/nɒtʃ ˈfɪltər/
Notch Filter is a type of EQ band that sharply attenuates a very narrow frequency range while leaving surrounding frequencies untouched. It is the primary tool for removing hum, feedback, ring, and resonance from audio signals.
Every mix has at least one frequency that doesn't belong — a room mode turning your kick into a cardboard box, a mic cable feeding 60 Hz into your vocal, a snare ring that follows you through every channel. The notch filter is the scalpel that removes it without disturbing anything else.
A notch filter — also called a band-reject filter or band-stop filter — is a filter type that reduces the level of a very narrow band of frequencies while passing all frequencies above and below it with minimal change. Unlike a standard parametric EQ cut, which carves a relatively broad bowl-shaped dip into the spectrum, a notch filter is defined by its extreme Q value: it can remove a band as narrow as a fraction of a single hertz, or as wide as a musically meaningful semitone. In practice, the term is used to describe any sharp, deep cut applied to a specific problem frequency, though technically a true notch filter achieves near-infinite attenuation at its center frequency — a behavior that distinguishes it from a conventional parametric band.
The fundamental utility of a notch filter in music production is the elimination of unwanted tonal artifacts that originate outside the musical content itself. These include mains hum (50 Hz in Europe and Australia, 60 Hz in North America) and its harmonic series (100/120 Hz, 150/180 Hz, and so on), feedback frequencies in live reinforcement systems, room resonances captured during tracking, tuning artifacts from acoustic instruments like the wolf note on a cello or the body resonance of a guitar, and the characteristic ring of drums — particularly snare wires — that fights for space in a mix. Because these artifacts are usually tonal and stable in frequency, a notch filter is the correct tool: broad-band cutting would sacrifice the musical content sitting immediately adjacent.
In the signal chain, a notch filter is most commonly implemented as a dedicated band within a parametric EQ. When you pull a narrow band to its deepest attenuation — often −18 dB, −24 dB, or infinity depending on the equalizer — and raise the Q to its maximum value (commonly 10, 20, or 100 depending on the plugin), you have effectively created a notch. Some equalizers, particularly hardware units designed for broadcast or live sound, include dedicated notch filter circuitry that operates independently of the main EQ bands, allowing the engineer to patch out hum without consuming a standard EQ band. In the digital domain, notch filters are also implemented as biquad IIR filters using a specific coefficient configuration that sets the filter's gain to zero at the center frequency.
For producers working primarily in the box, the notch filter appears in multiple contexts beyond traditional equalization. Dynamic EQs and multiband compressors can use notch-like bands to reactively suppress resonances only when they exceed a threshold, a technique popularized in mastering and acoustic guitar mixing. Linear-phase notch filters are available in mastering-grade EQs for situations where phase coherence across the stereo field must be maintained. And notch filters implemented in the feedback path of synthesizer and effects processors create entirely different behavior — phase notching from comb filtering, flanging, and phasing effects all derive their character from the interaction of notch-like cancellations at harmonically related frequencies.
A notch filter is defined by three parameters: center frequency (the frequency at which maximum attenuation occurs), bandwidth or Q factor (how narrow or wide the affected band is), and depth (how much gain reduction is applied at the center frequency). In the transfer function of a biquad IIR filter, the notch is achieved by setting the numerator coefficients so that the filter's gain approaches zero — or mathematically exactly zero in the ideal case — at the target frequency, while both the passband below and above return to unity gain. This means energy at the center frequency is phase-cancelled rather than absorbed, which has implications for how narrow a notch can become in practice: an infinitely narrow notch attenuates only an infinitely thin sliver of the spectrum, which in a digital system corresponds to a single frequency bin.
The Q factor determines the selectivity of the notch. Q is defined as the ratio of the center frequency to the bandwidth (Q = f₀ / BW), where bandwidth is measured between the two −3 dB points on either side of the notch. A Q of 1 produces a broad, one-octave-wide cut — more like a dip than a notch. A Q of 10 produces a bandwidth of roughly 1/10th of an octave, affecting only the immediate neighborhood of the target frequency. A Q of 100 or higher creates a scalpel-thin excision that is virtually inaudible on broadband content but completely removes a specific tone. For mains hum removal, engineers typically use Q values between 10 and 30, which is narrow enough to leave the musical bass frequencies untouched but wide enough to capture slight fluctuations in the hum's actual frequency. For feedback suppression in live sound, Q values of 20 to 60 are common.
The phase response of a notch filter is an important technical consideration. Like all minimum-phase IIR filters, a conventional notch filter introduces phase rotation in the frequencies surrounding the notch — the closer a frequency is to the center, the more its phase is shifted. At the exact center frequency, the phase relationship is undefined (since the gain is zero), but immediately adjacent frequencies experience phase rotation of up to ±90 degrees. In most mixing contexts, this phase shift is inaudible and inconsequential. In mastering, or when notch-filtering a signal that will later be summed with an unfiltered copy, the phase difference can create partial cancellations in the passband. Linear-phase implementations (available in FIR-based equalizers such as FabFilter Pro-Q 3 in linear-phase mode, or the linear-phase option in iZotope Ozone EQ) avoid this issue at the cost of pre-ringing and increased latency.
Harmonic notching is a more advanced application in which multiple notch filters are stacked at the fundamental frequency and each of its harmonics — typically 2nd, 3rd, 4th, and 5th. This is the standard technique for eliminating mains hum completely, since electrical hum is a complex tone with a full harmonic series rather than a pure sine wave. A single notch at 60 Hz leaves the 120 Hz, 180 Hz, and 240 Hz harmonics intact and audible, which is why the hum still sounds colored even after the fundamental is removed. Dedicated hum removal plugins (iZotope RX's Hum Removal, Waves X-Hum) automate this multi-notch process and often add adaptive tracking to follow drift in the hum's frequency.
Understanding notch filters at this level allows producers to make deliberate choices: whether to use a single deep notch or a series of shallower ones, whether minimum-phase or linear-phase processing is appropriate for the material, and whether a static notch is sufficient or a dynamic notch responding to signal level will produce more transparent results. These distinctions separate routine hum removal from genuinely transparent restoration work.
Diagram — Notch Filter: Notch filter frequency response curve showing deep narrow attenuation at 60 Hz, 120 Hz, and 180 Hz for harmonic hum removal, with Q factor and center frequency labeled.
Every notch filter — hardware or plugin — operates on the same core parameters. Know these and you can work with any implementation.
Center frequency sets where the notch sits in the spectrum, typically expressed in Hz. For mains hum removal, this is precisely 50 Hz or 60 Hz (and their harmonics). For resonance control, it is found by sweeping a narrow, boosted band to identify the problem frequency before notching. Even small errors in center frequency — as little as 2–3 Hz off-target — can leave an audible artifact while attenuating adjacent musical content unnecessarily.
Q is the ratio of center frequency to the −3 dB bandwidth. Low Q values (1–3) produce wide, shallow dips; high Q values (10–100+) produce surgical notches. For 60 Hz hum removal, Q values of 10–30 are standard. For live feedback control, Q values of 30–60 minimize the musical impact. In general, use the highest Q that fully captures the artifact — wider notches remove more musical content than necessary.
Depth determines the magnitude of attenuation at f₀, measured in negative dB. For complete elimination of a tonal artifact (hum, feedback), the deepest available setting — often labeled −∞ or 'notch' — is appropriate. For resonance control in acoustic instruments or room treatment, a shallower cut of −6 dB to −18 dB may be sufficient to tame the problem without creating an audible hole, especially when using a moderate Q where deeper cuts affect more surrounding frequencies.
Bandwidth is expressed in Hz or octaves and defines the range between the two −3 dB points of the notch. BW = f₀ / Q, meaning a 60 Hz notch with Q=20 has a bandwidth of 3 Hz — extremely narrow. Some hardware and plugin interfaces present bandwidth directly instead of Q, particularly broadcast and live sound tools. Understanding the BW directly helps when working on harmonic stacks, since higher harmonics need proportionally wider bandwidths to cover the same musical interval.
In minimum-phase mode (the default for most EQs), a notch filter introduces phase rotation in adjacent frequencies. This is acceptable in the vast majority of mixing contexts but can cause issues when the filtered signal is summed with an unfiltered copy. Linear-phase mode, available in plugins like FabFilter Pro-Q 3 and iZotope Ozone EQ, eliminates phase artifacts at the cost of pre-ringing and increased latency — making it appropriate for mastering and post-production but impractical for real-time live use.
Dynamic notch filters (available in tools like FabFilter Pro-Q 3, TDR Nova, and iZotope Neutron EQ) only engage attenuation when the target frequency exceeds a set threshold. This is especially useful for snare ring or guitar body resonance that is only problematic at louder transient peaks, not throughout the signal. Setting the threshold slightly above the instrument's steady-state level ensures the notch remains transparent during normal playing and only activates during problem moments.
Session-ready starting points. These values are starting points — always sweep to identify the exact problem frequency before committing to a notch depth and Q.
| Parameter | General | Drums | Vocals | Bass / Keys | Bus / Master |
|---|---|---|---|---|---|
| 60 Hz mains hum | 178 Hz, Q 20, −∞ | 60 Hz, Q 20, −∞ | 60 Hz, Q 20–30, −∞ | 60 Hz, Q 15, −∞ | 60 Hz, Q 25, −∞ |
| 50 Hz mains hum (EU) | 50 Hz, Q 20, −∞ | 50 Hz, Q 20, −∞ | 50 Hz, Q 25, −∞ | 50 Hz, Q 15, −∞ | 50 Hz, Q 25, −∞ |
| Snare body ring | sweep 180–400 Hz | 200–350 Hz, Q 8–15, −6 to −12 dB | — | — | avoid on bus |
| Room resonance / mode | sweep 80–300 Hz, Q 5–12, −6 to −18 dB | sweep kick region, Q 8, −9 dB | 200–400 Hz, Q 6–10, −6 dB | 80–160 Hz, Q 8, −9 dB | low Q, −3 to −6 dB only |
| Feedback (live sound) | sweep to find, Q 30–60, −∞ | varies, Q 40, −∞ | 2–8 kHz range, Q 40–60, −∞ | varies, Q 40, −∞ | Q 50, −∞ if needed |
| Guitar body resonance | sweep 80–200 Hz, Q 10, −6 to −12 dB | — | — | 80–160 Hz, Q 8–12, −9 dB | — |
| Harmonic hum stack | f₀ + 2nd + 3rd harmonic, each Q 20, −∞ | 60+120+180 Hz, Q 20 | 60+120+180 Hz, Q 20–25 | 60+120 Hz, Q 15–20 | use dedicated hum plugin |
These values are starting points — always sweep to identify the exact problem frequency before committing to a notch depth and Q.
The conceptual foundation of the notch filter predates electronic audio: the twin-T notch network, first described in the 1930s, used a passive RC circuit to create a frequency-specific null in a signal by phase-cancelling a target frequency against a delayed copy of itself. By the 1940s and 1950s, active notch filters built around op-amps were standard tools in telephone engineering, used to suppress powerline interference in long-distance transmission systems. The earliest broadcast audio applications were direct descendants of these telecommunications tools — by the mid-1950s, NBC and BBC engineering departments were deploying rack-mounted notch filter units at the inputs of tape recorders to suppress 60 Hz (US) or 50 Hz (UK) hum introduced by poorly shielded equipment and long cable runs.
In professional studio recording, notch filtering became a formal technique in the 1960s as engineers began working with directional microphones in acoustically challenging rooms. Rupert Neve's console designs of the late 1960s — beginning with the Neve 8014 in 1970 and later the landmark 8078 — incorporated high-Q parametric bands that could be driven into notch behavior by combining maximum Q with maximum cut, though Neve's primary innovation was the musicality of his broad shelves and bell curves. The SSL 4000 G console, introduced in 1979, brought four-band fully parametric equalization to every channel with sufficiently flexible Q controls to achieve notch-like behavior. API's 550b, introduced in 1969, offered proportional-Q behavior that became narrower as gain was reduced, making deep cuts inherently more surgical — a design philosophy that influenced many subsequent hardware EQs.
The introduction of digital audio workstations and software equalizers in the 1990s fundamentally changed how producers accessed notch filtering. The Digidesign (now Avid) Pro Tools EQ II, released in the mid-1990s, offered a notch filter type as a discrete band option — distinct from the standard bell EQ — allowing infinite attenuation at the center frequency. Sony Oxford (later Sonnox) released the Oxford EQ in 1999 as one of the first software equalizers with genuine engineering-grade notch behavior. The iZotope RX suite, first released in 2007, introduced spectral repair tools including a dedicated Hum Removal module that automated multi-notch harmonic hum suppression, representing a significant leap in accessibility for audio restoration work.
FabFilter's Pro-Q 2 (2013) and Pro-Q 3 (2018) brought notch filtering into the modern era of visual, precise, and dynamically assisted equalization. The ability to create a notch band with a single click, see its exact frequency response in real time, and optionally engage dynamic behavior transformed notch filtering from a specialized corrective tool into a routine step in the mixing workflow. By the mid-2010s, mixing engineers such as Chris Lord-Alge and Andrew Scheps were publicly discussing notch filtering as a standard part of their drum and vocal processing chains, signaling the technique's migration from corrective necessity to proactive tonal sculpting.
Vocals: Mains hum on vocal tracks is one of the most common problems producers encounter, particularly in home studios where ground loops between audio interfaces, computers, and monitors are endemic. A notch at 60 Hz (or 50 Hz in Europe) with Q 20–30 and full attenuation is typically the first insert on a problematic vocal chain. More musically interesting is using a notch on vocal resonances — the proximity effect buildup in the 200–400 Hz range that makes a close-miked vocal sound boxy, or the 2–4 kHz nasal honk that certain singers produce at forte dynamics. A dynamic notch in this region, triggered only when the vocalist pushes hard, keeps the sound natural during normal passages while controlling the artifact during emotional peaks.
Drums: Snare drums are notch filter territory. The body ring of a snare — a pitched resonance typically in the 180–350 Hz range that sustains after the initial transient — can build up across multiple channels: the snare top mic, the snare bottom mic, room mics, and even the overhead mics all contribute to the same ring at slightly different levels and phases. Identifying the exact ring frequency (sweep a narrow boost across the snare channel until the ring becomes unbearable, then notch it out) and applying consistent notches across all contributing channels can transform a cluttered, pitchy snare sound into a dry, punchy one. Kick drum resonances around 60–80 Hz, where the drum's tuning interacts with room modes, respond similarly.
Electric guitar and bass: Single-coil guitars are notorious for picking up mains hum, and while the standard solution is to switch to a hum-cancelling pickup configuration, recordings already made with single-coils require notch treatment. A harmonic stack of notches at 60, 120, and 180 Hz (or 50, 100, 150 Hz) removes the hum cleanly when the guitarist is not playing. Bass guitars with piezo pickups or direct box recordings often carry a mid-bass resonance around 80–160 Hz that corresponds to the instrument's body resonance; a notch here improves low-end definition without sacrificing weight. For acoustic guitar, the wolf note — an overpowered resonance where the body's air resonance aligns with a fretted note — typically appears in the 80–120 Hz range and responds well to a moderate notch of −6 to −9 dB at high Q.
Synthesizers and electronic elements: Notch filters are not exclusively corrective. Applying a swept notch to a synthesizer pad — either via automation or an LFO-modulated filter cutoff — creates phaser-like movement without the complex phase relationships of a true phaser circuit. A static notch at a harmonic frequency of a synth bass or lead can thin out an otherwise congested frequency region, making space for another element without a broad EQ cut that would affect the instrument's entire character. When layering multiple synthesizer sounds, narrow notches can reduce the masking between layers by targeting the specific frequencies where one element dominates, allowing both elements to project clearly in a busy mix.
One email a week. The techniques behind the terms — curated by working producers, not algorithms.
Abstract knowledge becomes practical when you can hear it in music you know. These tracks demonstrate notch filter used intentionally, at specific moments, for specific purposes.
Nigel Godrich and the Radiohead camp were early proponents of radical EQ surgery on organic recordings. The orchestral string arrangement on this track, recorded at Abbey Road, required extensive notch work to remove room resonances from the large recording hall that caused specific pitches in the string ensemble to bloom unnaturally. Listen to the lower register strings in the verse: despite the size of the room, there are no uncontrolled low-mid buildups. Godrich has discussed using notch filtering to treat individual microphone feeds rather than the mixed stem, preserving the room character while eliminating structural anomalies.
The opening kick and snare pattern on 'HUMBLE.' is a masterclass in drum treatment in modern hip-hop. The snare has an almost entirely suppressed body resonance — any natural ring from the source sample has been notched away, leaving only the crack of the transient and a controlled white-noise tail. A/B this snare against an unprocessed trap snare in the same frequency range and the difference is immediately apparent: Mike Will Made-It's mix has zero pitched energy in the 200–400 Hz snare ring region. This likely reflects a combination of sample selection and notch treatment.
Produced entirely in the FINNEAS O'Connell home studio in a converted bedroom in Los Angeles, 'bad guy' is a celebrated example of home-studio production achieving professional results through meticulous signal hygiene rather than expensive acoustics. FINNEAS has discussed in interviews using surgical EQ — including notch filtering — to remove room resonances from every recorded element, including Billie's vocal. The bass element sitting below the lead vocal has an unusually clean low-mid presentation with no boxy room character, consistent with notch treatment on room modes around 80–200 Hz.
Nile Rodgers' live rhythm guitar recording on 'Get Lucky' required careful frequency management to sit cleanly in a track with dense electronic elements. The guitar's characteristic body resonance and the studio's acoustic treatment combined to create a 100–150 Hz buildup that would have muddied the low end against the electronic bass. The finished mix reveals clean low-mid separation between the guitar and bass elements, consistent with notch filtering on the guitar track to remove body resonance. Rodgers and the Daft Punk team worked with engineer Florian Lagatta and mixed with Mick Guzauski, both known for precision EQ work.
A static, non-adjustable notch at a factory-set frequency. Common in power supply filtering, broadcast equipment, and some vintage channel strips where a permanent 50 Hz or 60 Hz notch was built in. In modern production, this concept is replicated by 'baking in' a notch via a plugin preset applied consistently to all tracks in a session. Appropriate when the problem frequency is known and constant.
The most common form in studio production — a fully adjustable notch where center frequency, Q, and depth are all user-controlled. Available as a discrete band type in most modern software equalizers (FabFilter Pro-Q 3, Logic Channel EQ, Pro Tools EQ3) and approximated in hardware by combining maximum Q with maximum cut on any parametric band. Offers the flexibility to target any problem frequency with any degree of selectivity.
A notch filter that only engages when the target frequency exceeds a user-set threshold, functioning essentially as a frequency-specific expander. Transparent during normal signal levels; active only when the artifact appears. Ideal for snare ring that only becomes problematic at loud dynamics, guitar body resonance that builds up only on sustained notes, or vocal resonances that peak on certain vowels or sung pitches.
Multiple notch filters placed at the fundamental frequency and its integer harmonics (2nd, 3rd, 4th, 5th and beyond), designed specifically for eliminating electrical hum in its complete form. Dedicated hum removal plugins automate the placement and tracking of these stacked notches and often include adaptive frequency tracking to follow slight drift in the hum's pitch. Manual implementation requires placing individual notch bands in a parametric EQ at each harmonic, which is straightforward but time-consuming.
An FIR-based implementation that achieves the same frequency-domain attenuation as a conventional notch filter without the phase rotation in adjacent frequencies. Critically important when the filtered signal will be summed with an unfiltered copy, or in mastering contexts where phase coherence across the stereo image must be preserved. The trade-off is pre-ringing (an artifact of FIR processing that creates a faint pre-echo before transients) and significant latency, making linear-phase notches unsuitable for live performance or latency-sensitive monitoring.
A notch filter whose center frequency is continuously modulated, typically by an LFO or envelope follower. This is the mechanism behind phaser and flanger effects: multiple notches sweep across the frequency spectrum, creating the characteristic 'whooshing' or 'jet' sound. While traditional phasers use an all-pass filter network to create comb-notch patterns, LFO-modulated notch filters in EQ plugins produce a related but distinct movement useful for adding animation to synth pads, guitars, and drum loops without the harmonic distortion introduced by analog phaser circuits.
Frequency conflicts — two instruments in the same range at similar levels — are the root cause of muddy mixes.
These MPW articles put notch filter into practice — specific techniques, real tools, and applied workflows.