An active noise reduction system was developed for use in aircrew flying helmets in which the acoustic noise field inside the ear defender is detected using a miniature microphone and an antiphase signal is fed back to a communications telephone within the ear defender. Performance of the active noise reduction system in a laboratory trial simulating flight conditions is shown to be satisfactory

Dorey, T. P.

Pelc, S. F.

Rawlinson, D.

Wheeler, P. D.

English

NASA Technical Reports Server

29 
AN ACTIVE NOISE REDUCTION SYSTEM FOR AIRCREW HELMETS 
Peter D. Wheeler and David Rawlinson 
Wolfson Unit for Noise and Vibration Control, I.S.V.R. 
Sou t hamp ton Un ive r s it y 
Stephen F. Pelc and Tony P. Dorey 
Department of Electronics, Southampton University 
INTRODUCTION 
In a high noise environment conventional ear-defenders may be incapable of 
An active noise reduction system (ANR) ,roviding sufficient noise attenuation. 
has been developed for use in aircrew flying helmets (ref. 1) in which the 
icoustic noise field inside the ear defender is detected using a miniature 
iicrophone and an antiphase signal is fed back to a communications telephone 
Jithin the ear defender. The feed-back loop also affects the communications 
gignal which has to be compensated outside the loop in order to retain the 
)riginal transfer function from telephone amplifier to listeners ear. 
The concept was not new at the outset of the work reported. In 1953 Olsen 
and May (ref. 2) investigated the possibility of creating a noiseless zone in 
€Pee field conditions, but the technique had not apparently been successfully 
spplied to headsets. However, the Procurement Executive, Ministry of Defence 
had outlined a method which proved the basis of this present work. 
The major engineering development involved in integrating the technique 
into a conventional aircrew flying helmet was associated with the identification 
and characterisation of suitable microphones and telephones. At the time of 
?revious publication (ref. 1) the feasibility of active reduction had been 
iemonstrated though tests with random noise had not been undertaken. Some 
instability problems still existed and the performance with a communications 
signal had not been assessed. 
During the last two years the remaining engineering problems have been 
svercome and an extensive program of laboratory subjective trials has been 
completed. In-flight trials of the system are currently underway. Throughout 
this programme of work there has been active participation by the Royal Aircraft 
Establishment, Farnborough, and the development has been carried out with the 
support of the Procurement Executive, Ministry of Defence, U.K. 
PRINCIPLES 
One channel of the complete system is shown schematically in Figure 1. 
The design is based upon the existing ear defender shell and seal, and incorpo- 
rates a high-fidelity type moving-coil telephone and a miniature electret 
mi crophone. 
545 
https://ntrs.nasa.gov/search.jsp?R=19790002682 2020-03-22T02:08:33+00:00Z
A complete analysis of the system shows that the pressure at the ear (Pel 
is comprised of two components due to the signal 
field without ANR (np) 
(Sv) and the external noise 
where A,B,K,T1 and T2 are the transfer functions of the amplifier, feedback 
loop, acoustic cavity, telephone and microphone, respectively. 
It is apparent that the amplification of the two components is different 
and that if the signal 
by the feedback loop, and returned to its original level then an improvement of 
a factor (1 + ABKT1T2) can be achieved. 
Sv is pre-emphasised, to compensate €or its attenuation 
The product of the terms KT1T2 is of fundamental significance as it 
represents the overall electrical transfer function of the telephone - cavity - 
microphone system. To provide ANR, the product AB would ideally have a 
complementary variation with frequency. 
must be tailored to ensure that when the loop gain approaches unity the total 
loop phase shift does not approach 180'. 
In practice the frequency response 
The systems ability to match this transfer function KT1T2 will determine 
the degree of noise reduction which is obtained. 
EVALUATION 
The ANR system has been comprehensively tested in a series of laboratory 
trials, prior to flight trials by the Royal Aircraft Establishment. 
Two basic methods of measuring ANR have been used throughout the project. 
These are: 
a) An electronic computation method 
b) An external broadband noise method 
The computer method used a single frequency excitation which was analysed 
and continuously plotted its amplitude and phase as a function of 
by means of an analogue computer circuit that calculated the value of 
1 + ABKTITZ 
frequency. 
In the second method, preferred because of its close approximation to the 
real life situation, subjects wearing the ANR modified flying helmet 
5 46 
were 
zxposed to an external noise field similar to that experienced by pilots in a 
iigh performance strike aircraft. Comparisons of attenuation and speech 
intelligibility scores, with and without the ANR system in o 
md the modified helmet's attenuation was also checked 
€lying helmet, 
Objective measurements of helmet attenuatio 
nicrophones placed at the subject's pinnae, and o 
These two methods of measurement were carefully compared in a structured 
ieries of eight experiments to check that the more convenient laboratory 
mnputational method gave the same results as the noise excited experiment. 
Che intelligibility testing was carried out using anglicised modified Rhyme 
Pest material comprisingfour sets of fifty initial or final consonant words 
blus twenty five central vowel words. 
feception of a word by marking a score sheet showing all six rhyming alterna- 
:ives for a given word and, at the end of the experiment, the sheets were 
karked by the experimenter. For each condition - ANR on or off - two of the 
four word lists available were presented and the number of subjects was chosen 
:o give a fully balanced experiment with a satisfactory level of statistical 
iignificance. The speech signal presented to the ear was matched, for all the 
ielmet configurations tested, to within rt 1 dB(A) overall and -L 1 dB in each 
! I 3  octave band. 
The subject was required t o  score his 
Eighteen subjects were tested twice in each configuration and the mean 
.mprovement in attenuation was found to be 12 dB(A) for the noise field used 
:Figure 2). 
Figure 3 shows the ANR performance as a function of frequency, together 
6th a measure of the scatter of results. 
rttenuation values is shown in Figure 4 .  
The comparison between mean helmet 
The mean improvement in speech intelligibility with the ANR system in 
tperation was 21% relative to a baseline of 51% when the system was switched 
)ff. The individual performances of subjects are shown in Figure 5. 
All those tested commented upon the increased comfort provided by the 
iigher attenuation of the ANR system, particularly at lower audio frequencies, 
rhere the aircraft's noise energy is concentrated. 
CONCLUSIONS 
conditions has been shown to give encouraging objective and subjective results. 
The system is capable of application to a wide range of situations where high 
attenuation (with or without voice communications) against low frequency noise 
is required. The response of the ANR system may be engineered, to an extent, 
to optimise attenuation for particular noise fields, such as are experienced 
in rotary wing and fixed wing aircraft, and for certain applications, the 
system may be fitted into a small, battery operated, electronics package which 
may be easily worn on the person. 
The use of active noise reduction in a laboratory trial simulating flight 
547 
REFERENCES 
1. Dorey, A.P., Pelc, S.F., and Watson, P.R. 1975,  "An Active Noise Reduction 
System for use with Ear Defenders". 
Cranfield, 24-27. 
8th International Aerospace Symposium, 
2. Olsen, H.F., and May, E.G. 1953,  J.A.S.A. 25 , No.6. -
EXTERNAL 
\ NOISE 
EAR DEFENDER\\ .- - \ 
\ 
Figure 1.- A system block diagram. 
548 
110 
n a 
PI a 
100 
CY 
9) w 
F 4  a 
4 
v 
* 90 
& 
a0 
125 250 500 lk 2k 4k 
1/3 octave centre frequency, Hz 
Figure 2 . -  Simulated aircraft cockpit noise. 
ean for 18 subjects - .I-- 
e -- 
octave centre frequency, Hz 
Figure 3.- ANR performance. 
549 
40 
30 % 
3 20 
n z 
2 
EJ 10 H 
2 
0 
ANR-modified 
1 
I * *  1 :  - 1 . .  1 ' f  ' * I  
lk 2k 4k 125 250 500 
1/3 octave centre frequency, Hz 
Figure 4 . -  Mean helmet attenuation. 
ANR on - mean 72% 
ANR off - mean 51% a 
80 
70 
2 
2 
8 60 
cn 
@ 50 
40 
ae 
30 
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 
SUBJECT 
Figure 5.-  Word scores for 18 subjects, 
550 


An active noise reduction system for aircrew helmets

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