This invention relates to the area of headsets worn by a user in specific environments and, in particular, those areas where the background noise must be abated while simultaneously allowing speech through.

Generally, environmental noise can be broken into several areas such as noises of short duration with varying spectral characteristics such as conversations, other long duration noise with fixed or relatively fixed spectral characteristics such as operating pumps, fans, engines etc., and noises of very brief duration with "spike" characteristics such as something rattling, etc.

The instant invention is intended for use in environments found in industry, such as those requiring attenuation of low frequency noise as well as noise covering the speech band (300 - 3300 Hz).

Passive headsets have been employed and work extremely well at higher frequencies above 1000 Hz but don't perform at all in the lower frequency ranges. In these lower ranges active cancellation has been shown to work extremely well and the provision of high levels of protection are afforded users in the 50 to 1000 Hz range.

The instant headset provides protection at both the high frequency range (above 1000 Hz) as well as the low range (50 to 1000 Hz). In the past, attempts to combine the two protections, i.e., high and low frequency attenuation, has resulted in not only the noise being attenuated but also the speech that the wearer needs to hear. Some systems met only limited success with fixed or "near-stationary" noise but not with the other noise of either (a) varying spectral characteristics or (b) brief duration noises with "spikes". Examples of such a system is found in U.S. Patent No. 4,025,721, to Graupe et al and U.S. Patent 4,185,168 to Graupe et al.

The instant invention solves the problem now existant, that of total attenuation of the noise and speech, by providing a solution of an active/passive headset that can employ any of several selective algorithms such as that disclosed in U.S. Patent No.

5,091,953 to Tretter, hereby incorporated by reference herein. Alternatively, it can employ the algorithm disclosed in U.S. Patent No. 5,105,377 to Ziegler which is also incorporated herein by reference. In addition, it can employ other algorithms such as that disclosed in the application of Ziegler in U.S. Patent Application No. 07/421 759 which is hereby incorporated by reference herein.

The technique of actively controlling noise is disclosed in various patents to Barry Chaplin especially U.S. Patent 4,654,871 which is hereby incorporated by reference herein.

Accordingly it is an object of this invention to provide an active/passive headset that will attenuate a wide range of environmental noise while allowing speech to be heard.

It is another object of this invention to provide a headset that blocks unwanted noise but allows speech to pass through a speech filter or "in-wire" canceller and be combined with the output of an active noise controller. Still another object of this invention is to combine passive quieting with active noise cancellation in a headset and allow for passage of speech with simultaneous quieting of environmental noise.

These and other objects of the invention will become readily apparent when reference is had to the accompanying drawings in which Fig. 1 shows a typical active/passive headset system incorporating the instant invention.

Fig. 2 shows a typical active/passive headset system using a digital virtual earth controller.

Fig. 3 is a more detailed block diagram of the digital virtual earth and adaptive feed forward systems shown in Figures 1 and 2.

Fig. 4 is a detailed description of an adaptive speech filter system.

Fig. 5 shows an active/passive closed back headset that combines the cancellation system with the speech filter system. ^■

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In Fig. 1 there is shown an active/passive closed back headset system 10. It consists of a typical passive headset 11, loudspeakers 12 that drive the anti-noise and residual microphones 13 to sense any remaining noise near the ear and reference microphones 14 to send advanced information for feed forward approaches and a system controller 20 which synthesizes the anti-noise signal.

The headset shown has closed backs 21, 22 for passive attenuation. Without the speakers, microphones and system controller, this headset would be a typical passive hearing protector.

The system is designed to use various algorithms such as that of Ziegler in U.S. Patent 5,105,377 or an adaptive feed forward approach by using the reference sensor as an input to the cancellation algorithm. Both these algorithms use a reference signal as inputs. The digital virtual earth (D E) algorithm develops a reference signal by subtracting an equalized version of its own anti-noise signal from the residual signal. The adaptive feed forward uses the reference microphone as its input and is very effective on complicated noise environments that are broadband and random in character. The Least Means Square (LMS) adapter 24 shown in Fig 1 are Filtered - X versions which have inherent compensation for the effects of the feedback delays around the loop. Box "C" at 25 is the impulse response of the entire cancellation system. Feedback compensator 26 and cancellation filter 27 complete the component portions of the controller.

DVE is highly effective to use in simple noise environments having only a few harmonics even where the noise varies tremendously.

Actuators 12 of the headset are large enough to be capable of producing anti- noise at the same level as the noise to be canceled. They have little or no distortion and have a minimum of input-to-output delay as any delay in the feedback loop slows down the system adaptation rate.

Residual sensors 13 are typically small electret microphones mounted on the speaker frame near the ear. It must faithfully reproduce the sound that remains at the

ear after cancellation so that the controller can make further adjustments to the anti- noise signal.

Reference microphones 14 are small electret microphones attached to the outside of the headset at a distance from the ear canal. This reference microphone is used to provide advanced information about the noise. The higher the frequency of the noise the more advanced information is needed to effectively cancel the noise, however, the passive muff limits the need for cancellation above 1000 Hz.

Figure 2 shows a basic block diagram of a digital virtual earth system, phenomena (such as noise) is detected by a residual sensor which sends out a sensor signal. The sensor signal is affected by filters and other factors shown in Figure 3 before reaching the processor. The DVE processor produces an output cancellation signal y. This output signal is affected by filters, and other factors. The output from the actuator combines with the original phenomena and the residual is detected at a location by the residual sensor of Figure 2. The processor output for a specific sample k are given by χ _k = r _k -ι _{k λ} .c _k

Y _k = -A _k r _k where y _k is cancellation output value y is a vector of previous cancellation output values

C_ _k is a vector of filter coefficients approximating S x E where S is the impulse response of the system after the processor and E is the impulse response of the system prior to the processor where a x b is the convolution of a and b. x _k is a vector of estimates of the noise r _k is a vector of most recent values of the residual signal A _k is a vector of cancellation filter coefficients and a.b is the product of the two vectors.

When an LMS algorithm is used the cancellation filter coefficients are adapted as

where g. is the result of convolving Q _k and x _k

A _k+l is a vector of the most recent values of g _k and α is set for convergence.

Figure 3 shows in more detail the active cancellation system in either a DVE or

AFF configuration. The active cancellation system has an input that receives residual noise signals (and reference signals in an AFF configuration) from a residual sensor (and reference sensor in the AFF configuration) and provides an output signal to an actuator. The actuator produces canceling phenomena from an electronic waveform (the output signal) into the area to be controlled (the ear canal). The canceling phenomena combines with the original phenomena so that the two sum algebraically. Figure 4 shows in detail the adaptive speech filtering system. The system has as an input a reference signal that is composed of noise and speech from a reference sensor. From this input the adaptive speech filter separates the speech from noise and provides an output signal composed of speech only to an actuator.

Fig. 5 shows the instant invention with an adaptive speech filter 30 and the impulse "C" using a DVE cancellation approach. Reference microphones 14 not only pick up damaging noise but also pick up speech and warning signals. The embodiment of Fig. 2 is designed to provide protection against damaging noise which typically interferes with the speech band. It also attenuates noise in the audible range of the human ear effectively attenuating any warning siren. Using reference microphones 14 as inputs to a speech filter 30 that is then mixed with the anti-noise signal to be sent to the speaker is a means of allowing speech to pass through to the worker. If warning signals are outside the speech band and outside the frequency range of the active controller 20, these can be passed to the worker as well. Fig. 5 is a diagrammatic view

of the integration of active/passive hearing protection with speech (and warning signal) filtering so that local communication can be maintained for the safety of the worker.

The speech filter 30 separates speech from noise and output speech to the speaker with a minimal effect on intelligibility. The specific in-wire approach to use is dependent upon the noise but the DVE algorithm could be used. Various approaches have specific advantages in selectively filtering speech. Even the approach used in the Graupe patents can be used and since the output of filter 30 cannot interfere with the working of controller 20, it needs to be filtered from the control system as shown in Fig. 5 and, using the equations developed for the DVE algorithm, the output signal u^ is a specific sample k given by

where u _k is the cancellation and speech output value. w _k is the filtered speech value.

T_ _k is a vector of filter coefficients determined by the adaptive speech filtering algorithm. y _k is the cancellation output value as before. v _k is the reference signal (speech and noise) input vector. The residual sensor used to control the cancellation system also picks up the speech that is passed via the actuator. Therefore, the residual signal r^ must remove the speech signal w^ before input to the cancellation system so as not to interfere with the operation of the controller. It does so by modifying the residual signal rj _j by where z _k is the filtered residual input to the controller. r_ _k is the original residual input.

Q _k is a vector of filter coefficients that is the overall impulse response of the cancellation system, as before. w _k is the filtered output speech signal that is output to the actuator with the cancellation signal y _k .

The need to remove C_ _k w _k from the residual signal may not be required for all applications, for example, a siren canceling headset system will not try and cancel speech while the siren is in operation.

While the preferred embodiment of the invention is described it will be obvious to those of ordinary skill in the art that many changes and modifications can be made without departing from the scope of the appended claims.