Patent Publication Number: US-2018033419-A1

Title: Acoustically open headphone with active noise reduction

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application may be related to pending U.S. patent application Ser. Nos. 14/993,443 and 14/993,607, both filed on Jan. 12, 2016 
    
    
     BACKGROUND 
     Headphones are typically located in, on or over the ears. One result is that outside sound is occluded. This has an effect on the wearer&#39;s ability to participate in conversations as well as the wearer&#39;s environmental/situational awareness. It is thus desirable at least in some situations to allow outside sounds to reach the ears of a person using headphones. 
     Headphones can be designed to sit off the ears so as to allow outside sounds to reach the wearer&#39;s ears. This type of headphone is sometimes referred to as an open headphone. Two benefits of an open headphone are situational awareness and being un-occluded. 
     The value of these benefits diminishes as the external environment starts getting noisier and the user is not able to enjoy the audio that they are listening to. In noisy environments above, for example, 70 dBA (especially babble), the open headphone experience deteriorates rapidly. It is in these environments that the open headphone can benefit from active noise reduction (ANR). 
     SUMMARY 
     In general, in one aspect, a headphone includes an electroacoustic transducer and a support structure for suspending the transducer adjacent to a user&#39;s ear when worn by the user such that the headphone is acoustically open. A first microphone is coupled to one or more of the transducer and the support structure such that the first microphone is located in a substantially broadband acoustic null of the transducer. A processor is coupled to the headphone. The microphone receives sound pressure waves and outputs a related electronic signal to the processor. The processor uses the electronic signal to operate the transducer to reduce targeted sound pressure waves at the user&#39;s ear. 
     Implementations may include one or more of the following, in any combination. A second microphone is coupled to one or more of the transducer and the support structure. The second microphone is a feedback microphone located between the transducer and the user&#39;s ear. The second microphone receives sound pressure waves and outputs a related electronic signal to the processor. The processor uses these electronic signal to operate the transducer to reduce targeted sound pressure waves at the user&#39;s ear. The first microphone is located substantially at a periphery of a basket of the transducer. The headphone further includes one or more additional microphones which are also coupled to one or more of the transducer and the support structure such that the one or more additional microphones are also located in a substantially broadband acoustic null of the transducer. The one or more additional microphones receive sound pressure waves and output a related electronic signals to the processor. The processor uses these electronic signals to operate the transducer to reduce targeted sound pressure waves at the user&#39;s ear. The processor discontinues using the electronic signal to operate the transducer to reduce targeted sound pressure waves at the user&#39;s ear when a noise level in a vicinity of the headphone drops below a certain level. Acoustic impedances at a rear and front of the electroacoustic transducer are substantially the same. The headphone further includes a pair of baskets which surround a diaphragm of the electroacoustic transducer. Each basket has one or more openings such that acoustic impedances at a rear and front of the electroacoustic transducer are substantially the same. 
     In general, in another aspect, a headphone includes an electroacoustic transducer and a support structure for suspending the transducer adjacent to a user&#39;s ear when worn by the user such that the headphone is acoustically open. A first microphone is coupled to one or more of the transducer and the support structure. A processor is coupled to the headphone. The microphone receives sound pressure waves and outputs a related electronic signal to the processor. The processor uses the electronic signal to operate the transducer to reduce targeted sound pressure waves at the user&#39;s ear. 
     Implementations may include one or more of the above and below features, in any combination. The first microphone is a feed-forward microphone. 
     In general, in another aspect, an apparatus for creating sound includes an electroacoustic transducer and a first microphone coupled to the transducer such that the first microphone is located in a substantially broadband acoustic null of the transducer. A processor is coupled to the microphone. The microphone receives sound pressure waves and outputs a related electronic signal to the processor. The processor uses the electronic signal to operate the transducer to reduce targeted sound pressure waves at a user&#39;s ear. 
     Implementations may include one or more of the above and below features, in any combination. Acoustic impedances at a rear and front of the electroacoustic transducer are substantially the same. 
     All examples and features mentioned above can be combined in any technically possible way. Other features and advantages will be apparent from the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a front view of a person wearing a pair of headphones; 
         FIG. 2A  is a side view of one of the headphones of  FIG. 1  which faces away from a user&#39;s ear; 
         FIG. 2B  is a perspective view of the other side of the one headphone from  FIG. 1  which faces towards a user&#39;s ear; 
         FIG. 3  is a block diagram of a processor, two microphones, and an electroacoustic transducer; 
         FIG. 4  is a graph showing the magnitude of ANR relative to frequency; 
         FIG. 5  is a graph showing the dipole behavior for an electroacoustic driver with mesh over the back basket; 
         FIG. 6  is a graph showing the dipole behavior for an electroacoustic driver with mesh removed from the back basket; 
         FIG. 7A  is a bottom view of an audio unit for a headphone; and 
         FIG. 7B  is a cross-sectional view taken along line  7 B- 7 B of  FIG. 7A . 
     
    
    
     DESCRIPTION 
     The description below discloses open headphones that sit off the ears so as to allow outside sounds to reach the wearer&#39;s ears. One or more microphones are used to sense noise in an environment near the headphones. Microphone signals are then used by a processor to operate an electroacoustic transducer of the headphones to reduce noise that is heard by a headphone user. As such, even in noisy environments the user is able to more clearly hear the audio program they are listening to through their headphones. The ANR has an equivalent effect of turning the audio volume up and can make the headphone more suitable in noisy environments higher than 70 dBA. 
     Referring to  FIG. 1 , a pair of headphones  10 ,  12  each include an electroacoustic transducer (discussed in more detail below). The headphones are each connected to a support structure  14  for suspending the respective transducers adjacent to a user&#39;s ears  16  when worn by the user  18 . As such, the headphone is acoustically open which means that a headphone only minimally passively interferes with the user hearing sounds in their environment. This helps to maintain completely natural self-voice (the user&#39;s voice sounds natural to themselves) as well as situational awareness. 
     In this example the support structure  14  is in the form of a nape band which rests on a nape of the neck of the user  18 . The support structure  14  also loops over and rests above the pinna of each of the user&#39;s ears and then extends to support each headphone  10 ,  12  in a position slightly spaced from a respective ear of the user. This arrangement provides comfort while the user is wearing the headphones. Alternatively, the support structure could be a more traditional headband which extends across the top and sides of a user&#39;s head. 
     Turning to  FIG. 2A , a first microphone  20  is coupled to an electroacoustic transducer  22 . In this example the microphone  20  is a feed forward microphone which is connected to and located substantially at a periphery of a rear basket  24  of the transducer  22 . Alternatively or additionally, the microphone  20  can be connected to a portion of the support structure  14 . It is preferable that that the microphone  20  is located in a substantially broadband acoustic null of the transducer  22 . This means that the transducer  22  is located where the acoustic energy coming off of both sides of a moving diaphragm (discussed further below) substantially cancels each other out across a broad frequency band. The low frequency bandwidth limitation comes from the ability of the transducer to cancel noise (e.g. about 50 Hz). The high frequency feed forward bandwidth is governed by the bandwidth of the null (in  FIG. 6  this is about 4 kHz). So in this example the broadband acoustic null ranges from about 50-4000 Hz. One or more additional feed forward microphones (not shown) can be coupled to one or more of the transducer  22  and the support structure  14  such that the one or more additional microphones are also located in a substantially broadband acoustic null of the transducer. 
     With reference to  FIG. 2B , a second microphone  26  is coupled to a front basket  28  of the transducer  22 . In this example the microphone  26  is a feedback microphone. Alternatively or additionally, the microphone  26  can be connected to a portion of the support structure  14 . The microphone  26  is located between the transducer and the user&#39;s ear. Also visible are a diaphragm  30  and a surround  32  of the transducer  22 . The surround  32  is a suspension which allows the diaphragm  30  to vibrate in order to create sound waves. 
     Turning to  FIG. 3 , a processor  34  is electrically connected with the microphones  20  and  26 , and with the transducer  22 . The microphone  20 , being in a broadband acoustic null of the transducer  22 , picks up sound pressure waves in the vicinity of the headphone that are entirely or mostly not created by the transducer  22 . The microphone  20  outputs an electronic signal to the processor  34  which is related to the sound pressure waves that are picked up (i.e. environmental noise). 
     The microphone  26  also picks up sound pressure waves in the vicinity of the headphone but also picks up sound pressure waves created by the transducer  22 . The microphone  26  outputs an electronic signal to the processor  34  which is related to the sound pressure waves that are picked up. The processor  34  subtracts an electronic signal used to drive the transducer  22  from the signal sent by microphone  26 . The resulting signal represents environmental noise in the vicinity of the headphone. The processor  34  uses the electronic signals from the microphones  20  and  26  to operate the transducer  22  to reduce targeted sound pressure waves at the user&#39;s ear. This is known to those skilled in the art as an active noise reduction system. The processor uses the signals of microphones  20  and  26  as is known to those skilled in the art (see, for example U.S. Pat. Nos. 8,184,822 and 8,416,960). 
     When a signal from one or both of the microphones  20  and  26  indicates to the processor  34  that a noise level in a vicinity of the headphone has dropped below a certain level (e.g. about 65 dBA), the processor discontinues using the electronic signals from the microphone(s) to operate the transducer  22  to reduce targeted sound pressure waves at the user&#39;s ear. In essence, when the environment around the user is relatively quiet, it makes sense to shut off the active noise reduction system in order to conserve battery power. 
     Referring to  FIG. 4 , a graph shows the magnitude of noise reduction in dB relative to frequency for the nape-band style open headphone of  FIG. 1  as measured on a single human head. The dotted line shows the noise reduction using the feedback microphone  26  only. The solid line shows the noise reduction using both the feed forward microphone  20  and the feedback microphone  26 . This graph shows that the active noise reduction system is effective in the mid-high frequency region. If the dotted line is subtracted from the solid line, what remains is the noise reduction using the feed forward microphone  20  only. In this case, the noise reduction is &gt;10 dB from about 300 Hz to about 2 kHz. 
     Turning to  FIGS. 5 and 6 , graphs are shown of the dipole behavior of the transducer  22  with ( FIG. 5 ) and without ( FIG. 6 ) a cloth mesh  36  ( FIG. 2A ) on a rear basket  24  of the transducer  22 . The dipole behavior is represented by the acoustic energy exiting the front (solid line) and back dashed line) of the transducer  22  being substantially equal at varying frequencies. The off-axis acoustic energy is shown by the dotted line. The dipole bandwidth increases significantly (from a top end of ˜2 kHz to ˜4 kHz) by just removing the mesh on the back. These measurements were taken at 5 cm from the driver and hold true for what the feedforward microphone  20  sees. 
       FIGS. 7A and 7B  show another example with an audio unit  50  that can be used in a headphone. Audio unit  50  includes a driver (or transducer)  52  that includes diaphragm/surround  54 , magnet/coil assembly  62  and structure or basket  56 . Rear acoustic chamber  55  is located behind diaphragm  54 . Openings  58 ,  60  and  81 - 86  are formed in the rear side of basket  56 . There can be one or more such openings. The area of each opening, and the area of the openings in total, is selected to achieve a desired acoustic impedance at the rear of the driver. The openings may also comprise tubes, and the length of each tube may be selected to achieve a desired acoustic impedance at the rear of the driver. In non-limiting examples acoustic resistance material  59  is located in or over opening  58  and acoustic resistance material  61  is located in or over opening  60 . Typically but not necessarily each of the openings is covered by an acoustic resistance material, so as to develop a particular acoustic impedance at the rear of the driver. 
     In one example the acoustic impedances at the rear and the front of the driver are approximately the same to achieve a wider bandwidth of far-field cancellation. This can be accomplished by including a second basket or structure  66  located in front of and surrounding diaphragm/surround  54  such that acoustic chamber  65  is formed in the front of the driver. Basket  66  can be but need not be the same as basket  56 , and can include the same openings and the same acoustic resistance material in the openings, so as to create the same acoustic impedances in the front and rear of the driver. A feed forward microphone  67  is secured to the periphery of one or both of the baskets  56  and  66  in a broadband acoustic null of the transducer  52 . A feedback microphone  73  is secured to the transducer  52 . Openings  68  and  70  filled with acoustic resistance material  69  and  71  are shown, to schematically illustrate this aspect. The acoustic resistance material helps to control a desired acoustic impedance to achieve a dipole pattern at low frequencies and a higher-order directional pattern at high frequencies. However, the increased impedance may result in decreased low frequency output. 
     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 embodiments are within the scope of the following claims.