PREN2620 Environmental Impact Assessment and Monitoring Techniques

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PREN2620Environmental Impact Assessment and
Monitoring Techniques
Blasting and Environmental Research Group

• Dr. Mark Pegden

Department of Mining, Quarry Mineral
Engineering, University of Leeds
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Lecture 2Noise
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Noise Definitions

• Air Overpressure Known as Air Blast or
  Concussion is the airborne vibration consisting
  of both noise (audible) and low (sub-audible)
  frequencies.
• Sound Pressure Level (SPL) Amplitude of sound
  measured in Pascals or Decibels.
• Sound Power Level (SWL) Sound output from a
  source (static or mobile) measured in Decibels.
• Community Noise The noise that impacts upon the
  local population living in the vicinity of the
  Mine or Quarry.

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Community Noise

• Air Overpressure and Sound
• Whenever something disturbs the air, pressure
  variations are produced which may or may not be
  heard. E.g in a blast the firing of the
  detonators and the rock fragments falling on each
  other will produce air pressure changes which can
  be heard (sound). As the free face is pushed out
  into the void by the blast, pressure changes will
  be produced which are unlikely to be heard (air
  overpressure) but which may be significant enough
  to rattle a window in a distant property.
• Sound is pressure variations detectable by the
  human ear.
• Pressure variations have two characteristics,
  frequency and amplitude.
• Frequency is measured in Hertz, and amplitude in
  micropascals which is usually converted into
  decibels (dB, a log scale) because of the vast
  range. L20log10 (P/p0), P-amplitude in Pascals
  and p0-20micropascals (threshold of hearing).

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Community Noise

• NOISE
• Noise is unwanted sound, but the physical level
  of sound does not always correspond to the level
  of annoyance it causes.
• It is therefore important to distinguish between
  objective (physical) noise levels and subjective
  (perceived) loudness.
• The human ear is more sensitive to some
  frequencies than to others so as is annoyance
  which is important in EIA, the recorded sound
  levels are adjusted using the A-weighting curve
  to give a single figure index which takes account
  of the varying sensitivity of the ear.

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Community Noise

• WEIGHTING SCALES
• Several different scales exist
• A Most commonly used and intended for low sound
  pressure levels.
• Designed to respond in a similar way to the human
  ear. Measurements in dB(A) broadly agree with
  peoples assessment of loudness. A change of
  3dB(A) is the minimum perceptible in normal
  conditions. A change in 10 dB(A) corresponds
  roughly to a doubling or halving. Always used for
  community noise.
• B Medium Sound Pressure Levels
• C High Sound Pressure Levels
• D Specially developed for aircraft noise

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Community Noise

• MEASURING NOISE
• Noise levels are hardly ever steady they rise
  and fall with the activity taking place in the
  area. This means that some kind of summary has
  to be given of the sound levels over a period of
  time.
• This will usually include the maximum and minimum
  sound levels during this time (Lmax Lmin), an
  average sound level (Leq,T - the continuous sound
  pressure level which over time T would have the
  same acoustic energy) and also a number of Ln,T
  levels.
• These are the noise levels which have been
  exceeded for N of the time T. For example
  L10,1hr is the dB(A) level which is exceeded for
  10 of the 1 hour period (i.e. 6 minutes) and
  indicates the noisier sounds and is generally
  used for road traffic assessment. The L90,24hr
  is the level exceeded for 90 of a 24 hour period
  and indicates the noise levels during quieter
  periods - perhaps the background noise. From
  this you will see that L1,T level is likely to be
  very similar to the Lmax.

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Community Noise

• NOISE IMPACT FACTORS
• The level of noise from an activity is influenced
  by
• components of the source
• transmission paths
• receiver
• SOURCE
• Sound level,
• Frequency (human sensitivity differs and higher
  frequencies are attenuated more easily),
• Directive pattern. Where a source is free to
  radiate in all directions the sound energy is
  reduced by 6dB when the distance is doubled.

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Community Noise

• Adding together Sound Levels -
• SPL 10 x log1010 (SPL1)/10
  10(SPL2)/10... 10(SPLn)/10
• Example - add together 68 dB(A) and 56dB(A)
• SPL 10 x Log10(1068/10 1056/10)
• 68 . 3 dB(A)
• Q - at the boundary of a quarry, the Background
  Noise Level has been found to be 50 dB(A). The
  two items of plant that you want to operate would
  produce 72 dB(A) and 67 dB(A) respectively at
  that point. What sound pressure level would you
  expect to record?
• SPL 10 x Log10(1050/10 1072/10 1067/10)
• 73.2 dB(A)

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Community Noise

• Subtracting Sound Levels -
• SPL 10 x log1010 (SPL1)/10 -
  10(SPL2)/10...- 10(SPLn)/10
• Example - take away 56 dB(A) from 68dB(A)
• SPL 10 x Log10(1068/10 - 1056/10)
• 67.7 dB(A)
• Q - You are monitoring noise at a house close to
  a mineral processing plant and record 67 dB(A)
  whilst the plant is operating and 65 dB(A) whilst
  the plant is not working. Calculate the sound
  pressure level at that location due to the plant
  alone.
• SPL 10 x Log10(1067/10 - 1065/10)
• 62.7 dB(A)

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Community Noise

• Averaging Sound Levels -
• SPL 10 x log1010 ((SPL1)/10
  ...10(SPLn)/10)/N
• Example - average 67 dB(A) and 69dB(A)
• SPL 10 x Log10(1067/10 1069/10)/2
• 68 . 1 dB(A)
• Q - You have monitored the noise emanating from a
  water pump at a distance of 10 metres in three
  different directions follows 75 dB(A),
  77dB(A) 78 dB(A). Calculate the average sound
  pressure of the water pump.
• SPL 10 x Log10(1075/10 1077/10
  1078/10)/3
• 76 . 8 dB(A)

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Community Noise

• Noise Calculations
• Remember
• Before arithmetic operations
• 10dB/10 (antilog to base 10)
• then carry out simple maths
• Add
• Subtract
• Average
• After arithmetic operations
• 10 x log10

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Community Noise

• Noise Prediction
• (SWL) - Sound Power Level - is the parameter
  used to describe the sound being produced by an
  item of equipment. The Sound Pressure Level
  experienced at a location for static plant (a
  receiver or Target) due to a source depends on a
  number of factors
• Sound Power Level of the source
• Distance from source to receiver
• Barriers (remember line of sight)
• Type of Ground surface (between source and
  receiver)
• Sound prediction
• Predict SPL at receiver for each source
• Sum SPLs to give final SPL at Receiver

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Community Noise

• Noise Prediction for Static Equipment
• Distance
• In most mining or quarrying situations sources
  are on a flat(ish) surface and the equation used
  is
• SPL SWL - 20 x Log10(Distance) - 8
• this equation assumes a hard(worst case) surface
  and may need to be modified for natural surfaces.
• Sound Power Levels - obtain from Manufacturer,
  or Tables (British Standard BS5228) or
  derive by measurement at 10 metres
• Barriers - either calculate or use
  BS5228 subtract 5dB if plant just
  visible subtract 10dB if plant not
  visible

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Community Noise

• Noise Prediction for Mobile Equipment
• For static plant, the noise sources are assumed
  to be point sources. It can be shown that for a
  point source as the distance is doubled the noise
  level decreases by 6dB(A).
• For mobile equipment the factors used are
  slightly different. These really are line
  sources. Also not only has the Sound Power Level
  of the equipment and the Distance to be taken
  into account but also the Speed of the machine
  and the Flow Rate. The equation used is then
• SPL SWL - 33 10 x log10(Flow rate)
• - 10 x Log10(Velocity) - 10 x Log10(Distance)
• Other Line noise sources ? -
• Conveyors - for a line source as the distance
  doubles, the noise level decreases by only
  3dB(A)

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Community Noise

• Example - The mine design engineer has planned a
  haul road that will be at its closest 160 metres
  from the nearest property. It is intended that
  the dump trucks will average 20 trips per hour at
  an average speed of 30 kilometres per hour. If
  the sound power level of the dump trucks is 105
  dB(A). Calculate the sound pressure Level at the
  property.
• SPL SWL - 33 10 x log10Flow rate - 10 x
  Log10Velocity - 10 x Log10Distance
• SPL 105 - 33 10 x Log20 - 10 x Log30- 10
  x Log160
• 48.2 dB(A)

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Noise Prediction With Barriers

• Department of Transport - Calculation of Road
  Traffic Noise
• Shadow Zone
• Below Barrier i.e. No line of sight from
  receiver to source
• Illuminated Zone
• Above Barrier i.e. Source just visible from
  receiver
• Calculate Travel Path Distances
• Direct Path Without Barrier
• Path via Top of Barrier
• Calculate Difference Between Travel Path
  Distances
• Obtain Barrier Correction from Graph in dB(A)

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Noise Prediction With Barriers
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Controlling Noise

• NOISE CONTROL
• Three ways - Source, Pathway Target
• Source - Enclosures for static plant. Efficient
  silencers for mobile plant
• Pathway - Baffle Banks, Acoustic Fences, Working
  Voids
• Target - Secondary Glazing, Acoustic Double
  Glazing
• Most sites concentrate on the source first, then
  the pathway and try not to get involved with
  Target solutions - WHY?
• Remember - Time of day is very important -
  Special attention to operations at night or
  weekends

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Controlling Noise

• NOISE CONTROL -At Source
• Noise monitoring of the plant is the essential
  first step to determine which equipment is the
  noisiest
• Replace - noisy plant or process with less noisy
  alternative
• Modify - existing plant by adding damping
  material or more efficient exhausts
• Enclose - noise enclosure can be simple (hay
  bale wall) or more substantial (designed acoustic
  enclosure)
• Orientate - equipment so that noise is directed
  away from sensitive locations
• Maintenance - routine maintenance to the correct
  standard will often prevent noise problems
  arising.

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Controlling Noise

• NOISE CONTROL - Pathway (Transmission)
• Second Best to control at source but often
  primary method used
• Increasing the distance - of the source from a
  sensitive location is often the most effective
  means of noise control
• Hide the Source - Rule of thumb - noise is
  reduced if the source can not be seen
• Baffle Bank - usually constructed from Top or
  Subsoil. They help to screen sensitive locations
  both acoustically and visually. Should be as
  close as possible to the target and extend as far
  as possible either side
• Acoustic Fencing - Can be expensive but highly
  effective means of noise control. Can be made out
  of timber or concrete panels

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Controlling Noise

• NOISE CONTROL - Pathway (Transmission) cont...
• Basic Mine or Quarry Planning
• Site layout and Working Method - use the planning
  of the method of working to minimise noise
  problems for sensitive targets
• Overburden Dumps - Build overburden dumps in
  stages (Lifts), always constructing the outside
  edge nearest the sensitive noise target first.
  Then fill in the bench level behind it.
• Working Void - Extremely effective method of
  noise control is to plan the orientation of the
  working voids so that the working face acts as a
  noise baffle

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Problem Areas

• POTENTIAL PROBLEM AREAS
• Most operations at surface mines or quarries are
  carried out below original ground level and are
  therefore naturally screen from sensitive
  properties. However some operations cannot be
  screened - what are they?
• Soil stripping - by Motorscrapers with Pushers -
  noise can only be minimised by well maintained
  equipment and baffle banks
• Baffle Bank - Construction/Removal - carried out
  by Motorscrapers can give extremely high noise
  levels - should be done after consultation with
  Minerals Planning Officer - dispensation
  allowable under MPG11 providing done at
  appropriate time of day
• Overburden Dumps - Big problem on opencast coal
  sites as dump trucks often have to negotiate
  steep ramps well above original ground levels -
  solution is better mine planning!

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Problem Areas

• POTENTIAL PROBLEM AREAS cont...
• Surface processing plant - often with crushers,
  screens etc. can give rise to noise nuisance.
  Best option is to locate within the pit.
  Processing equipment should also be enclosed
• Traffic - causes many problems especially if
  routes are through local villages. Many Mineral
  workings have designated authorised routes
• Transient/Impulsive - noises are particularly
  noticeable
• Dumping Rocks into trucks or ground hoppers
• Dragline Chains - rattle during night working
• Reversing Warning Devices - All mobile plant must
  be fitted with such devices to give warnings to
  mine operatives. However this can become a
  nuisance to local residents

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