Why does my recording hum, hiss or buzz?

Four distinct noises with four causes — mains hum, preamp hiss, digital interference and room noise. Each has a different signature and a different fix.

Difficulty: intermediate
Time: 16 min
Read: 5 min
Safety: caution

Short answer

A low steady hum at 50Hz is mains related — usually an earth loop between two mains-powered devices, or a cable running near a power lead. A constant hiss is preamp gain set too high because the microphone is too far away. A buzz that changes with screen activity is interference from a computer or a phone. A steady rumble is the building itself.

Noise problems are diagnosable by ear before you change anything, because the four common causes sound genuinely different. Guessing and changing equipment at random is the slow route.

Safety

Never defeat the earth pin on a mains plug or use a so-called ground lift adaptor to cure a hum. The earth connection is what prevents a fault making the metal chassis of your equipment live, and removing it can be lethal. Cure earth loops by changing how equipment is connected, by using a proper isolating transformer designed for the purpose, or by getting the wiring checked — never by disconnecting protective earth.

Step by step

Listen to what kind of noise it is.A low steady hum with a musical pitch is mains related. A broadband constant hiss like rain is electronic noise. An irregular buzz or whine that changes as you use the computer or move a phone is radio interference. A low rumble with no pitch is traffic, ventilation or the building.
Halve the system to find the source.Record with nothing plugged in but the interface. Then with the cable but no microphone. Then with the microphone. The step at which the noise appears identifies where it enters the chain, and this takes two minutes.
Cure hiss by moving the microphone closer.Hiss is preamp noise amplified because the signal is weak. Halving the distance to the microphone roughly doubles the signal without changing the noise, so the gain can come down. This fixes far more hiss than any equipment change.
Deal with mains hum by changing the connection topology.Hum from an earth loop happens when two mains-earthed devices are connected by a signal cable, creating two paths to earth. Plug everything into the same socket or extension so they share one earth reference, use balanced XLR connections rather than unbalanced ones, and use a proper isolating transformer if it persists. Never remove an earth pin.
Move cables away from power leads.A microphone cable running alongside or coiled with a mains lead picks up hum by induction. Separate them, cross them at right angles where they must meet, and keep the microphone away from power supplies, dimmers and fluorescent fittings.
Track down digital interference.A phone near a cable produces a distinctive pulsing buzz. A laptop on charge, a cheap USB power supply, a monitor and unshielded cables all contribute. Move the phone away, try running a laptop on battery, and swap a suspect USB cable for a better shielded one.
Silence the room.Fridge, boiler, extractor fan, computer fan, aquarium pump, clock. Switch off everything you can and listen for a full minute with headphones on. You have stopped hearing these and the microphone has not.
Use a high-pass filter for rumble.Traffic, ventilation and building noise sit mostly below 80Hz, which is below the useful range of a voice. A high-pass filter at 80 to 100Hz removes a great deal of it with no audible cost.
Apply noise reduction last and gently.Software noise reduction works by learning a profile of the steady noise and subtracting it. A light touch removes hiss convincingly; a heavy hand produces watery, phasey artefacts around the voice that are worse than the noise was.

Tips

Record thirty seconds of silence at the start of every session. It gives noise reduction software an accurate profile and it tells you what the room sounds like before you commit.
Balanced XLR cables reject induced hum by design, which is one of the main practical reasons to prefer them over unbalanced connections for anything longer than a short run.
A hum that appears only when the laptop is charging is almost always an earth loop through the power supply. Running on battery for the take is a legitimate and immediate fix.

Common mistakes

Turning the gain up to compensate for a quiet microphone — It raises the preamp noise floor along with the signal, which is exactly the hiss being complained about. Move the microphone closer instead.
Using a ground lift plug or cutting the earth pin — It removes the protective earth that stops a chassis becoming live in a fault. It is dangerous and it is not a repair. Fix the topology, use balanced connections, or use a purpose-built isolating transformer.
Relying on noise reduction rather than fixing the cause — Heavy noise reduction leaves audible artefacts around speech and dulls the recording. It is a finishing tool, not a substitute for a quiet signal path.

If it doesn't work

Low steady hum with a pitch

Cause: Mains earth loop, or a cable near a power lead — Fix: Plug everything into one socket, separate signal from power cables, use balanced connections, and run a laptop on battery to test.

Constant broadband hiss

Cause: Preamp gain too high for a weak signal — Fix: Move the microphone closer, reduce the gain, and check the microphone is actually powered if it needs phantom power.

Pulsing buzz that comes and goes

Cause: Radio interference from a phone or a poor power supply — Fix: Move the phone well away, swap the USB cable, and try a different power supply.

Low rumble with no pitch

Cause: Traffic, ventilation or building vibration — Fix: High-pass filter at 80 to 100Hz, isolate the microphone stand from the floor, and record at a quieter time.

Crackling that comes and goes with movement

Cause: A failing cable or a dirty connector — Fix: Swap the cable, and clean the connectors. Cables fail far more often than microphones do.

Questions people ask

Why does the hum stop when I touch the microphone?

Your body is completing or altering an earth path, which confirms a grounding problem rather than a faulty microphone. It is a diagnostic sign, not a solution, and it points at the connection topology between your devices.

Should I buy a noise gate?

A gate silences the track between phrases, which hides noise rather than removing it — and the noise still appears under every word. Fix the source first; a gentle gate is a reasonable final polish on an already clean recording.