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Measuring Coastal Boating Noise to Assess Potential Impacts on Marine Life
Researchers Monitor Noise Levels in Shallow Marine Environments Produced by Various Motor Configurations in Small Recreational Boats

By Shari Matzner
Senior Research Engineer
Mark Jones
Senior Research Engineer
Pacific Northwest National Laboratory
Marine Sensing Group
Sequim, Washington

Underwater noise and its effects on marine life deserve attention as human activity in the marine environment increases. Noise can affect fish and marine mammals in ways that are physiological, as in auditory threshold shifts, and behavioral, as in changes in foraging habits. One anthropogenic source of underwater noise that has received little attention to date is recreational boating. Coastal areas and archipelago regions, which play a crucial role in the marine ecosystem, are often subject to high levels of boat traffic.

In order to better understand the noise produced by a small powerboat, a test was conducted in Sequim Bay, Washington, using an instrumented research vessel and multiple acoustic sensors. The broadband noise and narrowband peak levels were observed from two different locations while the boat was operated under various conditions. The results, combined with background noise levels, sound propagation and local boat traffic patterns, can provide a picture of the total boating noise to which marine life may be subjected.

The measured transmission loss of a one-kilohertz signal showed differences in sound propagation along two different channels at the test site.

Test Procedures
The testing consisted of two stages designed to characterize the environment and to generate boat sounds under different operating conditions. To measure the underwater sound propagation characteristics of the site, a Lubell Labs Inc. (Columbus, Ohio) LL9162T underwater speaker was mounted on the vessel and used to broadcast a test signal from known locations. For boat noise measurements, the vessel followed a predefined track operating in different configurations, completing multiple passes in each configuration.

The test site was at the mouth of Sequim Bay on the northern end of the Olympic Peninsula at the Pacific Northwest National Laboratory's (PNNL) Marine Sciences Laboratory in Washington. Sequim Bay opens into the Strait of Juan de Fuca and is home to harbor seals and various species of fish and shellfish. The site is a shallow, nearshore environment with water depths between five and 30 meters.

Testing took place on May 6 and 7, 2010, at times when tidal flow was minimal. The boat traffic through the test site can be heavy, especially during crab and other fishing seasons. To minimize the interference from other boats during testing, an observer on the dock would communicate to the boat operator when no other boats were visible within 500 meters of the test track.

The test boat was a 23-foot aluminum-hulled boat equipped with dual Yamaha F100 outboard engines (four cylinders and a 30:13 gear ratio), which is representative of many of the recreational and small-scale commercial fishing vessels found in the region. Both three-blade propellers and high-performance four-blade propellers were used to produce different operating configurations. The other aspects of the operating configuration that researchers varied were the engines used—dual, port only or starboard only—and the rotations per minute (RPM) of the engines.

The boat was outfitted with a laptop-based data-logging system to continuously record GPS position, heading and speed, engine RPM and speed through the water. The GPS was a Trimble Navigation Ltd. (Sunnyvale, California) Pro XR that performs real-time correction from a beacon signal to achieve sub-meter accuracy. The engine RPM was recorded directly from the line between the engine and the tachometer. The speed through the water was recorded from a paddle-wheel transducer mounted on the transom.

Acoustic Sensors
Three hydrophones were used during the test. Two were deployed on the seafloor in approximately seven meters of water at a depth of five meters and cabled to an onshore data acquisition system. One cabled phone, named Hydrophone North, was located north of the dock in an area somewhat protected by a large spit of land. The other cabled phone, Hydrophone East, was located east of the dock near the main boating channel between the marina and the strait. A third phone was suspended off the dock at a depth of five meters and connected to a laptop through the sound card. This hydrophone was used for data verification and allowed for data visualization during the experiment.

The two cabled hydrophones were Teledyne Benthos (North Falmouth, Massachusetts) AQ-1 hydrophones combined with the company's AQ-201 preamplifiers. The signals from the preamplifiers passed through a custom signal conditioning board that low-pass filtered the signal to four kilohertz. This band was chosen because it contains most of the spectral energy of boat noise. The filtered signal was digitized using a Measurement Computing Systems (Norton, Massachusetts) USB-1608HS data acquisition board, sampled at 10 kilohertz with 16-bit resolution. The sensitivity of the phones and signal processing chain was calculated to be -180 decibels referenced to one volt per micropascal.

Background Noise Level
To estimate the background noise level, a total of seven one-minute intervals of data were selected from different times over the course of the test days. The intervals were selected manually to be times when no boats were present near the hydrophones, and the average spectral levels were calculated separately for each phone. The average background noise at both cabled phones was greatest in the band below 100 hertz. The maximum noise at Hydrophone North and Hydrophone East was about 80 decibels and 100 decibels, respectively, referenced to one micropascal. Above 100 hertz, the noise dropped off sharply at both hydrophones.

At Hydrophone East, the background noise level was nearly constant at 65 decibels referenced to one micropascal for frequencies above 1,500 hertz. But at Hydrophone North, the noise level increased at 750 hertz and continued to rise to above 80 decibels referenced to one micropascal at four kilohertz.

Although the 'quiet' intervals were chosen from times when no boats were passing directly through the test site, there were many boats out in the strait and other parts of the bay, which contributed to the overall background noise level. The difference in background noise between the two locations is indicative of the localized nature of the sound propagation characteristics of the site. To continue this article please click here.

Shari Matzner is a senior researcher at Pacific Northwest National Laboratory's Marine Sciences Laboratory investigating the detection and classification of underwater acoustic signals from boats and marine mammals.

Mark Jones leads the marine sensing group at Pacific Northwest National Laboratory's Marine Sciences Laboratory in Sequim, Washington, where he is responsible for sensor and instrumentation development in the littoral environment.

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