Patent Publication Number: US-2016232889-A1

Title: Apparatus and method for non-occluded active noise shaping

Description:
RELATED APPLICATION INFORMATION 
     This patent claims priority from provisional patent application No. 62/113,977, filed Feb. 9, 2015, titled SYSTEM AND METHOD FOR NON-OCCLUDED ACTIVE NOISE SHAPING. 
    
    
     NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     1. Field 
     This disclosure relates to ear pieces that shape or suppress ambient sound. 
     2. Description of the Related Art 
     Active noise suppression headphones are effective at removing unwanted background noise while listening to music, taking phone calls, or resting quietly during travel or in other noisy situations. These head phones, whether in-ear, on-ear, or over-ear, universally employ the same successful recipe: passively attenuate high frequencies with structures, then actively cancel the low frequencies with analog and/or digital electronics. However, despite their relative success, these headphones suffer from the annoying and uncomfortable problem of occlusion. 
     Occlusion is the blocking and enclosure of the ear drum in its own pressurized volume. When this volume is relatively small, as is the case with ear buds, it exacerbates low-frequency fluctuations caused by motion and ambient pressure changes. Additional small fluctuations in pressure emitted by the ear bud&#39;s speaker and caused by imperfections in noise cancelling algorithms may add to the unpleasant vertiginous feelings many feel with occlusion. 
     Occlusion also comes with significant disappointments in auditory experience. Especially, sound from one&#39;s own voice does not travel by the usual air path into the ear canal but instead is conducted through bone and flesh. The voice is somewhat muted and high frequencies are attenuated, with the net result a feeling of isolation and introversion. 
     A further shortcoming of the traditional occluding devices is their inability to let desired sound pass un-attenuated. Because of the large broadband passive attenuation, any sound one intentionally desires to hear must be captured with an external microphone and replayed through the internal speaker. This works, but even the best electronics fail to achieve the clarity and enjoyment provided by a simply open ear canal. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a non-occluding active noise shaping apparatus. 
         FIG. 2  is a chart showing the phase shift of a speaker as a function of frequency. 
         FIG. 3  is a cross-sectional schematic view of a non-occluding speaker. 
         FIG. 4  is a perspective exploded view of a non-occluding speaker. 
         FIG. 5  is a perspective view of an assembled non-occluding speaker. 
         FIG. 6A ,  FIG. 6B , and  FIG. 6C  are a side view, a perspective view, and a partially sectioned view, respectively, of a serpentine acoustic delay line. 
         FIG. 7  is an exploded perspective view of a non-occluding active noise shaping apparatus. 
         FIG. 8  is a perspective view of the non-occluding active noise shaping apparatus. 
         FIG. 9  is a flow chart of a process for suppressing noise. 
     
    
    
     Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator. 
     DETAILED DESCRIPTION 
     Description of Apparatus 
     Simplifying for the sake of explanation, all active noise suppression systems seek to cancel sound by creating anti-sound that destructively interferes with the ambient sound in order to create silence. Typical active noise suppression ear pieces are occluding and subject to the previously discussed issues. 
       FIG. 1  is a block diagram of a non-occluding active noise suppression apparatus  100 . The non-occluding active noise suppression apparatus  100  includes an ambient microphone  110 , an audio processor  120 , a speaker  130 , and an acoustic delay line  160 , an optional passive low-pass filter  165 , and a battery (not shown), all of which may be contained within a housing  180 . The non-occluding active noise suppression apparatus  100  may optionally include an internal microphone  140 , and a wireless interface  150 . The non-occluding active noise suppression apparatus  100  may receive ambient sound  105  and output personal sound  170 . In this context, the term “sound” refers to acoustic waves propagating in air. “Personal sound” means sound (acoustic waves propagating in air) that has been processed, modified, or tailored in accordance with a user&#39;s personal preferences. When the non-occluding active noise suppression apparatus  100  is operating to cancel the ambient sound to the extent possible, the person sound  170  may be silence. The term “audio” refers to an electronic representation of sound, which may be an analog signal or a digital data. In  FIG. 1 , dashed arrows represent sound and solid arrows represent audio and other signals. 
     The housing  180  may be configured to interface with a user&#39;s ear by fitting in, on, or over the user&#39;s ear such that the ambient sound  105  (other than ambient sound that passes through the non-occluding active noise suppression apparatus  100 ) is mostly excluded from reaching the user&#39;s ear canal and the personal sound  170  generated by the non-occluding active noise suppression apparatus  100  is provided directly into the user&#39;s ear canal. The housing  180  may have at least one inlet  182  for accepting the ambient sound  105  and an outlet  184  to allow the personal sound  170  to be output into the user&#39;s outer ear canal. The housing  180  may be, for example, an earbud housing. The term “earbud” means an apparatus configured to fit, at least partially, within and be supported by a user&#39;s ear. An earbud housing typically has a portion that fits within or against the user&#39;s outer ear canal. An earbud housing may have other portions that fit within the concha or pinna of the user&#39;s ear. 
     The depiction in  FIG. 1  of the non-occluding active noise suppression apparatus  100  as a set of functional blocks or elements does not imply any corresponding physical separation or demarcation. All or portions of one or more functional elements may be located within a common circuit device or module. Any of the functional elements may be divided between two or more circuit devices or modules. For example, all or portions of the audio processor  120  and the wireless interface  150  may be contained within a common signal processor circuit device or may be divided between two or more circuit devices. 
     The non-occluding active noise suppression apparatus  100  provides two paths, an acoustic path  195  and an electronic path  190 , for sound to travel from the inlet  182  to the outlet  184 . To prevent occlusion, the acoustic path  195  couples ambient air pressure from the inlet  182  to the outlet  184 . Along the electronic path  190 , a first portion of the ambient sound  105  is converted to an ambient audio signal  112  by the ambient microphone  110 . The ambient audio signal  112  is processed by the audio processor  120  to provide a processed audio signal  122  that is converted into processed sound  132  by the speaker  130 . Along the acoustic path  195 , a second portion of the ambient sound  105  passes through the acoustic delay line  160 . The delayed ambient sound  162  from the acoustic delay line  160  and the processed sound  132  from the speaker  130  acoustically combine in a mixing volume  172  proximate the outlet  184  to form the personal sound  170 . The mixing volume  172  may be or include a small volume between the speaker  130  and the outlet  184  within the housing  180 . The mixing volume  172  may be or include a portion of the user&#39;s ear canal (not shown). A portion of the personal sound  170  may be converted into a feedback audio signal  142  by the internal microphone  140 . The feedback audio signal  142  may be provided to the audio processor  120 . 
     The audio processor  120  may be an analog processor that processes the ambient audio signal  112  and the feedback audio signal  142 , if present, to provide the processed audio signal  122 . Preferably, the audio processor  120  may include one or more digital processor devices such as microcontrollers, microprocessors, digital signal processors, application specific integrated circuits (ASICs), or a system-on-a-chip (SOCs). In this case, the audio processor  120  may include circuits (e.g. preamplifiers and analog-to-digital converters) to convert the ambient audio signal  112  and the feedback audio signal  142  into ambient and feedback audio streams. In this context, the term “stream” means a sequence of digital samples. Further, the audio processor  120  may include circuits (e.g. a digital-to-analog converter and an amplifier) to convert digital processed audio data into the processed audio signal  122  to drive the speaker  130 . 
     The audio processor  120  may include and/or be coupled to memory (not shown). The memory may store software programs, which may include an operating system, for execution by the audio processor  120 . The memory may also store data for use by the audio processor  120 . The data stored in the memory may include, for example, digital sound samples and intermediate results of processes performed on the ambient and feedback audio streams. The memory may include a combination of read-only memory, flash memory, and static or dynamic random access memory. 
     The wireless interface  150  may provide the audio processor  120  with a connection to one or more wireless networks using a limited-range wireless communications protocol such as Bluetooth®, WiFi®, ZigBee®, or other wireless personal area network protocol. The wireless interface  150  may be used to receive data such as parameters for use by the audio processor  120  in processing the ambient audio signal  112  to produce the personal audio signal  122 . The wireless interface  150  may be used to receive a secondary audio feed. The wireless interface  150  may be used to export the personal audio signal  122 , which is to say transmit the personal audio signal  122  to a device external to the non-occluding active noise suppression apparatus  100 . The external device may then, for example, store and/or publish the personal audio stream, for example via social media. 
     The audio processor  120  performs noise cancellation processing, which is to say the audio processor processes the ambient audio signal  112  and the feedback audio signal  142 , if present, to produce a processed audio signal  122  that causes the speaker  130  to form processed sound  132  that includes anti-sound to cancel at least a portion of the delayed ambient sound  162 . The audio processor  120  may perform other processes to enhance or modify portions of the ambient sound that are not cancelled. Processes that may be performed include filtering, equalization, compression, limiting, noise reduction, echo cancellation, and/or other processes. 
     To cancel all or a portion of the delayed ambient sound  162 , the anti-sound  132  emitted from the speaker  130  must destructively interfere. In overly simple terms, destructive interference occurs when the anti-sound  132  has a similar amplitude and opposite polarity as the delayed ambient sound  162 , which is to say the anti-sound results in air motion in the opposite direction to that of the delayed ambient sound. For a single frequency, destructive interference will occur if the anti-sound  132  and the delayed ambient sound  162  are equal in amplitude and shifted in phase by 180 degrees. To cancel noise over a frequency range, it is necessary for the phase shift between the anti-sound  132  and the delayed ambient sound  162  to be substantially 180 degrees over the frequency range. In this context, “substantially 180 degrees” means “sufficiently close to 180 degrees to provide significant cancellation.” For example, a ten degree phase error (i.e. a phase shift of 170 or 190 degrees) at a particular frequency allows cancellation of up to 97% of the noise power at that frequency. An eighteen degree phase error at a particular frequency allows cancellation of up to 90% of the noise power at that frequency. 
     A typical human ear can detect sounds having frequencies up to 20 kHz, which corresponds to a period of 50 μs. At this frequency, a ten degree phase error corresponds to a difference of only 1.5 μs between the transit time along the electronic path  190  and the transit time along the acoustic path  195 . However, as previously described, active noise cancellation systems commonly combine passive filters that eliminate high frequency components of the ambient sound with active cancellation of low frequency components of the ambient sound. The frequency range over which active cancellation is employed will be referred to herein as the “operating frequency range”. 
     Known algorithms and methods for active noise cancellation include feedforward cancellation, feedback cancellation, and hybrid cancellation. Feedforward cancellation operates based on an ambient audio signal, such as the ambient audio signal  112 . Feedback cancellation operates based on a feedback audio signal such as the feedback audio signal  142 . Hybrid cancellation operates based on both an ambient audio signal and a feedback audio signal. Any of these methods may be employed in the non-occluding active noise suppression apparatus  100 . In any case, the electronic path  190  is operative to provide a substantially 180 degree phase shift with respect to the acoustic path  195  over the operating frequency range. 
     An earbud housing is typically about 10 millimeters long from an outer distal end to a proximal end in the ear canal. Sound traveling in air will transit 10 millimeters in about 30 μs. It may be difficult, if not impossible for the electronic path  190  to generate anti-sound within this short time interval. To increase the delay time along the acoustic path  195 , and thus allow more time for the electronic path  190  to generate and deploy anti-sound, an acoustic delay line  160  may be incorporated into the acoustic path. The acoustic delay line  160  delays the propagation of the ambient sound along the acoustic path  195 , which is to say increases the time required for the ambient sound to propagate from the inlet  182  to the outlet  184  beyond the time required for sound to travel an equivalent linear distance in air. 
       FIG. 6A ,  FIG. 6B , and  FIG. 6C  are a top view, a perspective view, and a sectioned perspective view of a serpentine acoustic tube  600  suitable for use as the acoustic delay line  160 . An input port  610  to receive ambient sound is identified in  FIG. 6A  and  FIG. 6C .  FIG. 6C  shows a cross section, revealing the back-and-forth serpentine passages  630  through which sound flows from the input port  610  to the output port  620 . The path length from the input port  610  to the output port  620  via the passages  630  is substantially longer than the direct distance from the input port  610  to the output port  620 . 
     The serpentine acoustic tube  600  could be fabricated by 3D printing, or could be molded in multiple pieces then glued or welded together. The serpentine acoustic tube  600  could also be fabricated in such a way that it shares its outer walls with those of the device housing  180 , thereby enabling simpler construction. 
     An alternate or additional method to delay the ambient sound along the acoustic path  195  is to cause the ambient sound to pass through a reticulated material in which the speed of sound is slower than the speed of sound in air. In this context, “reticulated” means forming or formed like a network or a web. Suitable reticulated materials may include open-cell or closed-cell foams made of polyurethane, polyester, polystyrene, or other plastic. Other suitable reticulated materials include organic fibers like cotton, bamboo, and yarn. For example, the acoustic delay line  160  may be formed by a straight sound tube or a serpentine sound tube filled with a reticulated material in which the speed of sound is slower than the speed of sound in air. 
     The delay line  160  may increase the transit time along the acoustic path  195  from 50 μs to as high as 250 μs. 
     Referring again to  FIG. 1 , the acoustic path  195  may include one or more passive acoustics filters. For example, the acoustic path  195  may include a passive low-pass filter  165  to provide passive attenuation of high frequencies while transmitting low frequencies including ambient air pressure changes to eliminate occlusion. A cut-off frequency of the passive low-pass filter  165  may define an upper limit on the operating frequency range where active cancellation is employed, which is to say an upper limit on the frequency of the anti-sound generated along the electronic path  190 . The passive low-pass filter  165  may be in addition to, or integrated with, the acoustic delay line  160 . Structures for passive low-pass and other passive filters are described in U.S. Pat. No. US 9,131,308 B2, Passive Audio Ear Filters With Multiple Filter Elements. 
     Even with the transit time along the acoustic path extended by the delay line  160 , the elements along the electronic path  190  must be designed to minimize delay time. Most digital audio processing systems utilize sigma-delta analog-digital converters (ADCs) and digital-analog converters (DACs), both of which introduce hundreds of microseconds of delay. Although sigma-delta converters can be used to detect, predict, and cancel highly periodic low frequency sound, they are unsuited for high performance active cancellation of higher frequency, transient, and non-periodic sounds. Thus the audio processor  120  may contain ADCs and DACs that execute very fast conversions, and that operate with very high digital bus speeds. For example, a Texas Instruments ADS8864 ADC can capture and digitize an analog signal in less than 2 μs. Similarly, a Texas Instruments DAC8832 DAC can convert a digital value to an analog signal in less than 2 μs. While these components are capable of conversions at 500 kHz or higher rates, the actual audio sampling speed may be lower, such as 32 kHz or 44.1 kHz for example. Similarly, microphones, amplifiers, analog electronic filters, and algorithms executed within the audio processor must all be chosen or designed for low latency. 
     The largest single delay in generating anti-sound  132  to cancel a portion of the delayed ambient sound  162  is the speaker  130 . Inherently, the delay between an electrical signal applied to a speaker and the production of sound varies with frequency.  FIG. 2  shows a chart  200  including a graph  210  of the phase shift between the electrical signal applied to a speaker and the sound produced by the speaker. At low frequencies, the phase shift is small. As the frequency approaches the natural resonant frequency f 0  of the speaker, the phase shift approaches 90 degrees. Above f 0  the shift approaches 180 degrees. Increasing the natural resonant frequency of the speaker increases the frequency band over which the phase shift is low. If the resonant frequency of the speaker  130  is higher than a cut-off frequency of the passive low pass filter  165 , the speaker will have low phase shift over the operating frequency range where active noise cancellation is employed. 
       FIG. 3  is a cross-sectional schematic view of an exemplary speaker  300  suitable for use as the speaker  130  in the non-occluding active noise suppression apparatus  100 .  FIG. 4  is perspective exploded view of the speaker  300 , and  FIG. 5  is a perspective view of the assembled speaker  300 . The speaker  300  may be configured to have a resonant frequency between 2 kilohertz (kHz) and 9 kHz. 
     The speaker  300  includes a diaphragm  310 , a voice coil assembly  320 , a suspension ring  330 , a washer  340 , a magnet  350 , and a yoke  360 . All of these elements may be rotationally symmetric about an axis  305 . The speaker  300  may be assembled using pressure sensitive adhesive rings (not shown) between adjacent elements. The speaker  300  can be designed to have a resonant frequency between 2 kHz and 9 kHz. Further, the speaker  300  may optionally provide a central passage  370  through the yoke  360 , voice coil assembly  320 , and diaphragm  310 . When present, the central passage may form a portion of the acoustic path  195 . For example, delayed ambient sound may be introduced though the central passage  370  to combine or interfere with sound produced by movement of the diaphragm  310 . 
     The diaphragm  310  is generally planar but may include ribs or other structure to increase rigidity. The diaphragm  310  is sufficiently rigid to move as a piston over the entire operating frequency range, avoiding “cone breakup” and resonances that occur in many other speaker diaphragms. The diaphragm  310  is suspended by an annular suspension ring  330  made from an elastic foam material, such as the PORON® 4701-30 series of very soft microcellular urethane foam materials or the PORON®  4701 - 40  series of soft microcellular urethane foam materials, both available from Rogers Corporation. The foam suspension ring  330  provides higher elasticity than typical speaker suspensions. The washer  340 , the magnet  350 , and the yoke  360  form a magnetic circuit that generates a magnetic field in the annular gap between the washer  340  and the yoke  360 . The cylindrical voice coil assembly is affixed to the diaphragm and extends into the annular space between the washer  340  and the yoke  360 . When driven by an electrical current, the interaction between a magnetic field produced by the voice coil  320  and a magnetic field produced by the magnetic circuit (washer  340 , magnet  350  and yoke  360 ) causes the voice coil  320  and diaphragm  310  to move parallel to the axis  305 . The assembled speaker  300  may have, for example, a diameter of 8 millimeters and a thickness of 3 millimeters. 
     The speaker shown in  FIGS. 3-5  is an example of a high resonance frequency speaker suitable for use in the non-occluding active noise suppression apparatus  100 . Other types of speakers having high resonance frequency, such as balanced armature speakers, and speakers that do not exhibit resonance, such as electrostatic speakers, may be used for the speaker  130 . 
       FIG. 7  shows an exploded view of an exemplary non-occluding active noise suppression ear bud  700  which utilizes the speaker  300  (shown in  FIGS. 3, 4, and 5 ) and the serpentine acoustic tube  600  (shown in  FIG. 6 ). The non-occluding active noise suppression ear bud  700  also includes a housing formed as an outer portion  710 A, a bottom portion  710 B, and an inner portion  710 C; a flexible tip  720  configure to mate with a protrusion on the inner cover portion  710 c and fit into a user&#39;s ear canal; first and second circuit cards  730 ,  750 ; an ambient microphone  735 ; an internal microphone  760 ; and a battery  740 . The ambient microphone  735  and the internal microphone  760  may be connected to either the first circuit card  730  or the second circuit card  750  using wires or flexible circuits which are not shown in  FIG. 7 . 
     The outer portion of the housing  710 A includes one or more perforations to admit ambient sound. Note that some of the apparent perforations visible in  FIG. 7  may be decorative and not fully penetrate the outer portion of the housing  710 A. A portion of the ambient sound is converted to an ambient audio signal by the ambient microphone  735 . The ambient audio signal and a feedback audio signal from the internal microphone  760  are processed by an audio processor, which may be the audio processor  120 , distributed between the first and second circuit cards  730 ,  750 . The audio processor outputs a processed audio signal to drive the speaker  300 . 
     A second portion of the ambient sound admitted through the perforations in the outer housing  710   a , enters the distributed between the serpentine acoustic tube  600  through an aperture in the first circuit card  730 . Delayed ambient sound exiting the serpentine acoustic tube  600  is coupled into a central aperture ( 370  in  FIG. 3 ) to combine with sound produced by the speaker  300 . Destructive interference between the delayed ambient sound the sound produced by the speaker  300  may attenuate or cancel some or all components of the delayed ambient sound. The combination of the delayed ambient sound and the sound produced by the speaker  300  may be introduced into the user&#39;s ear canal through an aperture in the flexible tip  720  (not visible). 
     Description of Apparatus 
       FIG. 9  is a flow chart of a process  900  for suppressing noise. The process  900  may be performed by a noise suppression apparatus, such as the non-occluding active noise suppression apparatus  100 , enclosed in a housing having an inlet to admit ambient sound and an outlet to output personal sound to the ear of a user. The housing may be, for example, an earbud housing configured to fit, at least partially, within and be supported by a user&#39;s ear. 
     Although shown as a flow chart for ease of explanation, the actions of the process  900  are performed continuously and concurrently. Since the actions within the process  900  are performed continuously so long as the noise suppression apparatus is operational, the process  900  does not have a convention start and end. 
     Ambient sound  905  may be received via the inlet. A portion of the ambient sound may be conveyed along an acoustic path at  910 . Conveying the ambient sound along the acoustic path may include delaying the ambient sound at  912  and/or low-pass filtering the ambient sound at  914  as previously described. 
     Another portion of the ambient sound may be conveyed along an electronic path at  920 . Conveying the ambient sound along the electronic path includes converting the ambient sound to a signal at  932  using a microphone. The signal from  932  is then processed at  934 . The processed signal from  9343  is converted into processed sound at  936  using a speaker. 
     The sound from  910  (i.e. sound that has traversed the acoustic path) and the processed sound from  936  are combined or mixed at  940  to provide the personal sound  995  output via the outlet to the ear of the user. 
     The electronic path is configured to provide a phase difference between the process sound from  936  and the sound from  910  of substantially 180 degrees over an operating frequency range, as previously described. 
     Closing Comments 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     As used herein, “plurality” means two or more. As used herein, a “set” of items may include one or more of such items. As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of ” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.