Keith M. Groth, CIH, CSP IHI Environmental March 7, 2012 Salt 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 B A X dBA A X dBA 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. Sound Pressure Level Near Field Far Field Conceptual Depiction of Noise “Master Equation” Reverberant Field Direct Field Critical Distance Log r Free Field Direct Noise (Air-Borne) Normally Found By Inspection, If Necessary Use Sub-Paths Reverberant Noise Measure To Find It, Location, Can Occupy Lower Frequencies Flanking Paths Can 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 Notional Free Body Diagram Receiver V Flanking Path R R R V Reverb Field R R V= Vibration Source V Receiver R R=Radiating Source 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?