1. Gordon Breeze CEng, MICE, BSc, MSc
Drakelands Mine: Investigation of Low Frequency Vibration Infrasound and Strategies for Mitigation
Gordon Breeze CEng, MICE, BSc, MSc
Principal Consultant
Building Technology Group

2. Intoduction Why are BRE Involved? The BRE Team Infrasound
To provide authoritative, independent help and advice to Wolf Minerals to investigate and solve the infrasound problem.
The BRE Team
Gordon Breeze, Amaia Harries, Sergio Stolovas (structural dynamics)
Garry Timmins, Ian West, Stephanie King (acoustics)
Infrasound
Uncommon (but not unheard of) problem
Infrasound can be propagated large distances through air
Difficult to reduce

3. Overview of Presentation
The Story So Far…….
- Work BRE has undertaken
- Findings
- What we have done, and where we are going
- How you can help

4. BRE’s Involvement (since 6th Sept 2015)
Summary of Testing
2015
Dec Vibration measurements in and around WSB
2016
Jan, May (twice),Jul, Nov Acoustic surveys
Feb, Mar Panel vibration surveys
May-Aug Review of long-term acoustic data
Jul (twice) Roof vibration surveys
Jul Machinery vibration survey
Jul Added mass and TMD investigations
Nov Household visits
Dec Low frequency beating investigation
2017
Jan Roof vibration survey
Jan Acoustic Survey
Mar Machinery vibration survey -

5. Ground Level 16 Hz Vibration Measurements

6. Structural Element Model
All of the primary structural elements (13,200 elements) modelled (information provided to BRE by Wolf Minerals)
Model used to investigate global dynamic response
Model formed basis of subsequent more detailed models

7. BRE Laser System Experimental Measurements (No machines running)

8. Initial 4-Stage Model of Infrasound Generation

9. Strategies for Mitigating Infrasound1 2 3

10. Stiffening the Walls and Roof
After stiffening
Before stiffening

11. Stiffening the Walls & Roof
Baseline roof vibration Survey
Stiffened roof vibration survey
Baseline Bay 6-7 Panel vibration survey
Stiffened Bay 6-7 Panel vibration survey

12. Impact of Stiffening the Walls & Roof
Change in 16Hz Noise (dB)
5dB infrasound reduction ≡ 3-fold reduction of energy

13. Effect of Wind Direction
Normalised Noise Level
Relative Angle (degrees)
“Expected” Noise Behaviour
Normalised Noise Level
Normalised Noise Level
Relative Angle (degrees)
Relative Angle (degrees)
Baseline scenario
After stiffening

14. 3-Stage Model of 16Hz Infrasound generation
Normalised Noise Level
Log(distance)

15. When can 16Hz infrasound be heard?
NANR45 Average Noise Acoustic Threshold (limit of audibility)

16. When can 16Hz Infrasound be heard?
Long-Term wind records
(Windwhistle Farm Measurement Station, Night-time )
16Hz Harmonics

17. Analysis of Complaints
Increased Purity of “Tone”
Behaviour of infrasound before and after complaint received

18. Indoor/Outdoor Noise Like-for-Like Comparisons
Data collected early in 2016
Notes:
16Hz harmonics present in outdoor noise
Front & back walls can re-radiate 16Hz noise (potential reverberance making it worse)
16Hz external noise amplified indoors
Noise at higher frequencies reduced

19. Our present-day picture
The 16Hz infrasound is being transmitted acoustically (through the air), and not through the ground.
Evidence suggests that 16Hz infrasound is not being heard
16Hz infrasound excites small objects, which then vibrate at higher frequencies (which can be heard)
16Hz infrasound (and its harmonics) also excites large objects (e.g. floors & walls), which can then re-radiate infrasound at 16Hz.
Complaints not associated with sudden increases of 16Hz infrasound levels
Complaints seem to be associated with increased purity of tone
We now believe complaints and problems are due to 16Hz infrasound being felt and experienced as body vibration.
People are more sensitive to tones (both acoustic and vibrational)

20. What is being done about the problem?
What has been considered/tried already
Walls & roof stiffened (produced large dB reductions near to mine, but no measurable (or discernible) far-field changes
Helmholtz Resonators added (no change observed)
Tuned Mass Dampers & adding mass to panels (both concepts proven, but the more robust option of wall stiffening adopted)
Changing mounting frame springs on large machine (no change observed)
Relocating machines. Rejected as unlikely to have any significant effect, and could make situation worse

21. What is being done about the problem?
What is being done now?
Removing the source
Underpan investigations (ruled out)
Operating machines in anti-phase (going ahead on 1 pair!)
Enclosure (going ahead!)
Splitting tasks undertaken by large machine into tasks undertaken by smaller machines
Infrasound mitigation
Installation of additional and larger noise-attenuation bunds

22. Where are we now? Infrasound is not well understood
Over the past 1½ years we (BRE, Wolf Minerals and others) have made significant progress
Understanding the theoretical causes of infrasound and its propagation
Measuring and quantifying infrasound behaviour
Modelling infrasound behaviour
Implementing mitigation measures and measuring their effect
Understanding how infrasound is affecting people

23. How Locals are Helping
But, we do not understand everything - and the problems have not yet been fixed !
Understand where and how it is affecting people by gathering as much information as possible.
Understand whether it is audible sound or felt vibration, or both.
Tie in complaints to plant operation records and weather conditions, to establish whether the operation of a particular machine or a set of conditions is responsible for the problem.
Over the past 1½ years we (BRE, Wolf Minerals and