A low pass filter should typically be set to 80 Hz for most subwoofer and home theater setups, which is the most widely recommended crossover point in the audio industry. But the right setting depends entirely on your specific speakers, your equipment, and what you’re actually filtering. In audio, 80 Hz is a starting point. In electronics, medicine, and digital signal processing, the correct cutoff frequency follows different rules entirely.
Subwoofer and Home Theater Settings
Most people searching for low pass filter settings are adjusting a subwoofer crossover. The low pass filter tells your subwoofer the highest frequency it should try to reproduce. Everything above that frequency gets progressively quieter, letting your main speakers handle the midrange and treble instead.
The right setting depends on the size of your main speakers:
- On-wall or compact satellite speakers: 150 to 200 Hz
- Small bookshelf or surround speakers: 100 to 120 Hz
- Mid-size bookshelf or center channels: 80 to 100 Hz
- Large bookshelf or center channels: 60 to 80 Hz
- Tower speakers with 4″ to 6″ woofers: 60 Hz
- Tower speakers with 8″ to 10″ woofers: 40 Hz, or set them to full range
The logic is straightforward. Smaller speakers can’t reproduce low bass well, so you raise the crossover point to let the subwoofer handle more of the frequency range. Larger speakers with bigger woofer cones can reach lower on their own, so you set the filter lower and let the subwoofer focus only on the deepest bass.
If you’re unsure and just want a single number to start with, set it to 80 Hz. This works well for the majority of speaker combinations and is the default recommendation for most surround sound receivers.
Filter Slope Makes a Big Difference
The cutoff frequency isn’t a hard wall. A low pass filter set to 80 Hz doesn’t silence everything above 80 Hz instantly. Instead, frequencies above the cutoff fade out gradually, and the slope setting controls how fast that fade happens.
Slope is measured in decibels per octave (dB/octave). An octave is a doubling of frequency, so one octave above 80 Hz is 160 Hz. With a 12 dB/octave slope, your subwoofer playing at 80 Hz will still reproduce some sound at 100 Hz and go quiet around 110 Hz. With a steeper 24 dB/octave slope, the signal drops off faster: it may play slightly at 90 Hz and go silent by 100 Hz. At 160 Hz (one full octave up), a 24 dB slope has reduced the signal by 24 dB, which is barely audible.
A steeper slope gives a cleaner handoff between your subwoofer and main speakers. A gentler slope creates more overlap, which can sound either fuller or muddier depending on your room and speaker placement. If your receiver or amplifier lets you choose, try 24 dB/octave first. It keeps the subwoofer focused on bass and reduces the chance of hearing it “localize,” where you can tell exactly where the sub is sitting in the room.
Car Audio Low Pass Settings
Car audio follows the same principles, but the environment changes things. In a car, the small enclosed space reinforces bass naturally, and road noise masks some frequencies. For a car subwoofer, setting the low pass filter between 70 and 80 Hz is a common starting point. If you’re running a dedicated bass-heavy setup, some people push it to 100 Hz, but this risks making the subwoofer sound boomy or obvious.
For the high pass filter on your door speakers (which is the opposite setting, blocking low frequencies from reaching smaller drivers), 80 Hz is again a reasonable starting point. This protects your mids and tweeters from trying to play deep bass they can’t handle, reducing distortion and letting them play louder and cleaner.
What “Cutoff Frequency” Actually Means
The cutoff frequency is the point where the output signal drops to 70.7% of the input signal’s strength. Engineers chose this specific threshold because it corresponds to a 3 dB reduction, which is roughly the smallest volume change most people can notice. Below the cutoff, frequencies pass through at full strength. Above it, they get progressively weaker.
For a basic electronic low pass filter built from a single resistor and capacitor, the cutoff frequency is calculated as 1 divided by (2 × π × resistance × capacitance). So a 500-ohm resistor paired with a 7-microfarad capacitor produces a cutoff frequency of about 45 Hz. This formula applies to simple passive filters in electronics projects and is useful if you’re building a filter from components rather than adjusting a dial on a receiver.
Low Pass Filters in Digital Recording
When converting an analog signal to digital (recording audio, sampling sensor data, digitizing video), a low pass filter serves a completely different purpose: preventing aliasing. Aliasing happens when a signal contains frequencies higher than half the sampling rate, causing those frequencies to fold back and appear as distorted, phantom signals in the recording.
The rule comes from the Nyquist sampling theorem. To capture a signal accurately, you need to sample at least twice as fast as the highest frequency present. If your system samples at 44,100 times per second (the CD audio standard), the highest frequency you can capture is 22,050 Hz. An anti-aliasing low pass filter placed before the sampler removes everything above that limit. In practice, these filters are set just below half the sampling rate to provide a safety margin.
This is why you don’t choose the setting yourself in most consumer audio equipment. Your audio interface or digital recorder has a fixed anti-aliasing filter matched to its sampling rate.
Medical and Scientific Equipment
In medical diagnostics, low pass filter settings are standardized to preserve the features doctors need to see while cutting electrical noise. A diagnostic ECG (heart monitor) uses a low pass filter set between 100 and 150 Hz. This is high enough to capture the sharp spikes of the heart’s electrical activity but low enough to block muscle noise from interfering. A monitoring-mode ECG, used for continuous bedside tracking rather than diagnosis, uses a tighter range of 0.5 to 40 Hz because it prioritizes a clean, readable trace over capturing every subtle waveform detail.
EEG recordings (brain activity) vary more widely. Researchers use low pass filters anywhere from 40 Hz to 1,000 Hz depending on what brain signals they’re studying. There’s no single consensus, and published studies from the same research groups have used different filter settings across experiments.
Camera Sensors and Image Sharpness
Some digital cameras include a physical optical low pass filter mounted directly over the image sensor. This filter blurs the image very slightly to prevent moiré, the wavy rainbow patterns that appear when fine repeating details (like fabric weaves or building facades) clash with the grid pattern of the sensor’s pixels. The filter blocks spatial frequencies above the sensor’s Nyquist limit, which is determined by pixel spacing.
Unlike audio filters, you can’t adjust this one. It’s a fixed physical component. Many modern cameras with high-resolution sensors omit the optical low pass filter entirely because the pixels are dense enough that moiré rarely appears, and removing the filter produces noticeably sharper images. If you’re choosing between camera models, the presence or absence of this filter is a trade-off between guaranteed moiré prevention and maximum sharpness.