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?