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HEARING  PROTECTION  OF  EARMUFFS  WORN 
OVER  EYEGLASSES 

Charles  W.  Nixon,  et  ai 

Aerospace  Medical  Research  Laboratory 
W  r igh t - Pa t te r s on  Air  Force  Base,  Ohio 

June  1974 


DISTRIBUTED  BY: 


National  Technical  Information  Service 
U.  S.  DEPARTMENT  OF  COMMERCE 

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SECURITY  CLASSIFICATION  OF  This  RASE  fW7 t.n  Dal.  Knitted) 

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BEFORE  COMPLETING  FORM 

1.  REPORT  NUMBER  U.  GOVT  ACCESSION  NO 

AMRL-TR-74-61  j 

i 

3.  recipient's  catalog  number 

A.  TITLE 

HEARING  PROTECTION  OF  EARMUFFS  WORN  OVER 
EYEGLASSES 

S.  TYPE  OF  REPORT  A  PERIOD  COVERED 

Final  report 

6.  performing  org.  REPORT  NUMBER 

?.  authorc; 

Charles  W.  Nixon,  Ph  D 

W.  C.  Knoblach,  SSgt,  USAF 

s.  CONTRACT  OR  grant  numbers; 

»  PERFORMING  ORGANIZATION  NAME  ANO  AODRESS 

Aerospace  Medical  Research  Laboratory, 

Aerospace  Medical  Division,  Air  Force  Systems 
Command,  Wright -Patterson  Air  Force  Base,  OH 

10.  PROGRAM  ELEMENT.  PROJECT.  TASK 

AREA  A  WORK  UNIT  NUMBERS 

62202F  723103T& 

1  >.  CONTROLLING  OFFICE  NAME  ANO  ADDRESS 

17.  REPORT  OATE 

Tune  1974 

IJ.  NUMBER  OF  PAGES 

3/ 

4.  MONITORING  AGENCY  NAME  a  AOOAESV"  dll lor on i  from  Controlling  Ol/lco)  IS.  SECURITY  CLASS,  fo/  thf  toport) 

Unclassified 

15..  OECLASSI  FICATi'on/OOWNGRADING 
SCHEDULE 


I*.  OlSTttmuTlON  STATEMENT  (•!  Ihlm  Rrpatt) 

Approved  for  public  release;  distribution  unlimited 


17.  DISTRIBUTION  STATEMENT  (of  tho  obmtroet  on  frog  In  Block  20,  II  dllloront  from  Rmport) 


IS.  KEY  W090S  fCwilinvt  on  roooroo  oldo  It  nocomoory  ond  Idonllty  by  block  number) 

Hearing  Protection  Sound  Protection 

Ear  Protectors  Personal  Protection 

Earmuffs  Attenuation  Devices 

Earmuff  Compatibility 


20.  ABSTRACT  (Contlnvo  on  roooroo  otdo  It  nocoooocy  ond  Idonllty  by  6/oeJr  numb  or) 

The  hearing  protection  ordinarily  provided  by  earmuffs  is  reduced  when  worn  by 
persons  who  also  wear  eyeglasses  because  sound  enters  the  device  throught  air 
leaks  around  the  eyeglass  temple  -  earmuff  cushion  interface.  This  study  ex¬ 
amined  the  acoustic  fit  of  different  earmuff  protectors  and  various  types  of  eye¬ 
glass  frames  found  in  a  population,  measured  the  loss  of  attenuation  due  to 
programmed  air  leaks  and  measured  differences  in  earmuff  protection  for  subjec 
subjects  while  wearing  and  not  wearing  eyeglasses.  Results  demonstrated  that 


DD  l  JAN  7)  1473  EDITION  OP  I  NOV  $5  IS  OBSOLETE^ 


[j  SECURITY  Cl  ASSlf  1C  AT, ON  0*  ThiS  PAGE  Potm  Fniotod) 


iCCuni  f  .  ■•.  ..‘.Oiiqw  or  TSI5  PAGEfniiw  Cm 

earmuffs  worn  over  eyeglasses  lose  from  1  dB  to  10  dB  of  attenuation  at  individ-1 
ual  frequencies.  The  amount  of  loss  is  related  to  type  of  earmuff,  type  of  eye-  j 
glasses  as  well  as  frequency  of  the  sound.  Two  remedial  approaches  were 
identified  as  (1)  authorizing  for  use  by  eyeglass  wearers  only  earmuffs  that 
demonstrate  by  test  satisfactory  sound  protection  with  eyeglasses  and  (2) 
the  use  of  an  insert  or  pad  at  the  eyeglass  temple  -  earmuff  cushion  interface 
to  minimize  and  eliminate  the  acoustic  leak.  j 


10/ 


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FOR  THE  COMMANDER 


sff* — — i «-  (£-  Vvw 

HENNlfiwE. 


VON  GIERKE 


Director 

Biodynamics  and  Bionics  Division 
Aerospace  Medical  Research  Laboratory 


I 


t 


AIR  FORCE/S«7tO/J  S«pt»mo«r  1»?4  -  100 


SUMMARY 


Laboratory  evidence,  both  of  a  physical  and  psychophysical  nature, 
substantiates  informal  subjective  reports  from  operational  situations 
that  attenuation  of  earmuffs  is  reduced  when  worn  over  eyeglasses. 

This  reduction  ranges  from  about  1  da  to  10  dB  at  individual  frequencies 
and  is  shown  to  be  associated  with  air  leaks  created  when  the  eyeplass 
frame  keeps  the  earcup  seal  away  from  the  side  of  the  head.  The  amount 
and  the  patterns  of  the  losses  vary  from  earmuff  to  earmuff  and  with  type 
of  eyeglass  temple.  The  greater  the  distance  of  the  earcups  away  from 
the  head,  the  greater  is  the  air  leak  and  subsequent  attenuation  loss. 

Within  limits ,  the  size  of  the  air  leak  corresponds  to  the  amount  of 
attenuation  loss  with  larger  leaks  showing  larger  losses .  Attenuation 
losses  were  greater  at  the  low  and  high  frequencies  than  at  the  middle 
frequencies . 

Well-trained  subjects  demonstrated  via  psychophysical  methods  that 
the  standard  AF  issue  zylonite  eyeglass  frame  does  contribute  to  losses 
of  attenuation  when  worn  with  standard  AF  circumaural  ear  protectors. 

The  amount  of  the  loss  varies  with  the  particular  earmuff  worn  and  to 
some  extent  with  the  type  of  eyeplass  bow,  as  well  as  with  variations 
in  the  configuration  of  the  wearer's  head.  In  addition,  the  nominal 
amount  of  loss  for  an  earmuff-eyeglass  combination  at  each  of  the  individual 
test  frequencies  can  be  specified,  on  the  average.  The  incidence  of  eye¬ 
glass  users  in  the  population  is  relatively  high  and  of  sufficient  proportion 
that  the  loss  of  sound  attenuation  is  considered  an  operational  problem. 

The  critical  issue  is  whether  eyeglass  wearers  experience  more  noise 
induced  hearing  loss  (with  earmuffs;  than  non-wearers  as  a  result  of 
the  reduced  protection.  An  investigation  of  hearing  levels  of  AF 


1 


personnel  who  wear  the  earmuff-eyeglass  combination  vs  those  who  do  not 
wear  eyeglasses  but  work  in  the  same  noise  environs  should  br  conducted. 

AF  Forms  1490,  Hearing  Conservation  Data  and  1491,  Reference  Audiogram 
already  contain  information  regarding  the  use  of  earmuffs  as  well  as 
eyeglasses  (including  safety  glasses).  Therefore,  these  data  should  be 
available  for  investigation  from  the  USAF  Hearing  Conservation  Data 
Registry,  Brooks  AFD as. 

In  view  of  the  eyeglasses-earmuf f  hearing  protection  problem  discussed 
herein,  some  remedial  action  seems  desirable.  Of  the  various  alternatives 
dicussed,  two  appear  to  be  most  workable:  (1)  evaluation  of  earmuff 
performance  with  eyeglasses  would  be  routinely  accomplished  when  earmuffs 
are  initially  evaluated  for  potential  AF  use.  Those  items  showing  little 
attenuation  loss  with  eyeglasses  would  be  identified  for  use  by  eyeglass 
wearers  while  those  showing  significant  loss  would  not  be  approved  for 
eyeglass  wearers.  (2)  Another  approach  would  be  to  provide  removable 
inserts  or  pads  to  be  used  at  the  temples  of  all  earmuff-eyeglass 
wearers  to  effectively  minimize  or  eliminate  the  leakage-protection 
problem.  A  short-term  applied  research  effort  could  identify  suitable 
materials  and  configurations,  their  relative  efficiency  in  terms  of 
increased  protection  and  provide  guidance  for  implementation  by  earmuff- 
eyeglass  wearers. 


PREFACE 


This  study  was  accomplished  by  the  Bioacoustics  Branch,  Biodynamics 
and  Bionics  Division  of  the  Aerospace  Medical  Research  Laboratory,  Wright 
Patterson  Air  Force  Base,  Ohio.  The  research  was  conducted  by  Charles 
W.  Nixon,  Ph.D.  and  SSgt  William  C.  Knoblach  under  Project  7231,  "Bio¬ 
mechanics  of  Aerospace  Operations,"  Task  723103,  "Biological  Acoustics 
in  Aerospace  Environments,"  and  Work  Unit  16  ,  "Auditory  Responses  to 
Acoustic  Energy  Experienced  in  Air  Force  Activities."  Acknowledgement 
is  made  to  Mr.  Jack  Kelly,  formerly  of  the  University  of  Dayton  Research 
Institute, and  Capt  David  Krantz  of  the  Bioacoustics  Branch  for  their 
support . 


3 


TABLE  OF  CONTENTS 


PAGE 


Summary  . .  1 

Preface  .  3 

Introduction . 4 

Purpose  ...  .  .........  .  b 

Approach  .  ...........  5 

Lyeglass-Earmuff  Interaction  .  6 

Programmed  Air  Leaks . 10 

Psychoacoustic  Tests  .  16 

Discussion . - . .  23 

References  .........  .  26 


4 


LIST  OF  TABLES 


Page  Wo. 

Mean  Distance  of  Eyeglass  Temples  From  Side  of  Head  8 

When  Worn  With  Four  Different  Earmuff  Protectors 

Mean  Difference  Scores  Between  Earmuff  Attenuation  When  21 
Worn  With  vs_  Without  Eyeglasses 

Differences  in  Attenuation  of  Earmuffs  Worn  With  and  22 
Without  Eyeglasses  (In  Decibels) 


LIST  OF  ILLUSTRATIONS 

Page  No. 

Physical  System  Used  to  Measure  Effect  of  Various  12 

Programmed  Air  Leaks  on  Earcup  Attenuation 

Loss  of  Earcup  Attenuation  Due  to  Programmed  Air  Leaks  14 
In  Four  Different  Levels  of  Ambient  Noise 

Variation  in  Amount  of  Attenuation  Loss  Due  to  Different 
Sizes  of  Air  Leaks  As  A  Function  of  Earcup  and  Test  ID 
Frequency 

Differences  in  Attenuation  l'J 

Average  Attenuation  of  Several  Earmuf f-Eyeglass  Combinations 
With  and  Without  A  Foam  Insert  (Pad)  At  The  Temple  2b 


INTRODUCTION 


United  States  A.ir  Force  noise  sources  comprise  some  of  the  most 
intense  acoustic  environments  in  existence.  These  environs  require  major 
ongoing  programs  of  noise  control  and  hearing  conservation  to  insure  that 
Air  Force  personnel  are  not  unnecessarily  exposed  to  noise  levels  exceeding 
the  limits  specified  in  Air  Force  Regulation  161-35,  Hazardous  Noise 
Exposure* 27  July  1973  (7).  When  noise  control  measures  for  maintaining 
exposures  to  within  limiting  values  are  not  feasible,  personal  hearing 
protective  devices  are  required.  A  variety  of  earmuff  type  and  insert 
earplug  type  protectors  are  provided  to  individuals  routinely  exposed  to 
intense  noise.  Currently,  AF  standard  earmuffs  are  distributed  in  the  field 
as  personal  equipment  items  and  AF  standard  earplugs  are  dispensed  as  medical 
service  items.  Both  types  of  sound  protectors  are  in  widespread  use 
throughout  the  world. 

Observations  over  the  years  and  informal  subjective  reports  from 
personnel  working  in  noise  environments  have  indicated  that  some  of  the 
types  of  earmuffs  in  use  do  not  appear  to  provide  adequate  sound  protection 
for  persons  wearing  eyeglasses.  Presumably  air  leaks  occur  at  the  points 
where  the  earmuffs  fit  over  the  eyeglass  temples’''  and  these  leaks  result 
in  reduced  sound  protection. 

A  number  of  different  earmuff  protectors  are  found  in  the  USAF 
inventory  (5).  Earmuff  protectors  are  procured  by  the  AF  in  large  quantities 
through  a  central  nurchasinp,  procedure.  Procurement  is  accomplished  on  a 
competitive  basis  which  involves  the  selection  of  one  specific  device  for 
purchase  from  a  gioup  of  items  which  are  qualified  as  technically  acceptable 
by  test  (6)  and  are  included  on  a  Qualified  Products  List  (QPL) .  Each 

’•The  terms  eyeglass  "temple"  and  eyeglass  "bow"  are  used  interchangeably. 


0 


central  procurement  selects  one  earmuff  from  the  QPL  for  acquisition  and 
placement  in  the  world-wide  inventory.  Subsequent  procurements  may  and 
do  select  different  items  from  the  same  list.  As  a  consequence  of  this 
method,  which  employs  a  list  of  qualified  products,  several  different 
earmuffs  are  now  in  use  in  AF  operational  situations.  The  evaluation  of 
earmuffs  for  consideration  of  their  inclusion  on  the  QPL  and  possible  use 
by  the  AF  does  not  include  tests  of  their  effectiveness  when  worn  over 
eyeglasses . 

PURPOSE 

The  purpose  of  this  investigation  was  to  evaluate  the  hearing  protection 
performance  of  earmuff  protectors,  some  of  which  are  presently  on  the  AF 
Qualified  Products  List ,  when  they  are  worn  by  persons  who  also  wear  eye¬ 
glasses.  This  effort  considered  if  problems  existed,  the  nature  of  the 
problems  and  recommendations  for  remedial  action  where  appropriate. 

APPROACH 

Decrements  in  the  amount  of  protection  provided  by  earmuffs  when  they 
are  worn  over  eyeglasses  may  be  a  function  of  the  inability  of  the  earcup 
cushion  to  fit  closely  around  the  eyeglass  frame  and  form  a  good  acoustic 
seal  against  the  head.  The  degree  to  which  this  acoustic  seal  is  or  is  not 
accomplished  and  the  extent  of  the  resulting  air  leak  determines  the 
reduction  in  protection  from  that  obtained  when  eyeglasses  are  not  worn. 
Since  earmuffs  differ  in  shape,  size,  material  flexibility,  and  the  like, 
from  manufacturer  to  manufacturer,  some  may  be  better  than  others  when 
worn  over  eyeglasses.  This  effort  was  carried  out  in  three  phases,  each 
of  which  was  directly  concerned  with  determining  the  compatibility  of 


7 


earmuffs  with  the  use  of  eyeglasses,  and  it  attempted  to  quantify  the 
amount  of  difference  between  attenuation  of  earmuffs  when  worn  with  and 
without  eyeglasses. 

The  first  phase  of  the  study  considered  the  relationship  of  a 
muff-type  protector  to  the  various  types  of  eyeglass  frames  found  in  a 
typical  population,  primarily  the  fit  or  seal  of  the  muffs  to  the  head. 

The  second  portion  was  concerned  with  physical  measures  of  loss  of 
attenuation  for  earmuffs  due  to  programmed  leaks  created  by  using  various 
sizes  of  hollow  tubing  inserted  under  the  earcup  cushion.  The  third  phase 
involved  measurements  of  the  actual  differences  in  attenuation  provided 
with  the  QPL  earmuffs  for  the  same  subjects  while  wearing  and  not  wearing 
eyeglasses . 

EYEGLASS-EARMUFF  INTERACTION 

Typically  an  earmuff  protector  in  use  encircles  the  pinna  and  the 
earcup  cushion  rests  against  that  area  of  the  head  immediately  surrounding 
the  ear  to  provide  an  acoustic  seal  against  the  outside  noise.  Maximum 
sound  protection  demands  that  a  good  acoustic  seal  be  accomplished  and 
maintained.  Ideally,  an  earmuff  cushion  should  fit  equally  the  individual 
who  wears  eyeglasses  as  well  as  it  does  those  who  do  not  wear  them. 
However,  observation  and  experience  suggest  that  eyeglasses  do  interfere 
with  the  proper  fit  and  seal  of  the  earmuff  cushion. 

Eyeglass  Temple  Disp lacement  of  Earmuff 

Earmuffs  rest  against  the  bows  of  the  user’s  eyeglasses  just  in 
front  of  the  pinna.  Some  types  of  bows  appear  to  "bend"  inward  under 
the  weight  or  tension  of  the  muffs  and  to  rest  against  the  sides  of 


8 


the  head,  while  others  hold  the  earmuff  seal  away  from  the  head  creating 
an  obvious  air  leak  that  is  visible  to  an  observer.  The  actual  displace¬ 
ment  or  distance  of  the  eyeglass  bows  from  the  head  of  each  subject  was 
measured  with  various  earmuffs  in  place  on  the  head  and  compared  to  the 
same  measurements  when  no  earmuff  was  worn. 

All  subjects  who  participated  in  the  measurement  survey  normally 
wore  eyeglasses  and  measurements  were  taken  with  their  own  personal 
eyeglasses  which  had  been  professionally  fitted  to  them  by  their  own 
physicians.  Consequently,  the  data  are  representative  of  the  types  of 
frames  and  the  kinds  of  fits  that  might  be  expected  in  typical  populations. 
All  measurements  were  taken  by  an  individual  with  training  and  experience 
in  the  fitting  of  eyeglasses. 

The  eyeglass  temple  displacement  with  and  without  earmuffs  was 
measured  on  more  than  100  volunteers,  both  left  and  right  ears,  and  the 
various  types  of  eyeglass  bows  observed  were  tabulated.  Approximately 
80%  of  the  bows  were  of  various  sizes  and  thicknesses  of  plastic,  about 
10%  were  metal  and  about  10%  of  thin  wire.  The  mean  displacement  values 
measured  on  these  individuals  are  shown  in  table  1. 

It  was  assumed,  prior  to  initiation  of  the  measurement  survey,  that 
placement  of  the  earmuff  over  the  eyeglass  temples  would  reduce  the 
distance  of  the  bows  from  the  sides  of  the  head.  Contrary  to  this  assump¬ 
tion,  it  was  observed  that  three  of  the  four  earmuffs  measured  with  eye¬ 
glasses  showed  bow  displacements  from  the  side  of  the  head  that  were 
greater  with  the  earmuff  than  when  no  earmuff  was  worn  (table  1).  It 
appeared,  on  inspection,  that  the  muff  may  have  exerted  pressure  on  that 
poition  of  the  eyeglass  bow  behind  the  pinna  in  such  a  way  that  the 


9 


TABLE  I 


MEAN  DISTANCE  OF  EYEGLASS  TEMPLES  FROM  SIDE  OT  HEAD 
WHEN  WORN  WITH  FOUR  DIFFERENT  EARMUFF  PROTECTORS 


EYEGLASSES 

EYEGLASSES 

EYEGLASSES 

EYEGLASSES 

EYEGLASSES 

WITHOUT 

WITHOUT 

WITHOUT 

WITHOUT 

WITHOUT 

EARMUFF 

EARMUFF  A 

EARMUFF  B 

EARMUFF  D 

EARMUFF  E 

RIGHT  SIDE 

6.33* 

6.11 

6.61 

6.49 

6.78 

LEFT  SIDE 

6.30 

5.67 

6.75 

6.50 

6.68 

^DISTANCE  IN  MILLIMETERS 


10 


forward  part  of  the  bow  "bulged  out"  at  the  temporal  area  of  the  head 
and  the  effectiveness  of  the  seal  around  the  bow  in  front  of  the  pinna 
was  reduced. 

The  exception  to  this  finding  was  demonstrated  by  Earmuff  A,  which 
was  the  only  device  for  which  the  measured  displacement  of  the  eyeglass 
bows  was  less  with  than  without  the  earmuff.  The  earcup  opening  for  this 
unit  is  quite  large  and  the  cushion  is  relatively  narrow.  This  configur¬ 
ation  appeared  to  allow  the  cushion  to  seal  against  the  head  behind  that 
portion  of  the  bow  which  extends  behind  the  ear  of  the  subject  instead 
of  resting  against  the  end  of  the  frame.  The  other  earmuff s  examined 
have  smaller  openings  and  wider  cushions  which  press  against  the  end  of 
the  frame.  On  this  basis,  device  A  would  be  expected  to  show  the  least 
amount  of  attenuation  decrement  of  the  muffs  examined  when  worn  with 
eyeglasses. 

Earcup  Cushion  Material 

Perhaps  the  most  common,  and  possibly  most  important  source  of  air 
leak  when  earmuffs  are  worn  with  eyeglasses,  is  the  degree  to  which  the 
material  of  the  earcup  cushion  fails  to  conform  to  or  around  the  eyeglass 
bow.  The  more  compressible  and  flexible  materials  are  better  able  to 
mold  or  form  themselves  around  the  temples  providing  a  more  effective 
seal  than  with  the  less  conforming  cushions.  This  characteristic  and 
its  relationship  to  attenuation  loss  is  clearly  demonstrated  in  a  report 
by  Webster  and  Rubin  (4)  which  examined  earmuff  protection  for  individuals 
wearing  eyeglasses  as  a  function  of  three  types  of  cushion  material  on 
the  earmuffs. 


11 


Si ze  of  Eyeglass  Temple 


Another  factor  which  contributes  to  loss  of  attenuation  due  to  air 
leaks,  which  is  not  independent  of  cushion  material,  is  the  physical 
thickness  or  size  of  the  eyeglass  bow.  Generally,  the  greater  the  thickness 
of  the  bow,  the  greater  is  the  possibility  of  loss  of  attenuation  due  to 
air  leaks.  Effects  of  military  issue  type  frames  are  reflected  in  the 
psychoacoustic  measurements  which  appear  later.  The  effects  of  thin  wire 
bows  would  ordinarily  be  expected  to  be  negligible  in  front  of  the  pinna, 
all  other  variables  excluded. 

The  amount  of  air  leak  and  attenuation  loss  appears  to  be  a  function 
of  various  combinations  of  at  least  the  three  factors  mentioned  above, 
the  displacement  of  the  temples  from  the  sides  of  the  head,  the  ability 
of  the  earcup  cushion  material  to  conform  around  the  temples,  and  the 
thickness  of  the  temples  or  bows.  In  addition,  the  shape  of  the  head 
of  the  wearer,  the  amount  of  headband  tension,  the  degrees  of  freedom 
of  the  headband  suspension,  and  the  like,  may  all  contribute  singly  or 
in  combination  to  a  reduction  in  acoustic  seal  and  attenuation  of  an 
earmuff  worn  over  eyeglasses.  The  earmuff  itself  would  appear  to  be 
the  most  controllable  factor  of  those  identified. 

PROGRAMMED  AIR  LEAKS 

The  necessity  of  obtaining  a  good  acoustic  seal  with  circumaural 
devices  to  insure  maximum  hearing  protection  is  demonstrated  by  a  series 
of  physical  measures  of  attenuation  of  earmuffs  for  which  simulated  air 
leaks  were  created.  A  flat  plate  system  for  measuring  sound  pressure 
levels  inside  an  earcup  was  assembled  and  calibrated  in  accordance  with 


12 


figure  1.  The  condenser  microphone  in  the  flat  plate  system  recorded 
the  amount  of  sound  pressure  present  inside  the  test  earcups .  Attenuation 
of  four  different  test  earcups  was  measured  first  without  an  air  leak 
and  then  again  with  simulated  air  leaks.  The  sizes  of  the  air  leaks  were 
determined  by  selecting  plastic  tubing  with  inside  diameters  ranging  from 
0.046  mm  to  0.233  mm.  The  plastic  tubing  (3/4"  lengths)  was  positioned 
between  the  flat  plate  and  the  earcup  cushion  for  the  measurements.  Soft, 
clay  was  used  to  seal  around  the  plastic  tubes  and  assure  that  the  only 
air  leak  was  through  and  not  around  the  tube.  Care  was  taken  to  assure 
that  the  tube  was  not  collapsed  by  the  weight  of  the  earcup  or  by  the 
clay  used  for  sealing  around  the  tubes.  A  constant  static  pressure  of 
1000  grams  was  applied  to  each  earcup  during  the  measurements. 

Earcup  performance  with  and  without  the  four  simulated  air  leaks 
was  measured  for  various  test  frequencies  at  four  different  intensity 
levels  of  broad  band  noise  exposure:  70  dB,  80  dB,  90  dB,  and  100  dB  SPL. 
Observation  of  the  data  reveals  that  the  amount  of  attenuation  loss 
due  to  air  leaks  is  reasonably  constant  with  ambient  level  for  the  range 
of  measurements  recorded  and  that  the  attenuation  is  generally  the  same 
at  100  dB  as  it  is  at  70  dB,  particularly  at  the  frequencies  most  affected 
by  leaks.  This  "constancy"  characteristic  permits  us  to  discuss  loss 
due  to  air  leaks  in  terms  of  amount  of  loss,  test  frequency,  and  particular 
earmuff  involved,  without  specifying  the  various  intensity  levels  (within 
the  range  investigated). 

Attenuation  losses  due  to  programmed  air  leaks  were  examined  for 
tubing  with  inner  diameters  covering  a  wide  range  of  sizes,  however, 


13 


AMPLIFIER 

r 

ATTENUATOR 

OSCILLATOR 

L_ 

NOISE  GENERATOR 

LOUDSPEAKER 
SOUND  SOURCE 


CONSTANT  STATIC 
PRESSURE 
(1000  grains)  „ 


EARMUFF  UNDER 
EST 


CONDENSER  MICROPHONE 
AND  PREAMPLIFIER 


PROGRAMMED 
^X^AIR  LEAK 


I 


FLAT  PLATE 
COUPLER 


AUTOMATIC 

FILTER 


AUTOMATIC 
GRAPHIC  RECORDER 


PHYSICAL  SYSTEM  USED  TO  MEASURE  EFFECT  OF  VARIOUS 
PROGRAMMED  AIR  LEAKS  ON  EARCUP  ATTENUATION 

FIGURE  1 


14 


major  effects  were  observed  primarily  in  the  range  between  the  0.046  mm 
and  0.233  mm  openings.  Attenuation  was  not  significantly  affected  by 
leaks  smaller  than  0.046  mm,  and  it  changed  little  for  those  leaks  greater 
than  0.233  which  were  examined.  Consequently,  all  subsequent  measurements 
were  taken  with  four  sizes  of  air  leaks  within  the  0.046  to  0.233  mm 
range. 

Loss  of  attenuation  due  to  two  of  the  simulated  air  leaks  for  an 
earmuff  in  various  levels  of  noise  is  summarized  in  figure  2.  These  data 
clearly  demonstrate  that  the  amount  of  loss  is  about  the  same  in  the 
range  of  ambient  levels  from  70  dB  to  100  dB.  It  is  also  observed  that 
as  the  size  of  the  air  leak  is  increased  from  0.133  to  0.233  the  amount 
of  attenuation  loss  also  increases,  as  expected.  The  amount  of  attenuation 
loss  is  frequency  dependent  with  the  greatest  losses  occurring  at  the  low 
frequency  end  of  the  scale  (125  Hz  and  250  Hz).  The  frequency  dependency 
is  also  directly  related  to  the  individual  earmuf f s ,  as  shown  in  figure  3. 
It  can  be  seen  that  a  specific  air  leak  caused  different  losses  at  the 
various  frequencies  as  well  as  different  losses  among  the  various  earmuffs. 
The  extent  of  this  variability  is  such  that  general  trends  or  rules  of 
thumb  describing  amounts  of  loss  as  a  function  of  air  leak  sizes  are  not 
readily  formulated.  The  exception  to  this  statement  is  that  very  small 
air  leaks  do,  in  fact,  cause  substantial  losses  in  the  low  frequency 
attenuation  performance  of  earmuff  devices.  Further,  that  different 
muffs  show  differing  amounts  of  attenuation  loss  for  the  same  air  leak. 
Therefore ,  air  leaks  introduced  when  earmuffs  are  worn  with  eyeglasses 
would  be  expected  to  have  different  effects  on  the  attenuation  depending 
upon  which  earmuff  is  worn. 


15 


ATTENUATION  LOSS  IN  DECIBELS 


LOSS  Of  EARCUP  ATTENUATION  DUE  TO  PROGRAMMED 
AIR  LEAKS  IN  FOUR  DIFFERENT  LEVELS  OF  AMBIENT  NOISE 

FIGURE  2 
16 


VARIATION  IN  AMOUNT  OF  ATTENUATION  LOSS  DUE  TO  DIFFERENT 
SIZES  OF  AIR  LEAKS  AS  A  FUNCTION  OF  EARCUP  AND  TEST  FREQUENCY 

FIGURE  3 


An  earmuff-eyeglasses  combination  with  a  small  air  leak  could  act 
as  a  helmholtz  resonator  at  particular  test  frequencies ,  producing  a 
sound  pressure  level  under  the  earcup  that  is  higher  than  the  level 
outside  the  earmuff.  The  "earcup-hollow  tube"  arrangement  used  in  the 
programmed  air  leak  measurements  constitutes  such  a  resonator.  The 
resonance  effects  could  not  be  seen  in  these  data  because  measurement 
were  taken  only  at  specific  test  frequencies.  A  continuously  changing 
or  sweeping  test  signal  moving  across  the  frequency  range  of  interest 
could  have  identified  the  resonant  peaks.  Although  this  study  did  not 
consider  resonance  effects  of  eartnuff-eyeglass  combinations,  it  is  pointed 
out  that  these  effects  are  encountered  in  use  and  generally  reduce  the 
effectiveness  of  the  protector  under  those  conditions. 

Physical  data  from  the  air  leak  measurements  are  not  sufficient  and 
variability  is  too  great  to  permit  formulation  of  a  simple  scheme  for 
predicting  these  effects.  Consequently,  measurements  of  the  actual 
attenuation  provided  by  earmuffs  worn  by  persons  with  and  without  eyeglasses 
was  the  next  logical  consideration  of  this  study. 

PSYCHOACOUSTIC  TESTS 

Five  circumaural  earmuff  protectors,  some  of  which  appear  on  the  Air 
Force  QPE  and  are  known  to  be  in  use  in  the  operational  situation,  were 
evaluated  when  worn  with  eyeglasses.  The  method  of  measuring  attenuation 
closely  followed  the  American  National  Standards  Institute  (ANSI)  Method 
for  the  measurements  of  Real  Ear  Attenuation  of  Ear  Protectors  at 
Threshold  (1)  which  is  described  in  detail  in  an  earlier  report  (5). 


18 


With  this  method  subjects  actually  wearing  the  sound  protectors  determine 
the  amount  of  protection  provided  in  a  specified  sound  field.  This  is,  in 
effect,  a  real  life  test  even  though  it  is  conducted  in  the  laboratory  at 
very  low  sound  pressure  levels . 

Since  the  primary  purpose  of  this  investigation  was  to  evaluate  a 
potential  AF  problem,  all  subjects  were  personally  fitted  with  standard 
AF  issue  eyeglasses  with  standard  zy Ionite  frames  but  with  no  lenses.  A  medical 
technician  with  training  and  experience  in  this  special  medical  area 
individually  fit  each  subject  with  the  appropriate  size  frames  using  a 
"spectacle-fitting  kit"  which  provides  a  basic  selection  of  sizes.  The 
technique,  method  and  purpose  of  this  exercise  were  coordinated  with  an 
ophthalmologist.  All  subjects  were  judged  to  be  provided  proper  frames  for 
their  head  shape  and  configuration.  It  is  understood  by  the  investigators 
that  the  fitting  of  eyeglasses  is  somewhat  influenced  by  the  individual 
lenses  required;  however,  for  the  purposes  of  this  evaluation  the  procedure 
employed  was  considered  appropriate  and  correct.  Standard  AF  frames  were 
used  in  the  evaluation  in  order  that  findings  might  be  related  to  the 
actual  operational  situation. 

Subjects  who  participated  in  this  phase  of  the  study  were  male 
university  students  with  normal  hearing  at  the  audiometric  test  frequencies 
of  from  125  Hz  to  8000  Hz.  Each  subject  participated  in  all  tests,  that 
is ,  he  wore  the  same  eyeglass  frames  with  each  of  the  five  earmuffs 
investigated.  Subjects,  using  the  psychophysical  method  of  adjustment  (3), 
determined  their  thresholds  of  hearing  under  three  separate  conditions, 

(1)  open-ear  (eyeglass  frames  with  no  muff),  (2)  wearing  an  earmuff  (no 
eyeglass  frames),  and  (3)  wearing  eyeglass  frames  and  an  earmuff. 


19 


Differences  in  the  threshold  of  hearing  between  the  open  ear  condition 
and  the  two  earmuff  conditions  are  described  as  the  attenuation  attributed 
to  the  muff  or  to  the  muff  and  eyeglasses  combination  worn  in  that  condition. 
The  differences  in  attenuation  between  the  earmuff  and  the  earmuff-plus- 
eyeglass  condition  is  described  as  the  attenuation  loss  due  to  eyeglasses. 

Differences  in  the  attenuation  of  the  selected  earmuffs  worn  with  and 
without  eyeglass  frames  are  summarized  in  figure  4.  The  amount  of  area 
between  the  curves  and  the  zero  lines  represents  the  average  amount  of 
attenuation  loss  or  reduction  experienced  by  that  particular  muff  when 
it  was  worn  over  AF  eyeglass  frames.  Several  observations  may  be  made  from 
these  data. 

First,  the  attenuation  reduction  is  frequency  selective.  All  devices 
reveal  greater  losses  of  attenuation  at  the  low  and  high  frequency  regions 
of  the  spectrum  than  at  the  mid- frequency  range.  Also,  minimum  and 
maximum  reduction  values  occur  at  different  test  frequencies  for  each  of 
the  different  devices  tested.  Clearly,  both  attenuation  and  loss  of 
attenuation  due  to  air  leak  are  directly  related  to  the  frequency  of  the 
test  signal. 

Second,  all  earmuffs  show  losses  in  attenuation  at  all  frequencies 
when  eyeglass  frames  ara  worn.  Further,  the  amount  of  loss  varies 
significantly  from  earmuff  to  earmuff,  confirming  that  reduction  in 
attenuation  is  a  function  of  the  individual  earmuff.  None  of  the  items 
showed  improved  protection  with  eyeglasses  at  any  test  frequency. 


20 


REDUCTION  IN  ATTENUATION  (dB)  WHEN  EYEGLASSES  ARE  WORN 


EARMUFF  A 


125  250  500  1000  2000  3000  4000  6000  8000 

FREQUENCY  (Hz) 


FIGURE  4 

DIFFERENCES  IN  ATTENUATION 


21 


Third,  the  earmuffs  can  be  categorized  or  ranked  in  terms  of  their 
susceptibility  to  loss  of  attenuation  when  worn  with  eyeglasses,  or 
conversely  stated,  in  terms  of  their  efficiency  when  used  with  eyeglasses. 
Figure  4  ranks  the  muffs  by  inspection  from  the  best  at  the  top  to  the 
poorest  item  at  the  bottom.  When  the  difference  values  are  actually 
ranked  and  summed,  the  order  of  the  numerical  values  for  the  last  two 
items  are  reversed  with  the  item  E  showing  the  greatest  loss  of 
attenuation,  in  terms  of  percentage  change  in  reduction  of  attenuation, 
earmuff  A  shows  8.3%  loss,  earmuff  B  16.2%,  earmuff  C  21.1%,  earmuff  D 
18 . rJ% ,  and  earmuff  E  21.6%.  Clearly,  item  A  is  best  re  percentage  change 
when  worn  with  eyeglasses;  i.e.,  it  shows  the  least  attenuation  loss,  and 
item  C  is  the  worst,  although  items  C  and  D  are  very  close  to  item  E. 

Forty-five  t-tests,  on  30  measures  each  of  the  differences  between 
attenuation  of  earmuffs  worn  with  and  without  eyeglasses,  are  summarized 
in  table  2.  Differences  which  were  not  statistically  significant  are 
underlined  and  indicate  that  essentially  the  same  attenuation  is  provided 
with  the  eyeglasses  as  without  them.  This  statistically  significant 
difference  amounted  to  about  2.5  db.  It  is  clear  that  earmuff  A  is  least 
affected  by  eyeglasses  and  that  earmuff  E  is  most  affected.  At  the  test 
frequency  of  2000  Hz,  no  significant  differences  between  attenuation  were 
found  for  any  of  the  devices. 

Data  on  differences  in  earmuff  attenuation  with  and  without  eyeglasses 
as  reported  by  Webster  and  Rubin  (4)  and  by  Fletcher  and  Loeb  (2)  are 
summarized  in  table  3.  Items  V,  W,  and  X  show  rather  large  differences. 
Item  II  is  the  earmuff  with  foam- latex  cushions  which  was  essentially 
unaffected  by  the  eyeglasses. 


22 


TABLE  2 


MEAN  DIFFERENCE  SCORES  BETWEEN  EARMUFF 
ATTENUATION  WHEN  WORN  WITH  vs  WITHOUT  EYEGLASSES 


EARMUFF 

EARMUFF 

EARMUFF 

EARMUFF 

EARMUFF 

A 

B 

C 

D 

E 

TEST 

FREQUENCY 

125 

3. 351'* 

2.71 

5.26 

7.05 

4.48 

250 

3.23 

4.63 

5. 36 

4.88 

4.56 

500 

3.17 

3. 12 

5.19 

4.76 

5.34 

1000 

1.13 

1.37 

3.97 

3.56 

5.33 

2000 

0.17 

0.02 

0.99 

0.45 

1.72 

3000 

1.25 

4.08 

1.06 

1.89 

3. 38 

4000 

0.15 

1.79 

6.49 

3.09 

2.74 

6000 

0.30 

3.57 

4.32 

8.71 

3.32 

8000 

4.97 

2.71 

2.45 

6.76 

5.44 

•'‘Entries  are  t-test  s  ores.  Mean  differences  in  excess  of  2.46  are  statistically  significant, 
underlined  scores  indicate  that  essentially  the  same  attenuation  was  provided  with  the  eyeglasses 
as  without  them. 


TABLE  3 


DIFFERENCES  IN  ATTENUATION  OF  LARMUFFS  WORN  WITH 
AND  WITHOUT  EYEGLASSES  (IN  DECIBELS) 


TEST 

FREQUENCY 

V>‘ 

W*‘ 

EARMUFF 

x** 

Y*>V 

z** 

125 

6 . 1  »■»*•'< 

3.1 

9 

6 

0 

250 

7.0 

7.5 

9 

7 

0 

500 

6.1 

6.7 

5 

4 

-1 

1000 

0.9 

4.6 

5 

2 

0 

2000 

8.7 

5.1 

0 

0 

-1 

3000 

7.2 

-1.0 

- 

- 

- 

4000 

11.8 

7.0 

5 

5 

0 

6000 

8.2 

11.4 

12 

5 

0 

8000 

4.7 

9.3 

- 

- 

- 

•’‘FLETCHER  AND  LOEB 
••‘•'■WEBSTER  AND  RUBIN 

•■'•'‘^POSITIVE  ENTRIES  INDICATE  AMOUNT  OF  ATTENUATION  LOSS  DUE  TO  THE  EYEGLASSES 


2k 


DISCUSSION 


It  is  the  opinion  of  the  investigators  that  the  state-of-the-art 
of  earmuff  design  is  sufficiently  advanced  that  the  loss  of  earmuff 
attenuation  when  worn  over  eyeglasses  is  a  technically  solvable  problem. 

The  performance  of  earmuff  A  over  that  of  earmuff  E  demonstrates  that 
better  comoat ability  is  already  achievable.  Webster  (4)  found  that  an 
earmuff  with  foam-laxtex  cushions  had  little  effect  on  attenuation 
while  vinyl  covered  cushions  resulted  in  the  usual  noticeable  low- 
frequency  loss  when  eyeglasses  are  worn.  He  suggested  that  a  piece  of 
foam-latex  or  similar  material  be  placed  under/over  the  eyeglass  temples 
to  form  a  more  effective  seal  than  would  be  obtained  otherwise. 

The  problem  of  earmuff-eyeglars  compatability  may  be  approached  in 
a  number  of  different  ways.  Une  alternative  is  to  provide  special 
earmuffs  (clearly  marked  >n  the  units)  for  persons  who  work  in  noise  and 
who  also  wear  eyeglasses.  This  would  require  a  separate  performance 
specification  and  evaluation  for  these  earmuffs.  Another  alternative 
is  to  apply  a  correction  factor  to  current  earmuff  performance  specifica¬ 
tions  to  account  for  the  attenuation  losses  due  to  air  leaks.  Unlike 
the  first  approach,  this  correction  factor(s)  would  be  determined  for 
each  earmuff  at  the  time  of  its  evaluation  for  potential  AF  use  and 
would  be  reflected  in  the  performance  data.  No  personnel,  other  than  the 
evaluators,  would  be  directly  involved.  This  method  would  adequately 
protect  tne  eyeglass  wearer  and  would  overprotect  the  non-wearer  of  eyeglasses. 

Eyeglass  wearers  could  be  provided  with  foam-latex  (or  similar 
material  with  configuration  to  ue  determined)  inserts  or  applicators  to 
jt  used  at  the  eyeglass  temple;;  to  i*  '.ove  the  acoustic  3eal  with  all 


earmuffs.  A  commercially  available  pad  designed  for  this  purpose  was 
evaluated  recently  in  our  laboratory.  Its  effectiveness  in  minimizing 
the  attenuation  loss  is  seen  as  increased  average  protection  at  all  test 
frequencies  in  figure  i>.  The  use  of  some  material  at  the  temple  may 
well  be  the  most  practical  approach  since  it  would  be  applicable  to  all 
items  already  in  use  operationally,  to  those  in  the  inventory  and  to 
those  procured  in  the  future.  The  relative  cost  would  be  expected  to 
be  small.  An  investigation  into  types  of  materials  and  of  appropriate 
configurations  would  precede  final  selection. 

Thin  wire  eyeglass  temples  which  rest  close  to  the  head  have 
essentially  no  effect  on  the  attenuation  of  earmuffs  worn  over  them. 

Some  personnel  in  the  field  have  removed  or  Btripped  the  plastic  off 
the  temples  of  AF  standard  eyeglass  frames  leaving  only  the  thin  metal 
strip  to  minimize  and  eliminate  air  leaks.  A  brief  examination  of  this 
approach  in  our  laboratory  confirmed  that  temple-stripping  does  improve 
attenuation.  Earmuffs  which  seal  poorly  over  unstripped  temples  show  the 
greatest  improvement  and  as  might  be  expected,  earmuffs  which  initially 
esel  well  show  little  improvement  when  worn  over  stripped  temples . 

Finally,  the  current  procedure  for  the  selection  of  earmuffs  for 
use  by  AF  personnel  in  noise  does  not  contain  provisions  for  eyeglass 
wearers  who  usually  receive  less  protection  than  is  indicated.  In  general, 
if  the  attenuation  values  of  earmuffs  already  on  the  QPL  were  corrected 
(reduced)  for  protection  lost  due  to  air  leaks  around  eyeglass  bows,  the 
resulting  values  would  not  be  expected  to  satisfy  the  performance 
requirements  in  the  earmuff  specification,  MIL-P-3826BB.  The  implication 
for  eyeglass-earmuff  wearers  in  noise  is  clear. 


26 


REFERENCES 


1.  American  National  Standards  Institute,  "Method  For  The  Measurement 
of  Real  Ear  Attenuation  at  Threshold."  Z24.22,  1957. 

2.  Fletcher,  J.  and  M.  Loeb,  "Evaluation  of  Willson  and  American 
Optical  Under  The  Helmet-Type  Ear  Protective  Devices."  USAMRL 
Letter  Report  £ 3,  Psychology  Division,  Ft.  Knox,  Kentucky,  1964. 

3.  Hirsh,  I.  J. ,  Measurement  of  Hearing,  McGraw-Hill  Book  Company,  1952. 

4.  Webster,  J.  C.  and  E.  R.  Rubin,  "Noise  Attenuation  of  Ear-Protective 
Devices,"  Sound,  Vol.  1,  No.  5,  September-October,  1962. 

6.  Nixon,  C.  W,,  D.  T.  Blackstock  and  R.  G.  Hansen,  "Performance  of 
Several  Ear  Protectors,"  WADC  Technical  Report  58-280,  May  1959. 

6.  Military  Specification,  Mil-P-38268B ,  Protector,  Ear,  Sound  PRU-1A/P, 
3  August  1964. 

7.  Air  Force  Regulation,  AFR  161-35,  Hazardous  Noise  Exposure,  27  July 
1973. 


28 


r'U.S. Government  Printing  Office:  1974  -  657-013/5!