Acoustic near field refers to the closest region of sound space around a source, where waves from their source reflect, absorb or diffuse in various acoustic media that they pass through.
Recent works on selective directional couplings in longitudinal acoustics and optics have focused on couplings between incident waves or anisotropic structures and near-field directionality; however, such as electromagnetic Janus-Huygens dipoles. Such extensions provide further insights into acoustic sources.
Free Field
The acoustic near field refers to the region of sound field closest to its source, unobstructed by walls or ceilings and free from obstructions such as walls and ceilings. Usually this region follows Inverse Square Law which states that sound pressure level will decrease with distance away from source.
Acoustic engineering uses this zone of the sound field to model transducer performance and device characteristics such as reverberation. Such simulation is usually carried out within an anechoic chamber or similar device.
Simulation of loudspeakers involves partitioning the sound field into near and far fields that can be modeled with various algorithms to predict spatial distribution of sound power. Acoustic pressure and intensity distributions created using such algorithms are then compared with measurements obtained directly.
This study utilized four NAH algorithms to reconstruct sound pressure and intensity distributions within an acoustic near field, then compared these predictions against direct measurement results to gauge how closely they match up with each other.
These results demonstrate that four NAH algorithms agree well with direct measurements but underestimate total sound power, as their near-field sound pressure and intensity distributions propagate into far fields before being reconstructed later. This is expected, since near field distributions often propagate out into far fields before being reconstituted later.
Free Field Motion (FFM) is an important contributor to amplified ground motion during an earthquake. This phenomenon can be affected by dynamic properties of soil columns, geological site conditions and ground motion characteristics at a particular site; therefore this paper seeks to evaluate their influence on free field surface motion and acceleration response spectra of 100 near-fault strong ground motions of various magnitudes in Mumbai city.
Results show that dynamic properties of soil columns, rock strata types, and ground motion characteristics at any particular location have an enormous effect on free field surface motion and amplification. Furthermore, soil nonlinearity at higher peak ground accelerations also has an influence.
Diffuse Field
A diffuse sound field is an acoustic environment in which energy distribution within a room is uniform due to repeated reflections. These types of environments are commonly found in gymnasia, swimming pools and interior spaces with marble, concrete or glass walls.
Reverberant chambers can create diffuse fields to characterize acoustic materials and their properties, yet are often too large to be practical for transducer simulations. Therefore, it is necessary to devise an efficient method for creating near field simulations which produce reliable performance estimates for transducers.
Contrary to free fields, an acoustic near field contains energy which cannot travel in any direction or leave the room, because sound pressure and particle velocity are in phase, thus eliminating transmission or storage of any acoustic energy; its intensity being determined by both components individually.
The acoustic near field can be divided into two distinct regions: the source area and surrounding near field. The source area encompasses all sources and objects within a room, while surrounding near field consists of all surfaces in a room that reflect sound waves from sources.
As the near field area typically contains rougher surfaces than source areas, acoustic waves tend to scatter from sources as particles travel less distance towards them.
Once the distance between source and nearest surface increases, the acoustic near field begins to converge towards it and this phenomenon is known as acoustic convergence. This process may vary depending on individual circumstances such as body motion, environmental conditions or nearby objects or sound sources affecting it.
As part of this process, there is also an increase in evanescent waves trapped within the near field, known to shape human hearing – for instance through listening to sweeps on an Etymotic ER4S.
Near Field
Near field refers to an electromagnetic wave’s radiation within about half a wavelength from an antenna, in which electric and magnetic fields have yet to balance themselves out, or transition zone.
An antenna emits an electric field when powered, while its antenna current produces magnetic fields, both of which contribute to an electromagnetic field that radiates out from it and radiates toward its surroundings or towards a receiver nearby.
Electromagnetic fields vary with distance from an antenna, and can be split into near field and far field regions, each having different properties.
Nearfield electromagnetic fields exhibit rapidly decreasing electric and magnetic field components due to their evanescent properties. Their amplitudes reduce quickly with increasing distance from an antenna, due to electromagnetic evanescence.
As these fields reduce in amplitude with distance, their electromagnetic components responsible for power transfer (i.e. radiative near-field energy transfer) tend to dissipate much faster than their evanescent components – meaning a receiver will only feel its effects if in close proximity to an antenna and is excited by radiation from it.
This phenomenon resembles the induction coupling effect of a transformer; when energy stored within an inductive near-field effect is transferred, more power from its primary circuit is drawn upon by the transformer.
Note, however, that the near-field components of electric and magnetic fields in the near field do not self-propagate; that is, they do not radiate energy over extended distances, thus drawing power away from their transmitter unless excited by an antenna receiver nearby.
Due to radiation near-field coupling being out-of-phase with its source antenna’s field, E and H fields in this near field cannot effectively return inductive or capacitive energy from antenna currents or charges back through inductance or capacitance. Therefore, it may often be necessary to separate reactive from nonreactive electric and magnetic fields within an antenna.
Far Field
Far field refers to an area farthest away from an electromagnetic radiation source than near field. This area may also be known as Fraunhofer or Fresnel region depending on where its source lies.
Electromagnetic radiation consists of both electric and magnetic fields. These fields can be linked with free space impedance of their medium in which they propagate and the Law of Diffraction governs how they behave.
Radio antennas are devices designed to radiate electromagnetic energy for receiving and transmitting signals, known as antennae. Their near field area usually extends within an r l radius; wherein r equals antenna length.
The near field is a reactive region in which electromagnetic fields are in phase, leading to an energy transfer between them. Couplings to structures like power lines, plumbing pipes, metal drain pipes or any other objects will be very strong in this region and this could have serious ramifications on antenna power radiated and EMC concerns.
Due to its reactive nature, near field measurements of electromagnetic interference can be challenging due to different fields reacting differently and not producing a coherent wavefront.
When making measurements with radio waves, there are various techniques for identifying near field and far field regions. Ultimately, which one you use depends on factors like your application, certification or standard requirements, customer preference etc.
As with any measurement, understanding how different fields interact is particularly crucial in order to accurately interpret results. For instance, frequency varies with distance from speakers due to longer wavelengths being more sensitive to room acoustics and therefore leading to more reflected sound waves.
Another element that can impact acoustic measurements accurately is the phase difference between electric and magnetic fields, which depends on both radio type and antenna used to conduct them. Therefore, it is imperative that physical distance between radio and radiator be accurate – something near-field/far-field transformation on radio signal can do to determine its exact phase difference between electric and magnetic fields.