Coil noise or coil whine is a phenomenon in electronics which produces undesirable audible sound. In magnetic resonance imaging "coil noise" is that part of total system noise attributed to the receiving coil, due to its non-zero temperature.
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 varying magnetic field exists around the coil. Forces due to this magnetic field can cause vibration of the coil wires or the core of the coil. For some magnetic core materials, a (strong) varying field can cause vibration of the core due to magnetostriction. If the frequency of the current in the coil falls within the audible range, the resulting vibration may produce sounds audible at a distance from the coil, especially if magnified by a mechanical resonance in the components or in the equipment enclosure.
Coil noise can occur when the coil is poorly secured to the circuit board or is poorly damped. The effect is more pronounced as the signal passing through the coil increases in strength or nears the (mechanical) resonant frequency of the coil.
Devices generating coil noise
Any device with heavy currents passed through coils may produce acoustic noise. It can be objectionable in some classes of equipment. Fluorescent lamp ballasts are one example of devices that are designed to minimize coil noise.
The amount of coil noise in a device will often seem to increase as the devices age. This is often due to the degradation of the damping materials in the component. As an example, a layer of glue is often added atop coils in televisions. Over the years, this glue degrades; as a result, the coil is able to move more as it vibrates, and the sound level increases.
Reducing coil noise
Steps to reduce coil noise include:
- 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.
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