Notch Filter: What It Is, How It Works, and When to Use It

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Definition

A notch filter, also called a band-stop filter, is a signal processing tool designed to attenuate a very narrow range of frequencies while allowing others to pass. It removes specific frequency issues without affecting the rest of the signal.

Notch filters are known for their selective frequency rejection and tight bandwidth control. They are useful in both technical correction and creative sound design. Key characteristics include:

• Extremely narrow frequency attenuation
• Adjustable bandwidth (Q factor)
• High precision for isolating unwanted tones

How Notch Filters Work

A notch filter is used to remove a very narrow band of frequencies while leaving the rest of the audio untouched. It’s a precise tool often used to eliminate specific problems like electrical hum or feedback without affecting the overall tone.

Core Mechanism

The center frequency is the exact point the filter is designed to cut. For example, 60 Hz is a common problem area in North America due to power line interference, and engineers often notch it out to clean recordings.

The Q value, or bandwidth, controls how wide the filter reaches. A higher Q sharply targets only the problem frequency, while a lower Q creates a broader dip.

The attenuation depth sets how much the frequency is reduced. This can range from a gentle reduction to completely muting the target frequency.

Frequency Response Graph

In a response graph, a notch filter looks like a sharp dip at the chosen frequency. Everything outside that dip stays at full volume, showing how the filter isolates just a narrow range.

Phase Implications

Using notch filters, especially analog ones, can cause phase shifts near the affected frequency. This can change the feel of the sound. Digital linear-phase filters avoid this issue, which makes them better for mastering or critical listening.

Types of Notch Filters

Notch filters come in different types based on how they’re built and how they behave. Some are hardware-based and others are digital, but all aim to remove narrow problem frequencies without affecting the rest of the signal.

By Implementation Technology

Passive notch filters are built using only resistors, capacitors, and inductors. They don’t need a power source and are often used in simple circuits. While they’re effective for specific tasks, they offer limited control and typically can’t be adjusted on the fly. A common example is the Twin-T filter, used for removing fixed-frequency issues like electrical hum.

Active notch filters include operational amplifiers (op-amps) alongside passive components. These require external power but provide deeper attenuation and more flexibility. With adjustable frequency and bandwidth controls, they’re common in analog mixing desks and equalizer units where more precision is needed.

Digital notch filters are implemented through software or firmware, offering the most control. They are found in DAWs, audio plugins, hearing aids, and live DSP processors. Because they’re programmable, they can be static or adaptive and offer surgical accuracy for removing unwanted tones.

By Functional Behavior

Fixed notch filters are tuned to one frequency and are often used to remove constant problems, like a 60 Hz power line hum. They work best when the interference never changes.

Sweepable notch filters allow users to adjust the center frequency manually, which is helpful when identifying and removing feedback during live performances or recordings.

Parametric notch filters offer full control over the center frequency, bandwidth (Q), and depth of the cut. They’re ideal for precision editing in studio environments and are a standard tool in many EQ plugins.

Adaptive notch filters automatically follow changes in the problem frequency. These are especially useful in live sound setups where feedback or interference may shift during a performance.

Key Parameters & Controls

Notch filters offer a few key settings that control how they work. The most important is the center frequency, which is the specific pitch that gets cut. Some filters let you adjust this freely across the full 20 Hz to 20 kHz range. Others, like fixed filters, are set to common problem frequencies such as 50 or 60 Hz to remove electrical hum.

Another important setting is the bandwidth, or Q factor. This controls how wide or narrow the cut is. A high Q value targets a very narrow band, perfect for removing a single, sharp tone without affecting nearby frequencies. A lower Q spreads the cut across a broader range, which can solve bigger issues but may also reduce wanted parts of the sound.

Attenuation depth sets how much the targeted frequency is reduced. A shallow cut, like -6 dB, gently lowers the problem without making it vanish. A deep cut, up to -∞ dB, silences it completely. Use only the amount needed – too much filtering can strip away useful sound or create noticeable gaps.

Notch Filter vs. Other Filters

Notch Filter Implementations

Notch filters appear in both hardware and software, serving roles in live sound, studio work, and post-production. Some are built for constant problem-solving, while others offer dynamic control for more flexible audio shaping.

Audio Hardware

Sabine FBX Feedback Eliminators are well-known for their ability to detect and suppress feedback in real time. These units apply ultra-narrow notch filters automatically as soon as feedback starts to build, making them ideal for live sound engineers working in acoustically difficult venues.

Furman PL-Plus units include power conditioning and built-in notch filtering to remove electrical noise. This is particularly useful in stage and studio environments where inconsistent power sources can introduce hums or interference into the signal chain.

Software Plugins

iZotope RX Spectral Repair gives engineers a visual approach to fixing problematic audio. Users can isolate and remove unwanted frequencies, like hums or squeaks, by selecting them directly in a spectral view and applying precise notch filters with minimal artifacts.

FabFilter Pro-Q 3 offers dynamic EQ functionality that can act like a notch filter only when necessary. This means it won’t cut frequencies unless they cross a defined threshold, preserving musicality while still removing harsh tones or resonances.

Waves Q10 Equalizer is a versatile plugin with multiple bands and ultra-narrow Q settings. It’s commonly used to surgically remove specific frequencies such as whines, hums, or digital noise without affecting the rest of the track.

Together, these tools provide flexible solutions for noise removal, tonal correction, and creative shaping – all using notch filtering as a core technique.

Advanced Concepts

Some notch filter techniques go beyond simple fixed-frequency cuts. Comb filters are a common example. They occur when a signal combines with a delayed version of itself, creating a series of notches across the frequency spectrum. This often happens in untreated rooms or when two mics pick up the same sound out of phase.

Dynamic notching is a more modern approach. It only activates the notch when a problem frequency appears, usually triggered by volume or side-chain input. This makes it useful for vocals, live instruments, or unpredictable sources where constant filtering would damage the tone.

There’s also a difference in how digital notch filters are designed. FIR (Finite Impulse Response) filters preserve the phase of your audio and are often used in mastering. IIR (Infinite Impulse Response) filters are more efficient and respond faster, making them better for live sound and low-latency setups. Choosing the right type depends on your workflow and goals.

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