Audio Noise Reduction: Techniques for Clean Recordings
Background noise in audio recordings — hum, hiss, traffic, and room ambience — degrades listener experience and professionalism. Modern noise reduction techniques can clean recordings without introducing distracting artifacts.
Key Takeaways
- Consistent, predictable noise that does not change over time: air conditioning hum (60/120 Hz in the US, 50/100 Hz in Europe), computer fan noise, tape hiss, electrical buzz.
- The classic noise reduction technique.
- A noise gate mutes audio below a threshold level.
- Electrical hum at 50/60 Hz (and harmonics at 100/120, 150/180 Hz) is removed with notch filters tuned to the exact frequencies.
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Types of Audio Noise
Stationary Noise
Consistent, predictable noise that does not change over time: air conditioning hum (60/120 Hz in the US, 50/100 Hz in Europe), computer fan noise, tape hiss, electrical buzz. Easiest to remove because algorithms can learn the pattern.
Non-Stationary Noise
Intermittent, unpredictable noise: traffic, construction, dog barking, keyboard clicks, phone notifications. Harder to remove because the pattern changes. May require manual editing (cutting affected segments) rather than algorithmic removal.
Spectral Subtraction
The classic noise reduction technique. The process:
- Capture a 'noise profile' from a segment of pure noise (no speech or music)
- The algorithm learns the frequency spectrum of the noise
- It subtracts that spectrum from the entire recording
Effective for stationary noise. Over-application creates a 'underwater' or 'musical noise' artifact — metallic warbling in quiet passages.
Gate/Expander
A noise gate mutes audio below a threshold level. When no one is speaking, the gate closes and the noise disappears. When speech exceeds the threshold, the gate opens.
Settings: Set the threshold just above the noise floor. Use a fast attack (1-2 ms) so the gate opens before clipping the beginning of words. Use a moderate release (50-100 ms) so the gate does not chop off word endings.
De-Hum
Electrical hum at 50/60 Hz (and harmonics at 100/120, 150/180 Hz) is removed with notch filters tuned to the exact frequencies. Narrow Q values (high selectivity) remove the hum without affecting nearby musical content.
Best Practice Order
- De-hum first — Remove electrical noise before other processing
- Noise reduction — Spectral subtraction on stationary noise
- Gate — Clean up remaining low-level noise between speech segments
- EQ and compression — Apply after noise reduction for the cleanest signal