Podcasters and voiceover performers have recently been complaining a lot about problems with audio hum. I'm not so sure what is happening, whether there is a spate of poorly shielded microphones on the market, ratty devices putting out interference, or whatever else. But here is something which will help you suppress hum and keep your audio more clean and crystal clear. Follow these suggestions and you should avoid any problems due to hum or buzz in your audio.
Ground loops may well be the most common cause of powerline hum in audio systems. They occur when there are multiple paths to ground, creating a differential voltage between them. This voltage difference can result in an unwanted current flow, which can introduce hum and noise into the audio signal.
Here's a simplified explanation of how ground loops happen:
To minimize the effect of ground loops, here are some best practices:
If possible, use balanced audio cables and connections. Balanced audio connections (such as XLR) are less susceptible to ground loop interference because they use differential signaling, which cancels out common-mode noise.
The power supply is responsible for converting the 50 Hz or 60 Hz AC voltage from the mains outlet into a DC voltage that can be used by the audio equipment. During this conversion process, the AC is rectified and filtered into smooth DC. There may be residual ripple on the DC, which can be exacerbated by poor filtering or regulation.
Poorly shielded and filtered power supplies can introduce hum into the audio signal, especially if they are shared between components. A well designed power suply will have high value capacitors across the outputs and high value choke coils in seriess, which will smooth out the ripple left over from the power conversion from AC to DC. A faulty voltage regulator in the power supply can be also be a source of audio hum, as ripple in the regulator can be passed to the output.
Certain types of power supplies may be better than others at reducing audio hum. For example, linear power supplies (LPS) are often considered to be superior to switching power supplies (SPS) in terms of audio performance. LPS use a transformer to convert the AC voltage to a lower DC voltage, which is then regulated by a linear regulator. This design tends to produce less noise and ripple compared to SPS, which use a switching circuit to convert the AC voltage to DC.
However, it's worth noting that not all SPS are created equal, and some high-quality SPS can perform very well in audio applications. The key is to look for power supplies that have been specifically designed for audio use, and that incorporate high-quality components and robust filtering.
To reduce audio hum due to the power supply, here are some best practices:
Incorrect power distribution can lead to hum. For instance, using a power strip with many devices can create ground loops. When audio equipment is plugged into a power strip with too many other devices, it can cause buzzing or humming in the audio equipment due to the presence of ground loops and electrical interference.
Electrical interference can also be a cause of buzzing or humming in audio equipment. When too many devices are plugged into a power strip, it can create a noisy electrical environment due to the presence of electromagnetic interference (EMI) and radio frequency interference (RFI) from the other devices. These interference signals can be picked up by the audio equipment and introduced into the audio signal as noise or buzzing.
To reduce buzzing or humming in audio equipment due to a power strip with too many other devices, here are some best practices:
Transformers can sometimes couple hum into the audio signal, especially if they are not properly shielded or grounded. Magnetic coupling from power transformers can introduce audio hum into an electronic sound system due to the presence of stray magnetic fields that can induce a voltage in nearby audio cables. This voltage can then be amplified and introduced into the audio signal as hum.
When current flows through a power transformer, it generates a magnetic field around the transformer. This magnetic field can then couple with nearby audio cables, inducing a voltage in the cables. This voltage can be introduced into the audio signal as hum, especially if the audio cables are not shielded or are located too close to the power transformer.
To avoid transformer coupling, here are some best practices:
If there is hum due to transformer coupling, there are some steps which can be taken to reduce its impact. Re-route audio cables away from power transformers to reduce the impact of magnetic coupling. Ferrite cores can be useful too, and should be placed around audio cables to suppress magnetic coupling. One of the smartest things you can do is keep your audio cables away from power transformers (and AC power cables too). Keep your microphone separated from anything which carries high voltage AC.
Sometimes there are things outside of your control which are going to induce the AC voltage onto your audio. For example, one I often encountered in my flying career was 400 Hz AC hum picked up from the electrically heated cockpit windshields. We flew with them on all the time in order to make them less brittle and less likely to shatter in the event of bird strikes.
Electromagnetic interference (EMI) is typically the next layer of noise, below powerline hum. It can occur from external sources like fluorescent lights, motors, or other electronic devices. Noise from fluorescent lights and light dimmers can be coupled into an audio system due to the presence of electromagnetic interference (EMI) and radio frequency interference (RFI). These interference signals can be picked up by the audio equipment and introduced into the audio signal as noise or hum.
Fluorescent lights and light dimmers can generate EMI and RFI due to the rapid switching of electrical currents. When these currents switch on and off, they can generate electromagnetic fields that can interfere with nearby audio equipment. This interference can be introduced into the audio signal as noise or hum.
To reduce EMI from electrical equipment and lights, here are some best practices:
Radio frequency interference tends to be one of the lesser sources of interference or noise in the background. It can occur from strong broadcast radio signals or nearby two way radio equipment.
The signal from a mobile phone can cause noise in a microphone audio due to the presence of EMI and RFI. Mobile phones emit radio frequency signals that can interfere with nearby audio equipment, especially if the mobile phone is in close proximity to the microphone.
To reduce the impact of mobile phone signals on microphone audio, here are some best practices:
The broadcast sound from a mediumwave radio station can get into an audio system through electromagnetic interference (EMI) and radio frequency interference (RFI) caused by the radio station's transmitter. The transmitter generates a strong radio frequency signal that can be picked up by audio equipment, especially if the equipment is not properly shielded or grounded.
To prevent the signal from getting into your home studio microphone and mixer audio, here are some best practices:
The truth is that hum, EMI, and RFI are always present to some degree. In a professional production environment, they can all be brought down to 60 dB or 70 dB below full scale, and often much more, by following best practices. Remaining hum and interference can be further suppressed with digital signal processing. If you have junk getting into your audio, work the issues one layer at a time. Layer by layer, until things are as clean as you need them to be.