This ties in closely with the transient.
  • Decay: the time the audio takes for the subsequent run down from the attack level/amplitude to the designated sustain level/amplitude.
  • Sustain: the level of the sound/amplitude after the attack and decay, which is maintained until the key is released.
  • Release: the time taken for the level to decay from the sustain level to zero after the note is released.

The above graph shows an envelope as it relates to the amplitude of signal. Envelopes can also be generated to control other parameters.

In the case of an envelope filter circuit, the signal is filtered according to its transient/envelope information.


A Look At Different Audio Filters

Let’s have a quick look at a few audio filters to get a better idea of how an envelope filter shapes the sound.

When the term “filter” is used, it generally refers to the elimination of certain frequencies from a signal. In the case of an envelope filter, we generally will see low-pass, high-pass or band-pass filters.

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 formants natural to 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.

I write more extensively on vocal formants and their role in wah-wah circuits in my article on wah pedals: What Are Wah-Wah Guitar Effects Pedals & How Do They Work?

There are 5 basic filters we should understand in audio. Envelope filters will typically use [the first] 3 of the following so I’ll give a bit more explaination of the first 3 in the paragraphs below. The 5 filter types are:

High-Pass Filter

The high-pass filter is perhaps the most commonly-used pass-type filter in mixing.

High-pass filters “pass” the high-frequencies above their cutoff frequency while progressively attenuating frequencies below the cut-off frequency. In other words, high-pass filters remove low-frequency content from an audio signal below a defined cut-off point.

Sweeping a high-pass filter will either remove more or allow more low-end to pass through to the output. Here’s another look at a high-pass filter, this time with a resonant frequency at its cutoff. Remember that the resonant peak will produce the formant-like peak required to produce the wah-wah effect:

Low-Pass Filter

Low-pass filters are the opposite of high-pass filters. They “pass” the low-frequencies below their cutoff while progressively attenuating frequencies above their cutoff. In other words, low-pass filters remove high-frequency content from an audio signal above a defined cut-off point.

Sweeping a low-pass filter will either remove more or allow more high-end to pass through to the output. Here’s another look at a low-pass filter, this time with a resonant frequency at its cutoff. Remember that the resonant peak will produce the formant-like peak required to produce the wah-wah effect:

Band-Pass Filter

A band-pass filter is like a combination of a low-pass and high-pass filter. It passes a band of frequencies (a defined range with a low cutoff and a high cutoff) while progressively attenuating frequencies below the low cutoff and above the high cutoff.

Band-pass filters, then, pass a band of frequencies while removing those on either side.

Sweeping a band-pass filter one way will pass more high-end while removing more low-end. Sweeping it the other way will do the opposite. Here’s another look at a band-pass filter, this time with a resonant frequency at its cutoff. Remember that the resonant peak will produce the formant-like peak required to produce the wah-wah effect:

Band-Reject Filter

A band-reject filter, often referred to as a notch filter, is kind of like the opposite of a band-pass filter.

A band-reject filter works by removing frequencies in a specified band within the overall frequency spectrum. It allows frequencies below the low cutoff point to pass along with frequencies above the high cutoff point.

Band-reject filters can be used to great effect in EQ since they can remove problem frequencies within a signal. However, they are not used in envelope filters.

All-Pass Filter

All-pass filters are odd. They actually do not filter any frequencies in terms of amplitude. They pass all frequencies.

Rather, they work by affecting the phase of any given sinusoidal component (frequency) according to its frequency.

All-pass filters, in terms of guitar and bass pedal effects, are used in phaser pedals to affect the phase of the copied signal. We’ll discuss phaser pedals and their relation to EQ later in this article.

All-pass filters aren’t used in envelope filters or EQ.


What Are Envelope Filter Pedals & How Do They Work?

Envelope filter pedals are stompbox-type units designed to affect bass and guitar signals through they can also be utilized on other instruments.

Their basic circuits include an envelope detector and an adjustable filter.

The input signal is read by the envelope detector which produces a coinciding envelope that closely represents the characteristic attack and sustain of the note(s) being played.

The input signal is defined by the detector as its average change in voltage over time.

Here’s a simple illustration of a dynamic signal and its resulting envelope:

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

The Electro-Harmonix Q-Tron (link to check the price on Amazon) envelope filter pedal offers options between low-pass (LP), band-pass (BP), high-pass (HP) and a mix (BP + dry signal). The filter type is selectable via the “mode” dial.

Electro-Harmonix Q-Tron +

Electro-Harmonix is featured in My New Microphone’s Top 11 Best Guitar/Bass Effects Pedal Brands To Know & Use.
The Electro-Harmonix Q-Tron+ is featured in My New Microphone’s Top 13 Best Envelope Filter Pedals For Guitar & Bass.

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 have another look at that envelope and how it could affect a low-pass-type filter:

The range; envelope detector sensitivity; sweep direction and filter type can often be adjusted within the pedal. We’ll get to envelope filter pedal parameters shortly.

As we’ve discussed briefly in the section on filters, an envelope filter should require a resonant peak if it is to produce a wah-wah-like sound (which is what we typically want out of our envelope filter).

Envelope filters are often referred to as “auto-wahs” since they produce a wah-like effect without the need for an expression pedal. The amplitude envelope of the input signal effectively acts as our expression pedal in this comparison.

We’ll discuss wah pedals in more detail in the section Other EQ-Modulation Effects.

So, the basic circuit of an envelope filter would look something like this:

Let’s start with the easy part. Some envelope pedals have a mix/blend knob that allows some direct/dry (unaffected/unfiltered) signal to be mixed in at the output of the pedal. Other pedals may not allow for this but it’s worth mentioning.

So then, some envelope filters are even simpler, looking something like this:

Next, we have input signal go into an audio filter section and an envelope detector section. The signal simultaneously has an envelope created to represent its ampltude and has a filter applied to it that is controlled by the envelope.

As the envelope amplitude increases and decreases, the filter (along with its resonant frequency) moves up and down in the frequency spectrum.

Therefore, the harder we play, the more intense the effect becomes.

Note that some envelope filters offer 2 directional modes. In this case, the “up” mode is configured so that an increase in the envelope amplitude causes an upward motion in the sweep. Conversely, the “down” mode would be configured so that an increase in the envelope amplitude produces downward motion in the sweep.

The Maxon AF-9 (link to check the price on Amazon) offers adjustable sweeping directions (in the drive control) along with 3 different filter types; range selection; sensitivity, and peak controls.

Maxon AF-9

The Maxon AF-9 is featured in My New Microphone’s Top 13 Best Envelope Filter Pedals For Guitar & Bass.

Note that, in either mode, the filter can still be HP, LP or BP.


Controls On Envelope Filter Pedals

To really comprehend the inner workings of an envelope filter, we should understand the typical parameters we can control.

Common envelope filter pedal controls include:

Filter Type

As we’ve mentioned, envelope filters can be designed with 3 basic filter types: low-pass, band-pass and/or high-pass.

In envelope filters that offer multiple filter types, the choices will be selectable on the pedal. This is typically achieved via a toggle switch.

Once again, here are a few illustrations of the three main envelope filter types (each with a resonance frequency):

Response/Attack

The response or “attack” control will alter the way the envelope detector reacts to an incoming signal.

Increasing the attack/response will slow down the attack of the envelope and make the initial sweep slower as a note is struck.

So rather than having this:

We’d have something like this:

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

Speed/Decay

The speed control (sometimes referred to as “decay” or “release”) is similar to the attack control except it acts up the decay of the envelope.

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

So instead of having this:

This image has an empty alt attribute; its file name is mnm_Envelope_Filter_Envelope_Detector-2.jpeg

We’d have something like this:

Attack and decay/release controls can also be seen on compressor and other pedals.

To learn more about compressor pedals, check out my article What Are Compressor Pedals (Guitar/Bass) & How Do They Work?

Sensitivity

Sensitivity refers to how much the incoming signal will affect the envelope and, therefore, the filtering of the signal.

Lower sensitivity ratings require more input signal (harder playing or a louder guitar/bass) to trigger the same level of filtering. Higher sensitivity ratings will produce the same amount of filtering with less signal strength at the input.

Range

The range parameter refers to 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

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 (Up/Down)

The sweep direction (sometimes referred to as drive) is pretty self-explanatory. It switches the direction of the filter sweep between an upward sweep (the envelope triggers the resonant frequency to increase) and a downward sweep (the envelope triggers the resonant frequency to decrease).

Mix

The mix control is not always featured on an envelope filter.

This control (sometimes referred to a “blend” or “wet/dry”) controls the relative amount of the filtered signal and dry/direct signal at the pedal’s output.

The MXR M82 Bass Envelope (link to check the price on Amazon) 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 wit the mix).

MXR M82

MXR is featured in My New Microphone’s Top 11 Best Guitar/Bass Effects Pedal Brands To Know & Use.
The MXR Bass Envelope Filter is featured in My New Microphone’s Top 13 Best Envelope Filter Pedals For Guitar & Bass.

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

Mojo Hand Wonder Filter

The Mojo Hand FX Wonder Filter is featured in My New Microphone’s Top 13 Best Envelope Filter Pedals For Guitar & Bass.


Tips On Using An Envelope Filter Pedal

Envelope filter pedals can produce superb tones when used correctly. Get the most of of the pedal depends largely on your playing style and dialing in the settings appropriately.

That being said, there are a few tips I should share when using envelope pedals. They are:

Give It A Break

This seems counter-intuitive. However, because envelope filtering is such a noticeable effect (unlike, say, a static EQ or compressor), we can actually get more out of it by using it less.

Save the envelope filter for sections of the song that need it the most. Bringing the effect in and out will make it that much exciting when it is engaged.

Spend Some Time To Dial It In

From the envelope detector to the filters, these pedals are highly tunable.

The signal going into the pedal with be different from guitar to guitar and bass to bass. It will also be affected by all the other pedals that come before the envelope filter.

Take some time to really tune the pedal to suit your playing.

Play Fast And Play Slow

This ties into the previous point. If you’re playing slowly, try adjusting the response (slower attack and speed/delay setting) to suit your playing. Conversely, set response times faster when playing busier sections.

Try It After An Overdrive Or Compressor Pedal

An overdrive or compressor pedal will reduce the dynamic range of the signal going into the envelope filter which will, in turn, produce a more consistent signal level across all notes on the guitar or bass. This will trigger a more consistent envelope, which may sound better across a wider range of notes.

Overdrive pedals have the advantage of adding extra harmonics to the high-end of the signal, which gives the envelope filter more frequencies be filtered.

To learn more about overdrive pedals, check out my article Guitar Pedals: Boost Vs. Overdrive Vs. Distortion Vs. Fuzz.

Read The Manual

Reading the manual will, hopefully, teach you, in detail, what the pedal does and how to get the sounds you need out of it.

With an envelope filter, it’s best to understand the settings that work best for you. These pedals are bit less experimental since they sound lacklustre when not setup to suit their input signals.

Check out the manual for tips and tricks on your own envelope pedal to get the most out of it.


Where Should An Envelope Filter Pedal Go In The Signal Chain?

Envelope pedals typically sound best when put near the front of the pedal chain since they affect the EQ and tonal characteristics of the signal.

However, if you’re running utility (buffers, tuners), synth, and/or pitch-shifting pedals, an envelope filter will likely perform better when put after these pedals.

It’s also common to put envelope filter pedals after distortion-type pedals (including overdrive and fuzz) to get a fatter tone. However, putting the envelope filter before the distortion may allow for increased reactivity and more harmonic content in the high-end.

Envelope filter pedals generally sound better when put before modulation and time-based effects.

That all being said, experiment with the position of your envelope pedal within the pedal chain to find the best spot for your rig.

To learn more about ordering pedals in the signal chain, check out my article How To Order Guitar/Bass Pedals (Ultimate Signal Flow Guide).


Other EQ Modulation Effects

Now that we understand envelope filter pedals, let’s have a look at other effects that utilize variations in a signal’s frequency response:

Wah Pedals

Wah pedals are stompbox-style units with treadle-style expression foot pedals designed to receive guitar, bass or other instrument signals. Their circuits are designed to modulate the formants of the sound by modulating one or more resonance peaks in the signal’s frequency response. Wah pedals do the with the use of filters and EQ.

These pedals work in the same way an envelope filters except the sweeping is done manually via an expression pedal rather that automatically by a tweakable envelope detector.

The Vox V847A (link to check the price on Amazon) wah pedal.

Vox V847A

The Vox V847A is featured in My New Microphone’s Top 14 Best Wah Pedals For Guitar & Bass.

To learn more about wah pedals, check out my article What Are Wah-Wah Guitar Effects Pedals & How Do They Work?

Phaser Pedals

You may be thinking to yourself “but phaser is a phase-modulating effect”. Well, that’s the category it fits into and indeed a phaser does modulate the phase of its signal. However, the way it achieves this effect is largely based on modulation EQ.

Phaser pedals produce their distinguished effect by modulating a series of all-pass filters. Yes, all-pass filters are filters that pass all frequencies and do not actually really filter anything. This seems strange, I know.

The way an all-pass filter works, at least in a phaser, is by splitting the input signal in two; phase-inverting one copy, and then sending the dry signal through a low-pass filter and the inverted signal through a high-pass filter (or vice versa).

The result is that all frequencies pass through but the phase of these frequencies changes. Phasers string multiple all-pass filter “stages” together and modulate the cutoffs of the all-pass filters’ HP and LP filters to produce their effect.

So then, technically speaking, phasers are EQ-modulation pedals, too.

The Electro-Harmonix Small Stone (link to check the price on Amazon) is a simple example of a phaser pedal.

Electro-Harmonix Small Stone

The Electro-Harmonix Small Stone is featured in My New Microphone’s Top 11 Best Phaser Pedals For Guitar & Bass.

To learn more about phaser pedals, check out my article What Are Phaser Pedals (Guitar/Bass FX) & How Do They Work?


Do I need a compressor pedal? Compressor pedals help balance out the dynamic of our playing by reducing the dynamic range of the audio signal. They are by no means necessary, though they may help increase perceived loudness; add sustain; improve transients, and make the guitar or bass fit better in the mix.

Related article: What Are Compressor Pedals (Guitar/Bass) & How Do They Work?

Where should I put my preamp pedal in my chain? Preamp pedals will work well pretty much anywhere in the signal chain so long as they’re before the guitar or bass amp’s power amplifier. That being said, they typically work best at the front of the pedal chain where they can act on a relatively clean and low-level guitar/bass signal.

Related article: Top 9 Best Boost/Preamp Pedals For Guitar & Bass.

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