Jan152011

How can I prevent electrical hum when recording audio on AC power?

Sean asks:

I do a lot of vocal recording with a USB microphone and, when running on battery power, everything sounds good. If my laptop is using AC power a hum permeates every recording. Can I do anything about this other than continuously using the battery? Hum removal in post-production can only really go so far.


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  • Mike January 15, 2011

    If possible try using a USB port further away from where your power cable plugs into the laptop. Also make sure the transformer (the “black box” between the cable which plugs into the socket and the cable that goes into your laptop) is as far away as possible.

    Other than that “humming” can only be minimized by using high quality condenser microphones preferably with transformerless output.

    You can read more (or probably way too much) about this issue here:
    http://www.epanorama.net/documents/groundloop/

  • Anonymous January 16, 2011

    Hi

    Hum derives from the fundamental frequency of the AC power line, which in the USA is 60 Hz. However, most hum also contains harmonics of that fundamental frequency: 120 Hz., 180 Hz., and so forth. In fact, the only real difference between “hum” and “buzz” is that
    buzz has more harmonics, and those harmonics extend to a higher audible frequency.

    three basic ways that hum and other power line-related noises can get into an audio system.
    1)One is through radiation in the air, such as when you bring an electric guitar close to the power transformer in an amplifier. In this case the guitar’s pickup acts as an antenna that receives the 60 Hz. energy being radiated by the transformer. The whole point of using shielding to surround the center conductor on guitar wires and studio patch cords is to keep radiated hum from impinging on the wire inside the shielding.

    2)hum can be induced into an audio system is when a voltage difference exists between the common ground connections of two or more pieces of equipment. Third, clicking and buzzing noises can get into an audio device through the power wires.

    3)solid-state light dimmers can create havoc with audio gear. These devices work by changing the amount of time the full power line voltage is applied to the light being controlled, as opposed to actually raising and lowering the voltage. Although AC power is supplied by the power company as a pure sine wave (having no harmonics), the dimmer’s sudden switching of the voltage on and off actually generates harmonics which extend to very high frequencies. And these high frequency harmonics can get into audio gear both by radiation through the air and also by traveling through the power wiring directly into the audio circuitry.

    hum can often be greatly reduced by connecting all of the device cases together with heavy wire. Since ground loops occur when a voltage difference is present between the ground terminals of connected audio equipment, using heavy wire – which has a low resistance – reduces that voltage difference

    You can use some Humfrees
    http://www.danabgoods.com/Humfrees/
    http://accessories.musiciansfriend.com/product/Humfrees-Rack-Isolation-Tabs?sku=544710
    These clever gadgets physically isolate the metal case of each device from the rack’s metal rails, preventing the devices from touching each other. Another possibility is to buy ground lift adapters that disconnect the grounding pin from the power cord of each piece of equipment. However, one device must be grounded properly. Another solution – though this can become fairly expensive – is to buy audio isolation transformers and place them in the signal path between every piece of audio gear that uses unbalanced audio wiring

    As for software
    SoundSoap
    http://www.bias-inc.com/products/soundSoap2/

    DC Millenium audio cleaning
    http://www.enhancedaudio.com/millennium.htm

    Noise Reduction Effect
    http://www.blazeaudio.com/products/digital_noise_remover.html

    Audacity
    http://audacity.sourceforge.net/
    Under the Effect menu, you have Noise Removal, Click Removal, and a couple of selective equalization filters that you can mix and match to get rid of most noise.

    Remove a Hum with Adobe Soundbooth. Remove 60 Cycle Hum
    http://www.adobe.com/designcenter/premierepro/articles/prpcs3it_ciblesson13/lesson13.pdf

  • Beginner Guitarist February 3, 2011

    I always have some sort of low frequency hum when recording, but what I do is use the noise reduction tool in my audio program to get rid of the hum.

    It works wonders, and somehow doesn’t affect the normal recording at all. So it’s basically the best thing next to sliced bread :-) I use sony sound forge pro, but I think most audio programs have this tool.

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