Coil Noise is a phenomenon in electronics which produces both undesirable audible sound and electronic noise.
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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 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.
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. Fluorescent 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.
Some steps to reducing coil noise include:
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.