This document discusses an approach to underwater noise abatement using acoustic resonators. It describes how bubble resonance can be used to attenuate noise in the 30-200 Hz range by more than 10 dB. The approach involves using encapsulated bubbles that are tuned to resonate at target frequencies. Recent offshore demonstrations showed the system reduced impact pile driving noise by up to 36.8 dB. New resonator designs improve performance and reduce ballast requirements. The technology has potential applications for treating noise from seismic surveying, shipping, dredging, and explosives.

1. 9/28/2015Mark S. Wochner, Ph.D. | President & CEO
Advanced Approaches in Underwater Noise Abatement

2. Technology Background
BACKGROUND: OIL AND GAS NOISE
– Research started in 2009
– Drilling, seismic surveying, pipe laying, support ship
– All-purpose noise abatement system
PREVIOUS METHOD: FREE BUBBLES
– Low frequency attenuation requires unstable large bubbles
– Limited performance for small bubbles (~10 dB)
OUR APPROACH: BUBBLE RESONANCE
– Frequencies of interest: 30 Hz to 200 Hz
– Desired noise level reduction: >10 dB

3. AIR BEHAVES LIKE A MASS-SPRING SYSTEM
– larger bubbles → lower resonance frequency
– energy from the acoustic wave goes into resonating the bubble
SMALL BUBBLES DON’T RESONATE AT DESIRED FREQUENCIES
LARGE FREE BUBBLES ARE UNSTABLE, SO WE CAPTURE THE AIR
Bubble Resonance
Commander & Prosperetti,
JASA 85, 732-746 (1989)
Monodisperse distribution of
bubbles: Bubble radius = 6 cm
Void fraction β = 0.01

4. AIR BEHAVES LIKE A MASS-SPRING SYSTEM
– larger bubbles → lower resonance frequency
– energy from the acoustic wave goes into resonating the bubble
SMALL BUBBLES DON’T RESONATE AT DESIRED FREQUENCIES
LARGE FREE BUBBLES ARE UNSTABLE, SO WE CAPTURE THE AIR
Bubble Resonance
Commander & Prosperetti,
JASA 85, 732-746 (1989)
Monodisperse distribution of
bubbles: Bubble radius = 6 cm
Void fraction β = 0.01
Church, JASA 97, 1510-1521 (1995)

5. AIR BEHAVES LIKE A MASS-SPRING SYSTEM
– larger bubbles → lower resonance frequency
– energy from the acoustic wave goes into resonating the bubble
SMALL BUBBLES DON’T RESONATE AT DESIRED FREQUENCIES
LARGE FREE BUBBLES ARE UNSTABLE, SO WE CAPTURE THE AIR
a0 = 12 cm
a0 = 8.02
a0 = 6 cm Fixed Void
Fraction: 0.5%
0.5%
1%
2%
Fixed radius:
a0 = 8.02 cm
Two unique controls:
Control target
frequencies
Control amount of
attenuation
Bubble Resonance

6. SOURCES OF ENERGY DISSIPATION
– Heat transfer from gas inside resonator to surrounding fluid
– Work done by the resonator:
o Viscous losses in surrounding fluid
o Viscoelastic stresses in shell (fully or partly encapsulated resonators)
o Re-radiation of acoustic energy (phase incoherent)
 energy from sound wave goes into oscillating the resonator
 dominant loss mechanism for large resonators
NOTE: Higher Q resonators are better oscillators and better attenuators
Energy Losses

7. Acoustic Demonstration
Demonstration Video: Low-Frequency Version
(For High-Fidelity Sound Systems)
http://youtu.be/sKf4ibzJ-Jc
Demonstration Video: High-Frequency Version
(For Laptop Speakers, etc)
http://youtu.be/BHupQqkx3Hk

8. no bubbles
N = 150, β = 0.02
N = 70
β = 0.01
N = 35
β = 0.005
a0 = 8 cm
Results of lake tests in 35 m of water for continuous and impulsive noise
N = 35, β = 0.005
N = 70, β = 0.01
N = 150, β = 0.02
no bubbles
Continuous Noise Impulsive Noise
Validation

9. N = 35, β = 0.005
N = 70, β = 0.01
N = 150, β = 0.02
no bubbles
Continuous Noise Attenuation Prediction
Validation & Prediction

10. Helmholtz resonators’ behavior is dependent on a number of factors:
volume, neck length, and aperture size
Helmholtz resonance is often used in sound suppression:
Bass Traps
Car exhaust systems
Vair
L
A
Water
ADVANTAGES:
– More customizable & predictable
– Better performance at depth
– More rigging & manufacturing options available
– Lower ballast requirements
Helmholtz Resonance

11. 2014 Demo Panel
Noise Abatement Concept

12. Close up of system on
articulating pile gripper
System deployed
around pile under
pile gripper
Noise Abatement Concept
2014 Demo Panel

13. RECENTLY COMPLETED OFFSHORE DEMONSTRATION
– Butendiek Wind Farm
– Monopile diameter: 6.8 m
– Project developer: WPD
– Installation company: Ballast Nedam
– Crew, vessel, support supplied by Ballast Nedam
– Three AdBm personnel involved in test
INSTALLATION OF SYSTEM IN NORTH SEA
– Four installations of the noise abatement system occurred
– No installation issues; fast deployment
Offshore Demonstration

14. Offshore Demonstration
The IHC Merwede NMS

15. Three different resonator sizes
Offshore Demonstration

16. Deployed system in the water Experimental setup
Offshore Demonstration

17. Without Noise Abatement
With Noise Abatement
Pile Range (m) Number of Strikes Baseline (dB) Panel (dB) Difference (dB) Max Reduction (dB)
BU-21 285 668 183.3 ± 0.7 164.2 ± 2.3 19.2 ± 2.4* 36.8*
Offshore Demonstration

18. NEW ACOUSTIC RESONATORS
– Injection-molded, positively buoyant materials
– Multiple sizes for varying depths, resonance frequencies, etc.
– More consistent performance at depth
Our resonators vs.
Free Bubbles (f0 = 100 Hz)
1/5 the volume at surface
1/17 the volume at 40 m
Less air = less buoyancy → less ballast, simpler framework
Modern Noise Abatement System
1/50 the air volume of balloons at 40 m

19. 440 kg static load tested Slight plastic deformation afterward
NEW ACOUSTIC RESONATORS
Modern Noise Abatement System

20. RESONATOR FRAMEWORK
– Fully protected within metal deployment framework
– Framework organized as slats
– System stacks like Venetian blinds
– Resonator slats open below surface
– Functional elements protected
Aluminum/Steel
Framework
Chain guides
Resonators fully protected when slats are stacked
Modern Noise Abatement System

21. THE ASSEMBLED SYSTEM
– Panels arranged around pile and deployed using winches
– Mounted to pile gripper or directly to vessel
– Complete, tunable, passive system
SystemSlats
Resonators
Modern Noise Abatement System

22. SEISMIC SURVEYING
– metal Helmholtz resonators perform best
– well-suited to air gun noise abatement
– system of towed bodies can be designed
SHIPPING & DREDGING NOISE
– ship treated directly with hydrodynamic system
– system of fencing can be used to protect an area
– long-term, no energy solution
EXPLOSIVES
– UXO removal, explosive demolitions
(End View)
Noise abatement panels
Airgun
Noise abatement panels
Airgun
(End View)
Acoustic resonator
system absorption
Other Applications

23. Acoustic resonator systems have tunable, predictable performance
System has demonstrated up to 50 dB of noise reduction
New resonator design is more robust, easier to manufacture, has
better depth performance, and requires 25% the ballast
Can be used to treat other noise sources: ships, air guns, explosives
Summary