Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WO 93/25167 PCT/lJS92/04569
ACTIVE SELECTIVE HEADSET
This invenhon rela~es to a headset to provide active, as opposed to passive,
protecuon for a wearer/user.
S In indus~ial environments there is often a demand for active headsets that `
attenuate low frequency noise as well as noise covering the speech band (300 to 3300
Hz). While passive hearing protection works well at higher frequencies above 1000
Hz, active technology achieves sin~ilar levels of protection at the lower frequencies of
50 to 1000 Hz. The added problem of passive is that it also attenuates speech.
The instant invention provides a solution that provides the protection offered by
a passive headset in a light weight open-back design using any of several active control
algorithms that selectively cancel only the damaging noise with minimal effects on
speech and warning signals.
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 withfixed 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 snch 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.
Other systems that do not offer selectivity, i.e., allow speech and warning
signals to be heard, include the BOSE headset found in U.S. Patent No. 4,455,675 and
other headsets with similar design$ (e.g., CARME Patent No. 4,833,719).
The instant invention solves the problem now existant, that of total active
attenuation of the noise by providing a solution of an active headset that can employ
any of several selective algorithms such as those 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 algolithms such as
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WO 93/25167 Pcrlus92/o4~69
that disclosed in the application of Zie~ler in U.S. Patent AEplication No. 07/4'~ l 759
which is hereby incQrporated bv referenç_ herein.
Accordingly, it is an object of this invention to provide an active headset for
canceling QUt undesirable noise and allowing speech in.
S Another object of this invention is to provide an open back headset with active
selective cancellation of unwanted noise.
These and other objects of this invention will become apparent when reference
is had tO the accompanying drawings in which
Fig. 1 is a diagrammatic view of an active/passive headset system embodying
both passive and active techniques.
Fig,. 2 is a diagramrnatic view of the active only headset showing the control
system.
Fig. 3 is a more detailed description of the system of Figure 2.
Fig. 4 is a diagrammatical view of the headset using the digital virtual earth
controller.
In Fig. 1 there is shown an active/passive closed back headset system lO. It
consists of a typical passive headset l l, loudspeakers 12 that drive the anti-noise and
residual microphones 13 to sense any remaining noise near the ear and reference
rnicrophones 14 to send advanced information for feedforward control approaches and
a system controller 2~ which synthesizes the an~i-noise signal.
The headset shown has closed backs 21, 22 for passive attenuation. Without
the speakers, microphones and systern controller, this headset would be a typic.al
passive hearing protector.
Active noise cancellation is described in various patents to one Barry Chaplin~
with U.S. Patent No. 4,654,87 l being of particular interest in this case. ' r .` `
The system is designed to use various algorithms such as that of Ziegler in U.S.Patent 5,105,377, that of Tretter in U.S. Patent No. 5,091,953 or an adaptive feed
forward approach. These algorithms use a reference signal as inputs. The digitalvirtual earth (DVE) algorithm develops a reference signal by subtracting an equalized
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version of its own anti-noise signal from the residual signal. The adaptive feedforward uses the reference microphone as its input and is very effective on complicated
noise environments that are broadband and random in character. The Tretter algorithm
uses a sync signal as itS reference signal. The Least Means Square (LMS) adapter 24
5 shown in Fig. 1 are Fil~ered - 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.
Feedbaclc 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, ~.g., siren noise.
Speakers 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 haYe a minimum of input-to^output delay as any delay in the feedback loop slQws
down the system adaptation rate.
Residual microphones 13 are typically small electret microphones mounted on
the speaker frame near the ear. It must faithfully reproduce the sound tbat remains at
the ear after cancellation so that the controller can make further adjustments to the anti-
noise signal.
2n Reference microphones 14 are small electret rnicrophones 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.
The passive/active controller just described can be configured to selectively
cancel only the damaging noise with minimal effects on speech and warning signals. {--
Fig. 2 shows the active only open backed headset system 50 with headset 51,
speakers 52, residual microphones 53 and reference microphones 54. The controller 60
consists of adapters 61, 62, feedback compensation 63, impulse response 64 and
cancellation ~1lter 65.
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Wo 93/25167 Pcr/us92/04~9 -
Any of the aforementioned control approaches with the various algorithms can
work on the system by attenuating all noise at frequencies below the speech band or to
selectively cancel tonal noise within the speech band (300 to 3300 Hz). The active
controller attenuates noise in the band of interest and allows speech and warning
S signals to pass through the open back headset design. For noise below the speech band
or slightly higher (below 1000 Hz), the adaptive feed forwa~d described above would
attenuate all sounds below the cutoff frequency chosen (dependen~ upon the noise)
with minimal effects on the intelligibility of speech.
If the noise is comprised of tonal components below and well into the speech
bands, selective noise control approaches would be used. These include the
aforementioned digital virtual earth or a synchronous approach which is described in
U.S. Patent No. ~,654,871 tO Chaplin and which is hereby incorporated by reference
herein. Both of these algorithms selectively cancel tonal noise. Digital virtual earth
(DVE) selectively cancels the strongest tonal components in the frequency band of
interest. Tne sync'nronous controller cancels the fundarnental frequency based upon a
synchronous input (e.g., tachometer from a propeller aircraft) and a limited number of
it's harrnonics in the frequency band of interest.
The system shown in Figure 4 configured as a digital virtual earth controller
generates an output sample, Yk. given by
x~ = r~ -Yt l.Ck
y~ =-A~_~
where Yk is the cancellation output value
y~ I is a vector of previous cancellation output values
C t is a vector of filter coefficients representing the impulse
response of the entire cancellation system ~ `
x t is a vector of estimates of the noise
r,~ is a vector of most recent values of the residual signal
A~ is a vector of cancellation filter coefficients and
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a.b is the dot product of the two vectors.
The output Yk is an anti-noise signal that p;~sses through D/A, filters, and
arnplifier before driving the actuator in Figure 3. The input residual signal rk is
sensed from $he residual sensor, amplified, filtered and sampled before input to the
5 active controller.
DVE and tne syncnronous controller have been demonstrated on real systems to
work effectively with tonal cancellation such as siren noise, aircraft propeller noise,
and turbine noise. Adaptive feedforward has been demonstrated on simulated systerns
to work effectively doing broad~and cancellation of noise between 50-3000 Hz as well
10 as broadband noise below 1000 Hz.
