Abstract:
Embodiments are disclosed relating to an active noise reducing system and method for a headphone with a rigid cup-like shell which has an outer surface and an inner surface that encompasses a cavity with an opening. The system and method include picking up sound at least at three positions that are regularly distributed over the outer surface, and providing a first electrical signal that represents the picked-up sound. The system and method further include: filtering the first electrical signal to provide a second electrical signal, and generating in the opening of the cavity sound from the second electrical signal. Filtering is performed with a transfer characteristic that is configured so that noise that travels through the shell from beyond the outer surface to beyond the inner surface is reduced by the sound generated in the opening.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to EP Application Serial No. 15167002.3 filed May 8, 2015, the disclosure of which is hereby incorporated in its entirety by reference herein. 
       TECHNICAL FIELD 
       [0002]    The disclosure relates to active noise control (ANC) headphones and a method for operating ANC headphones. 
       BACKGROUND 
       [0003]    Headphones may include active noise reduction, also known as active noise cancelling (ANC). Generally, noise reduction may be classified as feedback noise reduction or feedforward noise reduction or a combination thereof. In a feedback noise reduction system a microphone is positioned in an acoustic path that extends from a noise source to the ear of a listener. A speaker is positioned between the microphone and the noise source. Noise from the noise source and anti-noise emitted from the speaker are collected by the microphone and, based on the residual noise thereof, the anti-noise is controlled to reduce the noise from the noise source. In a feedforward noise reduction system, a microphone is positioned between the noise source and the speaker. The noise is collected by the microphone, is inverted in phase and is emitted from the speaker to reduce the external noise. In a combined feedforward/feedback (hybrid) noise reduction system, a first microphone is positioned in the acoustic path between the speaker and the ear of the listener. A second microphone is positioned in the acoustic path between the noise source and the speaker and collects the noise from the noise source. The output of the second microphone is used to make the transmission characteristic of the acoustic path from the first microphone to the speaker the same as the transmission characteristic of the acoustic path along which the noise from the noise source reaches the listener&#39;s ear. The speaker is positioned between the first microphone and the noise source. The noise collected by the first microphone is inverted in phase and emitted from the speaker to reduce the external noise. It is desired to improve the known headphones in order to reduce the noise emitted by a multiplicity of noise sources from a multiplicity of directions. 
       SUMMARY 
       [0004]    An active noise reducing headphone comprises a rigid cup-like shell having an inner surface and an outer surface, wherein the inner surface encompasses a cavity with an opening. The headphone further comprises a microphone arrangement configured to pick up sound at least at three positions that are regularly distributed over the outer surface, and to provide a first electrical signal that represents the picked-up sound, and an active noise control filter configured to provide, based on the first electrical signal, a second electrical signal. Furthermore, the headphone comprises a speaker disposed in the opening of the cavity and configured to generate sound from the second electrical signal. The active noise control filter has a transfer characteristic that is configured so that noise that travels through the shell from beyond the outer surface to beyond the inner surface is reduced by the sound generated by the speaker. 
         [0005]    An active noise reducing method is disclosed for a headphone with a rigid cup-like shell which has a convex surface and a concave surface that encompasses a cavity with an opening. The method comprises picking up sound at least at three positions that are regularly distributed over the convex surface, and providing a first electrical signal that represents the picked-up sound. The method further comprises: filtering the first electrical signal to provide a second electrical signal, and generating in the opening of the cavity sound from the second electrical signal. Filtering is performed with a transfer characteristic that is configured so that noise that travels through the shell from beyond the convex surface to beyond the concave surface is reduced by the sound generated in the opening. 
         [0006]    Other systems, methods, features and advantages will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The disclosure may be better understood from the following description of non-limiting embodiments with reference to the attached drawings, wherein below: 
           [0008]      FIG. 1  is a simplified illustration of an exemplary feedback type active noise control (ANC) earphone; 
           [0009]      FIG. 2  is a simplified illustration of an exemplary feedforward type ANC earphone; 
           [0010]      FIG. 3  is a simplified illustration of an exemplary hybrid type ANC earphone; 
           [0011]      FIG. 4  is a simplified illustration of an exemplary earphone with a conventional single small (reference) microphone; 
           [0012]      FIG. 5  is a simplified illustration of an exemplary earphone with an areal (reference) microphone; 
           [0013]      FIG. 6  is a simplified illustration of an exemplary earphone with a (reference) microphone array that approximates an areal microphone; 
           [0014]      FIG. 7  is a simplified circuit diagram of a circuit connected downstream of the microphone array shown in  FIG. 6 ; 
           [0015]      FIG. 8  is a simplified illustration of an exemplary array of microphones regularly arranged over the shell of an earphone; and 
           [0016]      FIG. 9  is a simplified illustration of another exemplary earphone with a microphone array and a shell having a barrel-like shape. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a simplified illustration of an exemplary feedback type active noise control (ANC) earphone  100  (e.g., as part of a headphone with two earphones). An acoustic path (also referred to as channel), represented by a tube  101 , is established by the ear canal, also known as external auditory meatus, and parts of the earphone  100 , into which noise, i.e., primary noise  102 , is introduced at a first end  109  from a noise source  103 . The sound waves of the primary noise  102  travel through the tube  101  to the second end  110  of the tube  101  from where the sound waves are radiated, e.g., to the tympanic membrane of a listener&#39;s ear  104  when the earphone  100  is attached to the listener&#39;s head. In order to reduce or cancel the primary noise  102  in the tube  101 , a sound radiating transducer, e.g., a speaker  105 , introduces cancelling sound  106  into the tube  101 . The cancelling sound  106  has an amplitude corresponding to or being the same as the primary noise  102 , however, of opposite phase. The primary noise  102  which enters the tube  101  is collected by an error microphone  107  and is processed by a feedback ANC processing module  108  to generate a cancelling signal and then emitted by the speaker  105  to reduce the primary noise  102 . The error microphone  107  is arranged downstream of the speaker  105  and thus is closer to the second end  110  of the tube  101  than to the speaker  105 , i.e., it is closer to the listener&#39;s ear  104 , in particular to its tympanic membrane. 
         [0018]      FIG. 2  is a simplified illustration of an exemplary feedforward type ANC earphone  200 . The earphone  200  differs from the earphone  100  shown in  FIG. 1  in that a microphone  201  is arranged between the first end  109  of the tube  101  and the speaker  105 , instead of being arranged between the speaker  105  and the second end  110  of the tube  101  as is microphone  107  in the earphone  100  shown in  FIG. 1 . Furthermore, instead of the feedback ANC processing module  108 , a feedforward ANC processing module  202  is connected between the microphone, i.e., microphone  201 , and speaker  105 . The feedforward ANC processing module  202  as shown may be, for example, a non-adaptive filter, i.e., a filter with fixed transfer function, but can alternatively be adaptive in connection with an additional error microphone  203  which is disposed between the speaker  105  and the second end  110  of the tube  101  and which controls (the transfer function of) the feedforward ANC processing module  202 . 
         [0019]      FIG. 3  is a simplified illustration of an exemplary hybrid type ANC earphone  300 . Based on the headphones  100  and  200  described above in connection with  FIGS. 1 and 2 , the (reference) microphone  201  senses the primary noise  102  and its output is used to model the transmission characteristic of a path from the speaker  105  to the (error) microphone  107 , such that it matches the transmission characteristic of a path along which the primary noise  102  reaches the second end  110  of the tube  101 . The primary noise  102  and sound radiated from the speaker  105  are sensed by the (error) microphone  107 , inverted in phase using the adapted (e.g., estimated) transmission characteristic of the signal path from the speaker  105  to the error microphone  107  and is then emitted by the speaker  105  disposed between the two microphones  201  and  107 , thereby reducing the undesirable noise at the listener&#39;s ear  104 . Signal inversion, transmission path modeling (estimation) and, as the case may be, adaptation are performed by a hybrid ANC processing module  301 . For example, the hybrid ANC processing module  301  may include a feedforward processing module similar to the feedforward ANC processing module  202  shown in  FIG. 2  to process the signal from microphone  201 , and a feedback processing module similar to the feedback ANC processing module  108  shown in  FIG. 1  to process the signal from microphone  107 . 
         [0020]    In an exemplary earphone  400  (part of a feedfoward ANC headphone with two earphones) shown in  FIG. 4 , a rigid cup-like shell  401  has an inner, e.g., convex surface  402 , and an outer, e.g., concave surface  403  which encompasses a cavity  404  with an opening  405 . An electro-acoustic transducer for converting electrical signals into sound, such as a speaker  406 , is disposed in the opening  405  of the cavity  404  and generates sound from an electrical signal provided by an active noise control filter  407 . The active noise control (ANC) filter  407  is commonly supplied with an electrical signal from only a single (reference) microphone  408 , which picks up sound at only one position on the convex surface  402  of the shell  401 . The ANC filter  407  may, for example, be configured to provide feedforward type or hybrid type active noise control. Even if the microphone  408  has an omni-directional characteristic, a share  410  of the sound emitted by a noise source  409  may be picked-up by microphone  408  while another share  411  may be not. However, both shares  410  and  411  may reach the ear of a listener (not shown) wearing the headphones so that the sound picked-up by the microphone  408  and, thus, the electrical signal corresponding to the picked-up sound does not or does not fully represent the sound arriving at the listener&#39;s ear. How much the microphone signal corresponds to the sound perceived by the listener depends on the position and the directivity of the noise source  409 . As a consequence, the noise reduction performance of the headphones is, inter alia, dependent on the position of the noise source  409  relative to the position of the microphone  408  and the directivity of the noise source  409 . 
         [0021]    In an exemplary earphone  500  shown in  FIG. 5  which is based on the earphone  400  shown in  FIG. 4 , the microphone  408  is substituted by an areal microphone  501  (i.e., a microphone with an extended membrane area) that may cover more than 50%, e.g., more than 75%, more than 90%, or up to 100% of the area of the convex surface  401 . The areal microphone  501  may be made from any pressure or force sensitive film such as, for example, ElectroMechanical Film (EMFi) which is an electret material with a cellular structure. EMFi&#39;s advantage over other solid polymer electrets is based on its flexibility due to the voided internal structure combined with a strong permanent charge, which makes EMFi very sensitive to dynamic forces exerted normal to its surface. The base material may be low-priced polypropylene (PP). 
         [0022]    EMFi may consist of several polypropylene layers separated by air voids. An external force exerted to the film&#39;s surface will change the thickness of the air voids. The charges residing on the polypropylene/void interfaces will then move in respect to each other, and as a result a mirror charge is generated to the electrodes. The generated charge is proportional to the change of the film thickness. Because of the elasticity of the material, the generated charge is proportional also to the force (or pressure) acting on the film. The basic voided PP-film is manufactured by biaxially orienting a specially fabricated polymer, performed in a continuous process, that forms the cellular structure. More detailed description of the EMFi can be found, e.g., in U.S. Pat. No. 4,654,546 or Jukka Lekkala and Mika Paajanen, “EMFi—New Electret Material for Sensors and Actuators”, 10th International Symposium on Electrets, 1999. During the manufacturing process, the EMFi material is charged by a corona discharge arrangement. Finally, the film is coated with electrically conductive electrode layers, completing the EMFi structure. The film has three layers, of which the few microns thick surface layers are smooth and homogeneous, whereas the dominant, thicker mid-section is full of flat voids separated by leaf-like PP-layers. 
         [0023]    Alternatively, an areal microphone may be approximated by way of a multiplicity of microphones  601  each with a significantly smaller membrane area than the areal microphone to be approximated. Microphones  601  form a microphone array and are regularly distributed over the convex surface  402  and the directivities of the microphones  601  may be such that they overlap so that for any solid angle of a semi-sphere at least one of the microphones  601  directly receives the noise from a directional noise source at any position. 
         [0024]    For example, the microphones  602  may have an omnidirectional characteristic and their output signals may be summed up as shown in  FIG. 7  by way of a summer  701  to provide an output signal that may substitute the output signal of areal microphone  501  described above in connection with  FIG. 5 . Due to the summing-up of the microphone output signals, the array of the microphones  602  exhibit a similar directional behavior as the areal microphone, which means it can be seen as a sensor that acoustically captures the zeroth room mode. Furthermore, due to the summing-up of the microphone output signals, noise generated by the microphones is reduced by 10 log 10  (N) [dB], wherein N is the number of microphones used. On top of that, commonly the noise behavior of small membrane microphones  602  is already per se better than that of the areal microphone  501 . 
         [0025]      FIG. 8  is a front view of the array of the microphones  602 , a lateral view of which is shown in  FIG. 6 . As can be seen, the microphones are regularly distributed over the convex surface  402  which means that the microphones  602  may be formed, built, arranged, or ordered according to some established rule, law, principle, or type. Particularly, the microphones  602  may be arranged both equilaterally and equiangularly as a regular polygon (two-dimensional arrangement) or may have faces that are congruent regular polygons with all the polyhedral angles being congruent as a regular polyhedron (three-dimensional arrangement). For example, three microphones  602  may be used which can be arranged at the corners of an equilateral triangle. Other arrangements may have four microphones disposed in the corners of a square. A multiplicity of arrangements of regularly distributed three or four microphones or more may be combined to form more complex arrangements. For example,  FIG. 8  shows a rhombus-like arrangement of thirteen microphones  602 . 
         [0026]    The shell may have various forms such as, for example, a dish-like shape as in the headphone shown in  FIGS. 4-6  or a barrel-like shape as shown in  FIG. 9  (shell  901 ) where the microphones  602  are disposed on a bottom wall  902  as well as on a sidewall  903  of a barrel. The ANC filter  407 , e.g., in connection with a feedforward ANC or hybrid ANC processing module, may be of a conventional type whose basic adaptive and non-adaptive structures are described, for example, in Sen M. Kuo and Dennis R. Morgan, “Active Noise Control: A Tutorial Review”, Proceedings of the IEEE, Vol. 87, No. 6, June 1999. 
         [0027]    The description of embodiments has been presented for purposes of illustration and description. Suitable modifications and variations to the embodiments may be performed in light of the above description or may be acquired from practicing the methods. For example, unless otherwise noted, one or more of the described methods may be performed by a suitable device and/or combination of devices. The described methods and associated actions may also be performed in various orders in addition to the order described in this application, in parallel, and/or simultaneously. The described systems are exemplary in nature, and may include additional elements and/or omit elements. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed. 
         [0028]    As used in this application, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is stated. Furthermore, references to “one embodiment” or “one example” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. The terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects.