Patent Publication Number: US-2003228019-A1

Title: Method and system for reducing noise

Description:
FIELD OF THE DISCLOSED TECHNIQUE  
       [0001] The disclosed technique relates to audio systems in general, and to methods and systems for reducing background noise, in particular.  
       BACKGROUND OF THE DISCLOSED TECHNIQUE  
       [0002] Ambient noise from various sources creates a noisy environment that often amounts to a disturbance to a person or an acoustic receiver. The noise may considerably interfere with sounds that are required to be captured by an acoustic sensor, or distract a person who is required to concentrate on specific tasks. Tasks that require listening to sounds by a human ear or their capturing via a microphone, are particularly vulnerable to disruption by noise. In this context, the noise is an objectionable acoustic pressure impinging upon the eardrums of a person or upon the receiving means of an acoustic sensor.  
       [0003] Devices and methods in the prior art were designed to provide active attenuation of noise. U.S. Pat. No. 4,985,925 issued to Langberg et al., provides an active noise reduction based on a negative feedback electro-acoustical system. The electro-acoustical system consists of an electronic earplug seated in the concha fossa. The system combines active and passive noise reduction in the quiet zone of the ear, a bilateral transducer circuit, a shunt feedback control filter network, and a combined input noise-filter/feedback system. The bilateral transducer circuit drives a speaker as an acoustical velocity source. The shunt feedback control filter network improves stability and increases noise reduction.  
       [0004] U.S. Pat. No. 5,600,729 issued to Darlington et al., teaches the application of Active Noise Reduction (ANR) in an ear defender. The ear defender includes detector means (e.g., a microphone) for detecting the sound level in the proximity of the ear of the person. The ear defender further includes output means (e.g., a speaker) for generation of noise reduction signal within the ear shell. The ear defender also includes a digital feedback controller for generating a feedback signal derived from the output of the detector means and applying it to the output means. The ear defender also features estimation means for providing estimation of the ear shell transfer function and subtracting from the input to the feedback controller, a signal representing the estimated electroacoustic transfer function of the system. A second, analog or digital feedback controller provides an active noise control on the basis of an average configuration for the system.  
       [0005] U.S. Pat. No. 6,078,672 issued to Saunders et al., provides a personal noise attenuation system for attenuating both tonal and broadband sound in a noisy environment immediately adjacent to a user. The system includes a spatially adjustable acousto-electric sensor adapted to sense ambient noise, including certain preselected sounds. The system also has attenuation means including both feedback and feed forward components so as to provide a heteronomous attenuation and more complete active noise attenuation. The adjustable acoustoelectric sensor is spatially moved so as to adapt to the changing physical characteristics of spatial zones in different noise fields adjacent to the user.  
       [0006] U.S. Pat. No. 6,278,786 issued to McIntosh, provides an active noise cancellation aircraft headset system. A speaker is mounted within each earcup of a headset for receiving and acoustically transducing a composite noise cancellation signal. A microphone is also mounted within each earcup for transducing acoustic pressure within the earcup to a corresponding analog error signal. An analog filter receives the analog error signal and inverts it to generate an analog broadband noise cancellation signal. The analog error signal is also provided to an analog to digital converter, which receives the analog microphone error signal and converts it to a digital error signal.  
       [0007] A digital signal processor (DSP) takes the digital error signal and using an adaptive digital feedback filter, generates a digital tonal noise cancellation signal. A digital to analog converter then converts the digital tonal noise cancellation signal to an analog tonal noise cancellation signal so that it can be summed with the analog broadband noise cancellation signal to form a composite cancellation signal. The composite cancellation signal is provided to the speakers in the earcups to cancel noise within the earcups. The broadband analog cancellation is effective to reduce overall noise within the earcup. The DSP provides active control of the analog cancellation loop gain to maximize the effectiveness of the broadband analog cancellation. The DSP also uses the adaptive feedback filter/algorithm to substantially reduce at least the loudest tonal noises penetrating the earcup. The tonal noses include engine and propeller noises, as well as harmonic vibrations of components of the fuselage of the aircraft.  
       SUMMARY OF THE DISCLOSED TECHNIQUE  
       [0008] It is an object of the disclosed technique to provide a novel method and system for producing a noise-free sound signal of the voice of a person talking in a noisy environment, which overcomes the disadvantages of the prior art.  
       [0009] In accordance with the disclosed technique, there is thus provided a system for producing a substantially noise-free signal of an acoustic sound (e.g., the voice of a pilot transmitting to an air traffic controller). The system furthermore produces a sound which includes a desired sound (e.g., the voice of an air traffic controller transmitted to the pilot) and an anti-phase noise sound, the anti-phase noise sound being in anti-phase relative to a noise. The system includes an acoustoelectric transducer, a reference-acoustoelectric transducer and an audio controller coupled with the reference-acoustoelectric transducer and the acoustoelectric transducer.  
       [0010] The acoustoelectric transducer produces a noise bearing sound signal by detecting the acoustic sound and the noise, and the reference-acoustoelectric transducer produces the reference noise signal by detecting the noise in a noisy environment. The audio controller produces the substantially noise-free signal, according to the reference noise signal and the noise bearing sound signal.  
       [0011] The system further includes an electroacoustic transducer for producing the sound and an active noise reduction controller coupled with the electroacoustic transducer and the reference-acoustoelectric transducer. The active noise reduction controller produces a sound signal according to the reference noise signal and according to a desired sound signal respective of the desired sound. The electroacoustic transducer produces the sound according to the sound signal.  
       [0012] In accordance with another aspect of the disclosed technique, there is thus provided a system for producing a sound. The sound includes a desired sound (e.g., the voice of an air traffic controller transmitted to a pilot) and an anti-phase noise sound, the anti-phase noise sound being in anti-phase relative to a noise. The system includes an electroacoustic transducer, a reference-acoustoelectric transducer and an active noise reduction controller coupled with electroacoustic transducer and the reference-acoustoelectric transducer.  
       [0013] The electroacoustic transducer produces the sound and the reference-acoustoelectric transducer produces a reference noise signal by detecting the noise in a noisy environment. The active noise reduction controller produces a sound signal according to the reference noise signal and according to a desired sound signal respective of the desired sound, and the electroacoustic transducer produces the sound according to the sound signal.  
       [0014] In accordance with a further aspect of the disclosed technique, there is thus provided a system for producing an anti-phase noise sound. The system includes an electroacoustic transducer, a reference-acoustoelectric transducer for producing a reference noise signal by detecting noise in a noisy environment and a digital active noise reduction controller coupled with the electroacoustic transducer and the reference-acoustoelectric transducer.  
       [0015] The digital active noise reduction controller produces an anti-phase noise signal according to the reference noise signal, wherein the anti-phase noise signal is in anti-phase relative to the reference noise signal. The electroacoustic transducer produces the anti-phase noise sound according to the anti-phase noise signal.  
       [0016] In accordance with another aspect of the disclosed technique, there is thus provided a system for producing sound, the sound including a desired sound (e.g., the voice of an air traffic controller transmitted to a pilot) and an anti-phase noise sound, the anti-phase noise sound being in anti-phase relative to a noise. The system includes an electroacoustic transducer, a reference-acoustoelectric transducer, an error-acoustoelectric transducer, a feedforward element and a feedback element. The system further includes a first summing element, a second summing element, a third summing element, a first estimated plant response element and a second estimated plant response element.  
       [0017] The reference-acoustoelectric transducer produces a reference noise signal by detecting the noise in a noisy environment. The feedforward element is coupled with the reference-acoustoelectric transducer. The feedback element is coupled with the feedforward element. The first summing element is coupled with the feedforward element, the feedback element and with the electroacoustic transducer. The second summing element is coupled with the feedback element, the feedforward element and with the error-acoustoelectric transducer. The third summing element is coupled with the feedback element and with the second summing element. The first estimated plant response element is coupled with the second summing element and the second estimated plant response element is coupled with the third summing element and with the electroacoustic transducer.  
       [0018] The first summing element produces a summation signal, by adding a feedback signal received from the feedback element, a feedforward signal received from the feedforward element, and a sound signal respective of the desired sound. The electroacoustic transducer produces the sound according to the summation signal. The first estimated plant response element produces a first estimated desired sound signal, respective of the desired sound as produced by the electroacoustic transducer.  
       [0019] The error-acoustoelectric transducer produces an error signal by detecting the sound. The second summing element produces a first difference signal, by subtracting the first estimated desired sound signal from the error signal. The second estimated plant response element produces an estimated difference signal, according to the summation signal. The third summing element produces a second difference signal, by subtracting the estimated difference signal from the first difference signal. The feedback element produces the feedback signal according to the first difference signal and the second difference signal and the feedforward element produces the feedforward signal, according to the reference noise signal and the first difference signal.  
       [0020] In accordance with a further aspect of the disclosed technique, there is thus provided a method for producing a noise-free sound signal. The method includes the procedures of producing a noise bearing sound signal by detecting acoustic sound and noise, producing a reference noise signal by detecting noise, determining a correction signal according to the reference noise signal and producing the noise-free sound signal, according to the noise bearing sound signal and the correction signal.  
       [0021] In accordance with another aspect of the disclosed technique, there is thus provided a method for producing a noise-canceling sound. The method includes the procedures of producing a reference noise signal by detecting noise, determining a noise-canceling signal according to the reference noise signal and producing the noise-canceling sound according to the determined noise-canceling signal.  
       [0022] In accordance with a further aspect of the disclosed technique, there is thus provided a method for producing an audio-and-noise-canceling sound. The method includes the procedures of producing a reference noise signal by detecting noise, receiving an audio signal, determining an audio-and-noise-canceling signal according to the reference noise signal and the audio signal, and producing the audio-and-noise-canceling sound according to the determined audio-and-noise-canceling signal.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023] The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
     [0024]FIG. 1A is a schematic illustration of a system for producing a noise-free sound signal, constructed and operative in accordance with an embodiment of the disclosed technique;  
     [0025]FIG. 1B is a schematic illustration of a detail of the audio controller of the system of FIG. 1A;  
     [0026]FIG. 1C is a schematic illustration of the system of FIG. 1A incorporated with a head-mounted device;  
     [0027]FIG. 2A is a schematic illustration of a noise-canceling system, constructed and operative in accordance with another embodiment of the disclosed technique;  
     [0028]FIG. 2B is a schematic illustration of a detail of the analog ANR controller of the ANR controller of the system of FIG. 2A;  
     [0029]FIG. 2C is a schematic illustration of the system of FIG. 2A, incorporated with a head-mounted device;  
     [0030]FIG. 3A is a schematic illustration of a noise reduction system, constructed and operative in accordance with a further embodiment of the disclosed technique;  
     [0031]FIG. 3B is a schematic illustration of the system of FIG. 3A, incorporated with a head-mounted device;  
     [0032]FIG. 4A is a schematic illustration of a digital noise reduction system, constructed and operative in accordance with another embodiment of the disclosed technique;  
     [0033]FIG. 4B is a schematic illustration of the feedforward portion of the system of FIG. 4A;  
     [0034]FIG. 4C is a schematic illustration of the feedback portion of the system of FIG. 4A;  
     [0035]FIG. 5A is a schematic illustration of a method for operating the system of FIG. 1A, operative in accordance with a further embodiment of the disclosed technique;  
     [0036]FIG. 5B is a schematic illustration of a method for operating a noise-canceling system, operative in accordance with another embodiment of the disclosed technique; and  
     [0037]FIG. 6 is a schematic illustration of a method for operating the system of FIG. 3A, operative in accordance with a further embodiment of the disclosed technique.  
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
     [0038] The disclosed technique processes a background noise signal together with a signal containing the background noise and a desired sound, and produces a signal of the desired sound, substantially free of the background noise. The disclosed technique produces a noise-free signal from the voice of a person speaking in a noisy environment. The disclosed technique allows a person located in a noisy environment, to hear the desired sound, substantially free from noise.  
     [0039] The term “acoustoelectric transducer” herein below, refers to a device which converts acoustical signals to electrical signals (e.g., a microphone). The term “electroacoustic transducer” herein below, refers to a device which converts electrical signals to acoustical signals (e.g., a loudspeaker). An acoustoelectric transducer can operate based on principles of electrodynamics, electrostatics, piezoelectricity, magnetostriction, fiber-optics, stimulation of carbon particles, and the like. An electroacoustic transducer can operate based on principles of electrodynamics, magnetism, piezoelectricity, magnetostriction, hydraulic, and the like. The term “electric” herein includes all electromagnetic signals, such as electric, optic, radio, and the like, that can be transmitted by wire or other communication channels, or wirelessly.  
     [0040] The term “quiet zone” herein below, refers to a region in the vicinity of the ear-drum, the ear, or within the outer canal thereof, at which a sound at approximately 180 degrees out-of-phase relative to the ambient noise (anti-phase, or out-of-phase by π radians), cancels the ambient noise and as a result, the person does not hear the ambient noise. The locations “close to the ear” herein below, are approximate and refer to the quiet zone. The term “tonal noise” herein below, refers to a noise which is confined to substantially limited frequency range or ranges, such as the noise generated by the rotors of a helicopter.  
     [0041] Reference is now made to FIGS. 1A, 1B and  1 C. FIG. 1A is a schematic illustration of a system for producing a noise-free sound signal, generally referenced  100 , constructed and operative in accordance with an embodiment of the disclosed technique. FIG. 1B is a schematic illustration of a detail of the audio controller of the system of FIG. 1A. FIG. 1C is a schematic illustration of the system of FIG. 1A incorporated with a head-mounted device, generally referenced  150 .  
     [0042] With reference to FIG. 1A, system  100  includes acoustoelectric transducers  102  and  104  and an audio controller  106 . Audio controller  106  is coupled with acoustoelectric transducers  102  and  104 .  
     [0043] Audio controller  106  is a digital processor, which simultaneously samples two input signals at the same sampling rate and determines a transfer function for these two input signals, according to an adaptive filtering method. Audio controller  106  applies the transfer function on one of the input signals and subtracts the result from the other input signal. Audio controller  106 , then produces an output signal respective of the result of the subtraction.  
     [0044] Acoustoelectric transducer  102  detects acoustic sound. This acoustic sound can be a human voice, machine generated voice, and the like. If the acoustic sound is the voice of a person (not shown), then acoustoelectric transducer  102  is located close to the mouth (not shown) of the person. Acoustoelectric transducer  102  detects the desired sound (i.e., the voice) as well as the noise (i.e., an undesired sound) which is present in the environment surrounding the person. The noise is generated for example, by other persons and devices, such as engines, turbines, motors, and mechanical devices, hydraulic or pneumatic devices (e.g., tubing, actuators), electromechanical devices (e.g., electric motor), loud-speakers which surround the speaker, firing of ammunition, by environmental sources, such as wind, rain, ocean waves, thunderstorm, by animals, and the like.  
     [0045] Acoustoelectric transducer  104  and acoustoelectric transducer  102  detect different sounds, due to either a sound absorbing material (not shown), located between acoustoelectric transducers  102  and  104 , or the mere distance between acoustoelectric transducers  102  and  104 . Thus, acoustoelectric transducer  104  detects the noise and substantially none of the desired sound, while acoustoelectric transducer  102  detects the desired sound and noise.  
     [0046] Audio controller  106  receives signals  108  and  110  from acoustoelectric transducers  102  and  104 , respectively. Each of signals  108  and  110  is in analog format. An analog to digital converter (not shown) and herein below referred to as ADC, which converts an analog signal to a digital signal, is coupled with acoustoelectric transducer  102  and audio controller  106 . Another ADC (not shown) is coupled with acoustoelectric transducer  104  and audio controller  106 . Thus, audio controller  106  receives signals  108  and  110  which are in digital format.  
     [0047] Signal  108  includes information respective of a desired sound and noise. Signal  110  includes information respective of noise. Audio controller  106  determines a new reduced-intensity sound pressure level (SPL) for signal  110 , by employing an SPL converter (not shown). The SPL converter can be in form of a hardwired look-up table, a software look-up table, a hardwired transfer function, a software transfer function, an adaptive filter, and the like. Audio controller  106  subtracts the new determined SPL from the SPL of signal  108 , which corresponds to signal  110 . The noise detected by acoustoelectric transducer  102  is different from the noise detected by acoustoelectric transducer  104 , namely—it is usually at a reduced intensity and with a retarded phase (due to an acoustic insulation or acoustic insulating distance between acoustoelectric transducers  102  and  104 ). Thus, the new determined SPL corresponds to a reduced and retarded function of the SPL of signal  110 . Audio controller  106  produces a signal  112  respective of the result of the above subtraction operation. Thus, signal  112  includes information respective of the desired sound, substantially excluding the noise.  
     [0048] The form and the parameters of the SPL converter are determined in accordance with certain physical parameters, such as the hearing characteristics of a person, the voice characteristics of a person, the sound absorbing characteristics of a headset worn by a person, the dimensions of the headset, the relative distances between acoustoelectric transducer  102  and acoustoelectric transducer  104 , the acoustic properties of the environment which surround acoustoelectric transducer  102  and acoustoelectric transducer  104 , the acoustic properties of the sound absorbing material located between acoustoelectric transducer  102  and acoustoelectric transducer  104 , and the like.  
     [0049] With reference to FIG. 1B, system  100  includes acoustoelectric transducers  102  and  104 , audio controller  106  and analog to digital converters  114  and  116 . Audio controller  106  includes an adaptive filter  118  and a summing element  120 . ADC  114  is coupled with acoustoelectric transducer  102  and summing element  120 . ADC  116  is coupled with acoustoelectric transducer  104  and adaptive filter  118 . Alternatively, ADC  114  is integrated with either acoustoelectric transducer  102  or audio controller  106 . Similarly, ADC  116  can be integrated with acoustoelectric transducer  104  or audio controller  106 .  
     [0050] Acoustoelectric transducer  102  produces an analog signal  122  and sends analog signal  122  to ADC  114 . ADC  114  converts analog signal  122  to a digital signal  124 , sends digital signal  124  to summing element  120  and adaptive filter  118  produces a signal  130  according to signal  128 . Signal  130  is respective of the ambient noise detected by acoustoelectric transducer  104  at a reduced SPL (i.e., the SPL of the ambient noise close to acoustoelectric transducer  102 ). Summing element  120  produces signal  112  by subtracting signal  130  from signal  124 . Signal  112  is further provided to an interface (not shown) for further processing or transmission. Acoustoelectric transducer  104  produces an analog signal  126  and sends analog signal  126  to ADC  116 . ADC  116  converts analog signal  126  to a digital signal  128  and sends digital signal  128  to adaptive filter  118 . Signal  112  from summing element  120  is fed back to adaptive filter  118 , in a feedback loop  132 . If signal  112  includes any residual noise, then adaptive filter  118  detects this residual noise and adjusts signal  130  accordingly. Summing element  120  then subtracts this residual noise from signal  124 .  
     [0051] With reference to FIG. 1C, acoustoelectric transducer  102  is incorporated with head-mounted device  150 . Audio controller  106  is coupled with acoustoelectric transducers  102  and  104 . Head-mounted device  150  is in form of a helmet, a headset, and the like. Acoustoelectric transducer  102  is located at the mouth (not shown) of the user (not shown). Acoustoelectric transducer  104  is located external to head-mounted device  150  or externally mounted thereon, but acoustically insulated or remote from the mouth of the user.  
     [0052] Head-mounted device  150  can include a visual device (not shown), such as a head-up display, visor, liquid crystal display (LCD), field emission display (FED), mirror, and the like. Additionally, head-mounted device  150  can include one or more electroacoustic transducers.  
     [0053] If head-mounted device  150  is in form of a helmet, it can include sound absorbing material, such as mineral wool, fiberglass, and the like. In this case, acoustoelectric transducer  102  detects the voice of the user, while also detecting the background noise—but at a reduced SPL.  
     [0054] In case head-mounted device  150  is in form of a headset, due to the physical distance of acoustoelectric transducer  104  from the mouth of the user, acoustoelectric transducer  104  detects the ambient noise and substantially none of the voice of the user. However, acoustoelectric transducer  102  detects the voice of the user and the ambient noise. It is noted that even ambient air can effectively acoustically insulate, such as insulating acoustoelectric transducer  104  from the mouth of the user.  
     [0055] In case head-mounted device  150  is a helmet worn by a pilot (not shown), the ambient noise can be the noise generated by the engine (i.e., power-plant) of the aircraft, by the engines of other aircraft flying closeby, the voices of the aircraft crew, the sound of thunder, the sound of ice particles striking the windshield, the sound of firing ammunition, and the like. Acoustoelectric transducer  102  is attached to the inner portion of head-mounted device  150 , close to the mouth of the pilot and acoustoelectric transducer  104  is attached to the outer portion of head-mounted device  150 .  
     [0056] Head-mounted device  150  includes sound absorbing material, and acoustoelectric transducer  104  is farther away from the mouth of the pilot than acoustoelectric transducer  102 . Hence, acoustoelectric transducer  104  detects mostly the ambient noise and substantially none of the voice of the pilot. However, since the sound absorbing material of head-mounted device  150  absorbs only a portion of the sound, acoustoelectric transducer  102  detects the voice of the pilot, in addition to the ambient noise at a reduced SPL. Thus, signal  108  includes information respective of the voice of the pilot and an attenuated level of the ambient noise, while signal  110  includes information respective of the ambient noise at an SPL higher than that detected by acoustoelectric transducer  102 . The attenuation level of the ambient noise may depend on frequency.  
     [0057] The parameters of the SPL converter can be determined empirically, by measuring the SPL values of signals  108  and  110  in a selected frequency range, in response to sound corresponding to the SPL values and in the frequency range of the expected ambient noise. It is noted that these measurements are performed without the voice of the pilot in the same location within the aircraft, in which system  100  is employed. These measurements can be performed before flight as “pre-calibrations” or during speech pauses at flight time. In addition, audio controller  106  calibrates system  100 , at the beginning of every flight. Alternatively, the parameters of the SPL converter can be determined analytically, by computing the estimated attenuation of SPL values of the ambient noise in a selected frequency range.  
     [0058] It is further noted that the attenuated SPL value of the ambient noise detected by acoustoelectric transducer  102 , depends also on the physical distance between acoustoelectric transducers  102  and  104 . It is noted that due to the physical distance between acoustoelectric transducers  102  and  104  and a given value of the speed of sound, signals  108  and  110  can include information respective of the ambient noise waveform, which are out of phase. In order to subtract the correct portion of the ambient noise waveform from signal  108 , audio controller  106  takes this phase-shift into account, by referring to a respective look-up table, transfer function, and the like.  
     [0059] According to another aspect of the disclosed technique, a noise reduction system employs an active noise reduction (ANR) controller, to produce a noise-free sound close to the ear of a user. The ANR controller produces an anti-phase signal of the ambient noise, which is derived from the detection of ambient noise by an external acoustoelectric transducer.  
     [0060] Reference is now made to FIGS. 2A, 2B and  2 C. FIG. 2A is a schematic illustration of a noise-canceling system, generally referenced  170 , constructed and operative in accordance with another embodiment of the disclosed technique. FIG. 2B is a schematic illustration of a detail of the analog ANR controller of the ANR controller of the system of FIG. 2A. FIG. 2C is a schematic illustration of the system of FIG. 2A, incorporated with a head-mounted device, generally referenced  214 .  
     [0061] With reference to FIG. 2A, system  170  includes an ANR controller  172 , a reference acoustoelectric transducer  174 , an error acoustoelectric transducer  176  and an electroacoustic transducer  178 . ANR controller  172  includes a digital ANR controller  180 , an analog ANR controller  182  and a primary summing element  184 . Digital ANR controller  180  is a device which produces an anti-phase signal for an input signal, at a reduced SPL. Analog ANR controller  182  is a device which produces an anti-phase signal for an input signal, at the same SPL.  
     [0062] Digital ANR controller  180  is coupled with reference acoustoelectric transducer  174 , error acoustoelectric transducer  176  and with primary summing element  184 . Analog ANR controller  182  is coupled with error acoustoelectric transducer  176  and with primary summing element  184 . Primary summing element  184  is coupled with electroacoustic transducer  178 .  
     [0063] Electroacoustic transducer  178  and error acoustoelectric transducer  176  are located close to an ear  186  of a user (not shown). Reference acoustoelectric transducer  174  is located substantially remote from ear  186 . Alternatively, a sound absorbing material (not shown) is located between electroacoustic transducer  178  and error acoustoelectric transducer  176  on one side and reference acoustoelectric transducer  174  on the other. In both cases, reference acoustoelectric transducer  174  detects the ambient noise and substantially none of the sound produced by electroacoustic transducer  178 . Likewise, error acoustoelectric transducer  176  detects the sound emitted by electroacoustic transducer  178  and the ambient noise at a location close to ear  186 .  
     [0064] Following is a description of a loop L 1  formed by digital ANR controller  180 , primary summing element  184 , electroacoustic transducer  178  and error acoustoelectric transducer  176 . Digital ANR controller  180  continuously samples a signal  188  from reference acoustoelectric transducer  174 , respective of the ambient noise, and a signal  192  respective of a desired sound, from a sound source (not shown). The desired sound from the sound source can be a human voice, machine generated sound, mechanical voice, a sound signal, an acoustic sound (e.g., loud speaker), and the like.  
     [0065] Digital ANR controller  180  determines a reduced SPL for signal  188  by employing an SPL converter as described herein above in connection with audio controller  106  (FIG. 1C). The reduced SPL for signal  188  corresponds to the SPL of the ambient noise, at a location close to ear  186 . Digital ANR controller  180  produces an anti-phase signal (not shown) for signal  188  at the reduced SPL, and adds this anti-phase signal at the reduced SPL, to signal  192 , thereby producing a signal  194 . Electroacoustic transducer  178  produces a sound according to signal  194 .  
     [0066] It is noted that error electroacoustic transducer  176  is located sufficiently close to ear  186 , such that the anti-phase sound of the ambient noise at the quiet zone of ear  186 , which is emitted by electroacoustic transducer  178 , substantially cancels out the ambient noise at the quiet zone of ear  186 . Error acoustoelectric transducer  176  is located sufficiently close to electroacoustic transducer  178 , to detect the sound emitted by electroacoustic transducer  178 .  
     [0067] Digital ANR controller  180  receives a signal  190  from error acoustoelectric transducer  176 , respective of the sound emitted by electroacoustic transducer  178  (which includes the desired sound and the anti-phase of the ambient noise at a location close to ear  186 ) and the ambient noise at a location close to ear  186 . Digital ANR controller  180  modifies a portion of signal  194  respective of the anti-phase of the ambient noise at a location close to ear  186 , by processing signals  188 ,  190  and  192 .  
     [0068] It is further noted that since signals  188  and  190  are analog, two analog to digital converters (not shown), are employed to convert signals  188  and  190  to digital format. Alternatively, these analog to digital converters are integrated with each one of reference acoustoelectric transducer  174  and error acoustoelectric transducer  176 , or integrated with digital ANR controller  180 . Signal  192  can be either digital or analog. If signal  192  is analog, then another ADC (not shown) converts signal  192  to digital format. A digital to analog converter (not shown), and herein below referred to as DAC, converts signal  194  from digital format to analog format. Alternatively, this DAC is integrated with either digital ANR controller  180  or with primary summing element  184 .  
     [0069] With further reference to FIG. 2B, analog ANR controller  182  includes a digital portion  228 , an analog portion  230  and a secondary summing element  232 . Secondary summing element  232  is coupled with digital portion  228 , analog portion  230  and primary summing element  184 . Primary summing element  184  is coupled with electroacoustic transducer  178 . Analog portion  230  is coupled with error acoustoelectric transducer  176 . Analog portion  230 , primary summing element  184 , secondary summing element  232 , electroacoustic transducer  178  and error acoustoelectric transducer  176  form a feedback loop L 2  in system  170 .  
     [0070] Following is a description of feedback loop L 2 . Analog portion  230  receives signal  190  from error acoustoelectric transducer  176 , produces a signal  234  and sends signal  234  to secondary summing element  232 . Signal  234  is approximately 180 degrees out-of-phase relative to signal  190 . Due to the operation of analog portion  230  and gain losses between electroacoustic transducer  178  and analog portion  230 , signal  234  is attenuated. Digital portion  228  produces a signal  236  by attenuating signal  192  by the same amount that signal  234  is attenuated and sends signal  236  to secondary summing element  232 .  
     [0071] Secondary summing element  232  produces a signal  198 , by adding signals  234  and  236 . Since the desired sound portion of signal  234  is out-of-phase by approximately 180 degrees relative to signal  236 , the desired sound portion of signal  234  and signal  236 , substantially cancel out at secondary summing element  232 . Thus, signal  198  is substantially respective of only the anti-phase of the ambient noise at a location close to ear  186 . Primary summing element  184  produces a signal  200  by adding signals  194  and  198 . Electroacoustic transducer  178  emits a sound respective of the sum of signal  194  (which includes the desired sound, an anti-phase to the ambient noise at a location close to ear  186  and an adjustment according to signal  190 ) and signal  198  (which includes another anti-phase to the ambient noise at a location close to ear  186 ).  
     [0072] It is noted that the ANR controller can include only the digital ANR controller coupled with the reference acoustoelectric transducer, the error acoustoelectric transducer and with the electroacoustic transducer. Thus, the digital ANR controller makes adjustments to a signal which sends to the electroacoustic transducer, according to an error signal, which the digital ANR controller receives from the error acoustoelectric transducer. In this case, the digital ANR controller reduces mainly tonal noise.  
     [0073] With reference to FIG. 2A, it is noted that digital ANR controller  180  operates at a slower rate than that of analog ANR controller  182 , but digital ANR controller  180  is substantially more effective in producing anti-phase signals for tonal noise and for noise at substantially high frequencies. On the other hand, analog ANR controller  182  is more effective in producing anti-phase signals for noise in a substantially wide frequency range, albeit at substantially low frequencies. Thus, by combining digital ANR controller  180  and analog ANR controller  182  in ANR controller  172 , system  170  is capable to produce a desired sound in the presence of noise, both at a narrow (i.e., tonal noise) or a wide frequency range, as well as low or high frequencies. Digital ANR controller  180  and analog ANR controller  182  attenuate the same noise. Thus, the attenuated noise in signal  200  is substantially equal to the sum of the attenuation performed by digital ANR controller  180  and analog ANR controller  182 .  
     [0074] With reference to FIG. 2C, system  170  includes ANR controller  202 , reference acoustoelectric transducers  204  and  238 , error acoustoelectric transducers  206  and  208  and electroacoustic transducers  210  and  212 . ANR controller  202  is similar to ANR controller  172  (FIG. 2A). Each of error acoustoelectric transducers  206  and  208  is similar to error acoustoelectric transducer  176 . Each of electroacoustic transducers  210  and  212  is similar to electroacoustic transducer  178 . Error acoustoelectric transducers  206  and  208  and electroacoustic transducers  210  and  212  are coupled with head-mounted device  214 . Reference acoustoelectric transducers  204  and  238  are located external to head-mounted device  214  or externally mounted thereon, but acoustically insulated or remote from error acoustoelectric transducers  206  and  208  and electroacoustic transducers  210  and  212 .  
     [0075] Head-mounted device  214  is similar to head-mounted device  150 , as described herein above in connection with FIG. 1C.  
     [0076] Error acoustoelectric transducer  206 , electroacoustic transducer  210  and reference acoustoelectric transducer  238  are located adjacent to the right ear (not shown) of the user (not shown). Error acoustoelectric transducer  208 , electroacoustic transducer  212  and reference acoustoelectric transducer  204  are located adjacent to the left ear (not shown) of the user. Error acoustoelectric transducer  206  detects the sound emitted by electroacoustic transducer  210 , the ambient noise at a reduced SPL, and substantially none of the sound emitted by electroacoustic transducer  212 . Error acoustoelectric transducer  208  detects the sound emitted by electroacoustic transducer  212 , the ambient noise at a reduced SPL, and substantially none of the sound emitted by electroacoustic transducer  210 . Reference acoustoelectric transducers  204  and  238  detect the ambient noise and substantially none of the sound which is emitted by electroacoustic transducers  210  and  212 .  
     [0077] ANR controller  202  is coupled with reference acoustoelectric transducers  204  and  238 , error acoustoelectric transducers  206  and  208  and with electroacoustic transducers  210  and  212 . ANR controller  202  receives a signal  216  from reference acoustoelectric transducer  204 , a signal  240  from reference acoustoelectric transducer  238 , a signal  218  from error acoustoelectric transducer  206 , a signal  220  from error acoustoelectric transducer  208  and a signal  222  from a sound source (not shown). Signals  216  and  238  are similar to signal  188  (FIG. 2A). Each of signals  218  and  220  is similar to signal  190 . Each of signals  224  and  226  is similar to signal  200  and signal  222  is similar to signal  192 .  
     [0078] Signal  222  can be either a single channel sound signal (i.e., monaural), or a multi-channel sound signal, such as stereophonic, quadraphonic, surround sound, and the like. ANR controller  202  produces a signal  224  for electroacoustic transducer  210  and a signal  226  for electroacoustic transducer  212 . ANR controller  202  produces signals  224  and  226 , by processing signals  216 ,  238 ,  218 ,  220  and  222 , in the same manner that ANR controller  172  (FIG. 2A) processes signals  188 ,  192  and the signal received from error acoustoelectric transducer  176 , for producing signal  200 .  
     [0079] Each of electroacoustic transducers  210  and  212  produces a sound which includes the sound respective of signal  222  and an anti-phase of the ambient noise at a reduced SPL. Since the anti-phase of the ambient noise substantially cancels the actual ambient noise at the quiet zone of the respective ear, the user hears mostly a sound corresponding to signal  222  and substantially none of the ambient noise. If signal  222  is a single channel sound signal, then each of signals  224  and  226  is produced according to signal  222  and the anti-phase of the ambient noise at a reduced SPL. Hence, the user can hear a monaural sound.  
     [0080] If signal  222  is stereo, then signals  224  and  226  are produced for example, according to the right and the left channel of signal  222 , respectively, and according to the anti-phase of the ambient noise at a reduced SPL. Hence, the user can hear the sound which corresponds to signal  222  in stereo, without hearing the ambient noise.  
     [0081] Alternatively, more than two electroacoustic transducers and respective acoustoelectric transducers can be coupled to the ANR controller. In this case, if signal  222  is multi-channel, then the user can hear the sound which corresponds to signal  222  in multi-dimension, without hearing the ambient noise.  
     [0082] With further reference to FIG. 2A, the electroacoustic transducers are coupled with the primary summing element and the acoustoelectric transducers are coupled with the digital ANR controller. The digital ANR controller produces a signal for each one of the electroacoustic transducers, by processing the desired sound signal, the noise signal and the error signal received from the respective acoustoelectric transducer.  
     [0083] With further reference to FIG. 2B, the electroacoustic transducers are coupled with the primary summing element and the acoustoelectric transducers are coupled with the analog portion of the analog ANR controller. According to the desired sound signal, the digital portion estimates in real time, the SPL of the desired sound which each of the electroacoustic transducers produces and the digital portion produces these estimated desired sound signals. The digital portion sends the estimated desired sound signal respective of each of the electroacoustic transducers, to the secondary summing element.  
     [0084] The analog portion produces an anti-phase signal respective of each of the signals received from the acoustoelectric transducers and sends these anti-phase signals to the secondary summing element. The secondary summing element produces a signal respective of each of the electroacoustic transducers, by adding the respective anti-phase signal received from the analog portion and the respective signal received from the digital portion. The primary summing element produces a signal for each of the electroacoustic transducers, by adding the respective signal received from the digital ANR controller and the respective signal received from the secondary summing element.  
     [0085] Alternatively, the noise-canceling system of FIG. 2A, receives no signals respective of the desired sound and produces only an anti-phase noise sound, according to noise detected by a reference acoustoelectric transducer located away from the ear of the user. In this case, the noise-canceling system includes a digital ANR controller similar to digital ANR controller  180 , a reference acoustoelectric transducer and an electroacoustic transducer. The digital ANR controller is coupled with the reference acoustoelectric transducer and the electroacoustic transducer. The reference acoustoelectric transducer is located in a noisy environment away from the ear of the user and the electroacoustic transducer is located close to the ear of the user.  
     [0086] Additionally, the noise-canceling system includes an error acoustoelectric transducer coupled with the digital ANR controller. The error acoustoelectric transducer is located close to the ear of the user and sends an error signal to the digital ANR controller, respective of the sound emitted by the electroacoustic transducer. The digital ANR controller processes the error signal and the reference noise signal and makes adjustments to the anti-phase noise signal which sends to the electroacoustic transducer.  
     [0087] Additionally, the noise-canceling system includes an analog ANR controller similar to analog ANR controller  182  and a summing element. The analog ANR controller is coupled with the error acoustoelectric transducer and the summing element, and the summing element is coupled with the digital ANR controller and the electroacoustic transducer. The analog ANR controller produces an anti-phase noise signal approximately 180 degrees out-of-phase relative to the error signal. The summing element produces a signal for the electroacoustic transducer, by adding the anti-phase noise signals produced by the digital ANR controller and the analog ANR controller.  
     [0088] Alternatively, the error acoustoelectric transducer can be coupled only with the analog active noise reduction controller and not with the digital active noise reduction controller. In this case, only the analog active noise reduction controller makes adjustments to the anti-phase noise signal which the digital active noise reduction controller sends to the electroacoustic transducer.  
     [0089] According to another aspect of the disclosed technique, a noise reduction system produces a noise-free sound close to the ear of a user, and a noise-free signal corresponding to the voice of the user. The system produces a noise-canceling sound or a noise canceling signal, according to a noise reference signal.  
     [0090] Reference is now made to FIGS. 3A and 3B. FIG. 3A is a schematic illustration of a noise reduction system, generally referenced  250 , constructed and operative in accordance with a further embodiment of the disclosed technique. FIG. 3B is a schematic illustration of the system of FIG. 3A, incorporated with a head-mounted device, generally referenced  304 .  
     [0091] With reference to FIG. 3A, system  250  includes a noise controller  252 , a reference acoustoelectric transducer  254 , an error acoustoelectric transducer  256 , a voice acoustoelectric transducer  258  and an electroacoustic transducer  260 . Noise controller  252  includes an ANR controller  262  and an audio controller  264 . ANR controller  262  is similar to ANR controller  172  (FIG. 2A) and audio controller  264  is similar to audio controller  106  (FIG. 1A).  
     [0092] ANR controller  262  is coupled with reference acoustoelectric transducer  254 , error acoustoelectric transducer  256  and with electroacoustic transducer  260 . Audio controller  264  is coupled with reference acoustoelectric transducer  254  and voice acoustoelectric transducer  258 .  
     [0093] Electroacoustic transducer  260  and error acoustoelectric transducer  256  are located close to an ear  266  of a user (not shown) and voice acoustoelectric transducer  258  is located close to a mouth  268  of the user. Sound absorbing material (not shown) can be placed between electroacoustic transducer  260 , error acoustoelectric transducer  256  and voice acoustoelectric transducer  258  on one side and reference acoustoelectric transducer  254 , on the other. Such a sound absorbing material can be in the form of an earmuff, and the like, which encloses electroacoustic transducer  260  and error acoustoelectric transducer  256 . In addition, sound absorbing material acoustically insulates voice acoustoelectric transducer  258  and mouth  268  from electroacoustic transducer  260 , error acoustoelectric transducer  256  and ear  266 . Thus, error acoustoelectric transducer  256  does not detect the voice of the user and voice acoustoelectric transducer  258  does not detect sound emitted by electroacoustic transducer  260 .  
     [0094] Thus, reference acoustoelectric transducer  254  detects the ambient noise and substantially none of the voice of the user or the sound emitted by electroacoustic transducer  260 . Reference acoustoelectric transducer  254  sends a signal  274  respective of the detected ambient noise, to ANR controller  262  and to audio controller  264 . Error acoustoelectric transducer  256  detects the sound emitted by electroacoustic transducer  260  and the ambient noise at a reduced SPL and sends a respective signal  276  to ANR controller  262 . Voice acoustoelectric transducer  258  detects the voice of the user from mouth  268  and the ambient noise at a reduced SPL and sends a respective signal  278  to audio controller  264 .  
     [0095] System  250  can be divided to a hearing portion and a speaking portion. The hearing portion consists of ANR controller  262 , reference acoustoelectric transducer  254 , error acoustoelectric transducer  256  and electroacoustic transducer  260 . The speaking portion consists of audio controller  264  and reference acoustoelectric transducer  254  and voice acoustoelectric transducer  258 . Reference acoustoelectric transducer  254  is common to the hearing portion and the speaking portion.  
     [0096] The hearing portion of system  250  is similar to system  170 , as described herein above in connection with FIG. 2A. ANR controller  262  determines an anti-phase to signal  274  at a reduced SPL (i.e., the ambient noise at the quiet zone of ear  266 ). ANR controller  262  produces a signal  280  respective of the desired sound, according to a signal  270  from a sound source (not shown) and the anti-phase of signal  274  at the reduced SPL. Electroacoustic transducer  260  produces a sound according to signal  280 . Thus, the user hears the desired sound and substantially none of the ambient noise. ANR controller  262  makes adjustments to signal  280 , according to signal  276 .  
     [0097] Alternatively, the active noise reduction controller does not receive any signal respective of the desired sound. In this case, the active noise reduction controller sends a noise-canceling signal to the electroacoustic transducer and a is different electroacoustic transducer produces the desired sound according to the signal respective of the desired sound. Further alternatively, the desired sound reaches the ear from a sound source other than an electroacoustic transducer, such as the voice of another person, mechanical voice, machine generated sound, and the like.  
     [0098] Alternatively, the acoustoelectric transducer can be eliminated from the noise reduction system. In this case, the active noise reduction controller produces a noise-canceling signal only according to the reference noise signal, and without any error signal as feedback.  
     [0099] The speaking portion of system  250  is similar to system  100 , as described herein above in connection with FIG. 1A. Thus, audio controller  264  produces a noise-free voice signal  272 .  
     [0100] With reference to FIG. 3B, system  250  includes a noise controller  290 , a reference acoustoelectric transducer  292 , error acoustoelectric transducers  294  and  296 , a voice acoustoelectric transducer  298  and electroacoustic transducers  300  and  302 . Noise reduction system  290  is similar to noise reduction system  252  (FIG. 3A). Noise controller  290  is coupled with reference acoustoelectric transducer  292 , error acoustoelectric transducers  294  and  296 , voice acoustoelectric transducer  298  and with electroacoustic transducers  300  and  302 .  
     [0101] Error acoustoelectric transducers  294  and  296 , voice acoustoelectric transducer  298  and electroacoustic transducers  300  and  302  are located within head-mounted device  304 . Reference acoustoelectric transducer  292  is located external to head-mounted device  304  or externally mounted thereon, but acoustically insulated or remote the mouth of the user and from error acoustoelectric transducers  294  and  296  and electroacoustic transducers  300  and  302 . Error acoustoelectric transducer  294  and electroacoustic transducer  300  are located at a right ear (not shown) of a user (not shown). Error acoustoelectric transducer  296  and electroacoustic transducer  302  are located at a left ear (not shown) of the user. Voice acoustoelectric transducer  298  is located at a mouth (not shown) of the user.  
     [0102] Noise controller  290  receives a signal  306  from reference acoustoelectric transducer  292 , respective of the ambient noise and a signal  308  from a sound source (not shown), respective of a desired sound. Noise controller  290  receives a signal  310  from voice acoustoelectric transducer  298  respective of the voice of the user and the ambient noise at a reduced SPL.  
     [0103] Noise controller  290 , reference acoustoelectric transducer  292 , error acoustoelectric transducers  294  and  296  and electroacoustic transducers  300  and  302 , form the hearing portion of system  250 , as described herein above in connection with FIG. 3A. Electroacoustic transducers  300  and  302  produce sounds which include a desired sound carried by a signal  308  and another sound at anti-phase and at a reduced SPL relative to signal  306 . Thus, the user hears the desired sound and substantially none of the ambient noise.  
     [0104] Noise controller  290 , reference acoustoelectric transducer  292  and voice acoustoelectric transducer  298 , form the speaking portion of system  250 , as described herein above in connection with FIG. 2A. Thus, noise controller  290  produces a noise-free signal  312  of the voice of the user, according to signals  306  and  310 .  
     [0105] Alternatively, system  250  can include two reference acoustoelectric transducers similar to reference acoustoelectric transducer  292  and coupled with noise controller  290 . Each of these reference acoustoelectric transducers is located external to head-mounted device  304 , in a manner similar to that described herein above in connection with reference acoustoelectric transducers  204  and  238  (FIG. 2C).  
     [0106] According to another aspect of the disclosed technique, an active noise reduction system includes a digital feedforward portion which receives a reference noise signal and a digital/analog feedback portion, which receives a signal respective of a sound produced by the system at the quiet zone of the ear. The feedforward portion produces a signal respective of a desired sound, and an anti-phase of the background noise according to a desired sound signal and the feedback from the feedback portion.  
     [0107] Reference is now made to FIGS. 4A, 4B and  4 C. FIG. 4A is a schematic illustration of a digital noise reduction system, generally referenced  320 , constructed and operative in accordance with another embodiment of the disclosed technique. FIG. 4B is a schematic illustration of the feedforward portion of the system of FIG. 4A. FIG. 4C is a schematic illustration of the feedback portion of the system of FIG. 4A. It is noted that system  320  is a detail illustration of a digital ANR controller such as digital ANR controller  180  (FIG. 2A).  
     [0108] With reference to FIG. 4A, system  320  includes a reference acoustoelectric transducer  322 , an error acoustoelectric transducer  324 , an electroacoustic transducer  326 , estimated plant response (EPR) elements  328  and  330 , a feedforward element  332 , a feedback element  334 , and summing elements  336 ,  338  and  340 . Feedforward element  332 , feedback element  334 , EPR elements  328  and  330  and summing elements  336 ,  338  and  340  together, are equivalent to digital ANR controller  180  (FIG. 2A). Feedforward element  332  includes an EPR element  342 , an adaptive filter  344  and a least mean square (LMS) element  346 . Feedback element  334  includes an adaptive filter  348 , an LMS element  350  and an EPR element  352 .  
     [0109] An EPR element is an element which estimates the ratio of two sound signals according to predetermined information, applies this ratio to an input signal to the EPR element and produces an output signal, accordingly. One of these two sound signals can be for example, respective of a desired sound which is to be produced by an electroacoustic transducer, while the other sound signal is respective of the sound which the electroacoustic transducer actually produces. An LMS element is an element which updates the response of the adaptive filter, according to an LMS adaptive filter method. The combination of an LMS element and an EPR element is equivalent to a Filter X LMS (FXLMS) element, as known in the art.  
     [0110] Electroacoustic transducer  326  and error acoustoelectric transducer  324  are located close to an ear  354  of a user (not shown). A sound absorbing element (not shown) is located between electroacoustic transducer  326  and error acoustoelectric transducer  324  on one side and reference acoustoelectric transducer  322  on the other. Thus, reference acoustoelectric transducer  322  detects the ambient noise and none of the sound emitted by electroacoustic transducer  326 . Error acoustoelectric transducer  324  detects the sound emitted by electroacoustic transducer  326  and the ambient noise at a reduced SPL. Each of adaptive filters  344  and  348  is similar in principle to adaptive filter  118 , as described herein above in connection with FIG. 1B.  
     [0111] With reference to FIG. 4B, the digital feedforward portion of system  320  includes reference acoustoelectric transducer  322 , error acoustoelectric transducer  324 , electroacoustic transducer  326 , feedforward element  332 , summing elements  336  and  340  and EPR element  330 . Summing element  336  is coupled with feedforward element  332 , electroacoustic transducer  326  and with EPR element  330 . Summing element  340  is coupled with feedforward element  332 , error acoustoelectric transducer  324  and with EPR element  330 . Reference acoustoelectric transducer  322  is coupled with feedforward element  332 .  
     [0112] Reference acoustoelectric transducer  322  detects the ambient noise and sends a respective signal  356  to feedforward element  332 . Feedforward element  332  determines the reduced SPL of the ambient noise at the quiet zone of ear  354 . It is noted that the SPL reduction is generally sensitive to the frequency of signal  356 . Feedforward element  332 , determines a signal  358  which is at anti-phase to the ambient noise signal  356  at the reduced SPL and sends signal  358  to summing element  336 . Summing element  336  adds signal  358  to a signal  360 , and produces a signal  362  respective of the result of addition. Signal  360  is respective of a desired sound from a sound source (not shown). Thus, signal  362  includes the desired sound signal and the anti-phase of the ambient noise at the reduced SPL. Summing element  336  sends signal  362  to electroacoustic transducer  326 .  
     [0113] Electroacoustic transducer  326  produces the desired sound together with a noise-canceling sound, according to signal  362 . Since the anti-phase of the ambient noise at the quiet zone of ear  354  cancels the ambient noise at this quiet zone, the user hears the desired sound and substantially none of the ambient noise.  
     [0114] Error acoustoelectric transducer  324  detects the sound emitted by electroacoustic transducer  326  and sends a signal  364  respective of the detected sound, to summing element  340 . EPR element  330  receives signal  360 , determines a signal  366  which is an estimate of the desired sound emitted by electroacoustic transducer  326  at the quiet zone of ear  354 , and sends signal  366  to summing element  340 . Summing element  340  produces an error signal  368 , by comparing signals  366  and  364  (i.e., by subtracting signal  366  from signal  364 ) and sends error signal  368  to feedforward element  332  and to feedback element  334 . Error signal  368  represents the difference between the desired sound as received from the sound source and the noise-cancelled desired sound emitted at the quiet zone of ear  354 . Feedforward element  332  makes a correction to signal  358  according to error signal  368  and sends signal  358  to summing element  336 .  
     [0115] With reference to FIG. 4C, the feedback portion of system  320  includes electroacoustic transducer  326 , error acoustoelectric transducer  324 , feedback element  334 , EPR elements  328  and  330  and summing elements  336 ,  338  and  340 . Summing element  336  is coupled with feedback element  334 , EPR elements  328  and  330  and with electroacoustic transducer  326 . Summing element  338  is coupled with feedback element  334 , EPR element  328  and with summing element  340 . Summing element  340  is coupled with feedback element  334 , EPR element  330 , summing element  338  and with error acoustoelectric transducer  324 .  
     [0116] Summing element  336  produces signal  362  by adding signal  358 , which summing element  336  receives from feedforward element  332 , to signal  360 , which summing element  336  receives from the sound source. Thus, as described herein above in connection with FIG. 4B, signal  362  includes the desired sound signal and the anti-phase of the ambient noise at the reduced SPL. Summing element  336  sends signal  362  to electroacoustic transducer  326  and to EPR element  328 .  
     [0117] Electroacoustic transducer  326  produces the desired sound together with a noise-canceling sound, according to signal  362 . Since the anti-phase of the ambient noise at the quiet zone of ear  354  cancels the ambient noise at this quiet zone, the user hears the desired sound and substantially none of the ambient noise.  
     [0118] Error acoustoelectric transducer  324  detects the sound emitted by electroacoustic transducer  326  and sends a signal  364  respective of the detected sound, to summing element  340 . EPR element  330  receives signal  360 , determines a signal  366  which is an estimate of the desired sound emitted at the quiet zone of ear  354  and sends signal  366  to summing element  340 . Summing element  340  produces an error signal  368 , by comparing signals  366  and  364  (i.e., by subtracting signal  366  from signal  364 ) and sends error signal  368  to feedback element  334 , to summing element  338  and to feedforward element  332 . Error signal  368  represents the difference between the desired sound as received from the sound source and the noise-cancelled desired sound emitted at the quiet zone of ear  354 .  
     [0119] EPR element  328  produces a signal  370 , which is an estimate of a sound emitted by electroacoustic transducer  326  and as detected by error acoustoelectric transducer  324 . EPR element  328  produces signal  370  according to signal  362 . Summing element  338  produces an error signal  372 , by comparing signals  368  and  370  (i.e., by subtracting signal  370  from signal  368 ) and sends error signal  372  to feedback element  334 . Feedback element  334  produces an error signal  374 , by processing error signals  368  and  372  and sends error signal  374  to summing element  336 . Summing element  336  produces signal  362  by adding error signal  374  to signal  358  (for the ambient noise canceling signal) and signal  360  (for the sound source signal).  
     [0120] It is noted that the noise reduction system can include a plurality of electroacoustic transducers and a respective acoustoelectric transducer for each of the electroacoustic transducers. In this case, the system receives the desired sound in a plurality of channels and the user can hear the desired sound in multiple dimensions.  
     [0121] It is further noted that system  320  produces an anti-phase noise signal according to a signal received from an acoustoelectric transducer (i.e., reference acoustoelectric transducer  322 ), which is not affected by the sound emitted by the electroacoustic transducer (i.e., electroacoustic transducer  326 ) and adapts this anti-phase noise signal according to a signal respective of the sound emitted by this electroacoustic transducer (i.e., signal  364 ). The operation of the feedforward portion and the feedback portion of system  320  are similar. The difference between the two portions is that the input to the feedforward portion is the ambient noise devoid of any sound emitted by the electroacoustic transducer, while the input to the feedback portion is the sound which is actually emitted by this electroacoustic transducer.  
     [0122] Reference is now made to FIG. 5A, which is a schematic illustration of a method for operating the system of FIG. 1A, operative in accordance with a further embodiment of the disclosed technique. In procedure  400  a noise bearing sound signal is produced, by detecting acoustic sound and noise. With reference to FIG. 1A, acoustoelectric transducer  102  detects acoustic sound and noise and sends signal  108  respective of this detected acoustic sound and noise, to audio controller  106 .  
     [0123] In procedure  402 , a reference noise signal is produced by detecting noise. With reference to FIG. 1A, acoustoelectric transducer  104  detects the noise and sends signal  110  respective of this noise, to audio controller  106 .  
     [0124] In procedure  404 , a correction signal is determined according to the reference noise signal. With reference to FIG. 1A, audio controller  106  determines a reduced SPL for signal  110 .  
     [0125] In procedure  406 , a noise-free signal is produced according to the correction signal and the noise bearing sound signal. With reference to FIG. 1A, audio controller  106  produces signal  112  by subtracting signal  110  at the reduced SPL, from signal  108 .  
     [0126] Reference is now made to FIG. 5B, which is a schematic illustration of a method for operating a noise-canceling system, operative in accordance with another embodiment of the disclosed technique. This noise-canceling system employs a reference acoustoelectric transducer to detect the ambient noise, wherein the reference acoustoelectric transducer is located away from the ear of the user. It is noted that the procedure of detecting the ambient noise by this reference acoustoelectric transducer, is common to both of the methods according to FIGS. 5A and 5B. It is further noted that the methods according to FIGS. 5A and  5 B, can be combined into a single method which is herein below described in connection with FIG. 6.  
     [0127] With reference to FIG. 5B, in procedure  408 , which is similar to procedure  402 , reference noise signal is produced by detecting noise. The reference acoustoelectric transducer produces a reference noise signal, by detecting the ambient noise. In procedure  410 , a noise-canceling signal is determined, by processing the reference noise signal. An ANR controller similar to ANR controller  172  (FIG. 2A) determines a noise-canceling signal by processing the reference noise signal. The ANR controller determines a reduced SPL for the reference noise signal, corresponding to the SPL of the ambient noise at a location close to the ear of the user. Furthermore, the ANR controller determines a noise-canceling signal, which is approximately 180 degrees out-of-phase relative to the reference noise signal. An electroacoustic transducer similar to electroacoustic transducer  178  (FIG. 2A), produces a noise-canceling sound according to the determined noise-canceling signal (procedure  412 ).  
     [0128] Reference is now made to FIG. 6, which is a schematic illustration of a method for operating the system of FIG. 3A, operative in accordance with a further embodiment of the disclosed technique. In procedure  420 , a noisy voice signal is produced by detecting voice and noise. With reference to FIG. 3A, voice acoustoelectric transducer  258  detects the voice of the user from mouth  268 , together with the ambient noise at a reduced SPL and sends signal  278  to audio controller  264 .  
     [0129] In procedure  422 , a reference noise signal is produced by detecting noise. With reference to FIG. 3A, reference acoustoelectric transducer  254  detects the ambient noise and sends signal  274  to audio controller  264 .  
     [0130] In procedure  424 , a correction signal is determined according to the reference noise signal. With reference to FIG. 3A, audio controller  264  determines a reduced SPL for signal  274 .  
     [0131] In procedure  426 , a noise-free voice signal is produced according to the correction signal and the noisy voice signal. With reference to FIG. 3A, audio controller  264  produces signal  272  by subtracting signal  274  at the reduced SPL, from signal  278 .  
     [0132] In procedure  428 , an audio signal is received. With reference to FIG. 3A, ANR controller  262  receives signal  270  from the sound source. In procedure  430 , an error signal is produced, by detecting sound in the vicinity of the ear. With reference to FIG. 3A, error acoustoelectric transducer  256  detects the sound close to ear  266  and sends signal  276  respective of this detected sound, to ANR controller  262 .  
     [0133] In procedure  432 , an audio-and-noise-canceling signal is determined, according to the reference noise signal, the audio signal and the error signal. With reference to FIG. 3A, ANR controller  262  determines signal  280 , by processing signals  270 ,  274  and  276 .  
     [0134] In procedure  434 , an audio-and-noise-canceling sound is produced according to the determined audio-and-noise-canceling signal. With reference to FIG. 3A, electroacoustic transducer  260  produces sound according to signal  280 .  
     [0135] It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.