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Acoustics: Characterizing Noise Associated with Marine Energy

Characterizing underwater sound is a critical part of determining the effects of human-generated sounds on marine life. From analyzing noise characteristics near offshore construction sites and energy installations, to characterizing sound from marine renewable energy (MRE) devices, Integral Consulting Inc. performs studies that provide a clear understanding of sound emitted in the underwater environment.

 

One such study involves a new technology designed to improve upon traditional acoustics sensing techniques. Together with Sandia National Laboratories, Noise Control Engineering, and Proteus Technologies, Integral is developing an array of sensors that measure sound and locate sound sources in near real time. The array, called the “NoiseSpotter,” is intended to evaluate environmental effects of MRE installations. This system will distinguish MRE device noise from surrounding sounds, including flow noise, and relay noise metrics in near real time to a land-based observer using a satellite telemetry link.

 

NoiseSpotter is a compact device and thus can be deployed easily from small watercraft.

 

“Noise from MRE devices, such as wave energy converters, is largely unexplored and poorly characterized,” explains Integral Scientist Kaus Raghukumar. “This study involving development and field testing of the NoiseSpotter array will lead to an increased understanding of the noise characteristics of wave energy converters to help support future noise propagation modeling.”

 

Reduced Time and Cost of Measurements

 

By developing this cost-effective, compact array of sensors, Integral and its collaborating partners aim to reduce the time and expense of collecting extended measurements of noise produced by MRE devices. The acoustic measurement platform will also collect broadband acoustic data, which will make it easier to detect marine mammals and other marine organisms.

 

NoiseSpotter can characterize, classify, and identify the location of sounds unnatural to the environment—such as noise from MRE devices, boats, ship traffic, and underwater construction—as well as natural sounds from marine animals and precipitation. Further, NoiseSpotter’s ability to estimate the location of a device can assist in the investigation of potential failure modes of MRE devices by helping isolate device sounds from other sounds in the environment. This new platform, capable of measuring acoustic pressure and particle velocity, will allow Integral clients to learn more about the environmental effects of their underwater activities.

 

How It Works

 

Current techniques for locating acoustic sources require the deployment of large arrays of cabled hydrophones, or multiple independent hydrophones over a large area, to measure acoustic pressure. The NoiseSpotter features a small (2 × 2 × 6-ft) three-sensor array, with each sensor measuring acoustic pressure and particle velocity to geolocate sources of sound more efficiently.

 

The NoiseSpotter is ready for deployment.

 

This ability to locate MRE devices is key to characterizing the noises they emit, which generally are low intensity sounds in the frequency band of 125–250 Hz and are undetectable above the ambient noise outside this band, with other sources of sound likely to be present in the vicinity. The other sources can include boats, industrial activities, and natural sources, such as marine mammals, precipitation, breaking surface waves, and fish choruses.

 

Integral’s NoiseSpotter platform can improve upon traditional bearing estimation methods by using acoustic vector sensors that measure triaxial particle velocity in addition to acoustic pressure. Particle velocity is a vector quantity (unlike pressure, which is a scalar), and each vector sensor has the ability to determine the bearing of an acoustic source. With multiple time-synchronized vector sensors located on the array, the noise source can be geolocated by triangulation of particle velocity vectors measured across the array.

 

Characterizing noise generated from marine and hydrokinetic devices is key to alleviating risk associated with underwater sound from MRE installations.

 

Pressure levels and windows of active transmission indicating that the vector sensors effectively characterize underwater noise.

 

An initial field test of the NoiseSpotter indicates that vector sensors are well-suited to geolocating MRE devices and characterizing the noise they generate. Data were sufficient for developing the bearing estimate algorithm; bearing estimates were within 10 percent of true azimuth and 25 percent of true elevation, clearly demonstrating NoiseSpotter’s ability to geolocate a source of sound. Next steps are to expand upon the bearing estimation to provide full 3-dimensional depiction of the source location of sound.

 

The goal of the 3-year project is to enable autonomous operation of the NoiseSpotter over long-term deployments (on the order of weeks) with regular transmissions of the data (including geolocation information of sources of sound) via satellite telemetry. With this capability, the NoiseSpotter has the makings of a uniquely powerful and cost-effective tool for regulators, stakeholders, and developers seeking to gather the data necessary to alleviate risk from underwater noise associated with MRE devices.

 

For more information on the NoiseSpotter, contact Kaus Raghukumar at kraghukumar@integral-corp.com or Grace Chang at gchang@integral-corp.com.

 

This work is currently funded by U.S. Department of Energy, Energy Efficiency and Renewable Energy (Award No. DE-EE0007822).