How Does an Envelope Filter/Auto Wah/Synth Pedal Work?

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In this post, I will try to explain most simply, what is the proper way to work with envelope filter/auto-wah pedals as well as synth pedals. I will go through the basics of eq filtering and other settings that you can find on these kinds of pedals, and then apply these principles to popular pedals and their knobs.

What is an Envelope Filter?

An Envelope Filter is an effect that modulates the sound of your guitar by sweeping one frequency or a range of frequencies. It boosts the bass or the mid-range and moves it towards the treble range. It is exactly what you do with your foot when you’re using a wah-wah pedal. With Envelope Filter/Auto-Wah pedal, the frequencies will sweep dynamically according to your playing.
Different pedals offer different options for controlling this movement. Let’s review and explain the most common possibilities found on some of the most popular filter pedals.
I will go through knobs and switches that are found on these pedals:
Electro-Harmonix Bass Micro Synth (Buy on
Electro-Harmonix Micro Synth (Buy on
Electro-Harmonix  Q-Tron (Buy on

Mxr Bass Envelope Filter (Buy on 
Keely Neutrino Envelope Filter (Buy on
EarthQuaker Devices Spatial Delivery (Buy on
Boss Bass Synthesizer Syb-5
and Line 6 Filter Modeler, available in these Line 6 Products:
M5  –
M9 –
M13 –
HX Stomp –
HX Effects –
POD HD500X –
(Click Here for a detailed review of Line 6 Synth and Filter sounds).

A Playlist of me playing several envelope filters pedals

Mode / Filter / Shape

This switch sets which frequencies will be cut. Or in other words which frequencies will be filtered. (That’s why  we call it a filter).

If you chose to set it to LP – meaning Low Pass Filter, Then only the Low Frequencies (bass or sub) will go through. This will give you a more boomy feel. It usually works best for bass instruments, mainly because mostly you don’t need the high treble frequencies.

If you set it to HP – meaning High Pass Filter, Then only the High Frequencies (treble) will go through. This will allow you to lose all the mud from the sub and bass, and it is useful when you’re playing chords or when there are a lot of low frequencies coming from other instruments.

If you set it to BP – meaning Band Pass Filter, Then only the Frequencies that are inside the specific range you chose will go through. This will give you a mid-range tone (nosey).

Q / Resonance / Peak

The Q of the filter is the loudness of the filter. The more you boost the Q, the filter will be more narrow and louder.
The more you increase the Q the filter becomes more loud and narrow, and this will cause the wah sound the be more clear.

Frequency / Start Frequency / Stop Frequency / Range*

A Frequency knob sets the start frequency of the filter, While the Start/Stop knobs set the entire range where the filter moves.
In Electro-Harmonix Q-Tron Pedal the Range switch determines if the filter moves in the low or in the high-frequency area. The low range makes the wah effect, and the high range creates overtones.

* In the “EarthQuaker Devices Spatial Delivery” Envelope filter the Range knob is used for Speed and Sensitivity – check the next paragraphs for explanations.

Speed / Rate / Decay

There are some pedals which allow you to control the speed that the filter moves between the start and the end/stop frequency. The reason that this knob is sometimes called “Rate” is that rotating the speed knob will affect the amount of modulation that occurs. In other words, how much we add to the overall rate of the modulation. (If you don’t know what is modulation, think about effects like tremolo, flanger, etc.)
Decay will also set the amount of time that it takes the filter to reach the end frequency.

Attack / Attack Delay

If you turn on this knob, it will create a “fade in” effect.

Sensitivity / Threshold / Trigger

A Threshold is like a gatekeeper that only allows a loud enough signal to go through.
With this knob, you can set the amount of loudness that is required to operate the filter.
This may come in handy when you want softer notes to sound dry and louder notes to sound “wah-ish.” If you wish to affect all of your notes, turn this knob off.
Sensitivity and Trigger are different names of the same game.

Buy One of these Envelope Filters or a Synth Pedals on
MXR Bass Envelope Filter –
Electro-Harmonix Micro Q-Tron –
Electro-Harmonix Bass Micro Synth –
Electro-Harmonix Micro Synth –
Keeley Neutrino Envelope Filter  –
EarthQuaker Devices Spatial Delivery –
Line 6 M5  –

Line 6 M9 –
Line 6 M13 –
Line 6 HX Stomp –
Line 6 HX Effects –
Line 6 POD HD500X –

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Thank you.

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