CA1168589A

CA1168589A - Hearing protectors

Info

Publication number: CA1168589A
Application number: CA000377426A
Authority: CA
Inventors: Anthony G. Gorman
Original assignee: Racal Acoustics Ltd
Current assignee: Racal Acoustics Ltd
Priority date: 1980-05-14
Filing date: 1981-05-12
Publication date: 1984-06-05
Also published as: DK215581A; NO811555L; NO150787C; SE8102931L; NO150787B; DE3119260A1

Abstract

ABSTRACT OF THE DISCLOSURE:
Hearing protectors having an earshell and ear cushion enclose a volume of air. A membrane in the earshell has a predetermined stiffness and mass so as to flatten the attenuation response of the protector from low frequencies (200Hz or less) up to frequencies important for understanding speech tat least 2kHz). Alternate or additional structures for flattening the response include further membranes and barriers dividing the enclosed volume of the shell with interconnecting tubes. An ear plug is also disclosed.

Description

~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 ,.. ,.~ .

1 1~8589 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 i~ 3 -., ~1~8589 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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1 1~i858~

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 _ 5 _ ~ 68~89 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, _ __ ~ 6 -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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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, _ g _ .

-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 : . i s~-;

1 168~89 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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1~685~9 --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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A hearing protector having an attenuation frequency response which is flat, within a range of about 10dB, from no higher than 200Hz to at least about 2kHz, and comprising 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 200Hz, high frequency transmission means fitted in the body arranged to permit higher frequencies above 200Hz to at least about 2kHz to be transmitted to the enclosed volume to provide said flat response.

2. A hearing protector as claimed in claim 1, wherein said flat response extends down to 100Hz.

3. A hearing protector as claimed in claim 2, wherein said flat response extends down to 60Hz.

4. A hearing protector as claimed in claim 1, wherein said high frequency transmission means 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 at frequencies below about 200Hz.

5. A hearing protector as claimed in claim 7, including two or more said diaphragms having different predetermined stiffnesses and masses to provide reduced attenuation over an increased spread of frequencies above 200Hz.

6. A hearing protector as claimed in claim 1, and including barrier means dividing said enclosed volume into a first part in direct communication with the eardrum of the wearer and a second part, tube means communicating between said 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.

7. A hearing protector as claimed in claim 1, wherein the body defines an earshell for an external hearing protector fitting around or against the ear of the wearer.

8. A hearing protector as claimed in claim 1, wherein the body defines an earplug fitting in the ear canal of the wearer.