Patent Description:
In one aspect, an earphone includes an earphone assembly, a first electro-acoustic transducer and a second electro-acoustic transducer. The earphone assembly has an earphone body with an inner surface, an acoustic opening and a cavity defined by the inner surface and the acoustic opening. The earphone assembly further includes a first acoustic impedance branch extending from an open end at the inner surface to a closed end inside the earphone body. The first acoustic impedance branch includes a branch volume that reduces a magnitude of a resonance at a first resonance frequency for an occluded ear canal defined by the cavity of the earphone assembly and an ear canal of a user when the earphone is at least partially inserted into the ear canal. The first electro-acoustic transducer is disposed inside the earphone assembly and is configured to generate a first acoustic signal in response to a first electrical signal. The second electro-acoustic transducer is disposed inside the earphone body along a length of the first acoustic impedance branch and is configured to generate a second acoustic signal in response to a second electrical signal. The first acoustic impedance branch comprises an acoustic waveguide having a length equal to one quarter of an acoustic wavelength for the first resonance frequency.

Examples may include one or more of the following features:.

The first electro-acoustic transducer may be disposed at the inner surface of the earphone body. The second electro-acoustic transducer may be disposed at the closed end of the first acoustic impedance branch. The first electro-acoustic transducer and the second electro-acoustic transducer may be different sizes.

The earphone assembly may further include a second acoustic impedance branch extending from the inner surface into the earphone body. The first electro-acoustic transducer may be disposed inside the earphone body along a length of the second acoustic impedance branch. The first electro-acoustic transducer may be disposed at a closed end of the second acoustic impedance branch.

The first electro-acoustic transducer and the second electro-acoustic transducer may generate acoustic signals having different frequency content.

The above and further advantages of examples of the present inventive concepts may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of features and implementations. <FIG> show examples not falling under the scope defined by appended claims. Embodiments of the invention are shown in <FIG>.

As shown in <FIG>, a typical in-ear audio device, such as an earphone <NUM>, includes an electro-acoustic transducer <NUM> (e.g., speaker) inside an earphone assembly <NUM>. The earphone assembly <NUM> includes a body comprising a rigid body 14A and a compliant eartip 14B attached to the nozzle portion of the rigid body 14A. As used herein, the nozzle portion means the end portion of the rigid body 14A that is inserted furthest into the ear canal <NUM>. The rigid body 14A may be formed as a hard plastic material. For example, the material may be a thermoplastic polymer such as acrylonitrile butadiene styrene ("ABS"). The earphone assembly <NUM> has an inner surface <NUM> and an acoustic opening <NUM>. The inner surface <NUM> includes the inner surface 15A of the rigid body 14A and the internal surface 15B of the eartip 14B, and the acoustic opening <NUM> is at the open end of the eartip 14B. The eartip 14B is formed of a material that comfortably conforms to the entrance of an ear canal <NUM> of a user, such as silicone. The eartip 14B may be configured for removal from and re-attachment to the rigid body 14A. The speaker <NUM> is disposed inside the rigid body 14A such that an acoustic front cavity <NUM> is defined on the front side of the speaker <NUM> and an acoustic back cavity <NUM> is defined on the back side of the speaker <NUM>. The acoustic back cavity <NUM> may be sealed, as shown, or it may be shunted (i.e., "ported") by way of one or more acoustic impedance paths to an external acoustic environment, to the front cavity <NUM>, or to both the external acoustic environment and front cavity <NUM>. Although not shown, the earphone <NUM> may include one or more microphones located in the front cavity <NUM> and/or one or more microphones disposed on the external surface of the rigid body 14A. In some implementations, the microphones may be feedback microphones and/or feedforward microphones. In other implementations, one or more microphones inside the earphone <NUM> may be used to augment speech pickup from the user and one or more external microphones can be used for hearing assistance or ambient pass-through. By way of non-limiting examples, the earphone <NUM> may be used in different types of devices such as devices that provide for music playback, communications, hearing assistance and/or augmented reality.

As illustrated, the speaker <NUM> is provided at an acute angle with respect to a length of the front cavity <NUM> which extends from a region adjacent to the speaker <NUM> to the acoustic opening. In other examples, the speaker <NUM> is located at the end of the rigid body 14A such that the acoustic energy from the speaker is directed substantially along the length of the front cavity <NUM>.

The front cavity <NUM> is an open cavity while the earphone <NUM> is not worn; however, when the earphone <NUM> is inserted into the entrance of the ear canal <NUM>, the front cavity <NUM> and the ear canal <NUM> couple together to form an acoustic cavity referred to as an occluded ear canal. The occluded ear canal behaves substantially as an acoustic waveguide. The ear drum <NUM> is located at one end of the occluded ear canal and the speaker <NUM> and front cavity <NUM> are located at the other end of the occluded ear canal. The figure omits most of the length of the ear canal to accommodate for scale and for clarity of the illustrated features. By way of example, the length of the ear canal of a user is typically in a range from about <NUM> to about <NUM>.

A rigidly terminated acoustic waveguide is known to have a first resonance at a frequency where the length of the waveguide is equal to a half-wavelength of propagating acoustic waves. This first resonance frequency depends on several factors, including, but not limited to, the length of the ear canal, the earphone insertion depth and the volume of the front cavity <NUM>. For typical earphones the first resonance frequency is in a frequency range extending from about <NUM> to about <NUM>.

The first resonance of the occluded ear canal causes undesirable effects for a number of reasons. First, for passive noise-isolating earphones and active noise reduction (ANR) earphones, the resonance amplifies the transmission of external noise into the ear canal and therefore reduces the amount of passive noise attenuation around the first resonance frequency. The reduced attenuation is particularly noticeable in ANR earphones because the active components of noise reduction typically include a feedback system and feedforward system, both of which contribute to noise reduction primarily at frequencies below the first resonance frequency. Consequently, the total noise reduction is greatest at frequencies below the first resonance frequency, small around the resonance frequency, and moderate at frequencies that are greater than the resonance frequency. It should be recognized that there are higher order resonances defined by the occluded ear canal; however, these resonances occur at higher frequencies where hearing is less sensitive and these resonances interact with other dynamic features so their effects are not as consistently prominent as the first resonance.

<FIG> shows an example of the noise attenuation achieved by an ANR earphone as a function of acoustic frequency. A typical ANR earphone system includes one or more feedforward microphones disposed on the external surface of the earphone, one or more feedback microphones disposed in the front cavity of the earphone and a circuit in electrical communication with the microphones. The circuit generates an electrical signal to drive the speaker based, for example, on an audio playback signal. The electrical signal is also responsive to the feedback and feedforward electrical signals generated by the microphones that are used for noise reduction.

The figure shows the attenuation as a function of acoustic frequency. The total attenuation <NUM> represents the sum of the active (i.e. feedback and/or feedforward) attenuation and the passive attenuation <NUM>. The negative effect of the ear canal resonance on the passive attenuation <NUM>, and therefore on the total attenuation <NUM>, is evident as the "notch" <NUM> at approximately <NUM>.

It should be noted that a second undesirable effect of the occluded ear canal resonance is the amplification of the speaker response at and around the resonance frequency, which is generally problematic for audio playback.

Examples of earphones described below include an impedance branch in acoustic communication with the front cavity of the earphone to effectively modify the boundary condition of the waveguide defined by the occluded ear canal. The acoustic impedance branch yields a reduction in the undesirable effects from the first resonance frequency. Consequently, the quality factor (Q) of the occluded ear canal is reduced and a substantially flatter spectral audio response is achieved.

<FIG> shows an example of an earphone <NUM> in which an acoustic circuit, defined in part by the acoustic impedance branch, acts in an analogous way to an electronic series resistor and capacitor (RC) circuit. In particular, the acoustic impedance branch includes an acoustic resistive element <NUM>. The acoustic impedance branch also includes a "branch volume" cavity <NUM> which acts as a capacitance or compliance in the acoustic circuit. Examples of acoustic resistive elements <NUM> include an acoustic screen, a wire mesh, an acoustic fabric and other substantially planar acoustic resistive elements. In one example, the acoustic resistive element <NUM> is Saati Acoustex woven mesh available from Saati Americas Corporation of Fountain Inn, SC. In an alternative example, the acoustic mesh is available from Sefar AG of Heiden, Switzerland. The acoustic resistive element <NUM> is located at an impedance aperture where the front cavity <NUM> is in communication with the branch volume <NUM>.

The acoustic RC circuit has a corner frequency fc at which the impedance magnitudes of the acoustic resistance and acoustic capacitance are equal. When the corner frequency fc is set to be approximately equal to the first resonance frequency of the occluded ear canal, the acoustic resistance and acoustic capacitance are in balance to allow acoustic energy into the acoustic impedance branch and to dissipate that acoustic energy. Although the corner frequency fc is set to be approximately equal to (i.e., substantially equal to) the first resonance frequency, the acoustic RC circuit may be detuned by a small frequency offset so that the two frequencies are not exactly equal. For example, the corner frequency fc can be tuned to a value within a <NUM>% range of the first resonance frequency (i.e., at a frequency that is <NUM> to <NUM> times the first resonance frequency). If the corner frequency fc is detuned to be slightly less than the first resonance frequency, the effective first resonance of the occluded ear canal may shift to a frequency that is closer to the second resonance of the open ear. It will be recognized in connection with alternative examples described below that similar detuning with respect the first resonance yields similar beneficial effects.

As a result of the acoustic RC circuit, the Q of the first resonance is reduced. The size of the branch volume <NUM> relative to the volume of the occluded ear canal substantially determines how much the Q is reduced. By way of example, the branch volume <NUM> may be less than <NUM><NUM> to more than <NUM><NUM> while the volume of the occluded ear canal is dependent on the volume of an "open" ear canal (typically in a range between about <NUM><NUM> and <NUM><NUM>), the insertion depth of the earphone and the volume of the front cavity.

In one non-limiting numerical example, the branch volume <NUM> is <NUM><NUM>, the impedance aperture has a radius of <NUM> and the acoustic resistive element <NUM> is an acoustic screen having an acoustic resistance of <NUM> rayl.

<FIG> shows an example of an earphone <NUM> in which an acoustic circuit defined by the acoustic impedance branch acts in a similar way to an electrical resistor, inductor and capacitor (RLC) circuit. The acoustic impedance branch includes a branch volume <NUM>, an acoustic channel <NUM> (e.g. a thin tube) extending from the branch volume <NUM> to the front cavity <NUM>, and an acoustic resistive element <NUM>. The acoustic channel <NUM> acts as an inductor or mass and the branch volume <NUM> acts as a capacitance or compliance. This type of circuit is commonly referred to as a Helmholtz Resonator and has a resonance frequencyfhr.

The acoustic resistive element <NUM> may be at the impedance aperture defined at the boundary between the front cavity <NUM> and the acoustic port, as shown in the figure, or may be located at the boundary between the acoustic channel <NUM> and the branch volume <NUM>. Alternatively or in addition, an acoustic resistive element may be a volume acoustic resistive element disposed in at least a portion of the acoustic channel <NUM>. For example, an acoustically resistive foam may be provided which partially or fully occupies the acoustic channel <NUM>. By way of a specific example, the acoustically resistive foam may be melamine foam.

When the acoustic Helmholtz Resonator frequency fhr is tuned appropriately with respect to the first resonance frequency of the occluded ear canal (e.g., within a frequency offset range that is within <NUM>% of the first resonance frequency), a significant reduction in the first resonance occurs. Analogous systems are often used to manage mechanical vibrations and are referred to as tuned mass dampers or damped vibration absorbers where such systems are tuned to damp vibrations as is known in the mechanical arts. It should be recognized that the structure of the ear canal can vary for different users. Consequently, an earphone assembly that may be optimally configured for one user may be mistuned for another user so that the damping of the first resonance is less.

In one non-limiting numerical example, the branch volume <NUM> is <NUM><NUM>, the impedance aperture has a radius of <NUM>, the acoustic channel <NUM> has a length of <NUM> and the acoustic resistive element <NUM> is an acoustic screen having an acoustic resistance of <NUM> rayl.

In an alternative example to the Helmholtz Resonator configuration, a waveguide can be used in place of the acoustic channel and branch volume. For example, the waveguide may be formed as a channel in the rigid body 14A of the earphone assembly <NUM>. The waveguide may have a constant cross-sectional area. Alternatively, the waveguide may have a cross-sectional area that varies along its length, for example a conical or an exponential waveguide. To reduce the first resonance of the occluded ear canal, the length of the waveguide may be tuned to have a first resonance frequency that is approximately equal to the first resonance frequency of the occluded ear canal. For example, the length of a constant-area waveguide may be approximately one quarter of the wavelength for the expected first resonance frequency.

<FIG> shows an example of an earphone <NUM> in which a foam shunt <NUM> is acoustically coupled at the impedance aperture <NUM> to the front cavity <NUM> while the remainder of the foam shunt <NUM> is surrounded by the rigid body 14A. The foam shunt <NUM> acts as a fluid having a density and speed of sound which are generally complex-valued parameters in which the imaginary component is associated with an acoustic resistance. An appropriate foam has an acoustic resistance that is sufficient to allow acoustic energy to couple into the acoustic impedance branch and to dissipate the coupled acoustic energy. Melamine foam is one example of a foam that may be used to form the foam shunt <NUM>.

In a variation of the illustrated example, the foam shunt <NUM> can have a geometric form such that the foam shunt <NUM> acts as a waveguide having a first resonance tuned approximately equal to the first resonance frequency of the occluded ear canal. The waveguide may have constant or varying cross-sectional area along its length. In this configuration, the foam shunt <NUM> acts as a tuned mass damper that significantly reduces the Q of the first resonance.

The earphone examples described above illustrate how the Q of the first resonance can be reduced. As a result, the undesirable effects of the first resonance of an occluded ear canal on the passive noise attenuation can be reduced. An example of passive attenuation as a function of acoustic frequency is shown in <FIG> in which one response <NUM> corresponds to acoustic power received at the ear drum as a function of acoustic frequency for an open ear canal and the other response <NUM> corresponds to the acoustic power received at the ear drum as a function of acoustic frequency while a nominal earphone providing only passive attenuation is inserted into the entrance of the ear canal. The amount of passive attenuation is defined as the difference between the two responses <NUM> and <NUM>. The passive attenuation corresponds to a diffuse noise field and the responses are normalized to the acoustic power received at the ear drum for the open ear canal at zero frequency. The first resonance frequency is evident in the inserted earphone response <NUM> at a frequency of approximately <NUM>.

<FIG> graphically shows the normalized acoustic power at the ear drum as a function of acoustic frequency for the open ear and nominal passive attenuation earphone (responses <NUM> and <NUM>, respectively) as described above with respect to <FIG>. <FIG> also shows the acoustic power at the eardrum as a function of acoustic frequency for four different earphones with each earphone configured with an acoustic impedance branch having a planar acoustic resistive element and a branch cavity as described above for <FIG>. The four earphones have branch volumes of <NUM><NUM>, <NUM><NUM>, <NUM><NUM> and <NUM><NUM>. It can be seen that the first resonance decreases monotonically both in magnitude and in acoustic frequency with increasing values of branch volume with the response <NUM> corresponding to the branch volume of <NUM><NUM>. An upper limit to the branch volume typically is due to the available space within the earphone body.

<FIG> graphically shows the normalized acoustic power at the ear drum as a function of acoustic frequency for the open ear and nominal passive attenuation earphone (responses <NUM> and <NUM>, respectively) as described above with respect to <FIG>. <FIG> also shows the acoustic power at the eardrum as a function of acoustic frequency for an earphone constructed according to the acoustic screen and branch volume configuration of <FIG>, the Helmholtz Resonator configuration of <FIG> and the foam resonator configuration of <FIG> (responses <NUM>, <NUM> and <NUM>, respectively). All branch volumes are <NUM><NUM>.

One significant advantage of various examples of earphones described above relates to the ability to control, and specifically, flatten the amplification of the speaker response in the feedback system. More specifically, a feedback controller can include frequency response features that remove the amplifying effect of the ear canal resonance on the speaker-to-feedback microphone response. Decreasing the magnitude of the resonance with an acoustic impedance branch permits a more robust feedback system to accommodate the effects of an occluded ear canal resonance.

<FIG> is an example of how the speaker-to-feedback microphone response improves with increasing branch volume. Response <NUM> corresponds to a nominal passive-only attenuating earphone and responses <NUM>, <NUM>, <NUM> and <NUM> correspond to branch volumes of <NUM><NUM>, <NUM><NUM>, <NUM><NUM> and <NUM><NUM>, respectively.

As described above, in some earphones the acoustic back cavity <NUM> (see <FIG>) may be shunted (ported) by an impedance path to the acoustic front cavity <NUM>. Furthermore, the front cavity <NUM> may be shunted to the external acoustic environment. Such ports may be used for low frequency pressure equalization and referred to as PEQ ports. For example, a PEQ port may be implemented as a narrow tube. Alternatively, one or more PEQ ports may shunt between the acoustic impedance branch and the back cavity <NUM> and/or between the acoustic impedance branch and the external acoustic environment. These alternative configurations are possible because the acoustic impedance branch is in acoustic communication with the front cavity <NUM> and may be configured to be effectively open to the front cavity <NUM> below several kHz. Consequently, low frequency acoustic energy from the PEQ port(s) passes through the acoustic impedance branch to the front cavity <NUM> and vice versa. In addition, PEQ ports may be configured to be effectively closed above several hundred Hz. Thus, the PEQ ports have no substantial influence on the effect of the acoustic impedance branch on the first resonance of the occluded ear canal.

Although the various examples described above include the acoustic impedance branch as located within the rigid body of an earphone assembly, in alternative examples the acoustic impedance branch is partially or fully located in the eartip. For example, part of the branch cavity or the entire branch cavity may be formed in the eartip.

In the various examples described above, which do not fall under the scope defined by the appendend claims, a single electroacoustic transducer <NUM> for sourcing an acoustic signal is disposed inside the earphone assembly. In the following examples showing embodiments of the claimed invention, one or more additional electro-acoustic transducers may be included in the earphone assembly. In these examples, an additional electro-acoustic transducer is disposed in an acoustic impedance branch that is in the form of a waveguide. For example, <FIG> illustrates an earphone <NUM> that includes a first electro-acoustic transducer (speaker) <NUM> and a second electro-acoustic transducer <NUM> inside a rigid body 116A that is attached to a compliant eartip 116B. The first speaker <NUM> separates a back cavity <NUM> and a front cavity <NUM> and is configured to generate a first acoustic signal in response to a first electrical signal. Although shown as a long, thin cavity extending away from the nozzle end of the earphone <NUM>, the back cavity <NUM> can have a substantially different shape subject to the shape and size constraints of the rigid body 116A. The second speaker <NUM> is configured to generate a second acoustic signal in response to a second electrical signal. The first and second acoustic signals may differ such that the first and second speakers <NUM> and <NUM> generate acoustic signals having different amplitudes and frequency content. The earphone <NUM> has an acoustic opening <NUM> at the open end of the eartip 116B which, in combination with the inner surfaces 118A and 118B of the rigid body 116A and eartip 116B, respectively, define the front cavity <NUM> of the earphone. Optionally, the earphone <NUM> may include one or more internal and/or external microphones such as feedback microphones and/or feedforward microphones. When the earphone <NUM> is at least partially inserted into the entrance of the ear canal <NUM>, an acoustic cavity is defined by the occlusion of the ear canal. This acoustic cavity and includes the earphone front cavity <NUM> and the portion of the ear canal that is not occupied by the earphone <NUM>.

Two acoustic impedance branches in the form of acoustic waveguides 120A and 120B are formed in the rigid body 116A. Each waveguide <NUM> has a substantially constant cross-sectional area and a length that is approximately one quarter of the wavelength of the expected first resonance frequency. In some implementations, the lengths of the waveguides <NUM> are in a range from about <NUM> to about <NUM> and the total cross-sectional area of each waveguide <NUM> is in a range from about <NUM><NUM> to about <NUM><NUM>. Larger cross-sectional areas are sometimes preferred but are limited by the size of the rigid body 116A. Each waveguide <NUM> extends from an aperture at the inner surface 118A to a closed end inside the rigid body 116A. As illustrated, one of the waveguides 120A is shunted to the external environment through a port <NUM>. Optionally, the other waveguide 120B can instead be shunted to the external environment or both waveguides 120A and 120B may be shunted.

The first speaker <NUM> is disposed near or at the nozzle end of the rigid body 116A close to the acoustic opening <NUM> such that acoustic energy generated by the first speaker <NUM> couples directly into the ear and propagates substantially down the length of the ear canal <NUM>. The second speaker <NUM> is disposed inside the rigid body 116A along a length of the other waveguide 120B and includes an acoustic aperture <NUM> through which acoustic energy generated by the second speaker <NUM> is introduced into the waveguide 120B which couples at one end into the acoustic cavity of the occluded ear canal. As illustrated, the acoustic aperture <NUM> is located away from the closed end of the waveguide 120B by approximately one quarter of the waveguide length. In other examples, the second speaker <NUM> and acoustic aperture <NUM> are located at a different position along the length of the waveguide 120B.

<FIG> is a cross-sectional view of the earphone <NUM> of <FIG> looking inward from the nozzle end and shows the two waveguides <NUM> relative to the first speaker <NUM>. The cross-sectional shape of each waveguide <NUM> is a lune, i.e., the area formed by the intersection of two non-concentric circular arcs. The lune shape efficiently addresses space constraint at the nozzle end of the rigid body 116A where the first speaker <NUM> is also present. Other cross-sectional shapes with similar or different areas can be used within the available space of the rigid body 116A.

Preferably, each speaker <NUM> and <NUM> provides an acoustic signal having different frequency content. For example, the first speaker <NUM> may provide substantial bass coverage and the second speaker <NUM> may provide higher frequency content. The speakers <NUM> and <NUM> may be of different size. By way of non-limiting numerical examples, one or both speakers <NUM> and <NUM> may be a balanced armature speaker and can range in size from smaller than a <NUM> x <NUM> x <NUM> device to a greater than <NUM> x <NUM> x <NUM> device. Alternatively, one or both speakers <NUM> and <NUM> may be a dynamic (moving coil) speaker and can range in size from a substantially cylindrical device that is smaller than a <NUM> diameter and <NUM> length device to larger than a <NUM> in diameter and <NUM> length device. In still another example, one or both speakers <NUM> and <NUM> may be piezoelectric transducers.

In other examples, waveguides may have a different shape, such as a circular or rectangular cross-section. In addition, the number of waveguides may be different. For example, a single waveguide may be used (e.g., waveguide 120A may be absent). Alternatively, three or more waveguides may be included in the earphone.

The number of speakers may be different. In contrast to <FIG>, each waveguide <NUM> may have a speaker disposed along its length. More specifically, a third speaker may be coupled to the waveguide 120A or the first speaker may be relocated to a position along the length of the waveguide 120A. In still other examples, the number of waveguides <NUM> may be different and the number of speakers coupled to waveguides may be different.

The performance of a speaker that is coupled into a waveguide depends on the location of the speaker along the length of the waveguide. <FIG> graphically illustrates the magnitude of the speaker-to-feedback microphone response for a balanced armature transducer as a function of acoustic frequency for different locations of the speaker aperture along the length of a constant cross-sectional area waveguide having a length of <NUM> and a circular cross-sectional area of approximately <NUM><NUM>. The location of the speaker associated with each plot corresponds to the distance of the acoustic aperture (e.g., acoustic aperture <NUM> in <FIG>) from the closed end of the waveguide.

Response <NUM> corresponds to the speaker located at the closed end of the waveguide, responses <NUM>, <NUM> and <NUM> correspond to the speaker positioned at <NUM>, <NUM>, mm and <NUM> from the closed end, respectively, and response <NUM> corresponds to the speaker at the open end of the waveguide (similar to the location of the first speaker <NUM> in <FIG>).

The response magnitudes are nearly identical for acoustic frequencies less than about <NUM>. At higher frequencies, the peaks and notches in the magnitude responses vary according to speaker position along the waveguide.

The available space for speakers within an earbud or other types of earphone is often limited, especially in the region nearest to the earphone nozzle. As described above, if the earphone includes one or more acoustic impedance branches to modify the ear canal first resonance frequency, a speaker can be coupled to the acoustic impedance branch at a location away from the nozzle where more space is available. The additional space allows a larger sized speaker to be used and, in some implementations, allows for an increased number of speakers. Thus, a waveguide or other form of acoustic impedance branch used to achieve a desired modification to the occluded ear canal resonance can also be used for the additional independent advantage of coupling an acoustic signal from one or more additional speakers to the ear canal.

Claim 1:
An earphone comprising:
an earphone assembly (<NUM>) having an earphone body with an inner surface (<NUM>), an acoustic opening (<NUM>) and a cavity (<NUM>) defined by the inner surface (<NUM>) and the acoustic opening (<NUM>), the earphone assembly (<NUM>) further including a first acoustic impedance branch (120A) extending from an open end at the inner surface (<NUM>) to a closed end inside the earphone body, wherein the first acoustic impedance branch (120A) includes a branch volume that reduces a magnitude of a resonance at a first resonance frequency for an occluded ear canal defined by the cavity (<NUM>) of the earphone assembly and an ear canal (<NUM>) of a user when the earphone is at least partially inserted into the ear canal (<NUM>);
a first electro-acoustic transducer (<NUM>) disposed inside the earphone assembly (<NUM>) and configured to generate a first acoustic signal in response to a first electrical signal; and
a second electro-acoustic transducer (<NUM>) disposed inside the earphone body along a length of the first acoustic impedance branch (120A) and configured to generate a second acoustic signal in response to a second electrical signal,
wherein the first acoustic impedance branch (120A) comprises an acoustic waveguide having a length equal to one quarter of an acoustic wavelength for the first resonance frequency.