Background:

Traditional underwater acoustic sensors such as hydrophones are pressure sensors responsive to oscillating pressure, a scalar quantity, in the field of the acoustic wave. A single hydrophone has no directionality. Vector sensors are responsive to water oscillatory velocity, a vector quantity, associated with the same acoustic wave. Thus, the vector sensor has directionality even if its size is much smaller compared to the acoustic wavelength. This makes a difference at ultra-low frequencies in the range of a fraction of a Hz to tens of Hz.

To measure particle velocity in the water, conventional vector sensors are  designed neutrally stay afloat in the water column. The real challenge is to develop highly sensitive, low-noise vector sensor in a small package that could stay afloat. Accelerometers used in conventional sensors are not suitable for sensitive measurements at ultra-low frequencies is that acceleration is proportional to f*v, where v is the particle velocity and f is the frequency. As the frequency decreases, the accelerometer's sensitivity decreases accordingly. It will be more advantageous to measure displacement, x, rather than acceleration, as the displacement is proportional to v/f. That is, for the given particle velocity, the displacement is increased as the frequency goes down.

Being able to set afloat, setting the sensor package on a particular location is also a challenge. The suspension element used to fixe the position restricts free motion of the sensor which might lead to improper measurements. Factors like water currents add unnecessary noise to readings and decrease the sensor’s efficiency especially at low frequencies.

Summary:

The problems and disadvantages associated with the prior art are overcome by the present invention, which includes an low frequency acoustic vector sensor for measuring acoustic wave particle velocities in a liquid, such as water. The acoustic vector sensor includes a housing, a horn positioned within the housing for amplifying the acoustic wave particle velocities, and a buoyant object positioned within the horn. The buoyant object is moveable between a stationary position and a displaced position in response to the amplified acoustic wave particle velocities. A sensor senses the displacement of said buoyant object. The buoyant object, which can consist of any desired shape and size (e.g., spherical, cylindrical, etc.) may be mounted to horn either mechanically or magnetically. and is neutrally or near-neutrally buoyant.

Benefits:

- Unlike hydrophones, vector sensors are responsive to water oscillatory velocity

- The suspension element in a conventional sensor restricts the free motion of the neutrally buoyant body which interferes with measurements

Applications:

- Underwater acoustic measurement devices

- Can be used to measure seismic activity, could help in predicting tsunamis, etc

Full patent: Low Frequency Acoustic Vector Sensor