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?