Notch Filter Calculator: Remove Hum, Feedback, and Ring
Master notch filtering for removing electrical hum, feedback, and unwanted resonances
1 What Is a Notch Filter?
A notch filter (also called a band-stop or band-reject filter) surgically removes a very narrow range of frequencies while leaving surrounding frequencies untouched. Unlike high-pass or low-pass filters that affect broad frequency ranges, notch filters target specific problem frequencies with precision.
Notch filters are essential problem-solving tools in audio production, used to remove electrical hum, feedback, resonances, and other unwanted tones without affecting the overall character of the sound. Mastering notch filtering is crucial for achieving clean, professional recordings.
Key Principle: A notch filter creates a sharp dip at a specific frequency. With a high Q (narrow bandwidth), it removes just the problem frequency. With a lower Q, it affects a wider range around the target.
2 Common Applications for Notch Filters
Removing Electrical Hum (60Hz/50Hz)
Electrical hum is one of the most common audio problems, caused by electromagnetic interference from power lines. In North America (and countries using 60Hz power), hum occurs at 60Hz and its harmonics (120Hz, 180Hz, 240Hz). In Europe, UK, and most other regions (50Hz power), the fundamental is 50Hz with harmonics at 100Hz, 150Hz, etc.
To remove hum effectively:
- Apply a narrow notch at the fundamental frequency (60Hz or 50Hz)
- Add additional notches at the 2nd harmonic (120Hz or 100Hz) if still audible
- Continue adding notches at higher harmonics as needed
- Use high Q values (8-30) to minimize impact on nearby musical content
Eliminating Feedback
In live sound, feedback occurs when a microphone picks up its own amplified signal from speakers. Notch filters on graphic EQs or dedicated feedback eliminators can remove feedback frequencies without significantly affecting the overall sound. Modern feedback eliminators use automatic detection and narrow notch filtering.
Removing Drum Ring and Resonances
Snare drums and toms often have resonant frequencies that ring unpleasantly. Rather than damping the drum physically or using wide EQ cuts, a narrow notch can remove just the offending frequency:
- Create a narrow EQ boost (+10-15dB, high Q)
- Sweep slowly while the drum rings out
- The ring frequency will become extremely obvious
- Flip the boost to a cut at the same frequency
- Adjust Q and cut amount to taste
| Problem | Typical Frequency | Recommended Q | Notes |
|---|---|---|---|
| 60Hz Hum (US) | 60 Hz | 8-16 | Add harmonics at 120, 180, 240Hz |
| 50Hz Hum (EU) | 50 Hz | 8-16 | Add harmonics at 100, 150, 200Hz |
| Snare Ring | 300-600 Hz | 8-16 | Varies by drum—sweep to find |
| Tom Ring | 200-500 Hz | 8-16 | Each tom has different resonance |
| Room Mode | 50-300 Hz | 4-8 | Based on room dimensions |
| Feedback | Varies | 16-30 | Very narrow to preserve tone |
| Monitor Buzz | 100-200 Hz | 8-12 | Often ground loop related |
3 Understanding Notch Filter Parameters
Center Frequency
The center frequency is the exact point where maximum attenuation occurs. Accurate identification of the problem frequency is crucial—even a few Hz off target means the notch won't fully remove the problem while unnecessarily affecting nearby frequencies.
Q (Bandwidth)
Q determines how narrow or wide the notch is. Higher Q values create narrower notches:
- Q 2-4 (Wide): Affects frequencies across roughly an octave. Useful for broad resonances.
- Q 8-16 (Narrow): Standard for most notch applications. Removes problem while preserving nearby content.
- Q 20-30+ (Surgical): Extremely narrow. Essential for hum removal and feedback elimination where even slight coloration is unacceptable.
Depth (Cut Amount)
Notch depth determines how much the target frequency is reduced. For complete removal of hum or feedback, deep cuts (-24dB to -48dB or complete notch) may be needed. For musical resonances, gentler cuts (-6dB to -12dB) often sound more natural.
4 The "Sweep and Destroy" Technique
Finding the exact problem frequency requires a systematic approach:
- Create a seek boost: Set up a parametric EQ band with +10 to +15dB gain and high Q (8-16).
- Sweep slowly: Move the frequency across the spectrum while listening. The problem frequency will become extremely obvious when you hit it.
- Mark the frequency: Note the exact frequency where the problem is worst.
- Convert to cut: Change the boost to a cut at the same frequency.
- Adjust parameters: Fine-tune Q and cut amount until the problem is resolved without affecting the overall tone.
Pro Tip: Use a spectrum analyzer alongside your ears. Hum and feedback show up as clear spikes on the analyzer, making identification easier. But always verify with your ears—the analyzer shows what's there, not necessarily what's problematic.
5 Harmonics and Multiple Notches
Electrical hum contains harmonics—integer multiples of the fundamental frequency. A 60Hz hum will have energy at 60Hz (fundamental), 120Hz (2nd harmonic), 180Hz (3rd harmonic), and so on. The strength of each harmonic varies depending on the source of the interference.
For effective hum removal:
- Start with a notch at the fundamental
- Listen for remaining hum—it may now sound "thinner" or "buzzier"
- Add a notch at the 2nd harmonic (this often contains the most energy after the fundamental)
- Continue adding harmonics until the hum is inaudible
- Most hum is effectively treated with 2-4 notches
6 Notch Filter vs. Other EQ Types
Notch vs. High-Pass Filter
A high-pass filter removes all content below a frequency, including musical content. A notch removes only the specific problem frequency. For hum on bass-heavy sources (bass guitar, kick drum), a notch preserves the musical low end while removing only the hum.
Notch vs. Bell/Parametric Cut
Bell curves are gentler and affect a wider range. They're better for musical EQ adjustments where you want to shape tone. Notch filters are more surgical and appropriate when you need to remove a specific frequency completely without coloring the surrounding range.
Notch vs. Dynamic EQ
Dynamic EQ only reduces a frequency when it exceeds a threshold. This can be useful for resonances that are only problematic at certain moments. However, for constant problems like hum, a static notch is more effective and simpler.
7 When Not to Use Notch Filters
Notch filters aren't always the answer:
- Musical tonal issues: Use broader EQ curves for tone shaping
- Noise floor problems: Use noise gates or dedicated noise reduction
- Broad frequency problems: Use high/low-pass filters or shelving EQ
- Clipping/distortion: Cannot be fixed with EQ
8 Conclusion
Notch filters are precision tools for removing specific frequency problems without affecting the rest of your audio. Whether you're dealing with electrical hum, feedback, drum ring, or room resonances, the ability to surgically remove problem frequencies is essential for professional audio production.
Use our Notch Filter Calculator above to quickly identify common problem frequencies and find appropriate Q settings for your specific needs. Remember: always verify with your ears, and use the minimum amount of processing needed to solve the problem.