Coil Noise

Coil Noise is a phenomenon in electronics which produces both undesirable audible sound, and electronic noise.

Description

Coil Noise is, as its name suggests, caused by Electromagnetic coils. These coils, which may act as inductors or transformers, have a certain resonant frequency when coupled with the rest of the electric circuit, as well as a resonance at which it will tend to physically vibrate.

As the wire that makes up the coil passes a variable current, a small amount of electrical oscillation occurs, creating a small magnetic field. Normally this magnetic field simply works to establish the inductance of the coil. However, this magnetic field can also cause the coil itself to physically vibrate. As the coil vibrates physically, it moves through a variable magnetic field, and feeds its resonance back into the system. This can produce signal interference in the circuit, and can produce an audible hum as the coil vibrates.

Coil noise can happen, for example, when the coil is poorly secured to the circuit board, is poorly damped, or if the resonant frequency of the coil is close to the resonant frequency of the electric circuit . The effect becomes more pronounced as the signal passing through the coil increases in strength, and as it nears the resonant frequency of the coil, or as it nears the resonant frequency of the circuit. Coil noise is also noticed most often when it is in the humanly audible frequency.

Coil noise is also effected by the irregularities of the magnetic material within the coil. The flux density of the inductor is effected by these irregularities, causing small currents in the coil, contaminating the original signal. This particular subset of is sometimes referred to as Magnetic Fluctuation Noise or the Barkhausen Effect. Coil noise can also occur in conjunction with the noise produced by Magnetostriction.

Devices Effected By Coil Noise

All but the simplest devices experience some level of Coil Noise, although it is usually far too slight to be noticed or deemed problematic. Ordinarily, the noise is simply a minor annoyance. In some devices however, such as audio video equipment, Coil Noise can present a major problem if it is not minimized in design. Video Cards are often effected by coil noise, often aggravating their users. Older Cathode ray tube televisions are also effected. Florescent lights are often blamed for such noise, although it is their electrical ballast or driving circuity that actually produces the sound.

The amount of Coil Noise in a device will often seem to increase as the devices age. This can be due to many factors, but is often due to the degradation of the damping materials in the circuit. As an example, a layer of glue is often added atop coils in televisions. Over the years, this glue degrades due to age and due to the years of vibration it has been subjected to. As a result, the coil is able to move more as it vibrates, and the sound level increases.

Reducing Coil Noise

Some steps to reducing coil noise include:

• As previously mentioned, damping is a critical step towards reducing coil noise.
• Low Noise Coils are specifically produced to avoid coil noise.
• Coil noise can be reduced in some instances by changing the shape of the coil to a figure eight rather than a traditional coil shape.

Coil noise can be reduced by damping and can be minimized by design, but in some cases, unacceptable levels will remain. To combat this, the coils are sometimes isolated from the rest of the device. They can be placed in such a way as to absorb and contain the noise and vibration that occurs.

See also

• Noise
• Signal noise
• Magnetostriction

References

• Landee, Robert (1957). Electronic designers' handbook, Volume 1. McGraw-Hill. 
• Self, Douglas (2006). Self on Audio. 
• Michaels, John (1996). Science, Volume 259, Issues 5095-5101. 
• Burgess, Ronald (1965). Fluctuation phenomena in solids. Academic Press. 
• The Journal of the Institution of Engineers (India)., Volume 36, Issues 7-12. The Institution. 1956. 
• Transactions of the American Institute of Electrical Engineers, Volume 69, Part 1. American Institute of Electrical Engineers. 1950. 
• Proceedings of the 18th Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Service Center. 1997. 
• Fundamental principles of vibrator power supply design. P. R. Mallory & Co. Inc. 1947.