What hum and buzz are
Mains electricity cycles at a fixed rate: 50 times a second in Europe, Africa, most of Asia and Australia, and 60 times a second in most of the Americas and in parts of Asia, such as South Korea, Taiwan, the Philippines and western Japan. Equipment that lets a little of that current leak into the audio records a tone at exactly that frequency: a low hum at 50 or 60 Hz, called the fundamental.
The leak is rarely a pure tone. It brings harmonics, tones at whole multiples of the fundamental (100, 150, 200 Hz and up, or 120, 180, 240 Hz and up), that climb into the range of the voice. When they are strong, the hum turns into a buzz, harsher and easier to hear on small speakers. Light dimmers and some lamps produce particularly harsh buzzes, because they chop the current into abrupt steps.
Common culprits in phone and home recording:
- Charging while recording, especially through an inexpensive charger or cable.
- Ground loops: two devices plugged into different outlets and linked by an audio cable, such as an external microphone running into a laptop on charge.
- Lighting: fluorescent tubes, some LED bulbs and dimmers close to the microphone.
- Power bricks and transformers sitting near the microphone or its cable.
- Long or poorly shielded cables between an external microphone and the recorder.
How the app removes hum
Noise Remover works on hum from both ends. The 80 Hz low-cut filter, on by default, sits in front of the neural model and turns down the 50 or 60 Hz fundamental before the model hears it: both frequencies lie below the filter’s cutoff, where a speaking voice has almost nothing. The model then treats the harmonics as the steady noise they are. In each of its 32 frequency bands, it turns down what does not behave like speech, which clears the buzz from your pauses and reduces it under your words.
In our lab test at full intensity, a 50 Hz hum with its harmonics dropped by 24 to 41 dB in the speaker’s pauses. The voice itself came out 0.3 to 1.6 dB quieter, depending on how loud the hum was: a hum 10 dB or more below the voice cost it less than 1 dB. The test measured the neural model without the low-cut filter in front of it. How we measured.
| Noise | Noise removed during pauses, in dB | Change in voice level |
|---|---|---|
| Electrical hum | 24 to 41 dB | −0.3 to −1.6 dB |
Settings for a humming recording
- Check that the low-cut filter is on
It is on by default. If you switched it off for this cleaning, switch it back on: the cleaning re-runs from the kept original, because the filter works before the model.
- Listen where the hum is alone
The end of a recording, after your last word, is where hum is easiest to hear. Play it in Cleaned, then in Original, to judge what is left.
- Raise the intensity if a buzz remains
Try 90 to 100%. Unless the hum was as loud as your voice, the higher setting costs the voice little; a sentence played both ways tells you.
- Trim the hum-only seconds
If the first or last seconds hold nothing but hum, cut them with the trim handles: the export keeps only what lies between them.
Remove the hum from your recording on the phone.
Free · Android 8.0 or later, 64-bit
How to stop hum before you record
- Unplug the charger. Record on battery: a phone or laptop on charge is a frequent source of hum.
- Move away from lights and power bricks. Keep the microphone and its cable clear of fluorescent tubes, dimmers, transformers and power strips.
- Try another outlet, or share one. If two connected devices hum together, plug both into the same power strip to break the ground loop.
- Use shorter, better cables. For an external microphone, a short shielded cable picks up far less than a long thin one, and audio cables should not run alongside power cables.
- Set dimmers to full, or switch the light off. Halfway is where many dimmers buzz loudest.
A filter set exactly on 50 Hz, or a noise gate, only solves part of the problem: noise reduction, noise gate and EQ compared explains why. The rest of a quiet setup is covered in recording clean audio with a phone.