NOAA_Bottom_Profiler_Airborne_Noise_Mitigation.pdf

NOAA Bottom Profiler Airborne Noise Mitigation

JOB MEMO

TO: Ken Nadeau

COMPANY NOAA

FROM: Nathan Jones {nathan@noise-control.com}

DATE: December 17, 2012

SUBJECT: OKEANOS EXPLORER Bottom Profiler Airborne noise mitigation

CC:

Noise Control Engineering Inc. of Billerica MA was retained by NOAA to evaluate methods for mitigating airborne noise generated the bottom profiler on the OKEANOS EXPLORER.

During the operation of the bottom profiler an audible airborne noise is generated that is found to be annoying by the crew.

Primary Solution

Based on the airborne noise measurements and photos provided by NOAA, NCE has developed a noise mitigation plan for the array. NCE evaluated a similar active sonar induced noise for the David Taylor Research Center. Based on that project, it is NCE’s experience that the majority of noise that is transmitted from the sonar and received in the vessel aurally is transmitted through the ship’s structure. It is most probable that this structureborne noise is introduced into the ship structure through the supports for the bottom profiling sonar array. It is much less feasible that the noise generated by the sonar is traveling though the water and coupling to the ship’s structure.

NCE recommends that the method of mounting be changed from a stanchion support from the array transducers (shown in Figure 1) to the hull to a rigid support on the face of the sonar dome. The dome mounting method will be stiffer than the stanchion support method as well as remove the connection to the tank above the transducers. The proposed dome mounting method is shown in Figure 2a-2c.

The recommended dome mounting method consists of welding an I-beam or box beam to the deck of the compartment that is the same height as the array mounting plate. An L-beam

(approximately 4”x4”x1/2”) is connected to both the array mounting plate and the I-beam. It is best to weld to both the mounting plate and the I-beam, but it may be more practical to bolt the L-beam to the array mounting plate (as shown in figure 2a-2c) since welding may damage the transducers.

Additional Treatment Options

Noise Control

Engineering, Inc.

799 MIDDLESEX TNPK.

BILLERICA MA 01821-3445

PHONE: 978-670-5339

FAX: 978-667-7047

E-mail: nonoise@noise-control.com

Noise Control December 13, 2012

Engineering, Inc.

Based on NCE’s experience the proposed mounting method will lower the audible noise of the profiling array into the background noise of the vessel. If additional noise reduction is required or desired NCE proposes the following treatments.

Isolation mounting the array: Using the deck mounting method it would be possible to add isolation mounts between the L-beam and the I-beam. The isolation mount should be a spring mount since such as the Christie & Grey Type ORS25 or ORS50 since an elastomeric mount could not withstand the flooded environment.

Decoupling treatment: To reduce the fluid borne noise path a decoupling material should be applied to the surfaces of the compartment containing the profiling array. The best method for doing this is the application of Rubatex closed cell elastomeric insulation. The Rubatex creates an impedance mismatch between the seawater in the compartment and the structure.

This mismatch reduces the acceptance of the fluid borne noise by the structure.

Figure 1. Existing mounts to be removed.

Remove Existing

Array Supports

Figrue 2a. Recommended mounting method

Figrue 2b. Recommended mounting method

Figrue 2c. Recommended mounting method