Auto-wah is a type of wah-wah

Wah-wah pedal

A wah-wah pedal is a type of guitar effects pedal that alters the tone of the signal to create a distinctive effect, mimicking the human voice...

 effects pedal typically used with electric guitar

Electric guitar

An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...

, bass guitar

Bass guitar

The bass guitar is a stringed instrument played primarily with the fingers or thumb , or by using a pick....

, clavinet

Clavinet

A Clavinet is an electrically amplified keyboard instrument manufactured by the Hohner company. It is essentially an electronically amplified clavichord, analogous to an electric guitar. Its distinctive bright staccato sound has appeared particularly in funk, disco, rock, and reggae songs.Various...

, and electric piano

Electric piano

An electric piano is an electric musical instrument.Electric pianos produce sounds mechanically and the sounds are turned into electrical signals by pickups. Unlike a synthesizer, the electric piano is not an electronic instrument, but electro-mechanical. The earliest electric pianos were invented...

 etc. When used, it creates an audio effect similar to that of the sitar

Sitar

The 'Tablaman' is a plucked stringed instrument predominantly used in Hindustani classical music, where it has been ubiquitous since the Middle Ages...

 employing a resonator.

The distinctive choppy rhythm guitar sound on many funk

Funk

Funk is a music genre that originated in the mid-late 1960s when African American musicians blended soul music, jazz and R&B into a rhythmic, danceable new form of music. Funk de-emphasizes melody and harmony and brings a strong rhythmic groove of electric bass and drums to the foreground...

 and disco

Disco

Disco is a genre of dance music. Disco acts charted high during the mid-1970s, and the genre's popularity peaked during the late 1970s. It had its roots in clubs that catered to African American, gay, psychedelic, and other communities in New York City and Philadelphia during the late 1960s and...

 recordings from the 1970s popularized the effect.

Operation

Instead of the effect being controlled by a foot pedal, as on a standard wah-wah, the effect alters in response to the volume of the input signal. Since the electronic circuits in the effect can respond much faster than a human musician can physically move a foot pedal, certain effects that a standard wah cannot achieve are only possible with auto-wah. The response of the effect is highly interactive with the dynamics of the input signal - this makes it possible to vary the response at will via slight adjustments to playing technique. Therefore it may take some practice before the response of the auto-wah can be wilfully controlled in order to achieve a consistent sound.

A typical auto-wah circuit uses an envelope follower to produce a voltage representing the overall volume of the input. This signal is then used to sweep the cutoff frequency

Cutoff frequency

In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...

 of a filter. The filter usually has a low-pass

Low-pass filter

A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...

 or bandpass response. The Boss AW-3 is an example of such a device.

There is a variant of auto-wah that utilises a low frequency oscillator (LFO) instead of an envelope follower to alter the effect. The filter response varies constantly with time and is not linked to playing dynamics. The Boss AW-2 is an example of such a device. The difference in sound is subtle, but careful listeners will notice the constant period of the filter sweep. The song "Falling Into Grace" by the Red Hot Chili Peppers

Red Hot Chili Peppers

Red Hot Chili Peppers is an American rock band, formed in Los Angeles in 1983. The group's musical style primarily consists of rock with an emphasis on funk, as well as elements from other genres such as punk, hip hop and psychedelic rock...

 is an example of an LFO controlled auto-wah applied to the bass guitar.

Typical controls on an auto-wah include a sensitivity control to adjust the input level to match the level expected by the envelope follower and other circuitry, a control for the initial cutoff point of the filter, and a control for the depth of the filter sweep. Some more sophisticated units offer controls for the resonance of the filter, multiple filter types, and options for sweeping the filter up or down.

Examples

Commercial examples include the MXR Envelope Filter, the Musitronics Mutron

Mutron

Musitronics, often shortened to Mu-tron, was a manufacturer of electronic musical effects active in the 1970s. Their product line focused on filtering and processing effects derived from synthesizer components...

 III and the Boss AW-2 Auto Wah (LFO controlled).

This effect can be heard in the songs "Stinkfoot" by Frank Zappa

Frank Zappa

Frank Vincent Zappa was an American composer, singer-songwriter, electric guitarist, record producer and film director. In a career spanning more than 30 years, Zappa wrote rock, jazz, orchestral and musique concrète works. He also directed feature-length films and music videos, and designed...

; "Going Mobile

Going Mobile

"Going Mobile" is a song written by Pete Townshend and originally released by The Who on their 1971 album Who's Next. It is one of the lighter songs on the album....

" by The Who

The Who

The Who are an English rock band formed in 1964 by Roger Daltrey , Pete Townshend , John Entwistle and Keith Moon . They became known for energetic live performances which often included instrument destruction...

; "Industrial Disease" by Dire Straits

Dire Straits

Dire Straits were a British rock band active from 1977 to 1995, composed of Mark Knopfler , his younger brother David Knopfler , John Illsley , and Pick Withers .Dire Straits' sound drew from a variety of musical influences, including jazz, folk, blues, and came closest...

; "What I Am" by Edie Brickell & New Bohemians

Edie Brickell & New Bohemians

Edie Brickell & New Bohemians is an alternative rock jam band that originated in Texas in the mid-1980s. The band is best known for their 1988 hit "What I Am" from the album Shooting Rubberbands at the Stars. Their music contains elements of rock, folk, blues, and jazz...

; "I Disappear" by Metallica

Metallica

Metallica is an American heavy metal band from Los Angeles, California. Formed in 1981 when James Hetfield responded to an advertisement that drummer Lars Ulrich had posted in a local newspaper. The current line-up features long-time lead guitarist Kirk Hammett and bassist Robert Trujillo ...

; "Mysterious Ways

Mysterious Ways (song)

"Mysterious Ways" is a song by the rock band U2. It is the eighth track from their 1991 album Achtung Baby and was released as the album's second single on 25 November 1991. The song reached the top ten of the singles charts in several countries, including Ireland, where it went to number one...

" by U2

U2

U2 are an Irish rock band from Dublin. Formed in 1976, the group consists of Bono , The Edge , Adam Clayton , and Larry Mullen, Jr. . U2's early sound was rooted in post-punk but eventually grew to incorporate influences from many genres of popular music...

; and
"Estimated Prophet" by The Grateful Dead. It is commonly used with a bass guitar and many examples can be found by listening to recordings by Bootsy Collins

Bootsy Collins

William Earl "Bootsy" Collins is an American funk bassist, singer, and songwriter.Rising to prominence with James Brown in the late 1960s, and with Parliament-Funkadelic in the '70s, Collins's driving bass guitar and humorous vocals established him as one of the leading names in funk...

. Another bass guitar line with envelope follower controlled auto-wah applied can be heard in "Sir Psycho Sexy" by the Red Hot Chili Peppers

Red Hot Chili Peppers

Red Hot Chili Peppers is an American rock band, formed in Los Angeles in 1983. The group's musical style primarily consists of rock with an emphasis on funk, as well as elements from other genres such as punk, hip hop and psychedelic rock...

.

Another notable example is "Higher Ground" by Stevie Wonder

Stevie Wonder

Stevland Hardaway Morris , better known by his stage name Stevie Wonder, is an American singer-songwriter, multi-instrumentalist, record producer and activist...

, where the main effect was created by playing a Clavinet

Clavinet

A Clavinet is an electrically amplified keyboard instrument manufactured by the Hohner company. It is essentially an electronically amplified clavichord, analogous to an electric guitar. Its distinctive bright staccato sound has appeared particularly in funk, disco, rock, and reggae songs.Various...

through an auto-wah.