Patent Publication Number: US-11653144-B2

Title: Open audio device

Description:
PRIORITY CLAIM 
     This application claims priority to, and is a continuation-in-part of, U.S. patent application Ser. No. 16/553,751, titled “OPEN AUDIO DEVICE,” filed on Aug. 28, 2019, the entire contents of which are incorporated by reference. 
    
    
     BACKGROUND 
     This disclosure relates to an open audio device. 
     Open audio devices allow the user to be more aware of the environment, and provide social cues that the wearer is available to interact with others. However, since the acoustic transducer(s) of open audio devices are spaced from the ear and do not confine the sound to the just the ear, open audio devices produce more sound spillage that can be heard by others as compared to on-ear headphones. Spillage can detract from the usefulness and desirability of open audio devices. 
     SUMMARY 
     All examples and features mentioned below can be combined in any technically possible way. 
     In one aspect, an open audio device includes an acoustic radiator that emits front-side acoustic radiation from its front side and emits rear-side acoustic radiation from its rear side, a front acoustic cavity that receives front-side acoustic radiation, a front transmission line that is acoustically coupled to the front acoustic cavity and comprises a first front sound-emitting opening, and a rear acoustic cavity that receives rear-side acoustic radiation and comprises at least a first rear sound-emitting opening. 
     Examples may include one of the above and/or below features, or any combination thereof. The open audio device may further comprise a second front sound-emitting opening that comprises a resistive element and is closer to the acoustic radiator than is the first front sound-emitting opening. The first rear sound-emitting opening may comprise a resistive element. 
     Examples may include one of the above and/or below features, or any combination thereof. The open audio device may further comprise a support structure that is configured to carry the acoustic radiator on a wearer&#39;s head such that the acoustic radiator is held proximate but not in an ear canal opening of the user. The open audio device may further comprise a housing that is carried by the support structure, wherein the housing contains the acoustic radiator and defines at least part of the front and rear acoustic cavities. At least part of the front transmission line may comprise a spout piece that is configured to be removably coupled to the housing. The spout piece may be configured such that when it is coupled to the housing it covers at least a portion of at least one rear sound-emitting opening. The rear acoustic cavity may comprise first and second rear sound-emitting openings, and at least one of the first and second rear sound-emitting openings may comprise a resistive element. The open audio device may further comprise a processor that is configured to provide audio signals to the acoustic radiator. The processor may be configured to modify the audio signals based on whether the spout piece is coupled to the housing. 
     Examples may include one of the above and/or below features, or any combination thereof. The front transmission line may be configured to locate the first front sound-emitting opening proximate but not in the ear canal opening. The front transmission line may have a length and is curved along its length such that it is configured to pass over the ear tragus and locate the first front sound-emitting opening near but not in the ear canal opening. The first front sound-emitting opening may be configured to direct sound generally near the ear canal opening. 
     Examples may include one of the above and/or below features, or any combination thereof. The open audio device may further comprise a second rear sound-emitting opening that is configured to be closer to the ear canal than and located apart from the first rear sound-emitting opening. At least one front sound-emitting opening may comprise a resistive element. A front resistive element may comprise a resistive screen. At least one rear sound-emitting opening may comprise a resistive element. A rear resistive element comprises a resistive screen. The open audio device may further comprise a resistive opening that acoustically couples the front and rear acoustic cavities. The open audio device may further comprise a housing that contains the acoustic radiator and is configured to be held on or proximate an ear of a user. 
     In another aspect, an open audio device includes an acoustic radiator that emits front-side acoustic radiation from its front side and emits rear-side acoustic radiation from its rear side, a support structure that is configured to carry the acoustic radiator on a wearer&#39;s head such that the acoustic radiator is held proximate but not in an ear canal opening of the user, a front acoustic cavity that receives front-side acoustic radiation and comprises a first front sound-emitting opening, a front transmission line that is acoustically coupled to the front acoustic cavity and comprises the first front sound-emitting opening, wherein the front transmission line is configured to locate the first front sound-emitting opening proximate but not in the ear canal opening, a rear acoustic cavity that receives rear-side acoustic radiation and comprises at least a first rear sound-emitting opening, and a housing that is carried by the support structure, wherein the housing contains the acoustic radiator and defines at least part of the front and rear acoustic cavities. At least part of the front transmission line may comprise a spout piece that is configured to be removably coupled to the housing. 
     According to another aspect, an open audio device includes an acoustic radiator that emits front-side acoustic radiation from its front side and emits rear-side acoustic radiation from its rear side, a front acoustic cavity that receives front-side acoustic radiation, and a rear acoustic cavity that receives rear-side acoustic radiation. At least one sound-emitting opening is acoustically coupled to the front acoustic cavity or the second acoustic cavity. The open audio device also includes a removable accessory that includes an acoustic transmission line that is acoustically coupled to the at least one sound-emitting opening when the removable accessory is attached to the open audio device. 
     Examples may include one of the above and/or below features, or any combination thereof. When the removable accessory is attached to the open audio device, the acoustic transmission line is configured to locate acoustic output of the at least one sound-emitting opening proximate but not in the ear canal opening. The acoustic transmission line includes a resistive element. The open audio device may include a processor configured to: provide audio signals to the acoustic radiator; and detect attachment of the removable accessory to the open audio device. The processor may be further configured to modify the audio signals based on whether the removable accessory is attached to the open audio device. The removable accessory may include an elastic material. In some implementations, the at least one sound-emitting opening includes a first front sound-emitting opening acoustically coupled to the front acoustic cavity and a first rear sound-emitting opening acoustically coupled to the rear acoustic cavity. The acoustic transmission line may be acoustically coupled to the front acoustic cavity and the first front sound-emitting opening when the removable accessory is attached to the open audio device, and the removable accessory may include an opening that is acoustically coupled to the rear acoustic cavity and the first rear sound-emitting opening when the removable accessory is attached to the open audio device. In certain implementations, the opening is sized and shaped to substantially match a size and shape of the first rear sound-emitting opening. The acoustic transmission line may be sized and shaped to substantially match a size and shape of the at least one sound-emitting opening. In some cases, the removable accessory includes a baffle. The removable accessory may be configured to cover at least a portion of the first rear sound-emitting opening when the removable accessory is attached to the open audio device. The acoustic transmission line may have a length and is curved along its length. In certain cases, the open audio device includes audio eyeglasses. In some examples, the removable accessory extends at an angle relative to a temple piece of the audio eyeglasses. The open audio device may also include a set of removable accessories of different sizes, each including an acoustic transmission line that is acoustically coupled to the at least one sound-emitting opening when the removable accessory is attached to the open audio device. 
     Yet another aspect features a removable accessory for an open audio device. The removable accessory includes an acoustic transmission line portion that includes a tube with an opening at a distal end and a sleeve portion defining a cavity and having a first opening at one end of the cavity and a second opening at an opposite end of the cavity. The first and second openings are sized to receive the open audio device. 
     Examples may include one of the above and/or below features, or any combination thereof. The removable accessory may also include a third opening in the sleeve portion. In some implementations, when the removable accessory is attached to the open audio device: the acoustic transmission line is acoustically coupled to a first sound-emitting opening of the open audio device; and the third opening in the sleeve portion is acoustically coupled to a second sound-emitting opening of the open audio device. The acoustic transmission line may be sized and shaped to substantially match a size and shape of the first sound-emitting opening, and the third opening may be sized and shaped to substantially match a size and shape of the second sound-emitting opening. In certain implementations, the acoustic transmission line includes a resistive element. The removable accessory may be formed of an elastic material. The removable accessory may include a baffle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an open audio earphone device on an ear. 
         FIG.  2    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  3    illustrates sound spillage from the open audio device of  FIG.  2   . 
         FIG.  4    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  5    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  6    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  7    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  8    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  9 A  is a cross-sectional illustration of an open audio device. 
         FIG.  9 B  shows the open audio device of  FIG.  9 A  mounted near an ear. 
         FIG.  10    is a schematic cross-sectional diagram of an open audio device. 
         FIG.  11    is a functional block diagram of an audio signal control system for an open audio device. 
         FIGS.  12 A and  12 B  are front and rear perspective views, respectively, of a pair of audio eyeglasses with a removable accessory for use with an open ear audio device. 
         FIG.  13 A  is a perspective view of a removable accessory. 
         FIG.  13 B  is a side view of the removable accessory of  FIG.  13 A . 
         FIG.  13 C  is an opposite side view of the removable accessory of  FIG.  13 A . 
         FIG.  13 D  is a rear view of the removable accessory of  FIG.  13 A . 
         FIG.  13 E  is a front view of the removable accessory of  FIG.  13 A . 
         FIG.  14 A  is a side view of an implementation of a removable accessory with a baffle for use with an open ear audio device. 
         FIG.  14 B  is an opposite side view of the removable accessory of  FIG.  14 A . 
     
    
    
     DETAILED DESCRIPTION 
     Open audio devices, such as those described in U. S. Patent Publication 2018-0167710, filed on Dec. 11, 2016 (the entire disclosure of which is incorporated herein by reference for all purposes) typically include an electro-acoustic transducer (i.e., a driver) with front and rear sides. In some non-limiting examples the front side sound exits the device near the user&#39;s ear canal, and the rear side sound exits farther from the user&#39;s ear canal. In other examples, the front side sound exits the device closer to the ear than does the rear side sound. At low frequencies, the sound from the front and rear sides are nearly equal in amplitude and out-of-phase (and so cancel in the far field), such that the device behaves approximately like a dipole. Accordingly, little sound is spilled to people who may be nearby. 
     Because the driver basket or the housing that contains the driver has some acoustic volume and at least one opening on each of the front and rear sides, resonances occur on both the front and the rear. When resonance occurs in the front or rear acoustic volume the sound pressure level (SPL) radiated from the opening from that volume increases. When resonances occur on the front and rear at substantially different frequencies, more sound radiates from one opening such that the dipole behavior no longer occurs at and above the resonant frequencies, and higher objectionable spillage occurs. 
     The present disclosure includes an open audio device of the type described in the patent application incorporated by reference. One manner by which low spillage can be accomplished in the open audio device with an acoustic transmission line coupled to an acoustic volume is with a housing that is configured such that the front and rear primary (i.e., fundamental) acoustic resonance frequencies are matched as closely as possible, given other product design constraints. In one non-limiting example the fundamental resonances are matched to some tolerance (e.g., within one octave of each other). For a simple dipole housing (e.g., with a single outlet opening in each of the front and rear acoustic cavities), this can be accomplished by adjusting the volumes and/or lengths of the front and rear acoustic cavities and the areas and/or lengths of their respective openings, so the resonances are nearly matched. Generally, though not necessarily, the front and rear cavity volumes are made small so that the overall device is compact, which can lead to greater user comfort. Generally, though not necessarily, the opening areas are often made as large as allowable so that resonances occur at as high of a frequency as possible (which thus maintains low spillage up to the resonance frequencies), while maintaining that the openings direct sound at the appropriate locations (e.g., the front opening at the end of the transmission line is near the ear canal, and the rear opening is substantially farther away from the ear canal so there is less sound cancellation at the ear). 
     An electro-acoustic transducer includes an acoustic element (e.g., a diaphragm) that emits front-side acoustic radiation from its front side and emits rear-side acoustic radiation from its rear side. A housing or other structure (e.g., the transducer basket) directs the front-side acoustic radiation and the rear-side acoustic radiation. A plurality of sound-emitting vents in this structure (at least one in the front and one in the rear) allow sound to leave the structure. One such vent or opening is in the acoustic transmission line that is coupled to the front or rear acoustic volume. The electro-acoustic transducer is able to achieve an appropriate ratio of sound pressure delivered to the ear to spilled sound. 
     This disclosure describes a type of open audio device with one or more electro-acoustic transducers that are located off of the ear. A headphone refers to a device that typically fits around, on, or in an ear and that radiates acoustic energy into the ear canal. Headphones are sometimes referred to as earphones, earpieces, headsets, earbuds, or sport headphones, and can be wired or wireless. A headphone includes an electro-acoustic transducer (driver) to transduce audio signals to acoustic energy. The acoustic driver may or may not be housed in an earcup. The figures and descriptions following in some cases show a single open audio device. A headphone may be a single stand-alone unit or one of a pair of headphones (each including at least one acoustic driver), one for each ear. A headphone may be connected mechanically to another headphone, for example by a headband and/or by leads that conduct audio signals to an acoustic driver in the headphone. A headphone may include components for wirelessly receiving audio signals. A headphone may include components of an active noise reduction (ANR) system. Headphones may also include other functionality, such as a microphone. 
     In an around the ear or on the ear or off the ear headphone, the headphone may include a headband or other support structure and at least one housing or other structure that contains a transducer and is arranged to sit on or over or proximate an ear of the user. The headband can be collapsible or foldable, and can be made of multiple parts. Some headbands include a slider, which may be positioned internal to the headband, that provides for any desired translation of the housing. Some headphones include a yoke pivotably mounted to the headband, with the housing pivotally mounted to the yoke, to provide for any desired rotation of the housing. 
     An open audio device includes but is not limited to off-ear headphones (i.e., devices that have one or more electro-acoustic transducers that are coupled to the head or ear (typically by a support structure) but do not occlude the ear canal opening), and audio devices carried by the upper torso, e.g., the shoulder region. In the description that follows the open audio device is depicted as an off-ear headphone, but that is not a limitation of the disclosure as the electro-acoustic transducer can be used in any device that is configured to deliver sound to one or both ears of the wearer where there are no ear cups and no ear buds. 
       FIG.  1    illustrates open audio device  20  mounted on ear  12  and/or the head proximate the ear. Device  20  may be considered an earphone. It includes acoustic module  22  that includes at least one electro-acoustic transducer, front acoustic volume sound-emitting opening  24  (which is close to but not on or in ear canal opening  14 ) and rear acoustic volume sound-emitting opening  26  (which is typically but not necessarily located as far as possible from front opening  24 ). Acoustic module  22  is carried by support structure  28 , which is configured to be mounted on ear  12  and/or the portion of the head proximate the ear. Open audio devices with an acoustic module located in front of the ear and carried by a support structure that is configured to be on the ear or head are known in the field and so are not further described herein in detail. 
     An exemplary dipole-like open audio device acoustic module  30  is depicted in  FIG.  2   . Module  30  includes transducer  32  that is located within housing  34 . Transducer  32  comprises diaphragm  44  that is moved by interaction of coil  46  with a magnetic field generated by the magnetic system, represented generally as structure  48 . Structure  48  may also include a basket and may be vented to the rear acoustic cavity  38 . Electro-acoustic transducer design and operation are well understood by those skilled in the field and so are not fully described herein. Front-side acoustic radiation enters front acoustic cavity  36  and rear-side acoustic radiation (which is out of phase with the front side radiation) enters rear acoustic cavity  38 . Sound exits front cavity  36  via opening  40  and sound exits rear cavity  38  via opening  42 . As described in more detail in the patent application incorporated by reference herein, since the sound exiting openings  40  and  42  is out of phase, it cancels in the far field. This dipole-like behavior leads to a reduction in spilled sound that can be heard by others who are near the user of device  30 . Also, since opening  40  is relatively close to the ear, its sound will mainly reach the ear before it is canceled by sound from opening  42 . Accordingly, audio device  30  is enabled to both deliver sound to the user and reduce spilled sound that is able to be heard by others. 
     As described above, front and rear cavities  36  and  38  and their respective openings  40  and  42  each behave acoustically to exhibit a fundamental resonance frequency. At and above this frequency the sound pressure exiting the cavity opening will increase. If the resonance frequencies of the two cavities are quite different this leads to imbalances in the SPL emitted from the front and rear openings, which leads to increased sound spillage. Exemplary spillage data is set forth in  FIG.  3   , wherein the sound spilled to bystanders (located one meter from the acoustic module) relative to the sound heard by the wearer (as dB spillage when 100 dB SPL is delivered to the ear) is plotted vs. frequency. The solid line plot is for when the rear resonance frequency is equal to the front resonance frequency, while the dashed line is for the rear resonance frequency much lower than the front, and the dash-dot line is for the rear resonance frequency much higher than the front. The best (lowest) spillage occurs when the resonance frequencies are nearly equal (i.e., equal to within about one octave or less). When the rear resonance frequency is much less, there is a broadband increase in spillage shown in the frequency range of about 500 Hz to 6 kHz in this example. When the rear resonance frequency is much higher, there is a peak in spillage shown in the frequency range of about 4 kHz and above in this example. 
     Note that either the front or rear openings may have a resistive element such as a screen, as with acoustic module  50 ,  FIG.  4   . Resonances can be damped by resistance elements, which can facilitate matching the front and rear acoustic radiation by making the resonant peaks less sharp so misalignment of resonant frequencies results in less difference between the front and rear acoustic radiation. Another manner of damping a resonance is with a Helmholtz resonator (not shown) coupled to a volume. In some examples, the resonator may include distinct port and volume elements or may be formed by a waveguide of either constant or non-constant cross-sectional area. The resonator may include a resistive element such as a resistive screen or porous foam. Acoustic module  50  includes transducer  52  that is located within housing  58 . Transducer  52  radiates front-side acoustic radiation into front acoustic cavity  54  and rear-side acoustic radiation into rear acoustic cavity  56 . Sound exits front cavity  54  via opening  60  and sound exits rear cavity  56  via opening  64 . Opening  60  is covered by resistive element  62  (which may be but need not be a resistive cloth) and opening  64  is covered by resistance element  66 . Note that only one of the openings might be covered by a resistance element. A resistance element can be beneficial for spillage, particularly if the rear opening has a resistive element, as the element can help damp the rear resonance and minimize additional sound radiated from the rear when the rear is not matched to the front resonance frequency. However, the resistance element in this example can also damp the transducer and reduce the efficiency at the transducer&#39;s resonance frequency. Beside adding resistance, either of screens  62  and  66  may be used primarily to prevent ingress of foreign material. 
     One or more openings may be used on the front and/or the rear sides. Using multiple openings in parallel can be a way to increase the resonance frequency to facilitate matching the front and rear. Also, a resistive element may be used on one or more of the multiple openings. It may be useful to use a higher resistance element on one of the multiple openings to help damp the respective cavity resonance without damping the transducer resonance. 
     An example is shown in  FIG.  5   . Acoustic module  70  includes transducer  72  that is located within housing  78 . Transducer  72  radiates front-side acoustic radiation into front acoustic cavity  74  and rear-side acoustic radiation into rear acoustic cavity  76 . Sound exits front cavity  74  via opening  80  and can also exit via opening  86  that is covered by a resistance element  88 . Sound exits rear cavity  76  via opening  84  and can also exit via opening  90  that is covered by a resistance element  92 . Note that only one of the openings  86  and  90  might be covered by a resistance element. Elements  88  and  92  help to damp resonances in cavities  74  and  76 , respectively. Also, one or both of the front and rear acoustic cavities may have more than one resistive opening. For example, there could be two smaller resistive openings instead of one larger resistive opening. For instance, circumferentially the main opening or nozzle may be located at zero degrees, with two resistive openings, one at +90 degrees and one at −90 degrees. In some examples, screens (not shown) may also be placed over either or both openings  80  and  84  to prevent ingress of foreign material. 
     There can be one, two, or more, openings in one or both of the front and rear acoustic cavities. One opening generally acts as the egress for sound pressure, although two or more (generally smaller) openings could replace a single such opening. Likewise, one opening may be resistive, to help damp cavity resonances, although two or more (generally smaller) resistive openings could replace a single such opening. For the front cavity, it is more important that the non-resistive or low-resistance opening (i.e., the nozzle) is close to the ear canal and that the resistive opening is farther from the ear canal but also (by necessity) away from the nozzle such that at resonance the resistive opening is in a high pressure location to be able to effectively shunt/damp the resonance. As such, the resistive opening could indeed be near the radiator (as with resistive opening  88 ,  FIG.  5   ), but it could also be along the circumference on the side opposite the nozzle opening  80 . Likewise, for the back cavity it is more important that the non-resistive/low-resistance opening is far from the ear canal, so as not to cancel bass at the ear, and that the resistive opening is away from the non-resistive opening, such that at resonance the resistive opening is in a high pressure location to be able to shunt/damp the resonance. The back resistive opening can also be located closer to the ear canal than the back non-resistive opening, in order to make a shorter dipole for better high-frequency spillage. As such, the resistive opening  90  could be near the radiator (as in  FIG.  5   ), but it could also be along the circumference on the side opposite opening  84 . 
     It is also possible to damp both the front and rear resonances with a resistance element within the housing and connecting the front and rear cavities, sometimes called a pressure equalization or PEQ port. PEQ ports are further described in U.S. Pat. No. 8,989,427, issued on Mar. 24, 2015. An example of a transducer with a PEQ port is shown in  FIG.  6   . Acoustic module  100  includes transducer  102  that is located within housing  108 . Transducer  102  radiates front-side acoustic radiation into front acoustic cavity  104  and rear-side acoustic radiation into rear acoustic cavity  106 . Sound exits front cavity  104  via opening  110 . Sound exits rear cavity  106  via opening  112 . Internal opening  114  connects cavities  104  and  106  and is covered by resistance element  116 . The resistance element  116  can be sufficiently resistive to prevent low frequencies from leaking between cavities  104  and  106  so bass output to the ear canal is maintained, but open enough to damp resonances in both the front cavity  104  and rear cavity  106 . In some examples, the opening  114  and resistance element  116  may be part of the housing  108  or part of the transducer  102 , such as a portion of the basket or as a portion of the diaphragm. In some examples, the opening  114  and resistance element  116  may be formed from an opening with an attached resistive screen or from a perforated section of material. 
     One or more of the openings in the front and/or rear cavities may be through an acoustic transmission line or waveguide in the housing. The transmission line may be beneficial in the audio device design as an element that can be smaller than the transducer and can direct either the front or rear side sound to a more optimal location. For instance,  FIG.  7    illustrates acoustic module  120  that includes transducer  122  that is located within housing  128 . Transducer  122  radiates front-side acoustic radiation into front acoustic cavity  124  and rear-side acoustic radiation into rear acoustic cavity  126 . Sound exits front cavity  124  via opening  130 . Sound exits rear cavity  126  via opening  132  which is at the end of acoustic transmission line  131  and so is farther from the transducer than is opening  130 . Second rear opening  134  is covered by resistance element  136 . An acoustic transmission line (with or without a second, resistive opening) can also or alternatively be coupled to the front acoustic cavity. The acoustic module topology is similar to the variable length dipole (VLD) disclosed in the patent application that is incorporated herein by reference. An aspect of the VLD is that, in addition to achieving the frequency-dependent dipole behavior, the optimal spillage is achieved by tuning to match the front and rear resonance frequencies as described herein. In this configuration, matching the front and rear resonance frequencies can be accomplished by adjusting the volumes and/or lengths of the front and rear acoustic cavities and the areas and/or lengths of their respective openings, so the resonances are nearly matched. Furthermore, the resistance of rear opening screen  136  can be adjusted to shift and damp the rear resonance. For instance, in the limiting case where resistance  136  was low to be effectively open, the total rear opening area is large leading to a higher resonance frequency, while in the limiting case where resistance  136  is high to be effectively closed, the total rear opening area is low leading to a lower resonance frequency. Adjustment of the resistance  136  to a moderate effective resistance can shift the rear resonance in between these extremes and damp it. In some instances, this resistance must also be balanced with its effect on the frequency-dependent dipole behavior. Generally, though not necessarily, the front and rear cavity volumes are made small so that the overall device is compact, which can lead to greater user comfort. Generally, though not necessarily, the opening areas are often made as large as allowable so that resonances occur at as high of a frequency as possible (which thus maintains low spillage up to the resonance frequencies), while maintaining that the openings direct sound at the appropriate locations (e.g., the front opening is near the ear canal and the rear openings are substantially farther from the ear canal so there is less sound cancellation at the ear). 
     The resistive element(s) disclosed herein can be used to damp the rear resonance in order to minimize sound radiated from the rear opening(s). Such damping can be particularly useful in a ported rear cavity design such as shown in  FIG.  7    since the port can lower the rear resonance frequency, which could otherwise lead to a greater front to rear resonance mismatch and so greater spilled sound. 
     As one non-limiting example of the use of a design like that in  FIG.  7   , the open audio device may be configured to place a small transducer in the cymba concha of the outer ear, with the front opening  130  very close to the ear canal. Rear port  131  is used to direct rear sound farther from the ear canal. Preferably but not necessarily, rear opening  132  is configured to be located such that it is not over the outer ear. A rear resistive element (such as element  136 ) may be needed on the rear side to increase and damp the rear resonance frequency in order to decrease spillage. 
     Desired matching of the front and rear resonances (e.g., to within the stated tolerance) can be measured using a probe microphone that measures the pressure at each of the openings while the transducer is excited to determine if the front and rear resonances were matched. Measurements could also be made by driving the transducer directly and measuring the resultant sound pressure per volt. Alternatively, the transducer cone movements could be measured by a laser, and the pressure per cone velocity could be measured to determine the resonances. 
       FIG.  8    illustrates acoustic module  150  that includes transducer  152  that is located within housing  158 . Transducer  152  radiates front-side acoustic radiation into front acoustic cavity  154  and rear-side acoustic radiation into rear acoustic cavity  156 . Sound exits front cavity  154  via sound-emitting opening  160  that is at the end of acoustic transmission line  159 . Front cavity  154  also includes optional second sound-emitting opening  162  with resistance element  164 . Sound exits rear cavity  156  via opening  166  that is covered by resistance element  168 . Transmission line  159  in part functions to deliver front side sound farther from the transducer. In one non-limiting example opening  160  can be located closer to the ear canal entrance than housing  158 . This allows the housing to be held off the ear while front side sound is still delivered close to the ear canal. 
       FIGS.  9 A and  9 B  illustrate acoustic module  170  that includes transducer  172  that is located within housing  190 . Transducer  172  radiates front-side acoustic radiation into front acoustic cavity  174  and rear-side acoustic radiation into rear acoustic cavity  176 . Sound exits front cavity  174  via sound-emitting opening  180  that is at the end of curved transmission line  178 . Opening  180  may have a screen over it that could be resistive, or it could prevent ingress of foreign material. Front cavity  174  also includes optional second sound-emitting opening  182  with resistance element  184  (e.g., an acoustically-resistive cloth) covering the opening. Sound exits rear cavity  176  via opening  186  that can be but need not be covered by resistance element  188 . Transmission line  178  in part functions to deliver front side sound farther from the transducer. As shown in  FIG.  9 B , housing  190  can be held just in front of ear  200 . The housing can be carried by support structure  192  that is configured to be coupled to or held against the ear and/or the portion of the head near the ear, as is known in the field. Transmission line  178  is directed toward ear canal opening  202 , and places opening  180  over or very close to the ear canal opening without blocking it. In one non-limiting example opening  180  can be located closer to the ear canal entrance than housing  190 . This allows the housing to be held off the ear while front side sound is still delivered very close to the ear canal. Also, increasing the distance between the front and rear openings, and locating the front opening very close to the ear canal, decreases cancellation of sound before it reaches the ear. 
     In cases such as depicted in  FIGS.  9 A and  9 B  where one of the front openings is in a transmission line in the housing, a purpose of this transmission line, or spout, is that it is smaller than the housing and can thus direct front sound closer to the ear canal. It can also do so without contacting the ear by hovering over the concha and thus still be comfortable, as in  FIG.  9 B . Being more proximate to the ear canal can more efficiently deliver sound to the ear, while still enabling the ear to be aurally open. Note that the transmission line can have any desired shape, length, or construction, and the front and/or rear openings/ports can be configured as shown in other drawings and as further described herein. In this design, a second resistive opening  182  is included on the front to shift the front resonance frequency higher (since the spout shifts it low), and opening  182  has a resistive element  184  to damp it. In other designs, this resistive opening  182  may not be present or there may be multiple front cavity openings/ports. Similarly, in this design a third resistive opening  186  is included on the rear, and opening  186  has a resistive element  188  to damp it. In other designs, this opening  186  may not have a resistive element  188  or there may be multiple rear cavity openings/ports (see, e.g.,  FIGS.  2 ,  4 - 7 ,  10   ). 
     The front transmission line or spout  178  may be an integral part of the housing  190 , as depicted in  FIG.  9 A . It may also be a separate piece that could be removed/replaced by the user. The separate piece could be but need not be made of a compliant material (like silicone or another thermoplastic elastomer) to be more comfortable in case it contacts part of the ear, such as the tragus. The spout typically makes up some or all of the front transmission line. A removable spout can also enable a dual function: with the spout removed, the device could operate like a dipole with front and rear resonances matched and the ear would appear more visually open. With the spout attached, the ear would not be as visually open, but would have improved bass, loudness and spillage. Since the spout would shift the front resonance frequency lower, part of the spout mounting structure could be used to block either a portion of the one rear opening or one of multiple rear openings so that the rear resonance frequency would also lower to match, as in  FIG.  10   . 
       FIG.  10    illustrates acoustic module  210  that includes transducer  212  that is located within housing  218 . Transducer  212  radiates front-side acoustic radiation into front acoustic cavity  214  and rear-side acoustic radiation into rear acoustic cavity  216 . Sound exits front cavity  214  via sound-emitting opening  222  that is at the end of transmission line  219 . Front cavity  214  also includes second sound-emitting opening  224  with resistance element  226 . Sound exits rear cavity  216  via opening  228  that is covered by resistance element  230 . A second opening  232  that is covered by resistance element  234  is also open to rear cavity  216 . Transmission line  219  in part functions to deliver front side sound farther from the transducer. In one non-limiting example opening  222  can be located closer to the ear canal entrance than housing  218 . This allows the housing to be held off the ear while front side sound is still delivered very close to the ear canal. 
     In the example of  FIG.  10    transmission line/spout  219  is a separate piece that is configured to be removably coupled to housing  218 . Spout  219  can be configured such that portion  221  covers opening  232 . As discussed above, this will help to lower the resonance of the rear cavity, ideally to match the lowering of the resonance frequency of front cavity  214  when spout  219  is coupled to housing  218  as shown in  FIG.  10   . While not shown in  FIG.  10   , spout  219  may be curved, as in  FIG.  9 A , or may take another shape. 
     In the dual function example of  FIG.  10   , the open audio device could be enabled to sense whether the spout is installed and adjust the audio accordingly for appropriate playback response; for example the audio signals may be adjusted by changing audio equalization or limiting parameters based on whether or not the spout is installed. Such adjustment may be necessary because the presence of the spout can change the transducer-to-ear transfer function and can load the transducer differently. Sensing of when the spout is coupled to the housing can be achieved in any desired manner. For example, an on-board microphone (not shown) either internal or external to the housing can be used to determine the transducer-to-microphone transfer function. Since the sound pressure increases around the cavity resonance frequency, a measure of this transfer function can be used to determine resonance and from this the state of the spout (on or off) can be inferred. Also, the transducer electrical impedance could be determined. At the cavity resonance the transducer impedance will have a peak that will shift in frequency as the resonance changes from the spout being either on or off. Therefore, the transducer impedance can be used as a measure of the resonance frequency and from this the state of the spout (on or off) can be inferred. Furthermore, various sensors could be used to determine if the spout is installed or not. For instance, a Hall effect sensor could be used to detect magnetic material in the spout to determine whether the spout is on or off. A capacitive sensor could be used to detect capacitive coupling to material in the spout to determine whether the spout is on or off. An optical sensor could be blocked or unblocked by the presence of the spout to determine whether the spout is on or off. A mechanical switch could be present that is triggered upon installation or removal of the spout. The housing could contain electrical contacts to form a circuit which is open when the spout is off and closed when the spout is on due to a conductive trace designed into the spout. 
       FIG.  11    is a functional block diagram of an audio signal control system  250  that can be used for sensing when the spout is installed or not and adjusting the audio. Audio signals are provided by audio signal source  254 . In an open-audio device the audio signals are typically (but not necessarily) provided from a source wirelessly (e.g., by Bluetooth or another wireless protocol). The audio signals are played by transducer  252 . Processor  256  (which may be but need not be a digital signal processor) is responsive to the audio signals and the transducer, and if a microphone is used to sense sound pressure in the front and/or rear cavity processor  256  would be responsive to the microphone as well. A computer program implemented by the processor can be used to determine cavity resonance and thus the state of the spout, and make appropriate adjustments to the audio signals. 
       FIGS.  12 A and  1 B  illustrate another example of an open ear audio device that can be used with a transmission line/spout. In the example of  FIGS.  12 A &amp;  12 B , open ear audio device comprises audio eyeglasses  300  having a front frame portion  302  and two temple pieces  304   a ,  304   b  (generally “ 304 ”) extended from the front frame portion  302 . Each temple portion  304  includes an acoustic driver or radiator  306  that emits front-side acoustic radiation from its front side and emits rear-side acoustic radiation from its rear side. The acoustic radiator  306  is built into a housing  308  of each temple portion  304 , and the housing  308  directs the front-side acoustic radiation and rear-side acoustic radiation. One or more sound-emitting openings  310   a ,  310   b  (generally “ 310 ”) in the housing  308  allow sound to leave the housing  308 . For example, sound exits a first sound-emitting opening  310   a  ( FIG.  12 B ), which in this example is located on the bottom of a portion of each temple piece  304 , and a second sound-emitting opening  310   b , which in this example is located on the side of a portion of each temple piece  304 . Openings  310  may include a screen element to prevent the ingress of foreign material and moisture. The screen element may optionally comprise an acoustically resistive material. While two openings  310   a  and  310   b  are shown in  FIGS.  12 A and  12 B , in other examples there may be fewer or additional sound-emitting openings in the housing. For example, the topology for the acoustic radiator  306  and sound-emitting openings  310  in the audio eyeglasses  300  may be as shown in any of  FIG.  2 ,  4 - 8  or  10   . 
     The right temple piece  304   a  is shown with a removable accessory  400  attached; however, it should be understood that such an accessory could be fitted with either or both temple pieces  304 .  FIGS.  13 A- 13 E  show various views of an example removable accessory  400 . Removable accessory  400  includes a transmission line/spout portion  402  that extends from the temple piece  304  at an angle relative to a length of temple piece  304 . Removable accessory  400  also includes a sleeve portion  404  that defines a cavity  406  with an opening at each end, each sized to receive a temple piece  304  of audio eyeglasses  300 , thereby enabling a user to slide the removable accessories  400  onto each temple piece  304 . The transmission line  402  includes a tube  408  with a first opening  410   a  at a distal end thereof. Transmission line  402  in part functions to deliver front side sound farther from acoustic radiator  306 , and with an outlet closer to the expected location of a user&#39;s ear. When removable accessory  400  is attached to the audio eyeglasses  300 , the first opening  410   a  at distal end of tube  408  is located proximate (but not in) a user&#39;s ear canal opening. Transmission line  402  of removable accessory  400  is directed toward ear canal opening, and places the first opening  410   a  over or very close to the ear canal opening without blocking it. In one non-limiting example, the first opening  410   a  can be located closer to the ear canal entrance than housing  308 . This allows the housing  308  to be held off the ear while front side sound is still delivered very close to the ear canal. Being more proximate to the ear canal can more efficiently deliver sound to the ear, while still enabling the ear to be aurally open. In some examples, removable accessory  400  increases the volume of sound heard at the ear by about 10 dB. Thus, attaching removable accessory  400  can result in power savings (as the sound is louder at the ear, audio can be played back at a lower volume) as well as reduced spillage (due to audio being played back at a lower volume compared to when the accessory is not attached). As with other examples described in this document, removable accessory  400  can enable a dual function: with the accessory  400  removed, the open audio device  300  could operate like a dipole with the ear appearing more visual open, whereas with the accessory attached, the ear would not be as visually open, but would have improved bass, loudness and spillage. 
     When removable accessory  400  is attached or installed on open audio device such as audio eyeglasses  300 , acoustic transmission line  402  is acoustically coupled to the first sound-emitting opening  310   a , enabling sound to exit from the first opening  410   a  at distal end of transmission line  402 . Removable accessory  400  may also include a second opening  410   b . When removable accessory  400  is attached or installed on open audio device such as audio eyeglasses  300 , the second opening  410   b  is acoustically coupled to the second sound-emitting opening  310   b . Acoustic transmission line  402  may be a variety of sizes and shapes, and in some examples may be sized and shaped to substantially match a size and shape of the first sound-emitting opening  310   a . Similarly, the second opening  410   b  may be sized and shaped to substantially match a size and shape of the second sound-emitting opening  310   b . In some examples, the second opening  410   b  may be sized and/or shaped differently from the second sound-emitting opening  310   b , such that a portion or all of sound-emitting opening  310   b  is covered when the removable accessory  400  is attached or installed on open audio device. Covering the second sound-emitting opening  310   b  completely may be beneficial in situations where a user desires loud volume playback but is not concerned about spillage (e.g., when cycling). 
     Removable accessory  400  may be made of a variety of materials, including elastic or compliant materials such as silicone, rubber, or another thermoplastic elastomer. Material should preferably allow for easy attachment and removal on open audio device. In addition, use of a compliant material aids in providing an interference fit with open audio device, enabling removable accessory to seal better to open audio device. Transmission line portion  402  and sleeve portion  404  may be integrally formed in a single removable accessory unit. Removable accessory  400  may be produced in a variety of colors and patterns, enabling users to customize the look of open audio device through the attachment of removable accessory. In addition, removable accessory could be produced in multiple sizes, enabling a user to select a size that best matches his or her head/ear geometry. For example, removable accessory  400  could have a transmission line portion  402  of varying lengths, or a transmission line  402  that extends at varying angles from sleeve portion  404  of removable accessory  404 . 
     Transmission line  402  may include a resistive element spanning a width of transmission line  402  to provide acoustic damping. The resistive element, which may comprise a resistive screen or cloth, may be located anywhere along a length of transmission line  402 . In some examples, resistive element is located at a distal end of transmission line  402 , near opening. Transmission line  402  may be substantially straight as shown in  FIGS.  13 A- 13 E , but also may be curved as shown in  FIG.  9 A , or may take any other suitable shape. 
     In some examples, removable accessory may also include a baffle or shield. An example of a removable accessory  500  with a baffle  501  (a/k/a “shield” or “flap”) is shown in  FIGS.  14 A and  14 B . Removable accessory  500  also includes a sleeve portion  504  that defines a cavity  506  with an opening at each end, each sized to receive a temple piece  304  of audio eyeglasses  300 , thereby enabling a user to slide the removable accessories  500  onto each temple piece  304 . In this example, a baffle  501  extends around the perimeter of a transmission line portion  402  of the removable accessory  500 , providing a barrier to shield the user&#39;s ear from external noise such as wind. Use of a baffle  501  further increases the volume of sound heard at the ear, in some examples by about 20 dB, and may reduce the impact of external noise such as wind on the user&#39;s ability to hear audio signals. The baffle  501  may be shaped so that is deflects wind away from the ear. An accessory with a baffle  501  may be beneficial in outdoor environments where background noise and wind are prevalent and may interfere with the user&#39;s ability to hear audio signals (e.g., when cycling, skiing, snowboarding, etc.). The baffle  501  may be made of any suitable material (e.g., silicone, rubber, or another thermoplastic elastomer) and may be integrally formed with the rest of removable accessory  500 . The baffle  501  may also include a secondary material such as microphone fluff to further act as a windscreen. The baffle  501  may take a variety of shapes, colors, and designs, enabling a user to customize the look of open audio device. 
     As described above, a processor on the open audio device could be used to sense whether the removable accessory is installed (using any of the techniques described above) and adjust the audio accordingly for appropriate playback response. For example, the audio signals may be adjusted by changing audio equalization or limiting parameters based on whether or not the removable accessory is installed. Where there are multiple sized/shaped removable accessories to accommodate differing head and ear geometries or accessories that can be used to selectively cover or expose certain ports, different audio equalization could be applied for each variant. In this case, there would need to be a method for detecting which specific removable accessory had been attached. This could be accomplished via a bar code or QR code that is scanned when the removable accessory is attached/installed, via a near field communication (NFC) tag and reader or any other suitable method. 
     Elements of  FIG.  11    are shown and described as discrete elements in a block diagram. These may be implemented as one or more of analog circuitry or digital circuitry. Alternatively, or additionally, they may be implemented with one or more microprocessors executing software instructions. The software instructions can include digital signal processing instructions. Operations may be performed by analog circuitry or by a microprocessor executing software that performs the equivalent of the analog operation. Signal lines may be implemented as discrete analog or digital signal lines, as a discrete digital signal line with appropriate signal processing that is able to process separate signals, and/or as elements of a wireless communication system. 
     When processes are represented or implied in the block diagram, the steps may be performed by one element or a plurality of elements. The steps may be performed together or at different times. The elements that perform the activities may be physically the same or proximate one another, or may be physically separate. One element may perform the actions of more than one block. Audio signals may be encoded or not, and may be transmitted in either digital or analog form. Conventional audio signal processing equipment and operations are in some cases omitted from the drawing. 
     Examples of the systems and methods described herein comprise computer components and computer-implemented steps that will be apparent to those skilled in the art. For example, it should be understood by one of skill in the art that the computer-implemented steps may be stored as computer-executable instructions on a computer-readable medium such as, for example, floppy disks, hard disks, optical disks, Flash ROMS, nonvolatile ROM, and RAM. Furthermore, it should be understood by one of skill in the art that the computer-executable instructions may be executed on a variety of processors such as, for example, microprocessors, digital signal processors, gate arrays, etc. For ease of exposition, not every step or element of the systems and methods described above is described herein as part of a computer system, but those skilled in the art will recognize that each step or element may have a corresponding computer system or software component. Such computer system and/or software components are therefore enabled by describing their corresponding steps or elements (that is, their functionality), and are within the scope of the disclosure. 
     A number of implementations have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein, and, accordingly, other examples are within the scope of the following claims.