keith m. groth, cih, csp ihi environmental march 7, 2012 salt lake city aiha local chapter
TRANSCRIPT
Keith M. Groth, CIH, CSPIHI Environmental
March 7, 2012Salt Lake City AIHA Local Chapter
Intro
Quick Review of Key Concepts
Understanding Matters
Where You Stand May Matter More
Where it Goes Matters Too
The Intent is Start the Process of Moving Beyond Ending Noise Assessments With, “For Noise Exposure Above 90 dBA OSHA Required Feasible Engineering Controls”
The Intent is Not Provide a Course in Acoustics Engineering.
Classical Approach is Employ Noise Control at: Source=>Path=>Receiver Sources: A Primary Source is Rarely Without
Secondary Sources Source Types: Vibrating Surfaces,
Compression, Combination Path: Contiguous volume of lowest sound
energy resistance between source and receiver. Usually “Paths” – One Usually Dominates for Each Source
Receiver: Fixed, Mobile, Task
Frequency Content Is Usually Important Can Help Identify Source of Concern
(Primary or Secondary) Determines Effective Noise Control Options
Noise Can Be Highly Directional
ROI and Doubling are Directly Related
Primary Noise Source Can Be Multi-Component
Motor, Articulating Members Cabinet, Mounts
Independent Versus Dependent
Secondary Sources Structure Borne Reverberant Noise Can Be a Significant Distance From Primary
What is Producing the Noise? Vibration
Rotating Source Turbulent Fluid/Vibrating Surface
Compression Rapid Air Movement/Change Blade Passage Frequency Overpressure/Blast
Evaluate Source SPL (Flat & A) & Octave Band
Sound Pressure Level Difference between 2 Sound Sources (dB)
Added Decibel to the Highest Sound Pressure
Level (dB)
0 3
1 2.5
2 2
3 2
4 1.5
5 1
6 1
7 1
8 0.5
9 0.5
10 0.5
>10 0
To add together more than 2 noise sources; start with the two largest. Combine the two largest and then third next. Keeping going until dB difference is greater than 10.
About 10 dB is the most you will ever add to the highest reading.
Not! X +3 dBA
X’ +3 dBA
X dBA
X dBA
AA
B
C
Measure Overall Noise at the Receiver(s) (A-Weighted, Octave Band or 1/3 Octave Band)
If Possible, Evaluate Sources Independently
Identify and Rank Order Sources (Usually Based on dBA)
Is Relative Source/Receiver Position Dynamic?
Which Noise Field is the Receiver In?
What Are the Noise Paths?
Are there Flanking Paths?
Near Field: Instantaneous pressure and velocity are not in phase. Normally occurs close to surface of radiating device.
Far Field: Instantaneous pressure and velocity are in phase. Typically starts far from source for low frequency and closer to source for high frequency.
Reverberant Field: Measured sound levels are dominated by reflected noise. Sound level is nearly constant with distance.
Near
Field
FarField
Direct Field
Critical Distance
Log r
Reverberant Field
Free Field
Sou
nd
Pre
ssu
re
Level
Conceptual Depiction of Noise “Master Equation”
Direct Noise (Air-Borne)Normally Found By Inspection, If Necessary Use Sub-Paths
Reverberant NoiseMeasure To Find It, Location, Can Occupy Lower
Frequencies
Flanking PathsCan be Hard To Find, Noise Intensity a Factor
Structure-Borne Typically Borne by Rigid Members, Frequency
Usually Different From the Source
Define The Problem Qualitatively Identify Source(s), Path(s),
Receivers(s) Free Body Diagram Evaluate and Rank the Sources Give Consideration to All Possible
Controls Select Combination of Controls for
Budget Apply Controls and Evaluate Results
V
ReverbField
Receiver
Receiver
V= Vibration Source R=Radiating Source
R
RR
R
R
RV
V
Notional Free Body Diagram
Flanking Path
Normally Will Provide the Most Benefit if Feasible
Lower Excitation Forces Alter Structure to Change Response to
Input Forces (Isolation, Dampening) Modification is most Practical in Design
(Newer Model or Retrofit Available)
Eliminate Path Start (Move Source, Move Receiver)
Alter Path to Reduce/Eliminate Energy Transfer to Receiver (Barriers/Walls, Enclosures)
Eliminated Secondary Sources By Path Elimination (Acoustical Treatment)
Enclose the Operator (Shields, Booths, Control Rooms)
Limit Transient/Collateral Exposures
DISCUSSION/QUESTION?