A low pass filter is a circuit or algorithm that allows signals below a specific frequency to pass through while blocking or reducing signals above that frequency. That specific frequency is called the cutoff frequency, and it’s the dividing line between what gets through and what gets attenuated. Low pass filters show up everywhere, from the crossover in your subwoofer to the noise reduction in your phone’s camera to the power supply inside your laptop.
How a Low Pass Filter Works
Every signal, whether it’s audio, an image, or electrical voltage, can be broken down into a combination of frequencies. Some of those frequencies carry the information you want (like the bass in a song), and some are unwanted noise or interference (like a high-pitched hiss). A low pass filter separates the two by letting low frequencies pass and progressively weakening everything above the cutoff point.
The simplest physical example is a resistor paired with a capacitor. At low frequencies, the capacitor barely conducts, so the signal passes through to the output. At high frequencies, the capacitor conducts more easily and effectively shorts the signal to ground, reducing what reaches the output. The cutoff frequency for this type of circuit is determined by the values of the resistor and capacitor: multiply the resistance by the capacitance, take the inverse, and you get the breakpoint frequency in radians per second.
Filter Order and Roll-Off
No low pass filter creates a perfect wall at the cutoff frequency. Instead, signals above the cutoff are gradually reduced. How quickly they’re reduced depends on the filter’s order. A first-order filter (one that uses a single resistor-capacitor pair, for example) rolls off at 20 decibels per decade. That means for every tenfold increase in frequency above the cutoff, the signal drops by 20 dB. A second-order filter doubles that to 40 dB per decade. Each additional order adds another 20 dB per decade of attenuation.
In practical terms, a first-order filter is gentle. It lets a fair amount of signal through just above the cutoff. If you need a sharper separation, like removing power supply noise without affecting nearby useful frequencies, you need a higher-order filter with a steeper roll-off.
Passive vs. Active Filters
Low pass filters come in two broad hardware categories. Passive filters use only resistors, capacitors, and inductors. They’re simple, reliable, and don’t need a power source. The tradeoff is that they can only reduce a signal’s strength, never boost it, and their performance can change depending on what’s connected to their output.
Active filters add an operational amplifier (op-amp) into the mix alongside those passive components. The op-amp can amplify the signal, which means an active filter can pass low frequencies without any loss in strength, or even add gain. Active filters also give designers more control over the shape of the frequency response, making it easier to build precise higher-order filters without stacking many stages of passive components.
Filter Response Types
Not all low pass filters with the same cutoff frequency behave identically. Different design approaches produce different tradeoffs in how cleanly the filter passes low frequencies and how sharply it transitions to blocking high ones.
A Butterworth filter provides the flattest possible response in the passband, meaning all frequencies below the cutoff come through at nearly equal strength. It’s the go-to choice when you want no distortion of the signal you’re keeping, though its transition from passband to stopband is relatively gradual.
A Chebyshev filter trades that flat passband for a much steeper roll-off. It achieves this by allowing small ripples in the passband, typically oscillating within a defined range (such as between 0 and 1 dB below the peak). A fifth-order Chebyshev filter can reach 40 dB of attenuation just a short distance past the cutoff, while a ninth-order Butterworth might still be rolling off gradually at the same point. If your priority is rejecting frequencies just above the cutoff as aggressively as possible, Chebyshev filters deliver that at the cost of slight passband variation.
Audio: Subwoofer Crossovers
One of the most familiar uses of low pass filters is in home theater and car audio systems. A subwoofer is designed to reproduce only bass frequencies, so a low pass filter strips out everything above a certain point before the signal reaches the subwoofer’s driver. The THX standard recommends a crossover frequency of 80 Hz, which works well for most mid-size bookshelf and surround speakers.
The ideal crossover point depends on your main speakers. Small satellite speakers that can’t reproduce much bass benefit from a higher crossover, around 150 to 200 Hz, so the subwoofer picks up more of the workload. Large tower speakers with 8- to 10-inch woofers can handle bass down to about 40 Hz on their own, so the crossover can be set much lower. A common rule of thumb is to set the low pass crossover about 10 Hz above the lowest frequency your main speakers can handle cleanly.
Power Supplies and Ripple Removal
When AC wall power is converted to the DC voltage that electronics need, the conversion process leaves behind small voltage fluctuations called ripple. In the United States, a full-wave rectified 60 Hz supply produces ripple at 120 Hz. For precision electronics, even a small amount of ripple can cause problems.
A low pass filter placed after the rectifier smooths out these fluctuations. The simplest approach is a single resistor-capacitor network, but demanding applications may need higher-order filters. One example from power electronics engineering reduced 120 Hz ripple from 1 volt peak-to-peak down to just 1 millivolt, a thousand-fold reduction, using a multi-stage low pass filter design that also settled quickly after voltage changes.
Digital and Software Filters
Low pass filters aren’t limited to physical circuits. In software, they work by mathematically averaging neighboring data points, which has the effect of smoothing out rapid changes (high-frequency content) while preserving slower trends (low-frequency content).
In image processing, the simplest digital low pass filter is spatial averaging: each pixel’s value is replaced by the average of its neighbors. This smooths out noise and reduces artifacts like the blocky patterns you sometimes see in heavily compressed JPEG images. The risk is that indiscriminate filtering blurs edges and fine textures, so practical implementations carefully adjust filter strength in different regions of an image to preserve important detail.
In audio, digital low pass filters serve the same purpose as their analog counterparts, removing hiss, electrical interference, or frequencies above the range of human hearing before recording or playback. They’re also critical before analog-to-digital conversion, where they suppress frequencies that could cause aliasing, a type of distortion that creates false tones or buzzing when a signal isn’t sampled fast enough.
A common software implementation is the FIR (finite impulse response) filter, which computes each output sample as a weighted average of current and past input samples. The weights determine the cutoff frequency and how sharply the filter rolls off. Tools like MATLAB include built-in functions for designing these filters, but the same math runs in everything from smartphone audio processing to medical imaging equipment.