Very low frequency | EPFL Graph Search

Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters (an obsolete metric unit equal to 10 kilometers). Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations (broadcasting time signals to set radio clocks) and for secure military communication. Since VLF waves can penetrate at least 40 meters (131 ft) into saltwater, they are used for military communication with submarines. Propagation characteristics Because of their long wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propag

Assessment of active electroacoustic absorbers as low-frequency modal dampers in rooms

Romain Boulandet, Hervé Lissek, Iris Rigas, Etienne Thierry Jean-Luc Rivet

In small to medium size rooms, natural room modes may severely strengthen the annoyance of low frequency noises, such as traffic or equipment noise. Moreover, there is an actual technological gap in the state-of-the-art concerning low-frequency treatments, and the only potential solutions basically take the form of heavy and bulky bodies. With a view to develop such low-frequency solutions (with compactness requirements), the concept of “shunt loudspeaker” has already been demonstrated to provide controllable electroacoustic dissipation to tackle one or a few modes around the loudspeaker resonance (in the low frequency range). The present paper extend this study to the concept of “electroacoustic absorbers”, based on the connection of loudspeakers to individual synthetic active electric loads, capable of achieving broadband sound absorption around the loudspeaker resonance, up to almost 200 Hz. This paper will especially investigate the optimization of the design, as well as discussion on practical spatial arrangement of electroacoustic absorbers, in the context of an experimental assessment in a test room, so as to provide significant damping of the low-frequency acoustic resonances in the [20-100 Hz]. The resulting modes attenuations range from -1dB to -14 dB along the bandwidth of interest.

2012

Contributions aux études sur le couplage électroacoustique dans les espaces clos en vue du contrôle actif

Pierre-Jean René

The aim of this work was to analyze the global behavior of a loudspeaker exciting a room in the frequency band of its first modes, for the purpose of active control applications. First, the loudspeaker in free field was studied. The characterization of a loudspeaker on the base of equivalent circuits has been established, showing the preponderant importance of the load impedance, determined by near field measurements. Then, the steady state has been modeled and the study has been set on the characterization of the wall impedance, which determines the eigenfrequencies and the amplitude of their modes, as well as charge impedance. The latter which modifies the volume velocity compared with the one in free field, has been characterized. The previous results enable the precise computation of the pressure field in a closed space and excited by a loudspeaker in steady state, according to the wall impedance. For active control purposes, transitory sounds are particularly important, therefore the behavior of a sound field in a room excited by several parameterized sounds has been studied in the time domain. The room response to the source activation could therefore be modeled. This response consists of a free and a transitory state, both of which are qualified through coherent assumptions and observations. Through simulation and systematic experimentation, the loudspeaker-room system has been characterized for a stationary as well as for a transitory sound, revealing the constraints affecting modal active control. Many experiments showed the efficiency of modal active noise control. Observations presented the importance of low frequencies in the annoyance of airplane noise inside rooms. Directives have been given to install active noise reduction system. The main part of this work constitutes an electro acoustical knowledge basis to perform modal active noise control.

EPFL2006

Corona discharge actuator as an active sound absorber under normal and oblique incidence

Thomas Humbert, Hervé Lissek, Stanislav Sergeev

In the majority of active sound absorbing systems, a conventional electrodynamic loudspeaker is used as a controlled source. However, particular situations may require an actuator that is more resistant to harsh environments, adjustable in shape, and lighter. In this work, a plasma-based electroacoustic actuator operating on the atmospheric corona discharge principle is used to achieve sound absorption in real-time. Two control strategies are introduced and tested for both normal in the impedance tube and grazing incidence in the flow duct. The performance of plasma-based active absorber is competitive with conventional passive technologies in terms of effective absorption bandwidth and low-frequency operation, however, it presents some inherent limitations that are discussed. The study reveals that the corona discharge technology is suitable for active noise control in ducts while offering flexibility in design, compactness, and versatility of the absorption frequency range.

2022

Contributions aux études sur le couplage électroacoustique dans les espaces clos en vue du contrôle actif

Pierre-Jean René

EPFL2006