Aucun titre de diapositive

1

• Proposal for a new Damage Risk Criterion for
  Impulse Noise
• Dr. A.L. Dancer
• French-German Research Institute of Saint-Louis -
  France
• and
• 
  Continuous
• Noise
• all
  Military
• Noises

2

• How things stand at present?
• To evaluate the hearing hazard due to weapon
  noises, a number of criteria have been proposed
• - the first ones (CHABA, MIL-STD, Pfander,
  Smoorenburg)
• use the peak pressure, the duration(s) and the
  number of impulses
• measured in the free field
• close to the subjects ear

NATO AC/243 (Panel 8/RSG.6)D/9, Feb. 1987
- 3 dB/oct.
3

• How things stand at present?
• - the second ones
• (Atherley and Martin, DTAT)
• are based on the measurement
• of the A-weighted acoustic
• energy
• They aim to evaluate
• the amount of acoustic
• energy which enters
• the cochlea (dosimetry)
• limit LAeq8 85 dB
• (comparable to ISO-1999)

Rosowski, 1994
Price and Kalb
Howitzer noise
4

• The threshold-of-hearing curve indicates how the
  acoustic energy is transmitted from the free
  field to the inner ear as a function of frequency
  (Rosowski, 1997).
• The A-weighting curve is the standardized curve
  the closest to the threshold-of-hearing curve. It
  is implemented in all dosimeters.
• N.B. the ear (middle ear, inner ear) is supposed
  to behave in a linear
• way up to the highest levels...

Threshold
A-weighting
5

• How things stand at present?
• - the third one (Price and Kalb, APG) uses a
  mathematical model
• of the auditory periphery and evaluates, for
  each impulse,
• the number of Auditory Damage Units (A.D.U.)
• this model takes
• into account the
• non-linearities
• of the middle ear
• and of the inner ear
• at the highest levels

6
Advantages and Shortcomings of the classical
DRCs Advantages - the classical DRCs (CHABA,
MIL-STD, Pfander, Smoorenburg) have been in
use for 40 years
- one needs to know only the
pressure-
time history of the impulses and their

number...
pressure-time histories are often
not
so simple and may be very tricky to

measure and to interpret precisely!
(signature of an anti-tank weapon
at the
shooters ear)
7
Advantages and Shortcomings of the classical
DRCs Shortcomings these DRCs (CHABA, MIL-STD,
Pfander, Smoorenburg) have been developed for
unprotected ears exposed to small weapon noises
(Coles et al., 1968) 1) they overestimate the
hazard of the large weapons (same Dp)

• 4 x 1 x
  ??
• 4
  small weapon noises have
• 
  more energy at medium
• 
  and high frequencies
• 
  larger hazard
• 
  for the ear

dB
100 Hz
1000 Hz
8

• For a given peak pressure,
• a longer A-duration brings
• more energy only on the low
• frequency side of the spectrum
• More rounds (x N)
  increase energy at all frequencies

9
Field experiment 24 soldiers have been
exposed (unprotected) to 20 howitzer rounds
(A-duration 9 ms) and 20 rifle rounds
(A-duration 0.25 ms) at the same peak pressure
1.8 kPa (159 dB) almost no TTS following
howitzer noise exposure significant
TTS following rifle noise exposure
incorrect evaluation of the
hazard by the classical DRCs
Howitzer
Rifle
10
Advantages and Shortcomings of the classical
DRCs Shortcomings 2) difficulties to assess
the actual efficiency of the HPs
a) to shift the limits by x dB (Pfander
25 dB and MIL-STD 29 dB for a
single hearing protection and
35.5 for a double) whatever the HP is
? (earplug, earmuff, linearity)

(N.B. these DRCs have been
developed for unprotected ears
exposed to small weapon noises)
x dB?
11
b) to measure the parameters of the pressure
wave under the HP and to introduce them into the
DRCs ? Field experiment WALK-UP STUDY
(Albuquerque) free field
(186 dB peak, A-duration 4 ms
N100) untested! under the modified
muff (180 dB peak, A-duration 7
ms N100) no TTS under the
standard muff (174 dB peak,
A-duration 8 ms N100) no TTS
190
180
x dB?
25 dB
12
Advantages and Shortcomings of the classical
DRCs Shortcomings 3) how to assess the hazard
of combined exposures (continuous and
impulse noises)? 4) the occupational and the
military criteria do not follow the same rules
(Employers Liability)! ...
13
Some of these shortcomings were known from the
beginning. The classical DRCs were designed in
that way because the technical limitations in
the sixties.
J. Acoust. Soc. Am., 43, 336-343, 1968
14
Advantages and Shortcomings of the mathematical
model of Price and Kalb (APG) Advantages -
this model/criterion aims to take into account
the actual behavior of the middle ear and of
the inner ear when exposed to high-level
impulse noises (of any kind) (protected or
unprotected...) the nonlinearity of the
middle ear (annular ligament) limits
the transmission of the high peak impulses
the damage mechanism inside the inner ear
depends on the amplitude of the basilar
membrane displacements in a nonlinear way
15
- it can explain paradoxical experimental
observations more damage from short
impulses than from long impulses
(despite the fact that long impulses
contain as much acoustic energy at middle
and high frequencies - and much more at
low frequencies - than the short ones) -
it can explain the actual efficiency of the
earmuffs (standard or modified see the
Albuquerque WALK-UP STUDY) at the highest
levels (despite the fact that the peak pressure
attenuation is limited 6 to 12 dB only, and
that the acoustic energy measured under the HP
exceeds the 85 dB LAeq8 limit) - it can be used
as a tool to engineer the noise shape of the
new weapons to reduce the hazard for
hearing...
16
Advantages and Shortcomings of the mathematical
model of Price and Kalb (APG) Shortcomings -
the model needs to know the very exact
pressure-time history of the impulse at the
subjects ear to work properly (variability...,
difficulties to perform precise measurements for
each exposure) - the parameters of the model
still need to be adjusted to fit the human
ear and the model needs to be validated for a lot
of impulse noise exposure conditions - the
model requires a specific equipment and a
specific procedure - the model seems not
directly applicable to continuous noise and
combined exposures - nothing similar is used for
occupational exposure...
17
Advantages and Shortcomings of the LAeq8 method
Advantages Many physical measurements,
animal experiments, and human observations (see
NATO RSG 6 and 29) have shown that - the LAeq8
method with a limit of 85 dB allows a limitation
of the hearing hazard comparable to that aimed
at by the other criteria - the LAeq8 methods
allows the assessment of the hazard for all
kinds of weapon noises. It can be apply as well
to impulses in free field and/or in
reverberant conditions (either for small or for
large caliber weapons), as to combined
exposures (impulse and continuous noise), -
the LAeq8 method reconciles the occupational and
the military criteria the auditory hazard is
evaluated along the same rules in military and
in occupational exposures,
18
Human studies - 20 subjects equipped with E.A.R.
foam plugs are exposed to 20 howitzer rounds
(175 dB peak, A-duration 8 ms, global
free-field LAeq8 109 dB) IL of the plugs 30
dB (i.e., corresponding unprotected exposure
LAeq8 of 79 dB) No significant TTS - 11
subjects equipped with nonlinear plugs are
exposed to 10 howitzer rounds (175 dB peak,
A-duration 8 ms, global free-field LAeq8 106
dB) IL of the plugs at that level 20 dB
(i.e., corresponding unprotected exposure LAeq8
of 86 dB) No significant TTS
(AEARO)
GUNFENDER (RACAL)
19
Human studies - 16 subjects equipped with
nonlinear plugs are exposed to 7 mortar rounds
(185 dB peak, A-duration 2.5 ms, global
free-field LAeq8 110 dB) IL the plugs at that
level 30 dB (i.e., corresponding unprotected
exposure LAeq8 of 80 dB) No significant TTS -
14 soldiers equipped with nonlinear plugs are
exposed to 6 Friedlander waves (Johnson and
Patterson, Albuquerque) (190 dB peak,
A-duration 1.5 ms, global free-field LAeq8 114
dB) IL the plugs at that level 30 dB (i.e.,
corresponding unprotected exposure LAeq8 of 84
dB) No significant TTS (in all but one soldier)
Ultrafit nonlinear (ISL/AEARO)
20
Advantages and Shortcomings of the LAeq8 method
Advantages - the LAeq8 method does not lead
to a too large overprotection and to an
unjustified restriction of the use of the weapons
as it is the case for the classical DRCs, -
the LAeq8 method allows to evaluate the
hearing protection (earplugs or earmuffs) from
Insertion-Loss data obtained by
Real-Ear-At-Threshold (REAT) or
Artificial-Test-Fixtures (ATF) methods in a
more accurate (i.e., non linear HPs, ANR HPs)
and less conservative way than the classical
DRCs,
ISL ATF
21
Advantages and Shortcomings of the LAeq8 method
Advantages - the LAeq8 method can be used
with the help of the measuring equipment and
the measuring procedures which are presently
available in many military facilities and in many
companies (compatible with ISO-1999), - the
LAeq8 method can be adapted for immediate use at
the lowest command level (pre-determination of
the allowable dose for each representative
military exposure and adding).
22
Advantages and Shortcomings of the LAeq8 method
Shortcomings - because of the existence of a
"critical level" (Kryter and Garinther)
unprotected ears should not be exposed to a peak
pressure larger than 140-160 (??) dB whatever
the A-weighted energy of the impulse(s) is, -
for protected ears exposed to large weapon noises
of very high peak levels, the LAeq8 method
based on Insertion Loss measurements may
underestimate (by 5 to 20 dB) the actual
protection efficiency of the hearing protector
(earmuffs) ( use of the results of
specific experiments WALK-UP STUDY, use of
the Price and Kalb model)
23
CONCLUSIONS The use of the A-weighted isoenergy
method with a limit of 85 dB (LAeq8) is
recommended as a Damage Risk Criterion for all
weapon noises as well for protected as for
unprotected ears (NATO RSG 29) - for the
unprotected ear, the measurements must be made
with the microphone located at the position
normally occupied by the head of the subject,
- for the protected ear, the Insertion Loss
value corresponding to the hearing protection
will be subtracted from the measurement
performed at the head location, - the Insertion
Loss of the HPs will be measured in comparable
exposure conditions (i.e., by means of an ATF)
to take into account nonlinear and or ANR
properties of the HPs.