CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present application is based on, claim priority to, and incorporates herein by reference in its entirety United States Provisional Patent Application No. 62/357,771, filed July 1, 2016, and entitled "Sound-fuse Earplugs for Preventing Blast Ear Injury."

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under Military Photomedicine grant (FA9550-11-1-0331) from Air Force Office of Scientific Research grant and Department of Defense. The government has certain rights in the invention.

BACKGROUND

[0003] The present disclosure relates generally to systems and methods for ear protection and, more specifically to hearing protection devices or earplugs to protect a user from sound-induced ear injuries.

[0004] Military personnel may experience substantial blast-induced injuries to the ear from improvised explosive devices, rocket propelled grenades, mortar rounds, etc. The most common injury is the rupture of the eardrum, which typically occurs at sound pressure levels (SPL's) above 180 dB (i.e., greater than 20,000 Pascal's). Once the eardrum is perforated, the subject's hearing ability is greatly compromised because sound coupling to ossicles is disrupted. High intensity sound conducted through an intact eardrum to the inner ear may cause acute and chronic damages to the hair cells in the cochlea. Identified by the Department of Veteran's Affairs as the second most common new disability among the U.S. forces, not only is the hearing loss a tactical risk that threatens combat effectiveness, it also deteriorates the future living quality of Soldiers and causes a significant financial cost.

[0005] Use of hearing protection devices may reduce the risk of hearing loss. However, passive damping earplugs tend to decrease the ability to determine the direction of a sound source and to hear common verbal communications. Some of these issues have been addressed by passive earplugs configured to allow low-level sounds to pass but attenuate or block impulsive sounds. A majority of these hearing protection devices, such as the Moldex BattlePlug® and 3M™ Combat Arms Earplug™, rely on a manual cap to switch between two modes of operations. This manual adjustment reduces the effectiveness especially due to the unpredicted nature of blasts in the field. Passive earplugs with nonlinear transmission filters, such as orifice filters and sound-induced movable plugs, have shown to provide some, but insufficient, level of protection and tend to suffer from unsatisfactory hearing qualities due to relatively large transmission loss and phase distortion leading to reduced awareness and directionality. Active electronic hearing protection devices also exist, providing dynamic range compression. However, these active devices require built-in power sources, are expensive, and do not reproduce all the qualities of natural sound waves.

BRIEF SUMMARY

[0006] The present disclosure provides systems and methods for hearing protection devices or earplugs to protect a user from sound-induced ear injuries including blast-induced hearing losses. In particular, the present disclosure provides systems and methods for a hearing protection device that may include a fuse arrangement that is configured to provide sound intensity-dependent transmission loss. The fuse arrangement may be operable in a low loss state where the hearing protection device transmits low-level sounds with low transmission loss and a high loss state where the hearing protection device provides a high transmission loss against strong blasts or loud sound waves. A transition between the low loss state and the high loss state may be achieved via reversible or irreversible changes in the structure or acoustic properties of the fuse arrangement, when the hearing protection device is exposed to impulsive sound at a predetermined intensity. This sound intensity- dependent transition between the low loss and high loss states may reactively prevent damages to the eardrum, middle ear, and inner ear of a user.

[0007] In one aspect, the present disclosure provides a hearing protection device that includes an acoustic blocker or damper having a canal extending therethrough, and a fuse arrangement. The fuse arrangement includes a first diaphragm, a second diaphragm, and a core arranged within the canal and acoustically coupling the first diaphragm to the second diaphragm. The fuse arrangement is reactively operable to transition from a low loss state with a first sound transmission loss to a high loss state with a second sound transmission loss in response a sound wave input to the first diaphragm with a predetermined threshold intensity, and wherein the first sound transmission loss is less than the second sound transmission loss. [0008] In another aspect, the present disclosure provides a hearing protection device including an acoustic blocker or damper having a canal extending therethrough, a first diaphragm, a second diaphragm, and a core arranged within the canal and acoustically coupling the first diaphragm to the second diaphragm. At least one of the acoustic coupling between the core and one of the first diaphragm and the second diaphragm and the core define a predetermined threshold intensity, and when a sound wave at the predetermined threshold intensity is input to the first diaphragm, a disruption is configured to form at the least one of the acoustic coupling between the core and one of the first diaphragm and the second diaphragm and the core to dampen transmission of the sound wave from the first diaphragm to the second diaphragm.

[0009] In another aspect, the present disclosure provides a hearing protection device including an acoustic blocker or damper having a first end, a second end, and a canal extending through the damper from the first end to the second end, and a fuse arrangement configured to transmit sound from the first end to the second end of the damper. The fuse arrangement is configured to provide variable sound transmission loss characteristics by reactively transitioning between a low loss state with a first transmission loss and a high loss state with a second, higher transmission loss in response to a sound wave input thereto with a predetermined threshold intensity.

[0010] The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The invention will be better understood and features, aspects and advantages other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such detailed description makes reference to the following drawings.

[0012] Fig. 1 is a graphical representation of a frequency response of a hearing protection device according to one aspect of the present disclosure.

[0013] Fig. 2 is a graphical representation of a sound intensity response curve of a reversible hearing protection device according to one aspect of the present disclosure. [0014] Fig. 3 is a graph representation of a sound intensity response curve of an irreversible hearing protection device according to one aspect of the present disclosure.

[0015] Fig. 4 is a schematic illustration of a hearing protection device according to one aspect of the present disclosure.

[0016] Fig. 5 is a side view of the hearing protection device of Fig. 4 in a low loss state.

[0017] Fig. 6 is a side view of the hearing protection device of Fig. 4 in a high loss state with a break occurring in a diaphragm of the hearing protection device.

[0018] Fig. 7 is a side view of the hearing protection device of Fig. 4 in a high loss state with a break occurring in a core of the hearing protection device.

[0019] Fig. 8 is schematic illustration of a reversible hearing protection device according to one aspect of the present disclosure.

[0020] Fig. 9 is a cross-sectional view of a core of the hearing protection device of Fig. 8 taken along line 9-9.

[0021] Fig. 10 is an enlarged view of a reversible core of the hearing protection device of Fig. 8 in a low loss state.

[0022] Fig. 11 is an enlarged view of a reversible core of the hearing protection device of Fig. 8 in a high loss state.

[0023] Fig. 12 illustrates a model used to calculate sound transmission through a diaphragm according to one aspect of the present disclosure.

[0024] Fig. 13 is a graph illustrating a dispersion curve of a bending flexural wave in a diaphragm according to one aspect of the present disclosure.

[0025] Fig. 14 illustrates a model of flexural vibration of a diaphragm according to one aspect of the present disclosure.

[0026] Fig. 15 illustrates a model of a hearing protection device according to one aspect of the present disclosure.

[0027] Fig. 16 is a graph of a calculated transmission loss as a function of a ratio of acoustic impedance (Z3/Z1) according to one aspect of the present disclosure.

[0028] Fig. 17 illustrates a prototype damper of a hearing protection device according to one aspect of the present disclosure.

[0029] Fig. 18 illustrates a process for inserting a core through a damper of a hearing protection device according to one aspect of the present disclosure. [0030] Fig. 19 illustrates a pair of fabricated polystyrene membranes used to form membranes of a hearing protection device according to one aspect of the present disclosure.

[0031] Fig. 20 illustrates a pair of fabricated polymethylmethacrylate (PMMA) used to form membranes of a hearing protection device according to one aspect of the present disclosure.

[0032] Fig. 21 is an enlarged view of an end of a prototype hearing protection device according to one aspect of the present disclosure.

[0033] Fig. 22 illustrates a plurality of prototype hearing protection devices according to one aspect of the present disclosure.

[0034] Fig. 23 is a graph illustrating measured output sound transmission levels through a prototype hearing protection device in low loss and high loss states for an input sound pressure level of 98 dB and a background noise level of 58 dB.

DETAILED DESCRIPTION

[0035] Before the present invention is described in further detail, it is to be understood that the invention is not limited to the particular configurations described. It is also to be understood that the terminology used herein is for the purpose of describing particular configurations only, and is not intended to be limiting. The scope of the present invention will be limited only by the claims. As used herein, the singular forms "a", "an", and "the" include plural configurations unless the context clearly dictates otherwise.

[0036] It should be apparent to those skilled in the art that many additional modifications beside those already described are possible without departing from the inventive concepts. In interpreting this disclosure, all terms should be interpreted in the broadest possible manner consistent with the context. Variations of the term "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, so the referenced elements, components, or steps may be combined with other elements, components, or steps that are not expressly referenced. Configurations referenced as "comprising" certain elements are also contemplated as "consisting essentially of and "consisting of those elements.

[0037] As will be described, the present disclosure provides an approach that may provide hearing protection against blast-induced hearing losses by reactively, or automatically, transitioning between a low loss state and a high loss state via reversible or irreversible changes in structure or acoustic properties. In general, a hearing protection device according to the present disclosure may be wearable by a user, similar to or within an earplug, to provide hearing protection against blast- induced hearing losses. The hearing protection device may include a fuse arrangement that is configured to provide variable sound transmission loss characteristics. For example, the fuse arrangement may be configured to operate in a low loss state with low sound transmission loss, and a high loss state with high sound transmission loss to protect against strong blasts or loud sound waves. A transition between the low loss state and the high loss state may occur reversibly or irreversibly in response to the occurrence of impulsive sound at a predetermined threshold intensity or sound pressure level.

[0038] Fig. 1 illustrates a frequency response curve for a hearing protection device according to the present disclosure. The hearing protection device may be reactively operable between a low loss state (illustrated by curve 100) and a high loss state (illustrated by curve 102). In the low loss state, the hearing protection device may be configured to transmit input sound therethrough with minimal energy loss. That is, the hearing protection device may operate with a first transmission loss in the low loss state and operate with a second transmission loss in the high loss state, where the first transmission loss is less than the second transmission loss. In some non-limiting examples, when the hearing protection device is in the low loss state, the sound transmission loss may be less than approximately 10 decibels (dB). In some non-limiting examples, when the hearing protection device is in the low loss state, the sound transmission loss may be less than approximately 8 dB. In some non-limiting examples, when the hearing protection device is in the low loss state, the sound transmission loss may be less than approximately 6 dB.

[0039] In the illustrated non-limiting example of Fig. 1, the transmission loss of the hearing protection device in the low loss state and the high loss state may be generally constant across the entire audible frequency range. However, in some non-limiting examples, the hearing protection device may define a variable transmission loss profile as a function of frequency in the low loss state and/or the high loss state. For example, in the low loss state and/or the high loss state, the hearing protection device may be configured to define a higher transmission loss at some frequencies and define a lower transmission loss at other frequencies.

[0040] In the high loss state, the sound transmission loss may be greater than the low loss state to attenuate input sound waves. The transmission loss or attenuation in the high loss state may be designed to be as high as possible to provide protection to the inner ear, the tympanic membrane and the middle ear of a user wearing the hearing protection device. In some non-limiting examples, when the hearing protection device is in the high loss state, the sound transmission loss may be greater than approximately 20 dB. In some non-limiting examples, when the hearing protection device is in the high loss state, the sound transmission loss may be greater than approximately 30 dB. In some non-limiting examples, when the hearing protection device is in the high loss state, the sound transmission loss may be greater than approximately 40 dB. In some non -limiting examples, when the hearing protection device is in the high loss state, the sound transmission loss may be greater than approximately 50 dB.

[0041] A transition between the low loss state and the high loss state may occur reactively, or automatically, when the hearing protection device is disrupted by input sound at a predetermined intensity or sound pressure level. In some non-limiting examples, the transition between the low loss state and the high loss state may be irreversible. That is, the disruption caused by input sound at a predetermined intensity may, for example, impart permanent physical damage to at least a portion of the hearing protection device, which dampens the sound transmission thereof causing the increased sound transmission loss. In these non-limiting examples, the hearing protection device may transition from the low loss state to the high loss state once and may be disposable.

[0042] In some non-limiting examples, the transition between the low loss state and the high loss state may be reversible. That is, the disruption caused by the input sound at a predetermined intensity may, for example, temporarily physically alter the hearing protection device to increase the sound transmission loss. In these non-limiting examples, the hearing protection device may be continually transitioned between the low loss state and the high loss state and be reusable.

[0043] Fig. 2 illustrates one non-limiting example of a nonlinear sound transmission loss profile as a function of sound pressure level for a hearing protection device according to the present disclosure. In the low loss state (illustrated by section 200), the hearing protection device may be configured to define a low transmission loss, or acoustic attenuation, over a predetermined range of sound pressure levels. In the high loss state (illustrated by section 202), the hearing protection device may be configured to define a higher transmission loss over a predetermined range of sound pressure levels, which are greater than the sound pressure levels of the low loss state. The transition from the low loss state to the high loss state may be gradual, for example, occurring from over a range of sound pressure levels. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 130 dB to a sound pressure level of approximately 140 dB. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 120 dB to a sound pressure level of approximately 140 dB. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 110 dB to a sound pressure level of approximately 140 dB. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 130 dB to a sound pressure level of approximately 150 dB. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 120 dB to a sound pressure level of approximately 150 dB. In some non-limiting examples, a gradual transition may occur from a sound pressure level of approximately 110 dB to a sound pressure level of approximately 150 dB.

[0044] In some non-limiting examples, a sharp, or step change, transition at a predetermined, or threshold, sound pressure level. In some non-limiting examples, the threshold sound pressure level may be at approximately 1 10 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 1 15 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 120 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 125 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 130 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 135 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 140 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 145 dB. In some non-limiting examples, the threshold sound pressure level may be at approximately 150 dB.

[0045] In the illustrated non-limiting example of Fig. 2, the hearing protection device may be configured to define a reversible transition between the low loss state and the high loss state. Alternatively, in the illustrated non-limiting example of Fig. 3, the hearing protection device may be configured to define an irreversible transition between the low loss state (illustrated by section 300) and the high loss state (illustrated by section 302). As illustrated in Fig. 3, once the hearing protection device is exposed to input sound at the predetermined threshold, the hearing protection device may transition to the high loss state and remain in the high loss state for all ranges of input sound pressure levels. Similar to the reversible transition, the irreversible transition may define a gradual increase in transmission loss, a step change increase in transmission loss, or another profile between the low loss state and the high loss state.

[0046] The design difference between a reversible transition and an irreversible transition between the low loss and high loss states may be governed by the application in which the hearing protection device is implemented. For example, in applications where non-disrupted ability to hear normal sounds is important, a reversible transition may be preferred over an irreversible transition. Conversely, in some applications, a disposable hearing protection device that utilizes a permanent irreversible transition may be desirable. For example, once permanently in the high loss state, the intensity response is linear and a user may perceive the situation more intuitively and naturally without the effects of a nonlinear intensity distortion.

[0047] Figs. 4 illustrates one non-limiting example of a hearing protection device 400 according to one aspect of the present disclosure. The hearing protection device 400 comprises a fuse arrangement 402 and an acoustic blocker or damper 404. The fuse arrangement 402 comprises a first diaphragm 406, a second diaphragm 408, and a rod or core 410 that acoustically couples the first diaphragm 406 and the second diaphragm 408. The first diaphragm 406 is arranged adjacent to a first end of the damper 404 and the second diaphragm 408 is arranged adjacent to a second end of the damper 404 opposite the first end. In some non-limiting examples, a coupling between the first and second diaphragms 406 and 408 and the acoustic blocker 404 may be a physical contact. In some non-limiting examples, a coupling between the first and second diaphragms 406 and 408 and the acoustic blocker 404 may be an acoustic coupling through an air gap in therebetween. In some non-limiting examples, a coupling between the first and second diaphragms 406 and 408 and the acoustic blocker 404 may be simply nominal without substantial mechanical or acoustic connection. The fuse arrangement 402 is configured to acoustically transmit input sound waves from the first diaphragm 406 along the core 410, and output sound waves at the second diaphragm 408.

[0048] In some non-limiting examples, the first diaphragm 406 and the second diaphragm 408 may be fabricated from a plastic, or polymeric, material (e.g., polystyrene, PMMA, PTFE, etc.). In the illustrated non-limiting example, the first diaphragm 406 and the second diaphragm 408 are in the form of a round disk. In some non-limiting examples, the core 410 may be fabricated from a plastic, or polymeric, material (e.g., polystyrene, etc.). In some non-limiting examples, the core 410 may be fabricated from an optical fiber material (e.g., silica, a polymer, etc.). In the illustrated non- limiting example, the core 410 is in the form of a cylindrical rod connected at opposing ends to one of the first diaphragm 406 and the second diaphragm 408.

[0049] The acoustic blocker or damper 404 is arranged between the first diaphragm 406 and the second diaphragm 408 and is configured to prevent acoustic cross-talk therebetween by reflection (blocker) or absorption (damper) of sound waves. In the illustrated non-limiting example, the damper 404 defines a generally cylindrical shape and includes a bore, or canal, 412 extending axially therethrough. The canal 412 may define a canal diameter D _c that is dimensioned to receive the core 410. In some non-limiting examples, the canal diameter D _c may be dimensioned to accommodate the core 410 therethrough, but a gap between the core 410 and the canal 412 may be minimized to prevent damping of the acoustic sound waves propagating along the core 410. In some non-limiting examples, the damper 404 may be fabricated from an acoustically absorbing porous polymeric, or foam, material (e.g., polyurethane foam, etc.). Alternatively, the acoustic blocker 404 may be fabricated from a reflecting metal, ceramic, or high-density solid material with large acoustic impedance so that it reflects sound waves coming through the air and, therefore, causes high acoustic transmission loss without the core 410.

[0050] In some non-limiting examples, the hearing protection device 400 may be configured to be wearable by a user and, in particular, to be placed at least partially into an ear canal of a user (in application a user may place one hearing protection device 400 in each ear). In some non-limiting examples, the hearing protection device 400 may be configured to be arranged within a commercial earplug to facilitate the placement within a user's ear canal. An outer diameter D ₀ of the damper 404 may be dimensioned to ensure that a user may at least partially arranged the hearing protection device within their ear canal. For example, the damper 404 may define an outer diameter D ₀ of less than 10 millimeters (mm). It should be appreciated that the design of hearing protection device 400 may be tailored to fit a variety of different ear canal geometries and/or commercial earplug designs. The diameter defined by the first and second diaphragms 406 and 408 may be less than or equal to the outer diameter D ₀ of the damper 404.

[0051] In operation, for example, a user may arrange the hearing protection device 400 at least partially into their ear canal such that the first diaphragm 406 is in acoustic communication with the user's surrounding environment and the second diaphragm 408 is in acoustic communication with the user's ear drum. Sound waves from the user's environment may be input to the first diaphragm 406 and transferred to the core 410 due to the mechanical coupling therebetween. The input sound waves may propagate along the core 410 until they reach the second diaphragm 408 where the mechanical coupling therebetween converts the mechanical energy of the core 410 into an output sound wave and finally toward the user's ear drum. As will be described, the design and properties of the fuse arrangement 402 enables the hearing protection device 400 to be reactively operable in a low loss mode (see Fig. 5) where the input sound waves are transmitted to the user as efficiently as possible (i.e., with a minimized transmission loss or damping), and a high loss mode (see Figs. 6 and 7) where the fuse arrangement 402 is configured to mechanically disrupt the input sound waves from transmitting to the user. As described herein, the hearing protection device may be operable with a first sound transmission loss in the low loss state and a second, higher sound transmission loss in the high loss state.

[0052] Fig. 5 illustrates one non-limiting example of the hearing protection device 400 in the low loss state. In the low loss state, the acoustic coupling between the first diaphragm 406, the core 410 and the second diaphragm 408 may be uninterrupted. That is, the mechanical coupling between the core 410 and the first and second diaphragms 406 and 408 may be in-tact and the core 410 itself may be uninterrupted, or continuous. The fuse arrangement 402 may be configured to reactively, or automatically, transition the hearing protection device 400 between the low loss state and the high loss state in response to an input sound wave at a predetermined threshold intensity. The design and properties of the fuse arrangement 402 may enable this reactive transition. For example, the coupling between the core 410 and at least one of the first diaphragm 406 and the second diaphragm 408 may be disrupted in response the input sound at the predetermined threshold intensity, as illustrated in Fig. 6. In some non-limiting examples, the core 410 may be coupled to the first and second diaphragms 406 and 408 via an adhesive material. The strength of the adhesive material may be optimized such that the coupling between the core 410 and at least one of the first diaphragm 406 and the second diaphragm 408 breaks at the predetermine threshold intensity.

[0053] In some non-limiting examples, the core 410 may be structurally designed to break once an input sound wave at the predetermine threshold intensity is transmitted therealong, as illustrated in Fig. 7. In any case, the fuse arrangement 402 is configured to provide a disruption in the acoustic transmission therealong in response to an input sound wave at the predetermined threshold intensity. As a result, the sound wave that is transmitted to the first diaphragm 406 is significantly attenuated, or dampened, in the damper 404. Thus, the hearing protection device 400 may reactively transition to the high loss state and thereby increase the sound transmission loss therethrough in response to, for example, a blast-induced input sound wave at the predetermined threshold intensity. Once in the high loss state, a portion of the input sound waves may be transmitted through the canal 412, however, this may be substantially reduced by the minimized canal diameter D _c for a given size of the core 410 and/or by introducing other loss mechanisms. [0054] This reactive transition provided by the fuse arrangement 402 of the hearing protection device 400 may provide protection from blast-induced hearing losses, for example, for military personnel, without the need for additional power supplies and/or manual switching mechanisms.

[0055] The non-limiting examples illustrated in Figs. 6 and 7 illustrate an irreversible transition between the low loss state and the high loss state. That is, once an input sound wave is transmitted to the fuse arrangement 402, the fuse arrangement 402 is configured to permanently disrupt the sound transmission therealong, for example, by breaking at an interface between the core 410 and the first diaphragm 406, or along the core 410 itself. Fig. 8 illustrates a non-limiting example of a hearing protection device 500 that may be configured to provide a reversible transition between the low loss state and the high loss state according to one aspect of the present disclosure. The hearing protection device 500 may be similar to the hearing protection device 400, except as described herein or is apparent from the figures. Similar components are identified using like reference numerals.

[0056] In the illustrated non-limiting example, the fuse arrangement 502 of the hearing protection device 500 comprises a core 504 that includes a mechanism to reversibly form a discontinuity at a point along the core 410 to reactively transition between the low loss state and the high loss state. With specific reference to Fig. 9, the core 504 includes a magnetic core 506 concentrically surrounded by an optical fiber material 508 (e.g., a polymer). In some non-limiting examples, the core 504 may be formed by a fiber thermal drawing process.

[0057] The core 504 includes a joint 510 that is configured to transition between an attached state and a dislocated state in response to an input sound wave at the predetermined threshold intensity, as illustrated in Figs. 10 and 11. In operation, the hearing protection device 500 may be configured to reversibly transition between the low loss state and the high loss state. Specifically, the joint 510 of the core 504 may be designed to be magnetically coupled (see Fig. 10) in the low loss state (i.e., when input sounds below to the predetermined threshold intensity are input to the first diaphragm 406). When an input sound wave above the predetermined threshold intensity is input to the first diaphragm 406, the joint 510 of the core 504 may be configured to dislocate (see Fig. 11) thereby causing a discontinuity along the core 504 and transitioning the fuse arrangement 502 to the high loss state. Once the input sound wave above the predetermined threshold intensity passes, the dislocated core 504 may magnetically re-attract at the joint 510 to transition the fuse arrangement 502 back to the low loss state. [0058] This reversible, reactive transition provided by the fuse arrangement 502 of the hearing protection device 500 may provide protection from blast-induced hearing losses, for example, for military personnel, without the need for additional power supplies and/or manual switching mechanisms.

[0059] As will follow, a modeled approach for the design and operating characteristics of the hearing protection devices is described in accordance with the systems and methods disclosed herein. The model here assumes an acoustically absorbing damper. However, a similar analysis is applied to devices using an acoustic reflecting blocker.

[0060] High Pass State Model

[0061] Initially, sound waves propagating through air and characterized by sound pressure and velocity are considered. Let z denote the characteristic acoustic impedance of a medium. The acoustic intensity / in the medium is given by:

and the particle speed u is related to the pressure p as: u -— P .

z v>

[0062] Referring to Fig. 12, the pressure reflectance r of the interface from the air to a second medium (e.g. the first diaphragm 406) can be calculated from continuity conditions for the pressures and particle velocities:

P ₊ + P- = P ₂

and

From equations (3) and (4) above, the pressure reflectance r and the transmittance t are given by:

p_ z ₂ - z.

(5) p. ^Z l + ^Z 2

and

The ratio of the intensity between the input and the transmitted is given by:

[0063] For the case of a diaphragm with a finite thickness /, the reflectance r is affected by the acoustic reflection from the back surface, as illustrated by Fig. 12. For the general case, a sound wave may be considered with a phasor term:

Ukx-cot)

^ (9) where k is the wavenumber and ω = 2πΐ is the angular frequency of sound. The reflectance r of the diaphragm can be calculated by considering the multiple interference of the acoustic wave in the diaphragm as follows:

p_ = rp ₊ = (r _x -t _x -t _x r _x e + ...)/> ₊ ,

(10)

(11) where

[0064] The transmittance ? is given by:

(13)

Solving for the transmittance t gives:

(14)

The transmitted intensity is expressed as:

(15)

[0065] When the thickness of a diaphragm is much shorter than the acoustic wavelength, the transmission approaches unity, and the diaphragm is lossless. The sound frequency at which the transmission is decreased to 0.5 (50% or 3 dB loss) is given by:

So,

or

(This result is consistent with an alternative derivation using boundary conditions for p+, p-, p2+, p2-, p3.) For example, considering a case using a PTFE diaphragm, zi (air) =400 Pa s/m, Z2 (PTFE) =2.97xl0 ⁶ Pa s/m, c ₂ (PTFE) = 1390 m/s, and / = 0.1 mm (this is similar to the thickness of the human tympanic membrane). Based on equation 17 above, the cutoff frequency is calculated to be approximately 600 Hz.

[0066] The above analysis may not account for the bending stiffness of the diaphragm, which may result in a discrepancy with human tympanic membranes that show low loss at up to approximately 8 kHz. To reconcile this, the excitation of flexural waves of the diaphragm and its role in transmission is considered.

[0067] Flexural Waves

[0068] For kl « 1, the diaphragm may be treated as a lumped element. For a thin diaphragm modeled as an Euler-Bernoulli plate, the bending stiffness is:

El ³

^{B =} \2(\ - v ² ) ⁽¹⁹⁾ where E is Young's modulus and v is Poisson's ratio. The wave number of a flexural wave in a rectangular plate is given by: k = {o) ^l m l Dy J/4 where m is the mass densit per unit area. The speed of flexural wave bending modes)

flexural

(21)

For

The flexural impedance z/is: z _f = Ρ _Λα V _f = (pc) ll .tfl / c - ^ζ „ _α VTSA .

[0069] For example, considering a case using a PTFE diaphragm, E = 0.5 GPa, p = 2200 kg/m3, c = 1390 m/s, z = 3xl0 ⁶ Rayleigh (rayl), and Vi= 536 m/s. Based on equations 21-24 above, for / = 0.1 mm, at 1 kHz a bending stiffness can be calculated to be B = 5.3xl0 ^"5 Nm and a flexural wave speed can be calculated to be Vf = 9.8 m/s. For a circular plate, the fundamental resonance mode frequency is given by:

For / = 0.1 mm,

For a radius of a = 4 mm, fo = -700 Hz.

[0070] The dispersion curve of the bending waves is shown in Fig. 13, where the x-axis corresponds to the sound frequency in Hz and the y-axis denotes the wavelength in meters. In the audible frequency range, the wavelength is close to or larger than the size of the diaphragm. This condition is necessary for diaphragm resonance to be excited efficiently from incoming planar-like sound waves. The efficiency is given by:

The efficiency drops to 0.5, i.e. 3 dB loss, when: k$— 7T 12. 28)

This indicates that some finite thickness may be beneficial to ensure efficient coupling to the input transverse waves.

[0071] For a 2D model, Newton's law for the diaphragm is given by:

where ξ denotes the displacement along the x-coordinate, and the second term is due to the bending stiffness that provides a restoration force (see Fig. 14). A 3D model can be analyzed by describing the second term with a radial coordinate.

[0072] When the bending mode is in resonance, as in the case for the longitudinal resonance, the diaphragm becomes acoustically transparent.

[0073] Solving with

or

and ά ² ζ du

m—— -m— = imcoii.

dt ² dt w

The transmittance is calculated to be:

and

The sound wave at each frequency can excite the resonance mode with Vt = Vf. Different frequencies excite bending waves that are in resonance. In this case, the transmission loss is zero. Therefore, a diaphragm with optimized thickness and size can be acoustically lossless.

[0074] Most of the transmitted sound wave may then be absorbed in the damper 404. Part of the wave can propagate through the canal 412 and reach the second diaphragm 408. An example case where the size of the diaphragm is much smaller than the sound wavelength is considered. In this case, the sound wave is diffracted over the entire solid angle. Therefore, the amount of sound energy at the end of a straight canal is:

^ = = %-; 101og ₁₀ (r„ _ra , ) = 24.1[ _i /B] * log io(^ ).

(35)

[0075] For example, for damai = 0.5 mm and L = 2.5 mm, the off-level attenuation is 16.8 dB. However, the attenuation may be considerably enhanced for a curved canal.

[0076] Low Loss State Model: Quasi Static Transmission Through a Core [0077] In the presence of a core (e.g., the core 410 or the core 504), the full calculation of acoustic transmission through a diaphragm (e.g., the first diaphragm 406) to the core requires numerical solving. Here an approximate analytic solution is provided. It is assumed that the diaphragm is rigid (or substantially stiff or semi flexible) and therefore distributes the pressure from the entering sound in the air to the solid core. Referring to Fig. 15, force particle velocity w, and mechanical impedance Z are considered:

Z ^<36) power =— f ₍₃₇₎ f = P - S; Z = z - S. ₍₃₈₎

Here S is the area of the diaphragm.

[0078] To analyze the input side (i.e., the first diaphragm 406), the continuity of force and particle velocities is applied in equations 39-44 below. Here force rather than pressure is considered because of the diaphragm being rigid. (The precise knowledge of the diaphragm impedance is not required. In fact, here it is assumed that the diaphragm impedance is the same as Zi by neglecting any impedance mismatch. Z2 is the impedance of the air at the backside of the diaphragm and, thus, should not be confused by the diaphragm impedance.) + /- = / J ₂ 2 + ¹ / J 33 (39)

+ u - u ₂ = u. (40)

Z, Z, Z ₂ Z ₃ CD z _a ,MD ² -d ² )

(42)

(Z _t +Z ₂ +Z ₃ ) (43) 2Z,

f ₊ (Z _t +Z ₂ +Z ₃ ) (44)

[0079] The power transmission is given

When the core is small, Zi = Z2. The transmission τ ₃ becomes maximum when:

Z ^3 = Z + ' Z ^2 ~2Z (46)

At this condition, the maximum transmission is 0.5 or 50%. It can be shown that 25% power is radiated to the air and 25% is reflected. Using the definition of the mechanical impedance, the minimum loss is achieved at:

For example, considering a case using a silica core, for D = 8 mm, Z ₃ (silica) =13 x 106 Pa s/m, the optimum core diameter is calculated to be do = 0.063 mm. However, it should be noted that the transmission is quite insensitive to the core diameter d. For example, when d= * do = 250 μηι, the transmission loss increases from 3 to 5 dB. The transmission loss, -10 log(i3), as a function of:

JC— . ^ /^ ^r _\ (48) is illustrated in the graph of Fig.16.

[0080] On the output side (e.g., the second diaphragm 408), the same method may be applied as follows:

/ ₊ +/_+/ ₄ =/ ₅ (49)

+ u_=u ₄ =u ₅

(Z _{i +} Z ₅ -Z ₃ )

(52) (Z ₃ +Z _t +Z _s )

2Z ₅

Λ (¾+z ₄ +z ₅ )

Z _x _ 4Z ₃ Z ₅

(Z ₃ +Z ₄ +zj (54)

The transmission loss is identical to that in the input side, since:

Thus, the minimum loss of 3 dB is obtained when:

¾ ^~ 2Z _{5 (56)}

[0081] Low Loss State Model: The Entire Hearing Protection Device

[0082] To calculate the transmission of the entire hearing protection device, acoustic resonance in both diaphragms and the core is considered. Now the impedance on the backside of the diaphragm has been modified by the presence of the core. The effect of acoustic resonance in the core can be analyzed using the same analysis method for the High Loss State Model described above. In principle, the transmission can be 100% for: k _j L « 1

The 3dB bandwidth is

(58)

For example, considering a case using PTFE diaphragms and a silica core, cs (silica) = 5700 m/s, L = 2.5 mm, D = 8 mm, d = 0.2 mm, zs (fused silica) =12.6 xlO ⁶ Pa s/m, z ₂ (PTFE) =3 xlO ⁶ Pa s/m, the 3 dB bandwidth frequency is calculated to be/jds greater than approximately 7300 Hz. Therefore, in the audible range, the core (e.g., the core 410 or the core 504) can be acoustically transparent, if the acoustic propagation loss in the core is negligible. Because there is no reflection from the core back into the diaphragm, the reflectance at the back interface is equal to, approximately,

D ² -d ²

^ ~ ^Γΐ (59)

As the difference is negligible, the impedance of the diaphragm is nearly unchanged. Therefore, at sufficiently low frequencies, the minimum loss of the entire device may be approximately 6 dB (3 dB at each side).

[0083] Based on the above-described modeling approach, a hearing protection device utilizing the properties and techniques described herein may be reactively operable between a low loss state with low sound transmission loss, and a high loss state with high sound transmission loss to protect against strong blasts or loud sound waves.

EXAMPLES

[0084] The following examples set forth, in detail, ways in which a hearing protection device according to the present disclosure may be used or implemented, and will enable one of skill in the art to more readily understand the principles thereof. The following examples are presented by way of illustration and are not meant to be limiting in any way.

[0085] Built on the modeled design principle described here, prototype hearing protection devices were fabricated. For the damper 404, several materials known in the art may be used, but for the prototype devices, polyurethane foam was utilized. To manufacture the hearing protection device, first, a cylindrical form of polyurethane foam was prepared by using a hole punch. The typical outer diameter was 8 mm, but smaller diameters as small as a few millimeters may be used. Then, the canal 412 was made by inserting a hot wire with a desirable diameter through the damper. It was found that submillimeter diameters may be appropriate. Fig. 17 illustrates a manufactured damper with a canal diameter D _c of 500 μιη.

[0086] Once the canal was formed, the core 410/504 is inserted into the canal as illustrated in Fig. 18. In some implementations, the core may be made with pulling thermoplastic polymer. In other implementations, typical optical fibers with silica waveguide and acrylic jacket may be used for the core.

[0087] To manufacture the first and second diaphragms 406 and 408, many different polymer materials, such as polystyrene, PMMA, and PTFE were used. Thin membranes with thicknesses typically less than 100 μιη are prepared first (see Fig. 19 for polystyrene and Fig. 20 for PMMA), and diaphragms with desired diameters are made by using a hole punch. The diameter of diaphragm is typically equal to or slightly smaller than the outer diameter of the damper.

[0088] Assembled prototype hearing protection devices are illustrated in Figs. 21 and 22. In the illustrated implementation, the diaphragms are made of ΙΟΟ-μιη-thick polystyrene membranes, the damper material is polyurethane, and the core was fabricated by either a 200^m-thick polystyrene fiber or a 125^m-diameter silica fiber. The core is glued to the pair of polymer diaphragms. The glue strength is optimized so that it breaks at specific blast sound levels (i.e., the predetermine threshold intensity). The entire fuse element may be inserted into a commercial earplug device.

[0089] The sound transmission efficiency (or loss) of the prototype hearing protection devices were measured using an acoustic chamber, loud speaker, and sound pressure level meter, over a frequency range from 100 Hz to 10 kHz. As illustrated in Fig. 23, in the low loss state at low sound pressure levels, the prototype hearing protection device transmits normal conversational sounds with an attenuation of between approximately 5-7 dB, when measured over a frequency range of 100 to 10,000 Hz at sound pressure levels lower than 110 dB. As described above, the modeled theoretical loss of the design is 6 dB.

[0090] When the core is broken, or disconnected from at least one of the diaphragms, the fuse assembly transitions to the high loss state where the sound transmission loss dramatically increases to approximately 30 dB, which is determined by the attenuation of the damper.

[0091] It is noted that the terms " substantially" (as in substantially different from or substantially the same as), "about," "approximately," etc. relative to specified values may indicate appreciably (or not appreciably) different from, within acceptable manufacturing tolerances, and/or without deviation that would significantly impact intended operational parameters. In certain implementations, acceptable values (that are substantially the same as, substantially different from, about, or approximately a specified value) may have, for example, a +/- 1 percent deviation, a +/- 5 percent deviation, or a +/- 10 percent deviation from the specified value, depending on the specific applications. Other acceptable deviations include, for example, at most 1 percent or at most 5 percent (in the case of substantially the same as, about, or approximately), or at least 5 percent or at least 10 percent (in the case of substantially larger/smaller than).

[0092] Within this specification embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the invention. For example, it will be appreciated that all preferred features described herein are applicable to all aspects of the invention described herein.

[0093] Thus, while the invention has been described above in connection with particular embodiments and examples, the invention is not necessarily so limited, and that numerous other embodiments, examples, uses, modifications and departures from the embodiments, examples and uses are intended to be encompassed by the claims attached hereto. The entire disclosure of each patent and publication cited herein is incorporated by reference, as if each such patent or publication were individually incorporated by reference herein.