Low-pass filter (LPF)

A low-pass filter is a filter that passes signals below a specified cutoff frequency and blocks signals above it.

Common uses in hi-fi

• Subwoofer crossover – LPF at 80 Hz sends only bass to the sub
• DAC reconstruction filter – LPF at half sample rate removes ultrasonic images from digital-to-analog conversion
• Speaker crossover woofer section – LPF at 2-3 kHz sends bass to the woofer, blocks midrange

Filter slopes

• -6 dB/octave (1st order) – gentle, minimal phase shift
• -12 dB/octave (2nd order) – most common in passive crossovers
• -24 dB/octave (4th order) – steep, used in most subwoofer LPFs