lesson
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You record a stunning acoustic guitar track, but when you check your master meter, the red peak lights are flashing even though the guitar sounds quiet. What is silently devouring your mix headroom?
The culprit is almost always infrasonic energy — massive acoustic vibrations below 40 Hz from air conditioners, passing trucks, or footsteps that your speakers cannot reproduce, but your meters definitely register.
How do we carve out this dead weight without making the instrument sound thin and lifeless?
Anatomy of a High-Pass Filter
A high-pass filter (HPF), also called a low-cut filter, attenuates (reduces the volume of) all frequencies below a specified target while letting higher frequencies pass through untouched.
The cutoff frequency (fc) is defined as the exact frequency where the signal has been reduced by 3 dB (the half-power point).
Below fc, the volume drops at a continuous rate called the filter slope (or roll-off), measured in decibels per octave (dB/oct), where an octave is a doubling or halving of frequency.
📊Interactive diagram
Before you twist any knobs, where exactly does this unwanted low-end garbage come from in a real studio session?
The Three Main Culprits of Low-End Clutter
The first culprit is mechanical vibration, which travels through the floor and up the metal microphone stand whenever a performer taps their foot or shifts weight (concentrated between 10 Hz and 35 Hz).
The second is environmental rumble, caused by HVAC air conditioning ducts, exterior road traffic, and subway lines vibrating building foundations (typically 20 Hz to 60 Hz).
The third culprit is vocal plosives — turbulent bursts of air from consonants like 'P' and 'B' that slam into the microphone diaphragm and produce massive pressure spikes between 30 Hz and 90 Hz.