Active anti-noise transmitters and receivers

In general, the low-frequency capability of noise reduction of conventional anti-noise transmitters and receivers is not encouraging, but more and more sound energy of the modern high-intensity noise environments concentrates in that frequency range. Active anti-noise transmitters and receivers, which are developed from active ear defenders, supply the devices not only the benefit of advanced low-frequency anti-noise capability, but also a selectivity in sound reduction. The latter virtue ensures a high intelligibility of speech when the low-frequency noise is highly attenuated. On the basis of ref. [1], a thorough discussion on the principles, structures and experimental results of active anti-noise transmitters and receivers are given in this paper.

Plasma perturbations in the coexisting environment of VLF transmitter emission, lightning strokes and seismic activity

We analyzed plasma perturbations occurring in the coexisting environment of powerful VLF transmitter emission, intense lightning strokes and strong seismic activity during pregnant period. The results suggest that anomalous electron bursts with energy dispersion in the range of ~100–350 keV, forming the "wisp" signature, are due to cyclotron resonance of electrons with monochromatic waves from the powerful NWC VLF transmitters during nighttime. The intense broad band VLF emissions (up-going 0+ whistlers) are observed while the DEMETER satellite goes through the region of intense thunderstorm activities at mid-latitudes. However, the effects of intense lightning activity and pregnant earthquake have little impact on this kind of stable energy-dispersed electron structures, despite the fact that they are presumably two primary reasons for the particle precipitation in the ionosphere. The case studied here provides us a valuable opportunity to address the various sources triggering the anomalous plasma perturbations in the ionosphere.