Remove Hum in Audacity Using a Notch Filter

A notch filter is a specialized audio tool designed to eliminate a specific, narrow frequency band while leaving the surrounding audio intact. In audio production, it is most commonly employed to eradicate low-frequency electrical hum caused by alternating current (AC) power lines—typically 50 Hz in Europe and parts of Asia, or 60 Hz in the Americas. This guide explains what a notch filter does, why power hum occurs, and the exact steps to apply a notch filter in Audacity to cleanly cut unwanted electrical noise from your recordings.

What is a Notch Filter?

A notch filter—also known as a band-stop or band-rejection filter—attenuates a very narrow range of frequencies around a specified center frequency. Unlike standard low-pass, high-pass, or broad parametric EQ filters, a notch filter features a high "Q" factor (quality factor). This high Q ensures that only the targeted frequency is suppressed, preventing loss of natural bass response, vocal warmth, or instrumental body adjacent to the cut.

Why 50 Hz and 60 Hz Hum Occurs

Electrical hum enters audio recordings primarily through electromagnetic interference or ground loops. Mains electricity operates at an alternating frequency of 50 Hz or 60 Hz, depending on your region. Poorly shielded cables, ungrounded equipment, or audio interfaces placed too close to power transformers can easily pick up this baseline oscillation. The result is a persistent, low-pitched buzz across the entire recording.

Step-by-Step: Using the Notch Filter in Audacity

Audacity includes a dedicated Notch Filter effect designed specifically for targeted noise removal. Follow these steps to apply it:

  1. Identify the Hum Frequency: Determine whether your recording environment uses a 50 Hz or 60 Hz power grid. If you are unsure, inspect the audio using Audacity's spectrogram view (Analyze > Plot Spectrum) to locate the sharp spike at either 50 Hz or 60 Hz.
  2. Select the Audio: Use the Selection Tool (F1) to highlight the affected section of the track, or press Ctrl + A (Windows) / Cmd + A (macOS) to select the entire audio track.
  3. Open the Notch Filter: Go to the top menu and select Effect > EQ and Filters > Notch Filter (in older Audacity versions, it may simply be under Effect > Notch Filter).
  4. Configure the Parameters:
    • Frequency (Hz): Enter 50 or 60, matching the mains frequency of the recording location.
    • Q (Quality Factor): Set the Q value. A value between 2.0 and 5.0 is standard for general hum reduction. A higher value (such as 10.0) creates a narrower cut, protecting more of the nearby frequencies, but may leave traces of the hum if the interference fluctuates slightly.
  5. Preview and Apply: Click Preview to listen to the result. If the hum is gone and the audio still sounds natural, click Apply.

Dealing with Harmonics

Electrical interference often produces harmonic overtones at exact multiples of the fundamental frequency (e.g., 100 Hz, 150 Hz, and 200 Hz for a 50 Hz hum; 120 Hz, 180 Hz, and 240 Hz for a 60 Hz hum). If a faint buzz remains after cutting the primary frequency, re-apply the Notch Filter at these secondary harmonic intervals with a high Q setting until the recording is fully clean.