Proximity effect

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This article is about the physics phenomena of proximity effects. For other uses, see: Proximity Effect (disambiguation)

In physics, proximity effect is a term used for many different effects in which things behave differently when near, or proximate, to one another.

Contents 1 In atomic physics 2 In electromagnetics 3 In electron beam lithography 4 In audio 5 In superconductivity 6 See also 7 External links 8 References

[edit] In atomic physics

At the atomic level, when two atoms come into proximity, the highest energy, or valence, orbitals of the atoms change substantially and the electrons on the two atoms reorganize. One way to probe a correlated state is through the proximity effect. This phenomenon occurs when the correlations present in one degenerate system "leak" into another one with which it is in chemical equilibrium. See also quantum tunneling, Casimir effect, van der Waals force.

[edit] In electromagnetics

A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. When an AC current flows through an isolated conductor, it creates an associated alternating magnetic field. The alternating magnetic field induces eddy currents within the conductor, altering the overall distribution of current flowing through the conductor. As a result, an AC current preferentially flows through the outer portion (or skin) of the conductor, a phenomenon called skin effect. If similar currents are also flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the conductor will be constrained to smaller regions. The resulting current crowding is termed proximity effect. The combination of skin and proximity effects significantly increases the AC resistance of the conductor when compared to its resistance to a DC current. At higher frequencies, the AC resistance of a conductor can easily exceed ten times its DC resistance. The additional resistance increases electrical losses which, in turn, generate undesirable heating. Proximity and skin effects significantly complicate the design of efficient transformers operating at high frequencies within switching power supplies.

Proximity effect can also occur within electrical cables. For example, if the conductors are a pair of audio speaker wires, their currents have opposite direction, and currents will preferentially flow along the sides of the wires that are facing each other. The AC resistance of the wires will dynamically change (slightly) along with the audio signal. Some believe that this will potentially introduce distortion and degrade stereo imaging. However, it can be shown that, for reasonable conductor sizes, spacing, and length, this effect is so small as to have an immeasurable practical impact on audio quality.

[edit] In electron beam lithography

When an electron beam is incident on a material, the electrons are not destroyed but are scattered both elastically (with angle changes but without energy loss) and inelastically (with energy loss). The elastically scattered electrons generally have sufficient energy to travel a large distance. Those which head back toward the source are called the back-scattered electrons. The inelastically scattered electrons generate additional radiation quanta through their energy loss, including X-rays, Auger electrons, and low-energy ejected electrons (also called secondary electrons). The range of the back-scattered electrons is much larger than the range of the secondary or Auger electrons due to their higher energy.

Back-scattered electrons often cause features written by electron beam lithography to be wider in densely patterned areas. Most electron-beam lithography systems compensate for this pattern dependence by reducing the dose in densely patterned regions compared to isolated features. The compensation cannot completely remove the fundamentally large difference in dose sensitivity between isolated and nested features.

[edit] In audio

The proximity effect in audio refers to a change in the frequency response of a directional microphone as the sound source is brought within close proximity of the microphone. The result of the change is a disproportionate increase in the bass response of the microphone. The effect is found in directional microphones due to the particulars of its construction (described below) and, consequently, is not exhibited in omni-directional microphones.

To understand how the proximity effect arises in directional microphones, it is first necessary to briefly understand how a directional microphone works. A microphone is constructed with a diaphragm whose mechanical movement is converted to electrical signals (via a magnetic coil, for example). The movement of the diaphragm is a function of the air pressure difference across the diaphragm arising from incident sound waves. In a directional microphone, sound reflected from surfaces behind the diaphragm is permitted to be incident on the rear side of the diaphragm. Since the sound reaching the rear of the diaphragm travels slightly farther than the sound at the front, it is slightly out of phase. The greater this phase difference, the greater the pressure difference and the greater the diaphragm movement. As the sound source moves off of the diaphragm axis, this phase difference decreases due to decreasing path length difference. This is what gives a directional microphone its directivity.

In addition to the angular dependence described above, the response of a directional microphone depends on the amplitude, frequency and distance of the source. These latter two dependencies are used to explain the proximity effect.

As described above, the phase difference across the diaphragm gives rise to the pressure difference that moves the diaphragm. This phase difference increases with frequency as the difference in path length becomes a larger portion of the wavelength of the sound. (This frequency dependence is offset by damping the diaphragm 6 dB per octave to achieve a flat frequency response but this is not germane to the proximity effect so nothing more will be said about it here). The point to be made regarding the frequency dependency is that the phase difference across the diaphragm is the smallest at low frequencies.

In addition to phase differences, amplitude differences also result in pressure differences across the diaphragm. This amplitude component arises from the fact that the far side of the diaphragm is further away from the sound source than the front side. Since sound pressure decreases as the inverse of the distance from the source (it is sound intensity that drops as the inverse of the distance squared, for those familiar with the inverse square law), the amplitude of the sound will be slightly less at the rear of the diaphragm as compared to the front of the diaphragm. Since the pressure difference due to the amplitude component is dependent only on the amplitude differences across the diaphragm, it is independent of frequency.

The properties of the amplitude component that are applicable to the proximity effect are that the contribution to the pressure difference is small and independent of frequency. At large distances between the source and the microphone, the amplitude component of the pressure difference is negligible compared to the phase component at all audio frequencies. As the source is brought closer to the directional microphone, the amplitude component of the pressure difference increases and becomes the dominant component at lower frequencies (recall that the phase component is relatively small at the low frequencies). At higher frequencies, the phase component of the pressure difference continues to dominate for all practical distances between source and microphone.

The result is that the frequency response of the microphone changes, specifically, increases at the low frequency (bass) end, as the audio source is brought within close proximity of the microphone. This is the proximity effect as it pertains to audio.

[edit] In superconductivity

The term "proximity effect" is used in the field of superconductivity to describe phenomena that occur when a superconductor (S) is placed in contact with a "normal" (N) non-superconductor. Typically the critical temperature T_c of the superconductor is suppressed and signs of weak superconductivity are observed in the normal material. The superconducting proximity effect (SPE) is caused by diffusion of Cooper pairs into the normal material, and by the diffusion of electronic excitations in the superconductor. As a contact effect, the SPE is closely related to thermoelectric phenomena like the Peltier effect or the formation of pn junctions in semiconductors. The proximity effect enhancement of T_c is largest when the normal material is a metal with a large diffusivity rather than an insulator (I). Proximity-effect suppression of T_c in a superconductor is largest when the normal material is ferromagnetic, as the presence of the internal magnetic field weakens superconductivity (Cooper pairs breaking).

The study of S/N, S/I and S/S' (S' is lower superconductor) bilayers and multilayers has been a particularly active area of SPE research. The behavior of the compound structure in the direction parallel to the interface differs from that perpendicular to the interface. In type II superconductors exposed to a magnetic field parallel to the interface, vortex defects will preferentially nucleate in the N or I layers and a discontinuity in behavior is observed when an increasing field forces them into the S layers. In type I superconductors, flux will similarly first penetrate N layers. Similar qualitative changes in behavior do not occur when a magnetic field is applied perpendicular to the S/I or S/N interface. In S/N and S/I multilayers at low temperatures, the long penetration depths and coherence lengths of the Cooper pairs will allow the S layers to maintain a mutual, three-dimensional quantum state. As temperature is increased, communication between the S layers is destroyed resulting in a crossover to two-dimensional behavior. The anisotropic behavior of S/N, S/I and S/S' bilayers and multilayers has served as a basis for understanding the far more complex critical field phenomena observed in the highly anisotropic cuprate high-temperature superconductors.

[edit] See also

• Skin effect
• Electromagnetism
• Andreev reflection

[edit] External links

• Proximity Effect music technology glossary
• Proximity Effect in Directional Microphones Shure Pro Audio Technical Library
• Skin Effect, Proximity Effect, and Litz Wire Electromagnetic effects
• Skin and Proximity Effects and HiFi Cables

[edit] References

• Terman, F.E. Radio Engineers' Handbook, McGraw-Hill 1943 -- details electromagnetic proximity and skin effects
• Superconductivity of Metals and Alloys by P.G. de Gennes, ISBN 0-201-40842-2, a textbook which devotes significant space to the superconducting proximity effect.