What Is a Notch Filter?
A notch filter is a filter that weakens signals in a small range of frequencies and allows all other frequencies to pass through unchanged.
Notch filters are effective at removing interfering signals at specific frequencies. In audio and other signal applications, the background hum produced by the electrical grid (often 60 Hz) can be removed by designing a notch filter for that frequency. In software-defined radios, interfering FM broadcast signals can be removed with notch filters. A notch filter is a type of bandstop filter made from a combination of high-pass and low-pass filters. Notch filters are also referred to as “band-rejection filters.”
Magnitude response of a notch filter in the Filter Visualization Tool in MATLAB.
FIR Filters
FIR filters are an attractive option because they are inherently stable. They can be designed to have a linear phase that introduces a delay in the filtered signal while maintaining the waveform shape. Nonetheless, these filters can have long transient responses and might prove computationally expensive in certain applications. FIR filters are useful in audio, biomedical, radar, and other applications where the waveform shape provides useful information. Common design methods for low-pass FIR-based filters include Kaiser window, least squares, and equiripple.
Magnitude response of an FIR notch filter designed with MATLAB.
IIR Filters
IIR filters are useful when computational resources are at a premium. IIR filters reduce hardware requirements by using fewer filter coefficients. Stable, causal IIR filters, however, do not have a perfectly linear phase. IIR filters are commonly used in applications, such as audio equalization, biomedical sensor signal processing, IoT/IIoT smart sensors, and high-speed telecommunication/RF applications. Design methods for IIR-based filters include Butterworth, Chebyshev (Type I and Type II), and elliptic.
Magnitude response of an IIR notch filter designed with MATLAB.
Filter Design in MATLAB
The bandstop function in Signal Processing Toolbox™ is particularly useful to quickly filter signals. You can use designfilt and other algorithm-specific (e.g., butter, fir1) functions when more control is required on parameters such as filter type, filter order, and attenuation. For more information on filter design, see Signal Processing Toolbox .
Examples and How To
Software Reference
Notch Filter FAQs
A notch filter is a bandstop filter that attenuates a narrow range of frequencies while allowing all others to pass. It is commonly used to remove unwanted interference, such as power-line noise at 50 or 60 Hz, without affecting the rest of the signal spectrum.
Use a notch filter when you need to suppress a specific frequency or a very narrow band of frequencies without altering the overall signal bandwidth. Low-pass or high-pass filters affect broad frequency ranges, making them unsuitable for precise frequency rejection.
A bandstop filter attenuates a wider range of frequencies, whereas a notch filter targets a very narrow band. Engineers often choose notch filters for applications such as eliminating single-frequency interference while preserving signal integrity.
To design a notch filter, determine the center frequency to reject and the desired bandwidth. Then select filter parameters, such as quality factor (Q) and order.
The quality factor defines how narrow or sharp the notch is. A higher Q means a narrower rejection band, which is ideal for removing a single interfering tone without affecting adjacent frequencies.
Notch filters introduce phase shifts near the notch frequency and attenuate amplitude within the rejection band. Outside this band, the signal remains largely unaffected, making notch filters suitable for precision filtering.
Yes. A notch filter tuned to 60 Hz (or 50 Hz in some regions) is commonly used to eliminate power-line interference in biomedical signals, audio systems, and instrumentation without degrading other frequency components.
Notch filters are widely used in audio processing, communication systems, biomedical signal analysis (e.g., ECG), and instrumentation to suppress narrowband interference while preserving signal fidelity.
In MATLAB, you can design and apply notch filters using functions such as designfilt or designNotchPeakIIR. These tools allow you to specify center frequency, bandwidth, and filter order for precise control.
Notch filters are effective for narrowband interference but can distort signals if the notch is too wide or if the target frequency varies over time. For dynamic interference, adaptive filtering may be a better solution.
See also: DSP System Toolbox, low-pass filter, filter design, quantization, high-pass filter, bandpass filter
