Noise reduction system

In order to overcome problems in an active noise reduction system of sound buffets at low frequency and signal enhancement caused by imperfect transfer functions of a noise cancelling sound generator and a microphone, one or more high pass filters for reducing low frequency signals are provided in a feedback loop between the sound generator and microphone. A low pass filter is provided for extending the bandwidth of the system but which does not introduce unduly large phase shifts.

This invention relates to systems for reducing the level of acoustic noise 
fields within ear-defenders or earphone structures worn by personnel 
(e.g., pilots, vehicle drivers, military personnel) in high noise 
environments. 
Known active noise reduction (ANR) systems for reducing the acoustic noise 
filed in ear-defenders comprise noise pick-up microphones and 
noise-cancelling sound generators (usually known as loudspeakers) mounted 
within the internal cavities or enclosures of the respective 
ear-defenders. The noise pick-up microphones produce electrical signal 
outputs in response to the acoustic noise fields within the cavities and 
these signal outputs are phase inverted, filtered and amplified in a 
feedback loop and fed to the noise-cancelling sound generators which 
produce noise-cancelling acoustic signals of substantially the same 
amplitude but of opposite phase to the acoustic noise field waveforms. The 
design considerations underlying such ANR systems are described in "Some 
transducer design considerations for earphone active noise reduction 
systems", Twiney et al., Vol. 7, part 2, pp. 95-102, Proc. Spring 
Conference, 1985, York, Institute of Acoustics. 
Problems arise through inherent imperfections in the pick-up microphones 
and sound generators, by way of unwanted phase changes producing signal 
enhancement or by way of failure to cope with large amplitude signals in 
certain frequency regions. 
One problem which occurs is that of large pressure pulses (buffets) which 
occur inside an ear-defender or earphone structure due to relative 
movement between the human head and the earphone, or propagate to the 
earphone from a device that causes a rapid pressure change, e.g. a gun, 
helicopter, vehicle, explosive device. These pulses are very high in 
amplitude, and create large signals in the feedback loop as a result of 
high system loop gain. Due to the inadequacy of the sound generator to 
produce enough sound output, drive voltages appear at the sound generators 
which are higher than the maximum input voltage, and may overdrive the 
sound generator and cause permanent failure. 
Another problem which arises is that of signal enhancement at certain 
frequencies within the bandwidth of the feedback loop wherein due to 
imperfect transfer functions of the noise pickup microphone and sound 
generator the ANR will, at certain frequencies be feeding in-phase (i.e. 
positive feedback) signals rather than anti-phase (i.e. negative feedback) 
signals to the sound generator. 
A further problem which occurs is that due to the imperfect transfer 
functions of both the microphone and generator, the total bandwidth for 
feedback signals having an appropriate phase is limited, being bounded by 
regions in which positive feedback occurs. It is usual to employ in 
feedback systems in general a lowpass first order filter operating at a 
high frequency in order to stabilize the loop. However such first order 
low pass filters are not appropriate for filtering out sound energy 
frequencies in ANR systems because of the large phase changes which occur 
in the cut-off regions which give rise to problems of positive feedback 
and signal enhancement. 
It was previously thought, as appears from the article referred to above, 
that electronic processing to overcome problems in ANR systems had limited 
application because of the causal relation between amplitude and phase 
response of electronic filters. 
Nevertheless it has now been found as a result of careful investigation 
into the problems arising in feedback loops of ANR systems, that 
electronic processing may be used to advantage. 
It is an object of the present invention to overcome one or more of the 
above problems. 
Accordingly the present invention provides in a first aspect an active 
noise reduction system comprising: 
a noise-cancelling sound generator, a microphone acoustically coupled to 
said generator, a feedback loop connected between the microphone and the 
generator, the feedback loop including loop stabilisation means for 
filtering and inverting the phase of the microphone signal and means for 
amplifying the microphone signal, and the feedback loop further including 
high pass frequency filter means for filtering out low frequency sound 
energy from high pressure sound pulses arising from buffets at low 
frequency. 
Thus this aspect of the invention is based on the recognition that the 
major part of sound energy in high pressure pulses is present at low 
frequencies say below 100 Hz and thus the provision of low frequency 
filter means in the feedback loop can reduce a major part of the sound 
energy in the pulses. Such further filter means is conveniently preferred 
to as an anti-buffet filter (ABF). The amount of ABF correction is limited 
because stability of the feedback loop must be maintained, the total loop 
gain being kept below unity where the total phase shift may cause 
constructive interference. 
Said further filter means may be used in conjunction with a voltage 
limiting means, which prevents the generator from being overdriven by 
amplification of high pressure sound pulses. Such voltage limiting means 
may comprise a non-linear amplifier or zener diode arrangement. 
It is also an object of the present invention to overcome the problem of 
signal enhancement with a simple and effective mechanism. 
In a further aspect, the present invention provides an active noise 
reduction system comprising: 
noise cancelling sound generator, a microphone acoustically coupled to said 
generator, and a feedback loop connected between said microphone and said 
generator, wherein said feedback loop comprises: 
loop stabilisation means for inverting the phase of microphone signals and 
filtering the microphone signals, and means for amplifying the phase 
inverted and filtered signals; and, 
further filter means coupled between the phase inverting means and the 
amplifying means for increasing loop gain and/or adjusting phase shift by 
predetermined amounts within one or more predetermined frequency bands. 
Thus in accordance with the invention, the provision of further filter 
means increasing gain or adjusting phase shift, preferably both, prevents 
enhancement of the signal in the feedback loop arising from imperfect 
transfer functions of the microphone and generator. The further filter 
means is conveniently termed an anti-enhancement filter (AEF). The amount 
of AEF correction is limited by the need to maintain stability of the 
feedback loop (the total loop gain must be kept below unity when the total 
phase shift may cause constructive interference). 
This further aspect of the invention is based on our discovery that 
enhancement problems caused by transducer imperfections arise in a 
frequency region centered at about 500 Hz where the gain decreases whereas 
the phase lag in this area increases to about 3.pi./2. Thus a high pass 
filter which adjusts the gain in this region whilst providing a phase 
advance compensating phase shift can significantly reduce the problems of 
signal enhancement. 
In a particularly preferred form of the invention, it has been discovered 
as a result of careful investigation into the operability of ANR systems 
that the functions of the ABF and AEF, which operate at different 
frequencies and with different transfer functions can be accomplished by 
the use of f single high pass filter (ABEF) for attenuating frequencies 
below a predetermined frequency, the ABEF having appropriate transfer 
characteristics to prevent phase shifts harmful to loop stability. 
As mentioned above, it is normal to employ in ANR systems loop 
stabilisation filters which include low pass filters for reducing the gain 
at high frequencies to prevent loop instability. It has now been 
discovered that the problem of low pass filters producing unduly large 
phase changes at the high end of the feedback loop bandwidth can be 
avoided and the present invention provides in a further aspect an active 
noise reduction system comprising: 
a noise-cancelling sound generator, a microphone acoustically coupled to 
said generator, a feedback loop connected between the microphone and the 
generator, the feedback loop including loop stabilisation means for 
filtering and inverting the phase of the microphone signal and means for 
amplifying the microphone signal, and the feedback loop further including 
low pass frequency filter means for filtering out high frequency sound 
energy, the gain of the filter in the cut-off region having a step shape, 
decreasing from a relatively high constant gain region to a relatively low 
constant gain region in a transitional region where the gain decreases 
continuously from the high region to the low region. 
By providing a cut-off filter characteristic having a step function in the 
cut-off region, the phase change will be kept much smaller than that which 
should occur with a first order low pass filter and by careful application 
the ANR bandwidth can be increased whilst signal enhancement kept to 
acceptable levels. 
As preferred speech signals are injected at a single point in the feedback 
loop between the AEF and the amplifying means, in order that the speech 
signals are substantially uncoloured by the AEF and other filters. It will 
be understood that the speech signals are in a frequency range which is 
for the most part above the frequency range in which is for the most part 
above the frequency range in which the ANR is operative and the speech 
signals are not therefore reduced. They may however be affected by higher 
frequency filters in the feedback loop.

Referring to FIG. 1 of the Drawings, the active noise reduction system 
illustrated comprises a generally cup-shaped circumaural earphone 
structure 1 arranged to enclose the wearer's ear 2. The rim of the 
structure 1 is cushioned against the side of the wearer's head 3 by means 
of a compliant ring cushion 4. The earphone structure 1 embodies a small 
noise pick-up microphone 5, which detects the noise within the earphone 
adjacent to the wearer's ear 2 and provides an electrical output dependent 
upon the detected noise. This output signal from the microphone is passed 
through an anti-buffet filter 6, a loop stabilisation unit 7, a low-pass 
filter 8, an anti-enhancement filter 10 and amplifier 12, to noise 
cancelling sound generator (loudspeaker) 14 which is mounted on a baffle 
16 within structure 1. Loop stabilisation unit 7 includes a phase inverter 
72, a loop stabilizing filter 74 (which may be incorporated in low-pass 
filter 8 as in FIG. 6) for filtering out very high frequencies, and a 
voltage limiting circuit 76 comprising a zener diode switching arrangement 
for limiting high amplitude input signals. Filter 6 is placed first in the 
feedback loop in order to minimise signal values in the loop. The effect 
of the anti-enhancement filter is to reduce noise effects arising from 
imperfect transfer functions of microphone 5 and generator 14. 
A speech signal is injected between anti-enhancement filter 10 and 
amplifier 12 at an input node 18. The introduction of the speech signal at 
this point allows the speech signal to be substantially uncoloured by the 
loop filters. If desired the speech signals may be pre-emphasised by 
amplification where they may be attenuated by the ANR system. 
Referring to FIG. 2, the ABF 6 comprises an amplifier 20 having a negative 
feedback loop with a resistor R1 connected to its inverting input, which 
receives an input signal from a resistive/capacitive network R2, R3, C1. 
The non-inverting input of the amplifier is connected through a resistor 
R4 to ground. 
The characteristics of ABF 6 are shown in FIG. 3, whence it may be seen 
that the filter has a loss factor of about 8 db up to about 100 Hz at 
which frequency the loss reduces continuously until at about 500 Hz the 
filter exhibits a small gain factor. 
The phase shift introduced by the filter is an advance with increasing 
frequency rising in the transitional region from the base level of 
substantially 180.degree. (the filter includes an inverting amplifier) to 
a maximum at about 200 Hz of about 215.degree.. This phase shift must be 
taken into account when considering the overall loop stability. The effect 
of the ABF 6 on the overall feedback loop transfer function is to 
attenuate the low frequency end of the function whereby noise in the 
frequency range up to 200 Hz is severely attenuated. 
The preferred form of AEF is shown in FIG. 4 as comprising two cascaded 
stages 21, 22, each stage comprising an amplifier 24 with a resistor R1 in 
a negative feedback loop and with the inverting amplifier input being 
connected to ground via the series combination of a resistor R2 and 
capacitor C1. The filter characteristics are shown in FIG. 5 with the gain 
having an step for, being roughly 0 db up to 100 Hz and then rising to 10 
db gain at 1 kHz. The phase shift, a phase advance with increasing 
frequency, rises in the region in which the gain changes, from a base 
level of substantially 0.degree. to a maximum value of 25.degree. at 
roughly 500 Hz. 
Because of the precise transfer functions of the microphone and generator, 
the gain reduces to a minimum value at about 500 Hz whereas the phase 
shift in this area rises to a maximum of about more than 3.pi./2. By 
providing AEF, the transfer functions are modified in this area to reduce 
phase shift and increase gain, thereby reducing signal enhancement. 
A circuit diagram of low pass filter 8 is shown in FIG. 6 as comprising a 
transitional second order filter including an amplifier 60 having a 
non-inverting input connected to a filter input via resistors R1, R2 and a 
capacitor C1 coupled between the amplifier input and ground. Two feedback 
loops are provided from the amplifier output to the non-inverting input: a 
first loop including a capacitor C2 and a second loop comprising resistors 
R3, R4, R5 and a capacitor C3 in series with a resistor R9 connected 
between resistors R4, R5 and ground. A further feedback loop is provided 
comprising a resistor R7 connected between the amplifier output and the 
inverting amplifier input. A further resistor R8 is connected between 
resistor R7 and ground. 
The characteristics of the filter are shown in FIG. 7 where the gain is 
close to 0 db up to about 1000 Hz and is about -30 db around 10,000 hz. 
The gain decreases between these regions relatively quickly in a cut-off 
region. 
The phase shift across the filter is roughly 150.degree. (the filter 
includes a non-inverting amplifier) in the region below 1,000 Hz and above 
10,000 Hz, but decreases to a minimum (a phase lag with increasing 
frequency) of about 45.degree. in the center of the cut-off region. Such a 
phase change of roughly 105.degree. is acceptable and is much smaller than 
180 degrees resulting from a conventional second order low-pass filter. 
Although a second order filter is shown, the filter could be a higher or 
lower order if desired. 
Referring now to FIG. 8, there is shown a high pass filter which combines 
the functions of the AEF and ABF and is herein referred to as an ABEF. The 
filter is a second order filter comprising tow filter sections connected 
in cascade, the filter sections being identical. (If desired a first order 
filter could be employed). Each filter section comprises an input port 80 
coupled to the inverting input of an amplifier 82 through a resistance R1 
connected in parallel with a capacitance C1 and a resistance R2. The 
non-inverting input of the amplifier is connected to ground via a 
resistance R3, and the output of the amplifier 86 is connected in a 
negative feedback loop to the inverting input of the amplifier via a 
resistor R4. 
Referring now to FIG. 9, the characteristics of the filter of FIG. 8 are 
shown where the gain is slightly greater than 0 db up to about 500 Hz and 
then rises to about 10 db at a frequency of 2 kHz in a transitional region 
between 500 Hz-2 kHz. The phase shift changes from a constant level of 
about 0.degree. to a maximum value of substantially 60.degree. at about 1 
kHz. 
It will be appreciated that although the particular embodiment specifically 
described is applied to a circumaural earphone structure, the invention 
may also be applied to other earphone structures such as the supra-aural 
type. 
It will also be appreciated that the filters shown may be replaced by 
digital filters, and the elements of the feedback loop may be digitised by 
employing a micro-computer with appropriate routines. The invention 
claimed is intended to cover both analog and digital systems.