Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~1~8589
The present invention is concerned ~ith hearing
protectors. ~learing protectors, also sometimes called ear
protectors or ear defenders, normally exhibit a non-uniform
characteristic of attenuation versus frequency. Using an
electric circuit analogue, the frequency response charac-
teristic of the attenuation of typical hearing protectors
approximates to that of an inductance-capacitance low-pass
filter, but with substantial insertion less at low fre-
quencies below a turnover frequency in the response. This
response characteristic arises from the physical form of
the normal hearing protector. Considering an external pro-
tector of the kind which fits around the ear of the wearer,
the protector usually comprises an earshell of substantially
rigid material enclosing a predetermined volume of air
exterior to the ear drum of the wearer. The shell has a
predetermined mass and is usually seated about the ear of
the wearer by means of an ear cushion. Using established
principles of electro-acoustic and electro-mechanical
analogies, it can be seen that the enclosed air volume
within the earshell is equivalent to a shunt capacitance,
whilst the mass of the earshell is equivalent to a series
inductance. The compliance of the ear cushion on the other
hand provides a series capacitance but of a relatively
large value so that its effect on the frequency response
characteristic of the attenuation is minimal except at
very low frequencies.
It can be seen, therefore, that the normal hear-
ing protector would have a frequency response characteristic
of attenuation with the attenuation increasing at the rate
3~ of 12 dB/octave. However, in practice, an upper limit to
attenuation is reached at higher frequencies because of
various flanking transmission paths. Normally, therefore,
a plateau of attenuation is reached at frequencies above,
say, lkHz. More particularly, for different forms and
types of prior art hearing protectoFs, it is normally found
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that -the attenuation in the frequency range between, say,
lkHz and 4kHz is greater than the attenuation at freguen-
cies below lkHz.
A problem with these prior art hearing protectors
is the difficulty of unders-tanding speech when wearing the
protectors. This difficulty arises partly because with prior
art hearing protectors typically the greatest attenuation
is at those frequencies which are mos-t important to speech
intelligibility, i.e. lkHz to 4kHz. On -the other hand, noise
in the environment at frequencies below lkHz typically in
the range 60 to 500 Hz is a-ttenuated least. There is a
well-known phenomenon known as upward masking whereby
relatively loud low frequency noise can mask rela-tively-
quiet higher frequency speech sounds. The result is that
speech is even less intelligible than would be expected
from considering the relative attenuation alone.
The resulting reduction in the intelligibility
of speech heard through hearing protectors is one important
- reason for the reluctance of people to wear hearing pro-
tectors.
According to the present invention, a hearing
protector is provided, which has an attenuation frequency
response which is flat, within a range of about 10ds, from
no higher than 200Hz to at least about 2kHz.
The hearing protector according to the invention
comprises a body arranged, when the protector is worn, to
enclose a volume of air exterior to the ear drum of the
wearer, said body having sufficient mass relative to the
enclosed volume, and/or the interconnection between the
body and the wearer being sufficiently stiff, such that
the protector has a predetermined attenuation at frequencies
about 200 Hz, and high frequency transmission means fitted
in the body arranged to permit higher frequencies above
200 Hz to at least about 2 kHz to be transmitted to the
enclosed volume to provide said flat response.
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With such a protector, the at-tenuation at the
frequencies important for understanding speech is not sub-
stanti.llly different from the a~tenuation at lower Erequen-
cies, so that the upward masking effect of low frequency
noise is reduced. Although the attenuation at the higher
Erequencies up -to at least 2 kHz may be less than with prior
art hearing protectors, satisfactory attenuation at these
frequencies is still achieved but wlth the great advantage
of improved speech intelllgibllity.
The hearing protector according to the invention
has an attenuation frequency response which is sufficiently
flat to reduce the upward masking effect of low frequency
nolse on the lntelligiblllty of speech when the protector
ls worn.
It will be appreciated that speech may be rendered
more intelligible with the hearing protector of the present
invention lf the frequency response of attenùation is flat
withln a 10 dB range up to the stated minlmum frequency
2kHz. Indeed, lf the environmental noise to be attenuated is
primarily hlgh frequency nolse, typlcally 3kHz and above,
then lt can be advantageous for the hearlng protector of
the present lnventlon to be formed with an increasing
attenuation above 2kHz. On the other hand, lf the environ-
mental nolse is mostly at low frequencies, i.e. 1 kHz and
below, then a useful Eurther lncrease ln the intelligibility
of percelved speech can be achieved if the substantially
flat, as defined, response ls extended to 3 kHz or more.
The attenuatlon frequency response of the protec-
tor must be flat, as deflned, down at least to a frequency
of 200 Hz in order to provide at least to tome degree the
stated advantages of this invention. However, desirably
the flat response may be extended to lower frequencies,
for example, 100 Hz, 60 Hz or less.
i In certain circumstances, the flat response of the
hearing protector may be extended above 2 kHz to as much as
8 kHz.
Preferably, the high frequency transmission means
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comprises a diaphragm in an opening through the body
between the outside environment and the enclosed volume,
the diaphragm having a predetermined stiffness-and mass
such as not substantially to reduce the attenuation of the
protector a-t frequencies below about 200 llz.
The protector may include two or more said dia-
phragms having different prede-termined stiffnesses and
masses to provide reduced attenuation over an increased
spread of frequencies above 200 Hz.
Alternatively, or as well, the hearing protector
may include barrier means dividiny said enclosed volume
into a first part in direct communication with the eardrum
of the wearer and a second part, tube means communicating
between sald first and second parts and defining a column
of air interconnecting said parts and having a predetermined
length and cross-section, the dimensions of said column of
air and the volume of said second part relative to said
first part being selected to reduce the effective size of
the enclosed volume of air at frequencies above 200 Hz so
as to reduce attenuation at such frequencies.
The present invention can be applied to the various
existing kinds of protectors, for example, earplugs, supra-
aural-type hearing protectors and circum-aural protectors,
and including hearing protectors attached to or built into
helmets or the like and protectors fitted with earphones
for communication purposes.
Thus, referring to the above examples of the in-
vention, the body may define an earshell for an external
hearing protector fitting around or against the ear of the
wearer. Alternatively, the body may define an earplug
fitting in the ear canal of -the weare-
The objects, advantages and other fea-tures of the
present invention will become more apparènt from the fol-
lowing non restrictive description of some preferred embo-
diments,made in connection with the accompanying drawings,wherein:
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Figure 1 is a typical frequency response charac-
teristic of a prior art hearing protector;
Figure 2 illustrates in cross-section a typical
form of prior ar-t hearing protector;
Figure 3 shows an equivalent electrical circuit
of the prior art hearing protector of Figure 2,
Figure 4 is a graphical representation of the
typical frequency response of attenuation of a hearing
protector embodying the present invention;
Figure 5 is a cross-sectional schematic diagram
illustrating one form of hearing protector embodying the
present invention;
Figure 5A is an analogous electrical circuit
equivalent to the protector of Figure 5;
Figures 6, 7 and 8 are cross~sectional schematic
diagrams of three further embodiments of the present inven-
tion; and
Figures 6A, 7A and 8A are corresponding analogous
electrical circuit diagrams equivalent to the embodiments
of Figures 6, 7 and 8, respectively.
The detailed shape and numerical values o~ the
frequency response as illustrated on Figure 1 for a prior
art hearing protector would vary for different forms and
types of prior art hearing protectors but, as stated above,
it is normally found that the attenuation in the frequency
range between, say, lkHz and 4kHz is greater than the
attenuation at frequencies below lkHz.
- The prior art hearing protector shown iII cross
section on Figure 2 has a rigid earshell 10 enclosing a
volume V of air. An ear cushion 11 enables the earshell 10
to be seated about the ear of the wearer. The head of the
wearer is illustrated in Figure 2 schematically at 12 with -
ear 13 and ear canal 14 leading to eardrum l5. The equiva-
lent electrical circuit of the prior art hearing protector
of Figure 2 is illustrated on Figure 3. The enclosed volu-
me V is equivalent to a shunt capacitance Cv. The mass of
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the earshell 10 is equi~alent to a series inductance LM
and the compliance of the ear cushion 11 is equivalent to
a series capacitance Cc.
As described above, the problem with the prior art
hearing protectors is the diEficulty of understanding speech
when wearing the protectors. This difficulty arises partly
because,as shown on Figure 1, the greatest a-ttenuation occurs
typically at those frequencies which are most important to
speech intelligibility, i.e. lkHz to 4kHz. As the noise in
the environment of the frequencies below lkHz typically in
the range 60 to 500 Hz is less attenuated, the above-
defined well-known ~<upward masking phenomenon occurs
resulting in that speech is even less intelligible than
would be expected from considering the relative attenuation
alone.
Referring to Figure 4, the attenuation of a hearing
protector embodying the present invention is shown to be
substantially flat, i.e. within a range lods about a low
frequency attenuation x at 100 Hz. The attenuation is
maintained within ~ 5d~ of the attenuation x up to a ~re-
quency of at least about 2 kHz. A-t higher frequencies, the
attenuation may increase as shown in Figure 4 and as normal
for hearing protectors due to the combined effect of the
mass of the earshell or body of the protector and the
enclosed volume of air.
In some arrangements it may be desirable to ex-
tend the f].at response region to higher frequencies than 2 kHz,
as indicated by the dotted line in Figure 4. Minor peaks or
troughs in the response over the substantially flat region
are of little consequence provided there is not a total-
variation of more than about lOdB. The frequency response
should be flat down to a frequency of no more than 200 Hz
- to obtain the advantages of -the present invention,
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flat re~non~ is no~nally e~en~d to l~r r~.Juenci~s, e.~.~lz or ~z.
Referrlng now to FIGURES 5 and 5~, an external or
circwn-aural ear protector is lllustrated in cross-sectlon.
~he ear protector has a body 21 definlng an earshell whlch
9 has a predetermlned mass. The protector iB provided with
an ear cushion 21 whlch allows the protector shell 20 to
flt against the head of the wearer around the ear providlng
- a satisfactory sound proof seal. A membrane or dlaphragm
22 1~ fitted ln an openlng 23 of the earshell 20.
10Referring to the analogous electrlc clrcult of
FIG~RE 5A, the mass of the earshell 20/t~e volul~ V of air
enclosed by the earshell and the compllance of the ear
cushlon 21 are represented by a serlc~s lncluctance Ls, a
shunt capacitance Cv and a furt~ler serles capacltance Cc
. 15 respectively. Thls cl.rcult asslmles that the earshell 20
ltself is substantlally rlgid so that, in the absence of
the di.aphragm 22, hlgh fr~quency no.l~e 1B gU~Stant1a1IY
excluded by the earshell from the enclosed voll~e and
lower frequencies are transmitted by bodlly vlbratlon of
the earshell 20 causin~ resilient deformation of the eàr
cushion 21.
~ The effect of the.dlaphra(Jm 22 ls to provide a s~cond
parallel path around the inductor LS and capacltor Cc.
The second path comprise~ a capacitor Cl ln series with an
inductance Ll. The capacltor C~ corr~sponds to the stiff-
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1168589
ness of the diaphragm 22. This sti.ffness, which is sub-
stant:i.ally less than that o.f the earshe],l 20 i9 ho~leVer
considerabl.y cJreater ( ty~ lly Lll.rec or fol,lr t:i~nes cJrerlter)than
the equivrl.lent stif-fness (Cc) proviclecl l.-y the ear c' ush,i,on 21. The
5 stiffness oE the cll~p'lr.a~TQ 22 and thus t,lle va'l.ue of the
equivalent capacitor Cl is cllosen so as to make tl~e irnpe~ance of the
second parallel path in the diac3ram of F~GURE 5A substantially
~Ixeater than that of the first path ( LS and Cc) at fre-
quenci.es below a selecte~l mini.mum frequency. This selected
1(3 minimum frequency normally corresponds to the low frequency
from which the hearin~ protector is desired to have a f lat
response. For example, the se].ected frequency may be 200 llz.
Then at frequencies below 200 ll~, the second parallel path,
and hence the dial?hra~m 22, have substantially no effect on
15 the attenuation provided by -the hearing pro-tector.
llowever, a t frequenc:ies ahove this minimum frequency,
the diaphragm 22 permits increased transmission of sound
into the enclosed air volwlle ancl thence a reduction in the
attenuation of the protector. rl'he reduction in attenuation
20 with frequency continues to frequencies at which the mass of
the diaphragm 22 becomes significan t. ThiS mass is repres-
ented by the inductor Ll al~cl has the effect of eventually
reducill~ the translnissi,on of higher frequency noi.se.
It can be seen there~ore that with proper selection of
25 the mass and stiffness of the didphra~rrl 22, the at~tenuation
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~1~8589
provided by the protec-tor of 1`ICU~E 5 can be reduced over
-a sel.ected frequency range, which for the present embodiment
may be Erom, say, ]50~1~ ul) to, say, 3kllz.
Referring now to FIGURES 6 and 6A, the hearingpro~ctor
,i].].ustratecl in FIGt)~r' 6; is si.l!~i,],ar to that of F'~GU~E 5 and
has tl-e same reference nu~erals for corresponding parts hu-t is
provided w,i-th a secondnlemb:Lane orc~a,3hrac~m 2~ fitted in a seoond
o~ening 25 of the earshe],l 20.The correspondi,nc3 analoyous electrical
circult of FIGUR~ 6~ il.],ustrates the effec-t of the second
mem~rane as a further parallel pa~ll around the inductor LS
and capacitor Cc and comprisin~ additional capacitor C2 and
induc-tor L2. The two-meml~ralles 22 and 2~ can have different
se].ectccl sti~fnesses arld ~nasses Lhere~y correspondin~ to
different values of capaci.tance and i.nductance in the
e].ectrical analogy. It call be seen therefore that the
' arrangement of FTGUR~ 6 call be used to extend the range of
frequencies over which -the attenuation of the protector is
maintained substant]a].ly I-la-t. For example, the diaphragm
2~ may provide the desirecl subst:antlally ~la-t response from
1501iz to 3~13z, and the di.aphra~lln 22 the flat response from
2 ]cllz u~ to, say, 8kllz.
FIGUR~ 7 i,llustrates a f~lr~her form of hearing
protector similar -to those of FIGUR~S 5 and 6. Again, the
same reference n~nera].s are usecl in l;lIGUR~ 7 for parts and
features corresponding -to those of FIG~R~S 5 and 6. 110wever,
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1 168589
i.n FIGUT~ 7 th-~ enclo~c-d volurnc of air within the earshell
20 ~ rs lividc~d in~() t wO l~artr; Vl Itl(l V2 ~ arrier 26. The
two p~rt volumes Vl and V~ are interconnected by means of a
tube 27 -throuyh the barri.er 26. Thé tube 27 is arranged to
deElne a column of ai.r 28 whlch has a predetermined length
and cross-section. Such a column of air has a predetermined
mass and providccl t.he dimens:ions o~ the tube 28 are less
than the wavelengths of the spectrum of sound energy under
considerat.ion, for the pre sure oF the vol~lme V2 of a:ir enrlosed by the
barrier 26 to be varied at the frequency of the sound, the
entire column of air 28 with its known mass must be vibrated
to and fro' along the tube 27. Thus as is known from
acoustic theory, the effective i.mpedance of the column of
air 28 interconnecting -the volume Vl and V2 rises with in-
creasincl frequency (as the column of air becomes moredifficult to move to and Ero'). r~ s reEerri.ng to the analogous
electrical circu.it in EI(;iJ~ 7~ it can be seen that the
barrier 26 and tube 27 have the effect of separating the
shunt capacitance Cv representing the entlre ~olume of air
enclosed by the earshell 20, into two capacitances Cvl and
Cv2. Cvl is representative of the volume Vl still in
direct contact with the eardr~ of the wearer of the
protector. Cv2 is represen-tative of the volume of air
enclosed by the barrier 26. Cv2 is in series with an
inductor LV2 which is representatlve of the column of air
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28. Thus, at ],ow frequencies the column of air 28 has
su~stant:Lal].y no effect so that the car protector of
FIGU~ 7 operates ln the same fashion as that of FIGURE 5.
However, at increasing frequencies, the effective enclosed
g volume of the earshell of FIGURE 7 is reduced by the
increaslng lmpedance of tlle column o~ air 28. T1,lis has the
efEect of reducing the attenuation of the ear protector at
higher frequencies. By proper selection of the volume V2
to be enclosed by the barrier 26 and the dimensions of the
colu~:n of air 28 as defined by the tube 27, a desired
flattening of the attenuation frequency response of the
:~ protector can be achieved in accordance with the present
invention. ~s illustrated in FIGUR~S 7 and 7A, both a
diaphragm 22 and barrier 26 with,-tube 27 may be provided,-
so that a desired flatteninc~ oE the response is producedby their combined effects.
Referring flnally to FIGURES 8 and 8A, FIGURE 8 shows
- . an enrplu-~ accordl.ncJ to tlle present lnven-ti.on. The earplug
- ~as a body 30 w11ic1l :Ls arranged to Flt in the ear canal of
the wearcr so as to occlude the ear canal and define an
enclosed volume of air between the pluy and tl~e eardrum of
the wearer. The body 30 oE the earp].ug has an annular
: : fIange 31 which presses resiliently against the walls of
the ear canal of the wearer providing a substan-tially ear-
'~ 25 tlght connection.
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Referri.ng to FIGUl~ 8A, the earplug of FIGURE 8 can
be represented by the same electrical conflguratlon of
shunt capacitance C and series induetanee L and C .
In thi~ ease, the inductance ~.cj is repr~sentatlve of the
ma~s of the body 30 of the.earplLIg, the eapaeitanee Cc is
representative of the compliance of the re~ilient flange
31 and the capaci-tanee Cv is representative of the volume.
oE air enelosed by the earplug.
In F'LGURE 8, a channe,l or opening 32 is provided
extendi.ng right through the earpl.ug but is closed by a
: diaphragm 33. The diaphraym 33 has an effect corresponding
- to t:he di.aphragm 22 o:E the ear protector of FIGURE 5.
By suitable selection of the resilience and the mass of
~le d:iaphragm 33, the attenua-tiorl provided by the earplug
can be substantial.ly flattellecl in accordance with the present
i.nvention. The stif~ness of the diaphragm 33 is represented
by a parallel connected capacitor C] in the diagram of
FIGUR~ 8~.
, Other forms and methods of flattening the respon'se of
~ hearing protectors i.n accorclance with the invention may be
achieved by different combinations of acoustical and or
~mechanical rner~ers.
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