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A Selective Noise Canceling Headset
For Use In Emergency Vehicles
by
Jeffrey N. Denenberg and David J.
Claybaugh
Noise Cancellation Technologies, Inc.
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ABSTRACT
Environmental noise has been a problem since the industrial
revolution. Noise affects our health
and safety, interferes with communication, and reduces our ability to enjoy
life at its fullest. The application of
Active Noise Cancellation to the control of noise was proposed over 50 years
ago but at that time was quite impractical.
Recent advances in computer technology, however, have now made active
noise cancellation a practical solution to many previously difficult problems
in environmental noise.
The application of active noise cancellation to a headset
for use in emergency vehicles produces a light weight headset that selectively
reduces the noise level at the ear by 10 to 15 dB. This enhances operator safety, reduces the possibility of hearing
damage, and allows effective communication within the vehicle.
INTRODUCTION
This technology does not mask the noise. Noise is reduced by generating a canceling
anti-noise signal that is equal to, but 180o
out of phase with, the offensive noise.
This anti-noise is then introduced at the ear such that it matches the
noise. The two signals then cancel each
other, effectively removing a significant portion of the noise energy at the
ear .
Active Noise Cancellation is
not a new idea. Creating a copy of the
noise and using it to cancel the original dates back to the early part of this
century. The first systems used a
simple "delay and invert" approach and showed some promise, but the
variability of real world components limited their effectiveness.
In the mid 1970's a major
step forward took place with the application of adaptive filters to generate
the anti-noise. This greatly enhanced
the effectiveness of the systems as they could continuously adapt to changes in
their external world as well as changes in their own components. A second breakthrough in the mid 1970's was
the recognition that many noise sources, including that made by the sirens used
in emergency vehicles, exhibit periodic or tonal noise. This tonal noise allows a more effective
solution as each repetition of the noise is similar to the last. The predictability of the noise allows
creation of an accurate anti-noise signal.
Practical application of
this technology still had to wait as the electronic technology available at
that time was not sufficient for implementation of Active Noise Cancellation
systems. Now digital computer
technology has evolved to the point where cost effective Digital Signal
Processing (DSP) microcomputers can perform the complex calculations involved
in noise cancellation. This technology advance has made it feasible to apply
Active Noise Cancellation to previously difficult noise problems at a
reasonable cost.
Additional
Applications For Active Noise Cancellation
Many other applications for this technology exist
(see Denenberg, 1992). They include:
Active Mufflers - Reduce exhaust noise from
internal combustion engines, compressors, and vacuum pumps without the
inefficiencies caused by back pressure.
Active Mounts - Contain vibration from
rotating machines to improve comfort, decrease wear, and reduce secondary
acoustic noise.
Active Enclosures - Control low frequency
noise with a lightweight enclosure while allowing openings for cooling air
flow maintenance access and other purposes.
Quiet Zones - Silent Seats and Cabin Quieting
to provide comfortable workstations for equipment operators.
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The noise in emergency vehicles comes from
several sources and can be quite strong.
We will concentrate here on the loudest of these, the electronic siren
(many of the other sources can also be canceled).
Figure 1: Fast Wail Modulation
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Noise in the cab of an ambulance (or other emergency
vehicle) varies as a function of siren placement, cab acoustical insulation,
and the condition of the windows (open or closed). Field measurements show that the noise level can range from 80 to
100 dBA. Noise this strong has
several negative affects on the vehicle operator and passengers. These include:
Rapid fatigue
Difficulty in communications
Failure to hear warning signals
Possible Hearing damage
SIREN NOISE
A typical emergency vehicle siren can generate several
different sound patterns. A typical
noise pattern from an electronic siren available in the Japanese market is the
“Fast Wail.” In this siren the signal
sent to the siren speaker is a high level square wave with a cyclically varying
frequency as shown in Figure 1.
The modulation cycle is over a four
second period with a fast rise time and a slower fall time. The frequency varies between 400 Hz and 800
Hz during each cycle.
When this drive signal passes through the band limited
speaker, the higher frequency harmonics are attenuated leaving behind a noise
as in Figure 2. (The noise frequency
has been reduced by a factor of 20 in Figure 2 for clarity)
Figure 2: The Siren Waveform
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Figure
3: A Siren Headset
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The
noise as heard in the vehicle cab is actually much more complex due to the
acoustics of the cab. Resonances and
multiple acoustical paths cause the noise to vary rapidly in amplitude as the
frequency slews. The resulting noise
waveform is shown in the section on results as measured in a vehicle cab.
ACTIVE NOISE CANCELLATION
This problem requires a system that can adapt at a rate
faster than the rate of change of the noise.
Most cancellation systems either suffer from a slow adaptation rate or
require a reference signal predictably in advance of the noise to be canceled. Since the only reference signal available is
the siren drive signal and the relationship between this signal and the noise
at the ear is non-linear and varies with head position, neither the classical
feed forward nor the synchronous noise cancellation algorithms can provide a
good solution.
A solution was found in NCT’s patented Digital Asynchronous
algorithm. When a single tone is much
stronger than other sounds, this feedback algorithm can adapt to that tone at a
rate that is on the order of twice the sum of the noise cycle period (between
1.25 and 2.5 milliseconds for this noise) and the total delay around the
feedback loop (about 0.5 milliseconds including the headset, analog filters and
a small number of digital sampling periods to allow algorithm processing
time). It therefore can typically adapt
to changes in this noise in 5 milliseconds which is much faster than the rate
of change.
Figure 3 shows the configuration of the active headset with
its digital controller. (Note that a
full system controls the noise independently at both ears, only one ear of
control is shown in Figure 3). The
elements of this system include:
Headset - A light-weight, open-back
unit allowing external signals to pass to the ear with little passive
attenuation. It has a broadband speaker
to minimize signal delay.
Residual Microphone -
Senses the remaining noise so that the digital controller can continuously
adjust the anti-noise waveform to minimize the noise at the ear
Equalizer - A filter that is equivalent
to the effect the anti-noise path has on the anti-noise. The result is added to the Residual signal
to derive the Reference, a signal that approximates the original noise at the
ear before cancellation.
Adaptive Filter - A Finite Impulse
Response filter. Its tap weights are
continuously adjusted to generate the anti-noise signal from the Reference
signal.
LMS Adapter - Correlates the Residual
signal with the Reference signal to determine updates to the tap weights of the
Adaptive Filter. (see Widrow and Stearns, 1985 for a mathematical analysis of
the LMS Algorithm)
Figure 4 is a block diagram of the DSP hardware required to
execute the cancellation algorithm. The
prototype hardware was jointly developed by Noise Cancellation Technologies and
Foster Electric. It is a complete one
board system that fits into a case that is only 2½ cm high by 20 cm wide by 16
cm deep and supports two channels of processing (one for each ear). It uses standard 12 volt power from the
vehicle.
Figure 4: Digital Controller Hardware
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The components of the
hardware are:
Anti-Aliasing Filters -
These filters limit the bandwidth of the residual signals to that of the noise
(400 to 800 hertz for the described siren signal). They are designed to reject energy outside this band to avoid
aliasing (the tendency for signals above one half of the sampling rate to get
translated in frequency by the sampling process) while minimizing the time
delay incurred in the filtering process .
A/D Converters - The
residual noise signals are converted to digital form by specially designed
Analog to Digital converters. They
operate at a 10 kHz sampling rate to minimize delay and have 12 bit accuracy to
have sufficient dynamic range to handle changing noise conditions.
16 Bit DSP - The prototype system uses
a TMS 320C25 DSP processor from Texas Instruments. This is one of several DSP chips that can achieve good
results. Others include the Analog
Devices 2100 series and the Motorola 56000 series.
Program ROM - The algorithm is written
in the assembly language of the DSP chip to maintain efficient use of the
processing resources. The resulting
machine language program is stored in an external Read Only Memory chip (Production
units are likely to use ROM memory that is on the DSP chip).
D/A Converters - The Digital to Analog
converters are used to generate the output anti-noise signals. They are also designed to minimize signal
delay and have 12 bit accuracy at the 10 kHz sampling rate.
Reconstruction Filters -
The output from the D/A converters contain copies of the desired anti-noise at
frequencies that are multiples of the sampling rate. These are removed by filters similar to those used for
anti-aliasing.
Power Supply - The power supply is a
high efficiency switching supply that accepts the vehicle battery voltage
(nominally 12 V) and generates the required set of precision voltages for the
analog and digital electronics in the controller.
CANCELLATION RESULTS
Figure 5
shows the results obtained by the Active Noise Canceling Headset as measured in
a vehicle cab. The upper half shows
five seconds (the modulation cycle is four seconds) of the original noise at
the ear with the active cancellation turned off. Note the complexity and rate of change of this noise due to the
complex acoustical path taken by the noise
The lower half shows the result when the active cancellation
system is turned on. The system
reliably reduces the siren noise by 10 to 15 dB with little impact on other
external sounds.
This
system has also been successfully tested on US standard electronic sirens which
have even faster variation. It is
currently under field test by a metropolitan area Emergency Medical Service on
the east coast of the US.
Figure
5: Measured Results
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CONCLUSIONS
The application of active noise cancellation to a headset
for use in emergency vehicles produces a light weight headset that reliably
reduces the noise level due to the electronic siren by 10 to 15 dB. Benefits include:
Enhanced operator safety since the operator’s ability to hear
external warning signals is improved.
Reduced possibility of hearing damage due to a lowered noise level.
Better intelligibility for communications within the vehicle since
the open back headset allows speech in with little attenuation.
ACKNOWLEDGMENTS
This work is the result of efforts by a large team over a
significant period of time. Some of
these were:
Ralph Busch
Graham
Eatwell
Robert Evans
John Hohman
Joe
Kulikauskas
Roy Scott
REFERENCES
Denenberg, J, “Anti-Noise,” IEEE
Potentials, Vol. 11, No. 2, pp. 36-40, (April, 1992)
Lemme, H and J. Denenberg, “Lärm Kontra
Lärm,” Elektronic Magazine, Vol. 41, No. 19, pp. 42-48, (September, 1992)
Widrow and Stearns, Adaptive Signal
Processing, Prentiss-Hall, 1985
