Directionality and gain of small acoustic velocity horns.

Acoustic Velocity Horns (AVHs) are acoustically small funnels open to incident acoustic waves from mouth and throat (for single horns) or both mouths for double horns. Unlike traditional pressure horns terminated at the throat, AVHs yield appreciable amplification of the particle velocity across a wide frequency range starting from extremely low infrasound frequencies. Such horns can be utilized to enhance the performance of conventional vector and acoustic intensity sensors. The present paper includes derivation of directional properties and gains for acoustic velocity as well as pressure for horns of various configurations: single and symmetrical double-horns, and symmetrical horns with inserts of various profiles. The study reveals that the conical double-horns provide the highest velocity gain as compared to horns with an exponential profile or with inserts. The maximum gain cannot exceed a horn's mouth-to-throat radii ratio. In addition to the velocity gain, AVHs offer dipole directionality for the particle velocity and omnidirectional response with no gain for acoustic pressure. These findings were experimentally validated using a water-submerged conical double-horn.

Electro-magnetically controlled acoustic metamaterials with adaptive properties.

A design of actively controlled metamaterial is proposed and discussed. The metamaterial consists of layers of electrically charged nano or micro particles exposed to external magnetic field. The particles are also attached to compliant layers in a way that the designed structure exhibits two resonances: mechanical spring-mass resonance and electro-magnetic cyclotron resonance. It is shown that if the cyclotron frequency is greater than the mechanical resonance frequency, the designed structure could be highly attenuative (40-60 dB) for vibration and sound waves in very broad frequency range even for wavelength much greater than the thickness of the metamaterial. The approach opens up wide range of opportunities for design of adaptively controlled acoustic metamaterials by controlling magnetic field and/or electrical charges.

Acoustic particle velocity horns.

The paper considers receiving acoustic horns designed for particle velocity amplification and suitable for use in vector sensing applications. Unlike conventional horns, designed for acoustic pressure amplification, acoustic velocity horns (AVHs) deliver significant velocity amplification even when the overall size of the horn is much less than an acoustic wavelength. An AVH requires an open-ended configuration, as compared to pressure horns which are terminated at the throat. The appropriate formulation, based on Webster's one-dimensional horn equation, is derived and analyzed for single conical and exponential horns as well as for double-horn configurations. Predicted horn amplification factors (ratio of mouth-to-throat radii) were verified using numerical modeling. It is shown that three independent geometrical parameters principally control a horn's performance: length l, throat radius R(1), and flare rate. Below a predicted resonance region, velocity amplification is practically independent of frequency. Acoustic velocity horns are naturally directional, providing maximum velocity amplification along the boresight.

Horns as particle velocity amplifiers.

Preliminary measurements and numerical predictions reveal that simple, and relatively small, horns generate remarkable amplification of acoustic particle velocity. For example, below 2 kHz, a 2.5 cm conical horn has a uniform velocity amplification ratio (throat-to-mouth) factor of approximately 3, or, in terms of a decibel level, 9.5 dB. It is shown that the velocity amplification factor depends on the horn's mouth-to-throat ratio as well as, though to a lesser degree, the horn's flare rate. A double horn configuration provides limited additional gain, approximately an increase of up to 25%.

Eddy-current non-inertial displacement sensing for underwater infrasound measurements.

A non-inertial sensing approach for an Acoustic Vector Sensor (AVS), which utilizes eddy-current displacement sensors and operates well at Ultra-Low Frequencies (ULF), is described here. In the past, most ULF measurements (from mHertz to approximately 10 Hertz) have been conducted using heavy geophones or seismometers that must be installed on the seafloor; these sensors are not suitable for water column measurements. Currently, there are no readily available compact and affordable underwater AVS that operate within this frequency region. Test results have confirmed the validity of the proposed eddy-current AVS design and have demonstrated high acoustic sensitivity.