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On the contrary, in Φ-OTDR-based sensors, the coherence length of lasers is longer than their pulse length. An event near the fiber generates an acoustic wave that affects the optical fiber by changing the phases of the backscattering centers.
An acoustic pulse is transmitted by the transceiver and detected by the subsea transponder, which replies with its own acoustic pulse. This return pulse is detected by the shipboard transceiver. The time from the transmission of the initial acoustic pulse until the reply is detected is measured by the USBL system and is converted into a range.
To infer information about the acoustic field it is necessary to sample the field in space and time. Temporal sampling has already been discussed in a previous section. The sensor array samples the spatial domain, while the beamformer integrate the sensor’s output in a special way to enhance detection and estimation performance of the system.
A type of active sonar, echo sounding is the transmission of an acoustic pulse directly downwards to the seabed, measuring the time between transmission and echo return, after having hit the bottom and bouncing back to its ship of origin. The acoustic pulse is emitted by a transducer which receives the return echo as well.
An electromagnetic acoustic transducer (EMAT) is a transducer for non-contact acoustic wave generation and reception in conducting materials. Its effect is based on electromagnetic mechanisms, which do not need direct coupling with the surface of the material. Due to this couplant-free feature, EMATs are particularly useful in harsh, i.e., hot ...
Surface acoustic wave sensors are a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena.
Data acquisition hardware for acoustic measurements typically utilizes 24-bit analog-to-digital converters (ADCs), anti-aliasing filters, and other signal conditioning. This signal conditioning may include amplification, filtering, sensor excitation, and input configuration. Another consideration is the frequency range of the instrumentation.
The sensor suite is combined (typically by use of a Kalman filter) to estimate the position of the vehicle. This may help to navigate submarines, autonomous, and remotely operated underwater vehicles. The AWAC (Acoustic Wave and Currents) is a type of ADCP specifically designed for surface wave height and direction.