Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS3306991 A
Publication typeGrant
Publication dateFeb 28, 1967
Filing dateJun 4, 1963
Priority dateJun 4, 1963
Publication numberUS 3306991 A, US 3306991A, US-A-3306991, US3306991 A, US3306991A
InventorsHomer J Wood
Original AssigneeHomer J Wood
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Protective hearing aid
US 3306991 A
Images(2)
Previous page
Next page
Description  (OCR text may contain errors)

Feb. 28, 1967 H. J. WOOD 3,3063% PROTECTIVE HEARING AID Filed June 4, 1963 2 Sheets-Sheet l INVENTOR HOMER J. WOOD mt dmmv Feb. 28, 1967 H. J. WOOD PROTECTIVE HEARING AID 2 Sheets-Sheet 2 Filed June 4, 1963 SOUND PQWERED MIC BATTERY R G E W F 7 U wmw L A 7 6 3 8 3 F\ P M E A? 3 P 6 3 HDMER J. WOOD INVENTOR.

ATTORNEY United States Patent 3,386,991 PROTECTIVE HEARING AID Homer 3. Wood, 14285 Valley Vista, Sherman Oaks, Calif. 91403 Filed June 4, 1963, Ser. No. 285,466 6 Claims. (Cl. 179-107) This invention relates to protective hearing aid apparatus and more particularly to apparatus for protecting the human ear from injurious noise, yet which will permit sounds of safe amplitudes to be heard between intermittent or unpredictable periods of excessive noise.

There are situations in industry, in the military services, and in commercial aviation, where persons must be exposed to sounds having levels in excess of those which may damage hearing. Under these conditions, damage to hearing can usually be avoided by having the individual exposed to the noise wear one or more ear protective devices. There are three general classes of ear protectors: (1) earplugs that fit into the ear canal; (2) carmuifs, covers or cushions that fit over the external ear and under which earplugs may be worn; and (3) rigid helmets that fit over the entire head and under which earplugs and earmuffs may be worn.

Investigations have shown that a great deal of deafness could have been avoided by proper protection from high noise levels. As a consequence, industry has recognized the need for protection against high intensity sound and it is common to provide employees required to work in areas of high ambient noise with the above-mentioned protective devices. The means involved heretofore have been of limited value; in particular, simple earplugs are of limited value because they do not inhibit sound transmission by bone conduction. In order to achieve adequate reductions in sound pressure levels it is necessary to have a good airtight seal between the ear protector and the surfaces of the ear or head against which it bears. Also, prior devices have inhibited normal hearing to the point that verbal communication is extremely difiicult if not impossible.

The amount of protection afforded by an ear protector or combination of protectors is measured in terms of the number of decibels it attenuates or reduces the intensity of sound that is transmitted through it. In general, the higher frequency components in a noise are attenuated more than are the lower-frequency components.

Ear protectors reduce the level of speech or other wanted signals to the same degree that they reduce the noise. In some situations, therefore, the reception and understandability of speech in the presence of other signals is adversely affected. To overcome this difiiculty various means have been attempted; for example, ground service personnel at airports, wearing ear covers or acoustical helmets, sometimes employ walkie-talkie radio transmitters for communication.

Hunters and especially skeet shooters, frequently suffer ear damage from gun blasts, but hearing impairment is often so gradual that it goes unrecognized until serious damage occurs and only recently has attention been given to it as a serious problem.

By the present invention, there is provided an acoustic barrier between the ear and the source of ambient noise and which has an electro-acoustical transmission system through which controlled amounts of sound may be transmitted to the ear. That is, the system includes means for limiting maximum excursion of the ampiltude of signals transmitted through the acoustic barrier. By this means, all sounds exceeding a given maximum amplitude may be attenuated to safe levels, whereas all sounds below the given amplitude level may be transmitted with negligible attenuation.

In a typical embodiment, the apparatus comprises a pair of protective ear covers having an electro-acoustic sound transmission system resembling, in general, a conventional hearing aid which will pick up external sounds via microphone located exteriorly of the ear covers and transmit the sounds to an earphone inside the ear covers. The dynamic range of the microphone-earphone system is limited so that only sounds below a given amplitude will reach the ear. Since the transmission characteristics of the apparatus are non-linear, low-level sounds will not be attenuated to any significant extent.

It is, therefore, a principal object of this invention to provide novel and improved means for protecting the ear against potentially harmful high-intensity noise yet which will not interfere with normal speech or other conimunication.

Another object of the invention is to provide novel and improved means for transmitting only those sounds through an acoustical barrier which do not exceed a given maximum intensity.

Another object of the invention is to provide novel and improved protective hearing aid having a microphone located exteriorly of an acoustic barrier for picking up ambient sounds and an earphone located at the ear within the barrier for reproducing the sound picked up by the microphone, and an amplifier therebetween for restricting the sound transmitted from the microphone to theearphone to levels below a given maximum intensity.

Still another object of the invention is to provide a novel and improved ear protector of the type to be worn over the external ear, and which may easily be moved into or out of place without the necessity of completely removing the apparatus from the head of the wearer.

Yet another object of the invention is to provide a novel and improved protective ear cover which may readily be formed or molded to conform to the particular head contour of an individual and thus provide a substantially air-tight seal between the ambient air and the ear.

Another object of the invention is the improvement of ear protectors, generally.

Other objects and features of the present invention will be readily apparent to those skilled in the art in the following specification and accompanying drawings wherein is illustrated the preferred form of the invention in which:

FIGURE 1 is a perspective view of the protective hearing aid of the invention showing the manner in which it is usually worn.

FIGURE 2 is a fragmentary front elevational view showing one half of the apparatus of the invention.

FIGURE 3 is a side elevational view of the apparatus of FIGURE 2.

FIGURE 4 is a top plan view of the apparatus of FIG- URES 2 and 3.

FIGURE 5 is a cross-sectional view apparatus of FIGURE 2.

FIGURE 6 is a fragmentary perspective view of the stilfening rib portion of the apparatus of FIGURE 5.

FIGURE 7 is a simplified block diagram of a first embodiment of an electro-acoustical transmission system which may be used in the invention.

FIGURE 8 is a simplified block diagram of a second embodiment of an electro-acoustical transmission system which may be used in the invention.

Referring now to FIGURE 1, the apapratus comprises an acoustical barrier in the form of a pair of ear covers 1 and 2 supported by headband 3. Ambient sounds are blocked by ear covers 1 and 2 so that they cannot reach the ears directly. However, microphone 4, carried within amplifier 5, wil pick up ambient sounds and transmit them, at controlled levels to an earphone located within the ear cover 1. It should be understood that either ear of 'a portion of the cover 1 or ear cover 2 may be provided with the microphone-amplifier-earphone combination, or if desired, both ear covers may be so provided and thereby permit binaural or directional hearing effects to be obtained.

In the preferred embodiment to be described hereinafter, the apparatus is the same for both ears and therefore is symmetrical about a vertical plane passing through the center of headband 3. For this reason, only one half of the device is shown in FIGURE 2, it being understood that the half not shown is substantially the same for the other ear.

Looking now at FIGURE 2, the apparatus comprises a headband 3, the undersurface of which is provided with a resilient cushion 6. Headband 3 is preferably fabricated from spring steel and is slidably attached to yoke 7 by means for a sliding telescopic joint including block member 8 and 'a plurality of rivets, two of which (9 and 10) are shown in FIGURE 2. This arrangement will permit the ear cover to be raised or lowered to lit the heads of various wearers. Yoke 7 is rigidly attached to slidable block member 8 and its lower end carries relief hinge 11 which pivot'ally supports bracket 12.

The lower end of headband 3 is attached to plate 15, the inwardly facing surface of which carries pads 16 and 17. Pads 16 and 17 are fabricated from 'any suitable resilient material, such as foam rubber or plastic, and may be adhesively bonded to plate 15. These pads (16-17) rest against the head of the wearer above the external ear and, together with cushion 6, support the device on the wearers head. Detent spring 13 is attached to yoke 7 via suitable fastening means. Detent spring 13 will engage the end of bracket 12 when yoke 7 is turned upward about the axis of relief hinge 11. This hinged arrangement permits the ear cover to be swung upward, as indicated by arrow 14, to the position indicated in dotted outline. Thus, the ear cover may be removed from the wearers ear while the remainder of the assembly continues to grip the head by means of the supporting pads 16-17. As can be seen in FIGURES 3 and 4, plate 15 and pads 16-17 extend rearwardly and downwardly to provide a relatively large surface 'area which bears against the wearers head. This arrangement makes the device more comfortable by distributing the weight of the device over a relatively large area and also serves to support the apparatus when the ear covers are turned upwardly or retracted from the ears. Ear cover 1 is supported from bail 18 which in turn is pivotally attached to yoke 7 by suitable fastening means such as rivet 19. Bail 18 is pivotally attached to bosses 21 and 22 (see FIGURE 3) on ear cover 1, by means of fasteners 23 and 24. The external shell portion of ear cover 1 may be fabricated from metal, Fiberglas, plastic or other materials capable of blocking the transmission of airborne sounds. The inwardly facing surface of ear cover 1 is provided with a cushion seal 25.

As stated hereinabove, to provide the necessary sound exclusion it is necessary to have an airtight seal between the ear cover and the head of the Weaver. Seals used in protective ear covers heretofore have not been altogether satisfactory. Cushion seal 25 of the present invention provides a comfortable yet eflicient seal between the ear cover 1 and the wearers head. Cushion seal 25 may be fabricated from foam rubber or plastic or other soft yieldable material. This se-al features a soft metal back-bone or stiffening rib 26 which is embedded as shown in FIGURES 5 and 6, and which permits the contour of the cushion seal 25 to be molded to fit the irregular contours of various wearers heads. That is, stiflening rib 26 is preferably fabricated from a relatively soft metal which can be readily bent or formed to establish the outline configuration of the cushion seal 25 within which it is embedded. Protective ear covers used heretofore represent compromises between the shell or ear cover profile and a flexible surface to fit 'a variety of head contours. Any deviations from the molded shape of the seal must therefore represent distortion of the sealing ring from its natural shape. In practice, it has been difficult with prior devices to provide sufficient flexibility in the seal without losing the ability to shield against blast and low frequency sounds. Also, a further disadvantage of prior devices is their inability to fit the contour deviation imposed by the frames of eyeglasses.

In the present invention, the cushion seal 25 is relatively soft and flexible yet will permit the basic seal shape to be modified by distorting the stiffening rib 26 which may be considered as a series of U-shaped tabs 27a, 27b, 270, etc. connected by metal strips 28a, 28b, 29a, 2912, etc. This rib arrangement provides the desired firmness of the overall seal, yet will allow small contour variations to be accommodated. Thus, the amount of elasticity in the seal required to fit regular contour deviations is minimized.

In the preferred embodiment the cushion seal behaves as if it were made of modeling clay, which is capable of resisting a substantial force after molding and would thereafter retain the molded shape. This cushion seal results in a substantially airtight juncture between the protective shell of the ear cover 1 and the head of the wearer with much less pressure on the wearers head than prior devices and contributes greatly to the wearers comfort.

A disadvantage of prior protective ear covers is that if the user desires to temporarily remove the ear covers, as in the temporary absence of high level ambient noise, the user will commonly pull the head set down around the neck; this results in discomfort and is otherwise inconvenient. The relief hinge feature described hereinabove, makes it possible to wear the apparatus comfortably in the protective position and yet very quickly uncover the ears when desired.

The interior of ear cover 1 is lined with a sound absorbent material 30 which may consist of foamed plastic, felt or other fibrous material, or other suitable sound absorbent substances.

Cushion seal 25 is provided with a central aperture of a size suflicient to encircle the outer ear. Cushion seal 25 is attached to ear cover 1 by means of an annular metal clip 26 which is partially embedded within the seal material and a portion of which extends around and engages a flanged portion of the periphery of ear cover 1, as shown in FIGURE 5. As will be apparent to those versed in the art, means other than clip 20 may be used to attach cushion seal 25 to ear cover 1. Sound absorbent material 30 has embedded therein an electro-acoustical transducer 31 which may be a miniature loudspeaker, an earphone, or the like.

The exterior of ear cover 1 is provided with a suitable mounting bracket 32 which is used to support the microphone 4 and amplifier 5. Leads 33 connect transducer 31 with amplifier 5. The amplifier housing includes microphone 4 which is used to pick up ambient sounds. The output of microphone 4 is amplified and transmitted to transducer 31 via leads 33. A combination switch and volume control 39 may be included in the amplifier circuit. A simplified block diagram of the electrical signal transmission system is shown in FIGURE 7. Microphone 4 connects to pre-amplifier 34 via lead 36; the outut of pre-amplifier 34 is supplied to limiting amplifier 35 via lead 36. The output of limiting amplifier 35 is supplied to transducer 31 via leads 33. Access door 41 is provided to permit replacement of the battery. Operating power for the apparatus is supplied from battery 37 via lead 38. Inasmuch as each of the functions of the units represented by a rectangle in the block diagram (FIGURE 7) may be any one of the numerous circuits for each respective function well-known in the art, it is deemed unnecessary to show circuit details. Preferably, the circuit embodies semiconductor elements in order to obtain the advantages of low-power consumption and very small size. It is believed that the description of the electro-acoustical signal transmission system is sufiicient for those versed in the art to practice it. However, reference may be made to Patent No. 2,327,321 for a detailed description of a suitable system which shows an automatic gain control in a hearing aid amplifier.

In this connection, it should be understood that practical limitations imposed on transducers such as earphone transducer 31 and/ or microphone 4 limit their dynamic range. This limitation in dynamic range will not make it possible to transmit sound pressure levels through the system which are of sufficient intensity to cause hearing impairment.

Two basic criteria have been obtained from research on hearing loss due to exposure to high intensity sound. One criterion applies to sounds that are pure tones or the sounds that contain most of their energy in narrow, so-called critical bands. Critical bands vary somewhat in width as a function of frequency but approximately 100 cycles per second wide for most of the audible range of sound frequencies. The second criterion applies to wideband noise, in which the energy is spread over an octave or several octave bands. The microphone-amplifier-earphone system ha practical limitations in transmitting either high-intensity narrow-band sounds or wideband sounds of any intensity. Based on industrial surveys, values have been determined for the maximum pressure level a sound can possess for both wideband noise and for pure tones or critical bands of noise, that a sound can possess if persons are not to suffer a possible hearing loss. These criteria are called damage risk criteria for hearing. While many persons can be exposed to greater intensities than the established damage risk criteria, without suffering any appreciable hearing loss, these values have been established as base for virtually all persons. In summary, it is the function of the electro-acoustical transmission system portion of the invention to attenuate sounds exceeding the established damage risk criteria for human hearing. The structural characteristics of the electro-acoustical transmission system may take a number of forms. For example, a microphone-amplifier-earphone system as described hereinabove may resemble a conventional hearing aid with the possible exception that the circuit may be provided with an automatic gain control or volume limiting feature. A circuit of the automatic gain control type continuously adjusts the gain or loss in the transmission system to restrict the transmitted sound energy to levels below the damage risk criteria. Circuits suitable to perform these functions are well known to those versed in the art.

Various modifications to the above described embodiments may be made. For example, the amplifier and battery may be omitted by using a sound-powered tele: phone type of system in which the microphone directly energizes the earphone. Since the area of the soundresponsive diaphragm in the microphone may be made larger than the area of the moving element in the earphone or the eardrum, transmission losses may be made negligible in a sound-powered system. Thus, it becomes feasible to construct a practical system without having a powered amplifier. There is shown in FIGURE 8 a simplified block diagram of a sound powered system comprising sound powered microphone 42 which is connected via line 43 to earphone 44. Limiting and/ or automatic gain control means may be readily inserted in line 43 of the sound powered system, if desired, as will be apparent to those skilled in the art.

In the foregoing description, reference has been made to an externally mounted microphone. It should be understood, however, that the microphone need only be responsive to ambient sounds or those sounds which originate exteriorly of the ear-enclosing barrier. Thus, the microphone may, if desired, be housed within the ear cover so as to have its sound responsive element or diaphragm exposed to ambient sounds through an aperture or port in the ear cover. Other modifications of the mounting arrangement of the microphone-earphone system will be apparent to those skilled in the art.

While preferred embodiments of the present invention have been specifically disclosed, it is to be understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpertation within the terms of the following claims.

What is claimed is:

1. A protective hearing aid comprising in combination:

a headband adapted to be Worn on the head;

a pair of sound excluding ear covers supported by said headband to enclose the outer ears of the wearer;

microphone means responsive to sounds originating exteriorly of said ear covers;

sound reproducing transducer means responsive to said microphone means to reproduce sounds picked up by said microphone means interiorly of at least one of said ear covers, a limiting amplifier to control said reproduced sounds within a finite dynamic range, the the upper limit of which is below the damage risk criteria for human hearing;

means on the external surface of one of said pair of ear covers for mounting said microphone means; and

means mounting said reproducing transducer means on the interior of said one ear cover.

2. A protective hearing aid as defined in claim 1 wherein said headband comprises:

a relatively fixed supporting member crossing over the top of the wearers head and a pair of relief hinges at either end of said member to which said ear covers are attached; and

detent means which, in conjunction with said relief gr ges, will permit said ear covers to be angularly positioned from a first position enclosing said outer ears to a second position in which said ear covers are retracted from said outer ears.

3. A protective hearing aid as defined in claim 1 where in said ear covers each include cushion seal means comprising:

a relatively soft, yieldable cushion, within which is embedded an annular stilfening rib adapted to be formed by external force to a realtively fixed shape which will establish the normal external contour configuration of said cushion.

4. A protective hearing aid comprising:

a headband adapted to be worn on and over the top of the head;

a pair of relief hinge means, one each of which is dependent from a corresponding end of said headband;

a sound-excluding ear cover pivotally supported by each of said hinge means and adapted to enclose the outer ears of the wearer;

a microphone responsive to sounds originating exteriorly of said ear covers;

bracket means secured to the external surface of one of said ear covers effective to mount said microphone above the horizontal center line of said one ear cover;

a sound reproducing transducer mounted within at least one of said ear covers;

a sound absorbent liner disposed adjacent the interior wall of said one ear cover for mounting said transducer; and

a limiting amplifier interconnecting said microphone and said reproducing transducer for transmitting 7 sound signals from said microphone to said transducer, said limiting amplifier having a maximum dynamic range which is less than the damage risk criteria for human hearing.

5. A protective hearing aid as defined in claim 4 including detent means adapted to selectively restrain said ear covers in a retracted position away from said outer ears.

6. A protective hearing aid as defined in claim 5 wherein said ear covers each include cushion seal means corm prising:

a relatively soft, yieldable annular ring within which is embedded an annular stiffening rib adapted to be selectively formed to establish the normal external contour configuration of said ring.

References Cited by the Examiner UNITED STATES PATENTS Adams-Randall 179156 Davison 179-156 Jansen et al. 179-156 Wadsworth et al. l79182 Wadsworth 179-107 X Dijkstra 179-107 X Beguin 179-182 KATHLEEN H. CLAFFY, Primary Examiner. WILLIAM C. COOPER, Examiner. S. J. BOR, Assistant Examiner.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US1087704 *Feb 19, 1913Feb 17, 1914Charles Adams-RandallHead-support for telephone-receivers.
US1182896 *Apr 27, 1915May 16, 1916Frank G DavisonHead-support for operators' instruments.
US2946860 *Jan 3, 1957Jul 26, 1960Rca CorpHeadset
US2946862 *Mar 21, 1955Jul 26, 1960David Clark Company IncEar protector and communication equipment
US3098121 *Sep 15, 1958Jul 16, 1963David Clark Company IncAutomatic sound control
US3124760 *Dec 12, 1960Mar 10, 1964 Transistor amplifies with variable
US3160717 *Mar 21, 1962Dec 8, 1964American Optical CorpEar protectors
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3461463 *Jun 9, 1967Aug 19, 1969American Optical CorpEar protector suspension devices and the combination with headgear
US3772478 *Dec 27, 1971Nov 13, 1973Instrument Systems CorpStethoscope headset
US3786519 *Nov 12, 1970Jan 22, 1974Gentex CorpHeadgear structure
US3890474 *Dec 26, 1973Jun 17, 1975Raymond C GlicksbergSound amplitude limiters
US3908200 *May 13, 1974Sep 30, 1975Gullfiber AbEar protecting device
US3919501 *May 21, 1974Nov 11, 1975Akg Akustische Kino GeraeteHeadphone construction
US3952158 *Aug 26, 1974Apr 20, 1976Kyle Gordon LEar protection and hearing device
US3968334 *Oct 10, 1974Jul 6, 1976Miguel PadillaAudiometric method and apparatus for testing the effectiveness of hearing protective devices
US4064362 *Sep 13, 1976Dec 20, 1977David Richard WilliamsHearing protector
US4066849 *Jun 11, 1976Jan 3, 1978Koss CorporationRotary-type control for headphone earpiece
US4224470 *Feb 26, 1979Sep 23, 1980Persson Leif A THeadset with limiter circuit
US4307385 *Nov 22, 1978Dec 22, 1981Sue Ann EvansNoise monitoring apparatus
US4315111 *May 29, 1980Feb 9, 1982Thomas Charles AHearing aid with remote momentary shut off switch
US4819270 *Jul 3, 1986Apr 4, 1989Leonard LombardoStereo dimensional recording method and microphone apparatus
US4928311 *Jul 1, 1987May 22, 1990Trompler Lyle DNoise limiting circuit for earmuffs
US5099519 *May 29, 1990Mar 24, 1992Yu GuanHeadphones
US5550923 *Sep 2, 1994Aug 27, 1996Minnesota Mining And Manufacturing CompanyDirectional ear device with adaptive bandwidth and gain control
US5764778 *Jun 7, 1995Jun 9, 1998Sensimetrics CorporationHearing aid headset having an array of microphones
US5790683 *Sep 16, 1997Aug 4, 1998Salzani; AugustoHeadphone including split earphones and coupling means for coupling it to the ears
US5987147 *Jul 29, 1998Nov 16, 1999Sony CorporationSound collector
US6406811Dec 23, 1999Jun 18, 20023M Innovative Properties CompanyBattery for headset assembly
US6754361 *Apr 17, 1997Jun 22, 20043M Innovative Properties CompanyErgonomic headset assembly
US6993292Feb 26, 2002Jan 31, 20063M Innovative Properties CompanySelf-monitoring radio network
US7103392Jan 15, 2002Sep 5, 20063M Innovative Properties CompanyWireless intercom system
US7120388Dec 16, 2002Oct 10, 20063M Innovative Properties CompanyWireless intercom system and method of communicating using wireless intercom system
US7292704Nov 26, 2003Nov 6, 2007Wayne LedererNoise attenuating headset
US7715799Oct 10, 2005May 11, 20103M Innovative Properties CompanySelf-monitoring radio network
US8130970Apr 26, 2006Mar 6, 20123M Innovative Properties CompanyEar cup
US8130985Jun 7, 2007Mar 6, 20123M Innovative Properties CompanyEar cup with bone conduction microphone
US8189801Apr 26, 2006May 29, 20123M Svenska AktiebolagEar cup
US8224011Apr 26, 2006Jul 17, 20123M Innovative Properties CompanyEar cup with microphone device
US8229740Sep 7, 2005Jul 24, 2012Sensear Pty Ltd.Apparatus and method for protecting hearing from noise while enhancing a sound signal of interest
US8243943 *Nov 23, 2004Aug 14, 20123M Svenska AktiebolagHearing protector with removable microphone, amplifier, and loudspeaker unit
US8559649 *Jun 24, 2002Oct 15, 2013Kurzweil Technologies, Inc.Sleep-aide device
US20100142718 *Dec 4, 2009Jun 10, 2010Sony Emcs ( Malaysia) Sdn. Bhd.Noise cancelling headphone
US20110011409 *Jul 17, 2009Jan 20, 2011Garrick AtkinsTitspamm
USRE43595Feb 28, 2011Aug 21, 2012Wayne LedererNoise attenuating headset
DE2813000A1 *Mar 21, 1978Oct 5, 1978Andreas PavelGuertel mit geraeten zum anhoeren von reproduzierter musik
DE2858302C2 *Mar 21, 1978Jan 31, 1991Andreas Mailand/Milano It PavelTitle not available
DE19743354A1 *Sep 30, 1997Apr 8, 1999Wolfram SpyraPortable, external noise recording and reproduction device e.g. for medical tests or speech therapy
DE19743354C2 *Sep 30, 1997Aug 5, 1999Wolfram SpyraTragbare Vorrichtung zur Aufnahme und Wiedergabe von Außengeräuschen
WO1987004065A1 *Feb 13, 1986Jul 16, 1987Lyle D TromplerNoise limiting circuit for earmuffs
WO2008122081A1 *Apr 4, 2008Oct 16, 2008Dyer JonathanHearing protection means
Classifications
U.S. Classification381/72, 381/371, 381/322
International ClassificationH04R25/00
Cooperative ClassificationA61F2011/145, A61F11/14
European ClassificationA61F11/14