Patent Publication Number: US-2010119077-A1

Title: Active hearing protection system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an active haring protection system comprising a left and a right earplug, each being adapted for providing for an acoustic attenuation of at least 10 dB averaged over the audible frequency range and comprising an active unit with a microphone for capturing audio signals from ambient sound and a loudspeaker. 
     2. Description of Related Art 
     U.S. Pat. No. 4,677,678 relates to a hearing protection system comprising two earmuffs or earplugs comprising each a microphone and a speaker. The system comprises two audio channels sharing a common sound level limiting unit. 
     US 2002/0080979 A1 relates to a hearing protection system comprising two earplugs into which an active unit comprising each a microphone and a speaker is inserted. The gain as a function of the sound input level is a three-segment piece-wise linear function including a first section providing maximum expansion up to a first knee point for maximum noise reduction, a second section providing less expansion up to a second knee point for less noise reduction, and a third section providing minimum or no expansion for input signals with high signal-to-noise-ratio (SNR) to minimise distortion. 
     EP 1 674 061 A1 relates to a hearing protection system comprising two earplugs including an active unit comprising a microphone and a speaker, wherein the audio signal processing mode is controlled by sound classification including level analysis. If the acoustic noise level is high, the gain may be significantly reduced or even completely eliminated. The system includes a transparent gain setting if acoustic noise is low, and a close speech mode if close speech in acoustic noise has been detected. 
     EP 1 674 057 A1, EP 1 615 468 A1 and DE 101 17 705 A1 relate to hearing protection systems comprising a microphone, an audio signal processing unit and a speaker at each ear, wherein in addition a wireless interface for an external audio source, such as a mobile phone or a remote active hearing protection system used by another person, is provided. 
     Hearing protection systems provided with an interface for wireless audio input from a remote audio signal source, but not having microphones at ear level, are known, for example, from EP 1 674 059 A1, GB 2 373 951 A, U.S. Pat. No. 5,426,719 and US 2003/0059071 A1. 
     EP 1 653 773 A2 relates to a hearing aid which is capable of automatically selecting, for example according to the SNR or according to the audio signal level, the audio signal input from one of a plurality of audio signal sources. The audio signal sources may be a T-coil receiver, a frequency modulation (FM) radio receiver or a direct audio input. The hearing aid also comprises a classifier for selecting the audio signal processing mode of the hearing aid according to the analysis of the auditory scene. A similar hearing aid is described in EP 1 443 803 A2. 
     U.S. Pat. No. 5,721,783 relates to a hearing instrument comprising two ear-pieces with a microphone and a speaker and a remote audio signal processing unit wirelessly connected to the ear-pieces. Audio signals from a phone can be provided via the remote audio signal processing unit to the ear-pieces. A switch is provided for switching between the microphone signals and the phone signals as the audio input to audio signal processing unit. The system may comprise active noise cancelling capability. 
     US2004/136522 A1 relates to an in-the-ear headset comprising an active noise reduction arrangement and a communication loudspeaker and a boom microphone. The communication loudspeaker and a boom microphone may be connected, via a controller, to a primary and a secondary communication unit for exchanging audio signals with another person. The controller detects the presence of the primary and the secondary communication unit and controls the weight of the audio signals from the primary and the secondary communication unit. 
     It is an object of the invention to provide for an active hearing protection system which provides for a particularly comfortable and flexible communication function. It is a further object to provide for a corresponding method for providing hearing protection to a user. 
     SUMMARY OF THE INVENTION 
     According to the invention, these objects are achieved by an active hearing protection system as defined in claim  1  and a method as defined in claim  39 , respectively. The invention is beneficial in that, by providing for a central unit having a digital audio signal processing unit having at least two channels and by automatically controlling the audio signal processing unit depending on whether the presence of an audio signal at the audio input of the audio signal processing unit has been detected or not, external audio signals can be combined for communication purposes in a particularly comfortable and flexible manner with the audio signal captured by the microphones of the system. 
     Preferred embodiments of the invention are defined in the dependent claims. 
     These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an example of an active hearing protection system according to the invention; 
         FIG. 2  is a block diagram of the electronic components of the system of  FIG. 1 ; 
         FIG. 3  is a block diagram of the system wherein operation in the communication mode is illustrated; and 
         FIG. 4  is a diagram showing an example of the acoustic gain model of the system. 
     
    
    
     The active hearing protection system shown in  FIG. 1  comprises a left ear earplug  10 , a right ear earplug  12 , a central unit  14 , a boom microphone  16  and a remote control unit  18 . The central unit  14  is connected to a communication device  20 . 
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferably the hearing protection earplugs  10 ,  12  have a hard shell with an elasticity from shore D85 to shore D65, which is customized, i.e. it has an outer shape according to the individual measured inner shape of the user&#39;s outer ear and ear canal. The hard shell  22  may be manufactured by layer-by-layer laser sintering of a powder material, for example, polyamide powder or by laser stereo-lithography or photo-polymerization. An overview regarding such additive layer-by-layer build-up processes for manufacturing customized shells of hearing devices can be found, for example, in US 2003/0133583 A1 or U.S. Pat. No. 6,533,062 B1. The inner shape of the user&#39;s ear canal and outer ear can be measured, for example, by taking an impression which then undergoes laser scanning or by direct laser scanning of the ear. Preferably the hard shell  22  is designed such that it provides for an acoustic attenuation of at least 10 dB averaged over the audible frequency range when worn by the user. 
     The shell  22  has a receptacle into which an active unit  24  can be inserted, preferably in a releasable manner. The shell  22  is provided with a sound channel  26  through which the active unit  24  is acoustically connected to the ear canal. The active unit  24  comprises a microphone  28  for capturing audio signals from ambient sound and a loudspeaker  30  for providing audio signals into the user&#39;s ear canal via the sound channel  26 . Such earplugs comprising an active unit are described, for example, in EP 1 674 059 A1. 
     The central unit  14  is to be worn at the user&#39;s body, e.g. by a loop  32  around the user&#39;s neck. The central unit  14  comprises an audio signal processing unit  34  for receiving and processing the audio signals captured by the microphones  28  and for supplying the loudspeakers  30  with audio signals to be reproduced to the user&#39;s ear. To this end, the active units  24  are connected to the central unit  14  via cable connections  36 . The remote control unit  18  is connected to the central unit  14  via a wireless link in order to serve as a user interface. 
     The boom microphone  16  may be connected to the central unit in order to supply audio signals captured from the user&#39;s voice (dashed lines in  FIGS. 1 and 2 ). Alternatively, it may be connected directly to the communication device  20 . The communication device  20 , for example, may be a mobile phone or a FM radio device, such as a walkie-talkie. Usually the communication device  20  and the central unit  14  will be connected by wires, although connection by a wireless link is conceivable. 
     In order to act as an active hearing protection system, audio signals captured from ambient sound by the microphones  28  are provided, after processing in the audio signal processing unit  34 , to the speakers  30  in order to provide sound impinging on the left ear to the left ear canal and sound impinging on the right ear to the right ear canal despite the acoustic attenuation provided by the shell  22  for relatively low sound pressure levels (for high sound pressure levels the gain will be progressively reduced in order to provide for a hearing protection function). Further, audio signals received from the communication device  20  are also to be provided to the user&#39;s ear via the central unit  14  and the speakers  30 . In addition, if bidirectional communication is desired, the user&#39;s voice can be captured by the boom microphone  16  and/or the microphones  28 ,  31  and will be supplied to the communication device  20 . 
     The structure and the functionality of the central unit  14  will be explained in more detail by reference to  FIGS. 2 to 4 . The central unit  14  comprises an input  38  for the left ear microphone  28 , an input  40  for the right ear microphone  28  and the audio signals from the communication device  20  and optionally an input  41  for the boom microphone  16 . Each of these channels is provided with a pre-amplifier  42  which preferably is adjustable. The pre-amplified signal is digitized in an analogue-to-digital converter  46  before it is supplied to the audio signal processing unit  34 , which has at least two outputs for outputting corresponding the processed digital audio signals to a digital-to-analogue converter  48 , with the respective analogue signal being provided to an output driver  50  driving the left ear speaker  30  and the right ear speaker  30 , respectively. A third output comprising a digital-to-analogue converter  48  and an output driver  50  may be provided for supplying audio signals from the audio signal processing unit  34  to the communication device  20 . 
     The central unit  14  comprises a further audio input  52  which is connected in the same manner as the input  40  to the communication device  20  and hence receives the same audio signals. The audio signals received at the input  52  are provided to a low speed analogue-to-digital converter  54 , with the digitized signals being provided to a detector  56  for detecting the presence of an audio signal from the communication device  20  at the audio input  52 . On the one hand, the detector  56  supplies a status signal to the audio signal processing unit  34  in order to select the audio signal processing mode depending on whether there is an audio signal from the communication device  20  or not. On the other hand, the detector  56  also acts on the audio input  40  which can be switched between the microphone  28  and the communication device  20  depending on whether an audio signal from the communication device  20  is present or not. 
     If no audio signal from the communication device  20  is detected by the detector  56 , the audio signal processing unit  34  operates in an “ambient mode” in which the right ear microphone  28  is selected as the audio signal source to the input  40  rather than the communication device  20 . In the ambient mode the audio signal processing unit  34  primarily acts as a stereo device, i.e. the audio signals from the right ear microphone  28  are supplied essentially to the right ear speaker  30 , and the audio signals from the left ear microphone  28  are essentially supplied to the left ear speaker  30 . However, there may be some transfer of audio signals between the two channels in order to enable binaural audio signal processing. Examples of binaural audio signal processing can be found in EP 1 320 281 A2, WO 99/43185 or US 2004/02 52852 A1. Apart from such binaural processing, both channels usually will be processed in the same manner, if the user is a normal hearing person. If, however, the user suffers from asymmetric hearing loss, also the audio signal processing in the “ambient mode” will be asymmetric in order to compensate for the hearing loss as far as possible. 
     In  FIG. 4  an example of the acoustic gain model implemented in the audio signal processing unit  34  is shown, wherein the output free field sound pressure level (SPL FF) is given as a function of the input SPL FF. In the example shown in  FIG. 4  it is assumed that the shell  22  provides for an attenuation of 30 dB. The curve with the diamonds represents the situation in which the gain is set to zero (i.e. no sound from the speakers  30 ). On this curve, the hearing protection system acts as a passive system. 
     The curve with the squares represents the situation in which the system has a “transparent” gain, i.e. below the knee point (which is set to 70 dBA) the user has the same sound feeling as if he was not wearing the earplugs  10 ,  12 . Different gain settings are applied below and above the knee point (which is set to 70 dBA in the example of  FIG. 4 ). Below this knee point the gain is constant, resulting in a linear curve. Above the knee point, i.e. for higher input levels, the gain is progressively reduced so that the output level remains constant. For even higher input levels the system will act as a passive hearing protector. 
     The system also comprises a manual volume control which is implemented on the central unit  14  or, alternatively or additionally, in the remote control unit  18 . By actuating the volume control, the value of the constant gain applied in the linear region below the knee point can be adjusted between a minimum value (circles in  FIG. 4 ) and a maximum value (triangles in  FIG. 4 ). When the maximum gain is applied, the maximum output free field sound pressure level is limited to 80 dBA SPL FF in the example of  FIG. 4 . 85 dBA SPL FF correspond to the maximum noise level tolerable for 8 hours a day according to Directive 2003/10/EC. Since the open ear gain (OEG) is around 10 dB, the sound pressure level in the ear canal is around 10 dB higher than the free-field values given in  FIG. 4 . 
     In  FIG. 4  the gain model is shown for one frequency. The gain model to be applied by the audio signal processing unit  34  may be frequency-dependant in order to compensate for the frequency-dependency of the acoustic attenuation provided by the shell  22  and/or a hearing loss of the user. 
     Beyond the volume control interface, the central unit  14  may comprise manual user interfaces such as e.g. on/off, audio signal processing mode (program) change, push-to-talk (PTT), call accept/reject or voice dialing. These manual control interfaces are designated by  35  in  FIGS. 1 and 2 . The same commands or a subset thereof may be provided on the remote control unit  18 . 
     The acoustic gain model may be automatically selected according to the result of an auditory scene analysis based on the audio signals provided by the microphones  28 . 
     Once an audio signal from the communication device  20  is detected by the detector  56 , the central unit  14  will switch into a “communication mode” in which the input from the right ear microphone  28  is replaced by the audio signals provided by the communication device  20 . Whereas in the ambient mode—apart from compensation of an asymmetric hearing loss—usually the same gain model is applied to both channels, in the communication mode preferably a different gain model now is applied to that channel to which the audio signals from the communication device  20  are provided to. In this “communication channel” the maximum output free field sound pressure level may be shifted to higher values, for example in such a manner that an output level of 100 dBA SPL FF is allowed (rather than 80 dBA SPL FF allowable for the channel of the left ear). This level limiting function in both channels is labelled “LIM” (“limiter”) in  FIG. 3 . In the communication mode the volume control will act only on the “ambient channel”, i.e. the left ear microphone channel. After having undergone respective audio signal processing, the two channels may be combined and then may be supplied as a mono signal to both loudspeakers  30 . 
     Such an example of operation of the system in the communication mode is schematically shown in  FIG. 3  (the right ear microphone  28  is omitted in  FIG. 3 , since it is blocked by action of the detector  56  on the input  40 ). According to this example in the communication mode a mixture of audio signals representative of the ambient sound as captured by the left ear microphone  28  and the audio signals provided by the communication device  20  is provided to both ears. The user can adjust the relative level of the “ambient channel” and the “communication channel” by actuating the volume control on the central unit  14  or via the remote control unit  18 , which in the communication mode acts only on the “ambient channel”. 
     The volume of the communication channel may be adjusted on the communication device  20 . 
     In case that the audio signals provided by the communication device  20  are speech signals the “communication channel” may undergo some speech enhancement filtering in order to improve intelligibility of the speech. Enhancement of the intelligibility of the voice in the “communication channel” can be achieved, for example, by reducing the audio band width and/or emphasizing speech frequencies (300 Hz to 3.4 kHz). However, such filtering also my be applied to the “ambient channel” or to both channels in the ambient mode if speech signals are received via the microphones  28 . The decision that speech is present may be made by performing auditory scene analysis in the audio signal processing unit  34  based on the audio signals captured by the microphones  28 . 
     As alternatives to the above-described example, in the communication mode the audio signals received from the communication device  20  may be provided completely or at least primarily to one of the left ear and right ear loudspeakers  30  and the audio signals captured by at least one the left ear and right ear microphones  28  may be provided completely or at least primarily to the other one of the loudspeakers  30 . 
     According to another alternative, in the communication mode the audio signals from the communication device  20  may be mixed to at least one of the audio signals captured by the left ear microphone  28  and the right ear microphone  28 , with, as in the ambient mode, the audio signals captured by the left ear microphone  28  being provided to the left ear speaker and the audio signals from the right ear microphone  28  being provided to the right ear speaker  30 . The mixing ratio may be different for the two channels. In this case the central unit would be equipped with a further fast analogue-to-digital converter, so that the audio signals from the communication device  20  could be processed in parallel to the audio signals from the left ear microphone  28  and the right ear microphone  28 . 
     Preferably the interface between the communication device  20  and the central unit  14  is bidirectional so that not only audio signals can be supplied from the communication device  20  to the central unit  14  but also audio signals and/or command signals can be sent from the central unit  14  to the communication device  20 . For example, the user&#39;s voice may be captured by the boom microphone  16  and/or the microphones  28  in order to be supplied to the communication device  20  via the third output (dashed lines). 
     The boom microphone  16  may be omitted if the central unit  14  is capable of performing blind source separation (BSS) on the audio signals provided by the microphones  28 ,  31  in order to separate the user&#39;s voice from background noise. Typically, the inner microphone  31  is required on only one of both sides. Preferably, each active unit  24  of the earplugs  10 ,  12  in this case may include a second microphone  31  which is acoustically oriented towards the ear canal in order to capture primarily the user&#39;s voice, whereas the microphone  28  is acoustically oriented towards ambience in order to capture primarily ambient sound. 
     Alternatively, one or two microphones  31  on the right and/or the left side may pick up the user&#39;s voice transferred by bone conduction to the ear canal (in-ear voice pickup). 
     Alternatively or in addition, the central unit  14  may be capable of performing acoustic beamforming on the audio signals captured by the microphones  28  in order to separate the user&#39;s voice from ambient sound/background noise. 
     The central unit  14  and/or remote control unit  18  preferably comprises a PTT (Push to Talk) element operable by the user to control transmission of audio signals, representing the user&#39;s voice, via the communication device  20 . 
     The central unit  14  and/or remote control unit  18  also may comprise a call accept/call reject element and a voice dialing activation element for the event that the communication device  20  is a mobile phone. 
     The communication device  20  also may serve to supply power to the central unit  14 . Alternatively, the central unit  14  may be provided with a primary or rechargeable battery (not shown in  FIGS. 1 to 3 ). 
     The audio signals representing the user&#39;s voice captured by the microphones  28  and/or the boom microphone  16  may be supplied not only to the communication device  20  but also to some extent to the right ear and left ear speaker  30  in order to reduce the occlusion effect caused by the acoustic attenuation of the shell  22 . 
     The central unit  14  may be capable of user preference learning in order to learn and store the preferred manual settings by the user. 
     Preferably, the audio signal processing unit  34  is realized as a single digital signal processing (DSP) chip. 
     The central unit  14  also may comprise a noise dosimeter functionality, with the sound exposure of the user being estimated from the level of the audio signals captured by the left ear and right ear microphones  28  or  31 , which is integrated over a given time period. The central unit  14  may comprise an alarm function wherein the user is alerted via at least one of the speakers  30  when a given maximum noise dose has been reached. Alternatively or in addition, the central unit  14  may comprise a protection function wherein the gain applied in the audio signal processing unit  34  is reduced when a given maximum noise dose has been reached. 
     While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. Therefore, this invention is not limited to the details shown and described herein, and includes all such changes and modifications as encompassed by the scope of the appended claims.