Patent Application
20130058493
This invention relates to in-ear devices incorporating active noise reduction. Such devices include but are not limited to earphones, “in-ear monitors”, hearing aids and similar assisted listening devices. Moreover, the term “in-ear” includes devices that may be partially located in the human auditory canal.
The active noise reduction functionality relevant to the present invention is realized using “feedback” or hybrid (a combination of feedback and feed-forward) control architectures, in which a or a plurality of sensors which include but are not limited to a microphone is located inboard (i.e. closer to the wearer's ear) of the “receiver” (miniature loudspeaker or driver) in the device. The output of the microphone is used to provide the observation required for feedback (or equivalent) control of the pressure in the ear. Those skilled in the art will understand that systems using pure “feed-forward” controllers do not require the presence of such an inboard microphone.
The in-ear device typically has a housing in which the driver and microphone are located, and which provides an acoustic path from the driver to the outlet of the in-ear device. The outlet is in use located in the ear canal, so that the acoustic signal from the outlet can be delivered to the tympanic membrane (also known as the ear drum).
Positioning of a sensing microphone inboard of the driver requires the microphone is located in the acoustic path between the driver and the outlet. Thus the sound generated by the driver is required to pass around the partial obstacle constituted by the microphone (the body of which is acoustically opaque) in travelling to the ear drum. In existing constructions sufficient space is left around the microphone so that there is no significant acoustic impedance.
The considerations at hand for designing in-ear devices having active noise reduction using feedback control are very different to those present with headphones, or feed-forward architectures. In particular, stability is an issue as the load condition of the device can greatly affect the open loop transfer function (OLTF). This dynamic is the main constraint during the design of the active noise reduction functionality performance of the device. In fact, the larger the dynamic of the OLTF the larger the stability margin of the closed loop system must be in order to ensure the robustness of the active noise reduction functionality.
Further miniaturisation is only exacerbating these issues. For example, commonly used electrodynamic drivers see their source impedance increasing with the inverse of the square of the diaphragm area. Additional measures must therefore be taken to ensure the stability and performance of the system since they become increasingly sensitive to their loads. In addition, moving the electronics required to create a feedback controller, or part thereof, inside the housing would require considerable miniaturisation of the acoustic system to achieve a decrease in overall size of the device and consequently a carefully tuned OLTF would be required to reduce the necessary controller complexity to design a high performance noise cancellation system. The use of acoustic impedances as described in embodiments of the present invention provides a solution for tuning the OLTF for a miniaturised in-ear device incorporating active noise reduction through feedback or hybrid control architectures.
It is an object of the invention to provide improved active noise cancellation performance in an in-ear device or to at least provide the public with a useful alternative to existing devices.
Other objects of the invention may become apparent from the following description, which is given by way of example only.
In one aspect the invention provides an in-ear device comprising:
In one embodiment the high acoustic impedance is such that the impedance of the device from the driver input to the microphone output over a selected audio frequency range is greater than the impedance of the driver over the selected audio frequency range.
In one embodiment the frequency range comprises the mid-range audio frequencies.
In one embodiment the frequency range is 1 kHz to 2 kHz.
In one embodiment the frequency range is 200 Hz to 2 kHz.
In one embodiment the frequency range is 1 kHz to 2.5 kHz.
In one embodiment the acoustic impedance is provided by a constriction in the acoustic path.
In one embodiment the acoustic impedance is provided at a periphery of the microphone. Preferably the impedance is provided between the periphery of the microphone and a wall of the device.
In another embodiment the acoustic impedance is provided between the microphone and the outlet.
In one embodiment the impedance comprises an acoustic resistance
In one embodiment the impedance is selected to improve stability of the device when used in an active noise reduction feedback or hybrid control architecture.
In one embodiment the impedance is selected to attenuate noise from a source external to the device, also referred to as passive attenuation.
In one embodiment the impedance comprises a plurality of pathways arranged around the periphery of the microphone. Preferably the pathways are parallel with the axis of symmetry of the microphone. Preferably the multiple pathways are disposed in a regular distribution around the circumference of the microphone.
In another aspect the invention provides an in-ear device comprising:
In one embodiment the high acoustic impedance is such that the impedance of the device from the driver input to the microphone output over a selected audio frequency range is greater than the impedance of the driver over the selected audio frequency range.
In one embodiment the frequency range comprises the mid-range audio frequencies.
In another aspect the invention provides a method of improving the stability of an in-ear device having:
In one embodiment the method includes the steps of determining the impedance of the driver over a selected audio frequency range, and selecting the acoustic impedance such that the impedance of the device from the driver input to the microphone output is greater than the impedance of the driver over the selected audio frequency range.
In one embodiment the frequency range comprises the mid-range audio frequencies.
Other aspects of the invention will be apparent from the following description.
One or more embodiments of the invention will be described below with reference to the accompanying drawings, in which:
The present invention relates specifically to the design of the acoustic path or conduit between the driver and the outlet of the device. In some embodiments the invention is realised in the design of the conduits/passageways through which sound is conducted around the microphone. The acoustic impedance of these elements may be designed so as to engineer the electro-acoustic transfer function between the input to the driver and the output from the microphone, which constitutes (a component of) the “open-loop transfer function” (OLTF) of the “system-under-control” or “plant” (to use the terminology of automatic control). This transfer function is a key determinant of system stability and noise cancelling performance. The desired improvement in robustness of the closed loop system is achieved by decreasing the dynamic (i.e. the variation) of the OLTF with regard to vulnerability and sensitivity of the earphone to varying load conditions. Those skilled in the art will understand that a device designed within the ambit of the present invention will exhibit improvements in the active noise reduction functionality performance compared to the same system outside it.
In one embodiment, the present invention teaches the deliberate design of acoustic path(s) around the inboard microphone of an in-ear device in order to introduce desirable properties to the overall system, specifically in terms of robust controllability.
It has been shown to be beneficial in many noisy environments that the device be capable of actively cancelling, (or at least substantially reducing) sounds that propagate by various paths from external ambient noise fields to the ear. This can usefully be achieved by control strategies in which there is direct observation of the pressures within the (partially) sealed system comprising the device and the remaining volume of the wearer's meatus. For convenience, such observation is provided by a microphone (6) incorporated within the body of the device.
The interaction between the acoustic network constituted by the acoustic output “port” or outlet of the device (4) (which embodies a substantially inductive acoustic impedance) and the volume of air in the meatus (3) (which behaves to first order of approximation as a compliance) is known to exhibit non-trivial acoustic behaviour, introducing a “Helmholtz” resonance. This has been identified as means to optimise the performance of an Active Noise Reduction (ANR) enabled system, by favourably influencing the transfer function between receiver input and microphone output (Vmic/Vreceiver of
The present invention addresses the path(s) by which sound is conducted around the microphone (7). These paths express a series acoustic impedance, the optimisation of which constitutes another means for adjusting the overall acoustic (and electro-acoustic) performance of the device, with consequent impact on system stability and performance. The existence of such paths is unique to those applications where an inboard microphone is present (typically those in which it is intended to apply feedback control, hybrid feedback/feed-forward control, or adaptive control).
There are a number of options available for modelling the OLTF of the system. One of the ways to model the OLTF of the system is further illustrated by
These impedance modifiers can be, but are not limited to, path constrictions, expressing both acoustic inductance and resistance. In order to make these impedances sufficiently large to have significance to the overall system dynamics, the paths typically have small cross-sectional area (with carefully defined aspect ratio) and specified length. They have been found to have one embodiment as a series of “slits”, regularly disposed around the periphery of the microphone, as discussed further below with reference to
The slits referred to above can be used in embodiments where the microphone faces toward the driver, see
Although some of the examples taught in this specification relate to the case in which the microphone is intentionally sensitive to pressures “upstream” of the impedance embodied by the acoustic path(s) around the microphone (as depicted in
An embodiment in which the acoustic path around the microphone is provided in the form of engineered slits is shown in
The addition of the high impedance in the acoustic path and having the microphone facing towards the driver dampens the Helmholtz resonance, providing a smoother, more regular and larger phase increase, whilst reducing the magnitude and phase differences in the OLTF for a number of key loading conditions, which are typically found during the use of such a product. The former aspect is illustrated in
As can be seen, the addition of the impedance increases the performance of the system and improves the stability, although the optimization of this last point is discussed next. Referring again to
This is illustrated in
It should be noted that the practical implementation of the “optimized” internal dimensions and transmittance properties must account for:
The embodiments described above teach the use of the inboard microphone as an obstructing object, around which we establish sound-carrying “conduits”, the acoustics of which are designed to optimize other features of the closed loop system and enhance its performance. In the embodiments described below these engineered impedances are not necessarily “around” the microphone, but are located at other (or additional) locations in the acoustic path between the driver and the outlet port.
Referring to
The addition of engineered acoustic high impedances in the acoustic path modifies the OLTF dynamics. It reduces the difference between the wearing load conditions, as illustrated in this example when coupled with the IEC711 ear simulator, and under closed pipe load condition. The controller can therefore be designed with smaller but still sufficient stability margins to cope with the reduced range of realisable OLTF and/or increase the useable feedback gain with a constant stability margin and/or widen the frequency range covered by the noise cancelation function.
As outlined above, the Two Port representation provides a convenient model to express the system in terms of transmittance and impedances, as approximations of the loads and source impedance of the different parts of the acoustic system can be easily calculated. As an example, in cases where a multiplicity of conduits are employed, their acoustic impedances will (to first order of approximation) act in parallel. It is convenient (though not necessary) that the dimensions (and, therefore, acoustic impedance) of each of a multiplicity of such pathways are equal.
An introduction to the TwoPort network method is shown in Table 1 below. Further information is available in M E Van Valkenberg Network analysis, 3rd ed., Prentice Hall (1974).
A TwoPort representation of an electrodynamic loudspeaker (other types of transducers have other Twoport representations) where the inputs are the usual electrical variables and outputs are the usual acoustical variables is:
In which:
The source impedance Zsource of the driver can be calculated as:
The two Port method can be used to characterise acoustic networks. Example of a Two Port of a uniform lossless acoustic waveguide of section S and length L, has the acoustic variables at each end related by a Two Port in which k is the wave number:
The elements of an unknown Two Port can be determined performing some measurements according to Egolf, D. P., and Leonard, R. G. (1977) “Experimental scheme for analyzing the dynamic behavior of electro-acoustic transducers,” J. Acoust. Soc. Am. 62, 1013-1023. Also, many acoustic elements and their lumped parameters equivalent circuits are showed in J. Borwick Loudspeaker and headphone handbook, 3rd ed., 2001, 9780240515786, p 588.
The introduction of one or more high impedance pathways within the body of an in-ear device increases the overall series impedance of the device. This has the generally beneficial effect of reducing the transmission of unwanted noise through the body of the device to the ear (i.e. it will increase the passive attenuation of the device). This is particularly important in the case of dynamic receivers, which may require openings to the rear of their diaphragm in order to avoid undesirable high-compliance loading on the diaphragm. Sound from an external ambient noise field can pass through these openings, through the diaphragm and onward to the ear. The introduction of high impedance obstacles in this subsequent path to the ear is seen to afford means to control the level of attenuation provided by this noise transmission path.
The introduction of the engineered acoustic conduits around the microphone increases the acoustic source impedance of the device. Note the small drivers used in these implementations have high source impedance (in the order of 5.6M Rayl compared to typically 415 Rayl for the air and 1.8M rayls for the IEC711) and thus are very sensitive to load variation hence care that must be taken in designing the acoustic conduits. This has known (and, potentially damaging) consequences to aspects of system performance (including sensitivity, leak sensitivity, stability and frequency response). Notwithstanding these consequences, the conduits offer overall benefit in giving the designer more control over the open loop response, Vmic/Vreceiver.
The paths or conduits may take simple form (such as a uniform section pipe) or more complex form (including for example bent pipes, concatenated pipes of changing cross-section, etc). In the case of the simpler, similarly simple models of the acoustic impedance (such as inductances and resistances) will appear as the first-order models of the conduit transfer acoustics. This may permit adequate parameterisation of the path to allow optimisation of aspects of overall system behaviour. More complete modelling of the acoustics of simple conduit forms—or modelling of the acoustics of more complex forms—both will motivate more sophisticated statements of the impedance (such as the generalized solutions that may arise from finite element analysis or similar numerical models). The conduits are then designed such that these generalized impedances confer the desired significant impedance compared to the driver source impedance.
Other forms of acoustic impedance may be used, apart from, or in combination with, the slits or constrictions described above. Thus for example an acoustically resistive mesh may be located at area 7 in the
Referring now to
The in-ear device (locus 20) in
The use of a high impedance load as described in this document between the error microphone and ear has the benefits that it:
1. Defines a large impedance, which becomes the dominant factor in the series of impedances as described above), and as such:
a. Reduces the sensitivity of the OLTF to loading conditions
b. Reduces the sensitivity to the design of the earphone itself around the encapsulated driver and microphone of the product
2. The increase in inductance lowers the Helmholtz resonance (the resonance described in international patent publication WO 2007/054807 “Noise Cancellation Earphone”).
3. The specified resistance will ensure that the transmission line between the driver and microphone is loaded by a large resistance at the Helmholtz resonance (i.e. it damps a resonance that is otherwise a core feature of in-ear device acoustics) thus:
The specified impedance is therefore ‘optimized’ by balancing these points and the other design parameters:
Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth.
Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the spirit or scope of the appended claims.
This is a utility application that claims the benefit of and priority from U.S. Provisional Patent Application Ser. No. 61/491,983, filed Jun. 1, 2011.
| Number | Date | Country | |
|---|---|---|---|
| 61491983 | Jun 2011 | US |
