The Acoustics Research Group at the Department of Physics and Astronomy, Brigham Young University, Utah, USA has chosen Spectrum Instrumentation’s leading-edge digitizers and signal generators to form the heart of its new underwater acoustics laboratory. The new lab is a big step forward in research on sound waves traveling through the water as it effectively provides a miniaturized version of the ocean. Experiments are possible on sound wave’s behaviour in different water layers and their reflections from the ocean’s most diverse ground materials such as rocks, sand or mud. Miniaturization means that the highest precision is needed from the measurement equipment as the experimental results are scaled up afterward to indicate what would happen in the real world.
The new laboratory water tank is rectangular and measures 3.6 m long by 1.2 m wide with a maximum water depth of 0.91 m. The research involves using a hydrophone for the signals or chirps generated by an Arbitrary Waveform Generator (AWG), the Spectrum model M2p.6546-x4. This PC-card generates signals with 24 V output swings that are then amplified before being broadcasted by the hydrophones. After travelling through the tank, the signals are detected by another hydrophone and processed by a Spectrum M2p.5932-x4 digitizer card. The transmitter and receiver are each held by a robotic arm that positions and orientates them within the water so that source and receiver can be positioned as required.
The tank enables experiments to be done on how the seafloor affects sound waves bouncing off it. A pure rock bottom will have a different effect than sand, mud, or layers of different materials. “It is even more complicated,” explained Dr Traci Neilsen, the professor in charge of the project, “because water is not homogeneous. Changes in temperature and salinity change the sound speed and cause the waves to bend, similar to how a mirage happens. We plan to examine the impact of water temperature changes on machine learning for localizing sound sources. These tank studies are more repeatable, efficient, and cost-effective than ocean experiments and will allow us to develop techniques that can then be tested on ocean data.”
The pair of Spectrum PCIe-cards are housed in an external PCIe chassis in the main control console and accurately synchronized together using a Star-Hub module by Spectrum. The setup has a second identical pair of cards in a second chassis that can be triggered into operation by the first chassis. This scaled experiment requires much higher frequencies in the kilohertz range than would be used in the ocean. The digitizers and AWG cards have a high resolution of 16-bit and can even sample and output at rates of 40 Megasamples per second, respectively. At the same time, the skew between channels is less than 100 pico-seconds. That delivers the high precision required for the experiments
The two UR10e robot arms, along with the signal generation and the data acquisition, are all controlled by a custom LabVIEW software program that Adam Kingsley created and referred to as “Easy Spectrum Acoustics Underwater” (ESAU).
A key part of the experimental setup is to model the open ocean, so special anechoic panels from Precision Acoustics on the sides of the tank reduce the reflections. A significant innovation was the design of a filtration and circulation pump by John Ellsworth, who is BYU Department of Physics and Astronomy’s Research Laboratories Supervisor. This pump keeps the water clean without creating bubbles in the tank, which are a significant source of noise. With all these preparations in place, impulse responses could be measured, making it easier to eliminate noise from readings when an experiment is being done. The precision of the Spectrum PC cards with a Signal to Noise Ratio (SNR) of more than 71 dB ensures that the impulse response elimination gives accurate experimental results.