An Introduction to Electromagnetic Interference
Electromagnetic Interference refers to unwanted electromagnetic waves that can be received and amplified by an audio system. EMI waves can sometimes be picked up by digital recorders infusing an undesirable BUZZ or HUM into the recording file. The most common sources of EMI include power lines, radio frequencies and cell phones.
GSM BUZZ (Global System for Mobile Buzz)
One of the most common forms of EMI is Global System for Mobile Buzz or GSM Buzz which is emitted from cell phones. This is the sound of digital data being transmitted to a mobile device. Although it’s the sound of the transmission is received at a very low level GSM has a pulse rate of 217 Hz making it very audible to a recording device. A cellular device attached to a headset or Bluetooth device will increase the likelihood of GSM buzz even more because the audio-frequency range increases.
Often times GSM buzz won’t be detected by the recording levels or even by monitoring the recording with headphones. It’s not until the playback of the recording does the audio artifact become noticeable.
The simplest way to deter GSM Buzz is to turn off any cellular or mobile device and keep them away from the recorder while the interview is in progress.
RFI (Radio frequency interference)
Radio frequency interference or RFI is another type of high frequency interference that can occur. RFI is often introduced by an external source such as a wireless router, a radio or television transmitter, a microwave or even fluorescent lights. Electromagnetic radiation emitted from an external source can sometimes cause interference in an audio system.
For instance, using wireless microphones to record can sometimes attract RFI while an improperly grounded or insulated audio cable can also sometimes act as an antenna and pick up unwanted frequencies. Although some RFI is above the range of human hearing these frequencies can be detected by digital audio recorders causing unwanted distortion or interference in the recording.
Electrical noise can also interfere with a recording. EN consists of fluctuating electrical voltages or current flow, static electricity, or some combination of these. This can occur with faulty wiring, active electronic circuits, poor insulation or faulty ground loops. The result can be a low-level hum or buzz that bleeds into the recording.
One way to eliminate or reduce electrical noise is to use an outlet that’s not being used or a powerbar with a surge protector. If you’re using an external microphone with a recorder make sure you you’re using a well-insulated noise canceling signal cable. Avoid layering cables and chords on top of each other. Regularly test your cords and cables. Keep your cords and cables as short as possible. Get rid of as many active electronic components in a recording chain as possible. For instance if you can plug in a recorder’s AC adapter with enough length away from the power supply without using an extension cord you’re more likely to avoid capturing electric noise on the recording.
Some vocal condenser microphones have built in pre-amps to boost the signal strength of the voice. This can sometimes create undesirable noise if the microphone is connected into a recording device with an additional voltage gain creating surplus output power.