U.S. Department 

of Transportation 

Federal Aviation 

Advisory 

Circular 

Subject: COCKPIT NOISE RND SPEECH Date* x/22/89 

INmmENCEBk=TwEENcR=B=S Initiated by: AIR-120 

AC No: 20-133 

Change: 

1 . PURPOSE. This advisory circular (AC) provides information about the 

relationship between flight cre-w cockpit voice communication and cockpit noise 

levels. Cuidmce, cn speech interference levels, noise measurent and 

masurmt systems, and methods to improve cockpit cmicatim, is provided 

for those manufacturers, Owners or operators who believe cockpit noise may be a 

problem m their aircraft. This guidance material is relevant to the operaticn 

of all types of civil aircraft. 

2 . B?+CKGROUYD. 

a. Many modem aircraft provide comfort, convenience, and excellent 

performance. At the s;rme time that the manufacturers have developed mre 

perful engines, they have tried to give the occupants better noise protection 

and control, so that many of today's aircraft are more powerful, yet quieter 

than ever. Still, the levels of sound associated with -red flight are high 

enough in same aircraft to raise concern about the effect these noise levels may 

have odn direct voice ccmnunication between flight crew members. 

b 

accident 

The National Transportation Safety Board (NTSB) investigation of an 

involving a twin-engine, small airplane, concluded that the cockpit 

noise levels of that particular airplane wzre loud enough to interfere with 

direct voice ccrrrmunicaticn. In the opinion of the NTSB, this cmunication 

interference could have affected crew coordination and contributed to the 

accident. The MTSB also believes that poor crew cmnication, because of high 

cockpit noise levels, may have contribted to other accidents. 

3 . DEFINlTIaJS. 

a. Noise - Any sound which is undesirable because it interferes with 

speech and hearing. 

b. Noise Spectra - The description of noise sound waves by resolution of 

their components, each of different frequency and (usually) different arrplitude 

and phase. 

c. Frequency(Hz) -- The number of oscillations per second of a sine-wave of 

sound. 

d Decibel(dB) -- The unit in which the relative levels of intensity of 

acouskcal quantities, such as sound pressure levels, noise levels and pawer 

levels, are expressed on a scale frclm zero for the average least 

perceptible level to about 130 for the average pain level. 
AC 20433 3122189 

e. A-Weighted Sound Level (dB(A)) -- A single event sound level which has 

been filterd or wighted to discriminate against the low and high frequency 

extremes to approximate the auditory sensitivity of the human ear. 

f octave Band -- All of the cwonents, in a sound spectrum, whose 

frequencies are between two sine wave (pure tone) cmaents whose ratio of 

frequencies exactly two, ie. separated by a octave, 

4 . DISCUSSICN. Today's large, jet-powered, air-transport airplanes present few 

speech-interference problems for flight crews. However, propeller or rotor 

driven aircraft, regardless of the power plant used, have noisier cockpits for 

several reasons. Much of the prcpeller or rotor noise energy lies in lower 

frequencies, which are much more difficult to attenuate than high-freqency 

sounds. In nonpressurized aircraft, constructian permits air leaks that are 

both sound transmitters and sound sources; propeller and rotor tips can travel 

at or near Mach 1, which means, in scme flight configuratims, small scnic b-s 

cmstantly banbard the aircraft. In addition, techniques for minimizing sound 

production or sound transmission require the addition of physical mass to the 

system, ad where payload determines the value or utility of the aircraft, 

adding enough mass to reduce noise, can cost severely in payload. Streamlining 

can be very costly in new design costs (to remove air leaks) and it may also 

require major changes in production methods. Sam of these methods require 

additional weight which reduces utility. 

a. Outside the aircraft, noise spectra vary greatly as a function of 

aircraft size and type and the variety of powerplant, bt the interactions of 

those spectra with the sound-insulation properties of the various airframes 

generally lead to strikingly similar spectra cm the inside. Cmkpit noise 

studies hawz shown the spectral shapes of cockpit noises vary only slightly 

from one type .,.. of fixed-wing aircraft to another, 

b 0 The primary energy in those noises lies in the low frequencies, 

.ranging rrw>stly from 100 to 300 Ha, with a rapid decrease as frequency increases. 

This spectral canfiguration may peak at different sound levels for different 

airplanes. The overall sound intensity varies from about 70 dB(A) to more than 

100 dB(AL Generally, the quietest cockpits are found in jet aircraft; the 

noisiest are found in open cockpit airplanes such as those used for aerial 

application in agriculture and in sots srrallmilitary jets that use 

afterburners. 

c. Within a general class of aircraft (for example, light, single-engine 

airplanes), the variations in cockpit noise level among airplanes of a 

single type may be atit as large as the variations found among all the types 

within the class. Age and history seem to be iFortant determinants of the 

cockpit noise level as much as the original design. Therefore, little is to be 

gained by looicing at a single sound spectrum from a single airplane as if it 

were typical of its tyE?c"- and would remain typical of its type. 

d The following sections present an overview of a mans to assess the 

level if cockpit speech interference due to noise and methods to measure and 

inprove co&pit c~unications. 

2 Par 3 
3/22/89 AC 20433 

(1) Speech Interference Level, This AC utilizes a noise interference 

metric known as-the perferred-frequency speech interference level (PSIL). The 

PSIL is an average of the unweighted noise sound pressure level of three octave 

bands at 500,lOOO and 2000 Hz and relates to an "A" weighted decibel measurement 

(dB(AL The PSIL has been accepted as a suitable predictor for a much more 

cqlex mzasure of speech intelligibility known as the articulation index (AI). 

The AI ranges from 0.0 to 1.0 with an increasing value indicating a more perfect 

cmunicaticn. The Armed Forces maintain that for cmunicaticns approximately 

3 feet apart, an AI between 0.2 and 0.3 represents an acceptable minim 

intelligibility level. The maximum PSIL for AL=O.2 is 83 and for AI=O.3 is 78. 

The FAA believes that in cockpits with noise levels above 88 dB(A) (PSIL=78), 

efforts should be made to aid comnunicatians by use of one or more of the 

methods discussed in this AC. The evolutim of speech intelligibility research 

and the develqt of criteria regarding speech interference is covered in sane 

detail in appendix 1. 

(2) Cockpit Noise Measurement. A portable sound 1evelxWzer (SLM) 

which indicates the sound output in "A" weighted decibels (dB(A)) is recunmended 

for the measurement of cockpit noise. 

(a) A quick noise survey of the cockpit can be made by observing 

the sound level for approximately 20 seco&lds while the aircraft is in stabilized 

flight. One or two repeat readings are recmnded to average the data. 

Readings should be taken in the takeoff, approach, cruise and descent modes of 

flight so that a corrqprehensive noise picture is obtained. 

(b) If the above tests indicate a noise problem or a borderline 

noise pr&lem exists, additional noise measurements should be taken and 

recorded, as discussed in appendices 2 and 3. Recording noise levels is 

desirable as this will allow a more canplete noise analysis to be made. In 

addition, a sample calculation of PSIL is shown in appendix 4. 

(3) Methods to Improve/Aid Cockpit Ccprmunication. When the noise 

level in the cockpit, exceeds 88 dB(A) (PSIIF~~), the noise will be of 

sufficient magnitude as to interfere with normal cockpit cmunications, i.e. 

voice and radio. Therefore, efforts should be made to aid ccrmmunications, The 

following methods are suggested to improve the signal (voice)-to-noise ratio, 

which will enhance the intelligibility of cockpit COrrmunications. Appropriate 

FAA approvals must be obtained for any type design changes resulting from any of 

the following methods qloyed: 

(a) Decrease the cockpit noise level. 

(i) Use of door seals 

(ii) Acoustical insulatim. 

(b) Increase the voice signal levels or modify the 

signal-to-noise ratio. 

(i) Increase the gain of intervening audio arrplifiers. 

(reference !lSO-C5Oc, Aircraft Audio and Interphone Arqlifiers) 

Par 4 
AC 20433 3/22/89 

(ii) Use of electronic headsets, noise cancelling or bocxn 

microphones and intercom systems. (reference TXH'Z57b, Aircraft Headsets and 

Speakers (for Air Carrier Aircraft) and TSO<58a, Aircraft Microphones(for Air 

Carrier Aircraft)) 

(iii) JQxopriate use of hearing protectors. 

(iv) Move the flight crewmzmbers closer together. 

e. Appendix 5 discusses in detail the advantages and disadvantages of the 

.methods described above to improve cockpit cmications. The coverall 

objective, of the modificatim should be to improve the intelligibility of 

carununications. The minimum goal should be to achieve an articulation index 

(AI) of 0.3., identifiable by a PSIL of 78 or a measured noise level of 88 dB(A) 

or less. 

f Regardless of the mzthad used to aid cclrmnuunications care should be 

takento assure that aural warnings (i.e. oversped, stall, and landing gear) 

can be heard with or without the canrpnunications aid in place. 

a 

pp 

M.C. Beard 

Director, Aircraft Certification 

Service, AIR-1 

Par 4 
3122189 

APPENDIX1 

AC 20433 

Appendix 1 

QW?NTIFYINGSPEFCH INTERFERENCE 

Several researchers have contributed landmark studies of the ways in which noise 

can interfere with the understandability or intelligibility of speech. 

It has been demonstrated that the frequencies necessary for 100 percent 

intelligibility of a speech signal cover the range frcxn about 300 Hz to about 

7000 Hz. 

A masure of that portion of the speech intelligibility range that is available 

in a specific comnunication situation is knm as the articulation index (AI). 

The AI was developed by French and Steinberg and is a number falling between 0 

and l,O,* AI accounts for the level and spectra of ambient noise, and describes 

the relative ease or difficulty of a particular ccrmnuunication situation. An AI 

of 1.0 is considered perfect, with lawer values indicating corrmunications of 

lesser quality. 

* French, N.R. and Steinberg J.f "Factors governing the intelligibility of 

speech sounds/' Journal of Acoustical Society of erica, 19,90-119, 1947. 

Researchers have devised a set of relationships between AI and speech 

intelligibility for several sorts of speech test materials ranging from nonsense 

syllables, in which the content is quite unpredictable, to sentences# which 

are, comparatively, perceptually redundant--if you hear part of a sentence, you 

have a reasonably good chance to guess correctly what the rest of it is. 

In 1947, Beranek published a report that serves as a further basis 

for determining hw noise interferes with speech.* The speech interference 

level(SIL) is an average of the octave-band noise levels at SCXE preselected set 

of center frequencies. In his original proposal, Beranek used the three 

octaves running from 600-4800 Hz. Later work, primarily by Webster and by 

Klwnpp and Webster, showed that the inclusion of different frequency bands in 

the averages leads to AI predictions that are accurate for different 

cmunication conditions.** Thus, an average of the octave band levels at 500, 

1000, and 2000 Hz seems well suited for predicting an AI of 0.2; i.e. a minimal 

ccnrmunication enviromt. An average of 500, 1000, 2000, and 4000 Hz 

corresponds fairly well with an AI of 0.5 and an average of 1000, 2000, and 4000 

Hz seems to go with an AI of 0.8. The 500, 1000, and 2000 Hz SIL has cc%ne to be 

known as the preferred-frequency SIL (PSIL), and it is often closely related to 

a dB(A) masurerrent of the sm noise, though the relationship is not perfect. 

JCL. L. Beranek, "The design of speech camnunication systems," 

Institute of Radio mqineers, 35, 880 f 1947. 

Proceedinqs of the 

**J. C. Webster, 'Relations between speech-interference contours and idealized 

articulation-in&x cOntours," Journal of the Acoustical Society of America, 

36, 1662, 1964; J. C. Webster, "Noise and Ccmnunication," in D. Jones and T. 

Chapman (editors), Noise and Society, Ladon: Wiley in preparation; R. G. 

Klwp and J.C. Webster, "Physical measurmts of equal speech-interfering navy 

noises," Journal of the Acoustical Society of mrica, 35, 1328, 1963. 
AC 20-133 

Appendix 1 

3122189 

The maximum PSIL for cormzunications approximately 3 feet apart for an AI of 0.2 

is 83. The maximum for an AI of 0.3 is 78. As will be shm belcw, these two 

AI% represent the range of acceptable minimum intelligibility levels. 

Therefore, when a cockpit has a noise level above a PSIL level of 78, talkers 

and listeners can 'be expected to have sm voice-cmunication problems. This 

prediction can be &if&d slightly by the fact that, in many cockpits, the 

pilot and copilot can 'be 'tTy>re or less than 3 feet apart. Hover, in the 

cockpits of aircraft likly to be relatively noisy, i.e. small aircraft, 

creembers would probably be seated at distances between 2 and 3 feet apart. 

The messages that are expected to be transmitted in aviation comnunications cm 

from a prescribed vocaklary. However, even when that vocablary is ignored, 

the mssages are spoken in context, which usually means that they are more 

intelligible. The md Forces have set acceptable levels of performance for 

comnunications equipment, and those performance levels can be transford into 

AI values: they range from 0.25 to 0.3. The Air Force, for example, defines an 

80 percent score on a rhyme test as passing and a 70 percent score as 

ccnditimally passing. In figure 1, it can 'be seen that the 80 percent 

criterion is almost exactly 0.3 and that the 70 percent criterion is very close 

to 0.25. 

Navy and Army limits of acceptability are approximately the same as the Air 

Force%. Webster and Allen specified an 80 percent rhp test score as (the 

Navy fence) the lest acceptable value*. They reasoned that "95 percent of 

standard test sentences will be understood over a system that will pass 80 

percent? of rhyme test words. Following identical reasoning, the FAA believes 

that, short of measuring human performance on rhyme tests in cockpit-noise 

environments, the choice of AI=O.3 is both reasonable and acceptable. This AI 

equates to a PSIL of approxim&zely 78 at a distance of 3 feet. 

*J. C. Webster and C, R. Allen, "Speech intelligibility in naval aircraft 

radios," Naval Electronics Laboratory Center Report, TR 1830, 1972. 

Rolrtion Bowtom Al and Varloua Yorwnr 

ot Spooch Intolllglblllty 

TYFE OF NATERIAL AK0 SKILL OF 

TALKEKS AK0 USTEYERS 

- 

I I I I I I 

From ANSI SW=1989 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

ARTICULATION INDEX 

2 
3/22/89 AC 20-133 

Appendix 1 

The follming table also corroborates the relationship between the various test 

results and Articulation Index: 

Table 1. Expected Word or Sentence Scores for Various Articulation 

Indices (AI) 

PERCENT INTELLIGIBILITY 

Articulation Phonetically* 

Index Balanced Test 

Modified** 

Rhyme Tksts 

Sskence* 

Test 

032 22 54 77 

0.3 41 72 92 

0.35 50 78 95 

0.40 62 86 96 

0.50 77 91 98 

0.60 85 94 98 

0.80 92 98 99 

*Fran Kryter and Whitman (1963) 

**Fran Webster and .Allen (1972) 

Assuming that pilots can cmmnicate visually with each other, an AI of 0.3 

actually can be elevated to 0.47 as indicated by the following chart 

(figure 2). 

0.8 7 

" 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

CALCULATED ARTICULATION INDEX 

Relation between calculated Al and 

effective Al for a communication system 

wherein the listener can see the lips and 

face of the talker (Sumby and Pollack, 

1954). 

FIGURE 2 
AC 20-133 

Appendix 1 

3129189 

Thus, an AI of 0.3, if aided by visual cues, can raise the intilligibility 

level to approximately 98 percent (as shown figures 1 & 2). Hmever, visual 

comnunication, while it can irrprove intelligibility, requires the persons to 

look directly at each other. This full-face orientation in the cockpit 'between 

the pilot and cqilot is an unusual occurrence. Cockpit noise levels in many 

rotorcraft and propeller-driven airplanes, especially the piston-engine types, 

can possibly excel the rnaximm practical PSIL values noted above. 

If one considers the distance between the heads of a pilot and copilot to be 3 

feet, then in a noise field whose intensity exceeds a PSIL of about 90 (about 97 

dB(A>), vocal effort cannot overce the intelligibility problem created by the 

noise. First, shouted speech is not as intelligible as speech produced with 

less effort (see figure 3). Second, in that much noise, human vocal systems 

are, an the average, just about at the limit of their loudness. (Reflexively, 

talkers raise their voices in order to be heard above the bkkground noise. In 

this instance,though, where noise levels are quite high, the reflex cannot lead 

to IIy>re intense speaking levels: the vocal system has already reached its 

physiological end point.) When PSIL = 90, AI approaches zero as does 

intelligibility--that PSIL condition is unacceptable at a 3-foot distance. 

FROM: Pickett, J.M.: Limits of Direct Speech Carmunication in Noise. J. 

Acoust. Sot. &rerica, vol. 30, no. 4, Apr. 1958, pp. 278-281. 

SPEECH INTENSITY, DB 

Relations between speech intelligibility in noise and vocal 

force. Vocal force measured as speech intensity one m from 

lips in a free field. Parameter, over-all signal-tenoise ratio, db. 

Noise, 70 db, flat spectrum. 

FIGURE 3 

4 
3122189 

APPENDIX 2

AC 20433 

Appendix 2 

TESTSETUP 

Measuremmt in the cockpit should be made at the typical head location of each 

flight crewmmber. The micrclphoe should be placed at the representative ear 

position on the side where speech cannunication is normally received and moved 

around slightly to obtain a spatial average of noise at the head positim. 

Wknever possible, the rmxwrement shall be made with the crewmmber absent frcun 

his location so as to minimize interference and shielding effects. During the 

masurements care should be t&en not to hold the microphone close to a 

sound-reflecting or sound-refracting surface. A cmnm recmmmdatim is to 

stay at least one foot away; in practical use, a 6-inch distance is probably 

adequate, 

TESTCCNDITICrJS 

The aircraft interior should be in a fully furnished -figuration for its 

intended use (passenger, cargo, other) with tie downs, carpets, seats, curtains, 

interior trim panels I etc., installed. Systems us&l for providing conditioned 

air (i.e., pressurizatim, cooling, heating, 1 should be operational. Cabin 

pressure should be noted so that adjustmmts for differences in air pressure may 

be made, if necessary. Cabin pressure can affect noise measurements taken on 

the ground or inflight. TIE difference between these measurements is abcxt 

0.25 dB(A9. On sane aircraft, windcms can be qen during flight and could 

adversely affect the noise level in the cockpit. If this case exists this 

ccmdition should also be tested. 

If a tape recorder is used, the acoustic sensitivity calibration can be recorded 

during flight to establish the reference acoallstic lewl for subsequent data 

processing and for cqarison with the preflight recording of 

acoustic-sensitivity signals. Recorded noise levels should be measured cm the 

ground and inflight to establish the proper gain to be used for recording above 

the backgramd noise levels. At least me reel of tape used during the test 

should have a recording of acoustic-sensitivity calibration signals. 

Where possible masuremnts should be made when all aircraft -rating 

conditions (such as altitude, airspeed and engine power settings) are stabilized. 

The aircraft cockpit noise should be tested in take-off, approach, landing, 

cruise, and descent at high speed. 

On multi-engine aircraft the use of engine synchronization is optional depending 

cm the test objectives. Installation and operation of engine synchronizers or 

propeller synchrophasers is frequently desirabk for increased passenger ccanfort. 

Operatim of such devices during accnstical testing is advisable if the test 

objective is to measure the optimum cabin environmnt. Hmzver# imperfect 

synchronizer operation may intmduce very low frequency beats which compranise 
AC 20433 Appendix 2 3122189 

the data, so that intentional operation out of sync may be necessary. In such 

cases, the engines should be set to produce a known beat frequency high enough 

to,allow reasonable length data records and minimize amplitude effects. 

The following flight data should be observed and noted while the 

acoustic data is being obtained: 

a. Flight Regimz? -takeoff, cruise, approach, landing, descent etc. 

b 0 Airplane pressure altitude. 

c. Airplane indicated airspeed and/or Mach number. 

d l Propeller RPM (if applicable). 

e. Engine power settings. 

f 0 Synchronizer or synchrophaser operation. 

g 0 External ambient air terrperature. 

h a Cabin pressure and teqerature. 

i. Cabin system operation modes. 

DATA ACQJISITICN 

If tape recording is used, the record length at each location should be at least 

2 l/2 tircres the data reduction integration period, ht in no case less'than 20 

seconds. If audible beats are present the record shall include at least 

3 complete beats. Sufficient precautias should be taken to ensure the data 

signals are not cqramised by inappropriate tape recorded gain settings. Data 

should be recorded with the sound level meter in the flat mde (unwzighted), 

W&n portable sound level meters are used for direct lrreasurent of sound 

pressure levels, (use the A-weighting network with Sm response setting) the 

data to be reported shall be the maximum reading noti on the rr&er. When 

audible beats are present the meter should be observd for a period of time long 

enough to include at least three beats, and the maximum meter reading noted 

shall be reported. If the sound level meter has integrating capability where 

the time period is qerator-controlled, the tim period used shall be at least 

lo-20 seccnds. If audible beats are present, the time period shall be 

sufficient to include at least 3 cmplete beats, but not less than 20 seconds. 

DATA RIZDUCI'ICN 

Data reduction, from the recording, when employed, should be perforlrvsd by time 

averaging data samples of at least 8 seconds duration. When audible beats arxz 

present, the integration period should be extended to include at least a 

three-beat period. 

2 
3/22/89 AC 20433 

Appendix 2 

Sound pressure levels should be obtained for the eight-octave bands center 

frequencies from 63 Hz to 8 KHz. overall smnd pressures shcmld be stained by 

sumning antilogarithmically the octave band data. Preferred speech interference 

level &IM should be calculated by algebraically averaging the unweighted 

levels in the 500, 1,000, and 2,000 Hz octave bands. 

Frequency weighting may be added to octave band sound pressure level data. 

weighting functim should correspcnd to that referenced in Intematiozal 

Electrmechanical Camissim UEXX 651. Frequency weighted overall sound 

pressure levels are obtained by antilogariUnnically s ummingthe octave-band 

after wighting is applied. 

Presentation of the acoustical data should inclclde at least the following 

information: 

The 

data 

1 0 Overall Aeighted sound pressure levels at each measurement location. 

2 l Preferred speech interference levels at each measuremnt location. 
3122189 

AC 20433 Appendix 3 

APPENDIX 3

A portable sound level mster (SIM) and a portable battery--red FM retarder 

are rem-mended to measure cockpit noise. The SLM includes the micrqhone, 

amplifier, rectifier and a rrreter which gives a somd output directly in decibels. 

A cmnecting jack is provided so the amplifier output can also be recorded on a 

magnetic recorder for further study. 

M-t sound level rcleters also include weighting networks selected by a panel 

switch. The "flat" positim sums all frequencies evenly. Tzle "C" position is 

almost the same as "flat" and one or the other may be cm&ted on cheaper 

instruments. The "A" and *'B" weightings are designed to approximate the ear's 

response and to give a truer approach to lotiness of complex sounds. (The "B" 

scale is little used today, while the "A" weighting is used extensiwly. The 

designation "dE3(A) I' (p:, less properly, "dBA*', indicates the reading with the "A" 

wzighting.1 

More expensive mters include, either as an attachmmt or in&mmlly, a series 

of band pass filters, usually of one octave width. Eight such bands will cover 

the usual masuremmt range of 50 to 10,000 Hz. Such filters provide a 

convenient mans for a quick evaluation of the frequency structure of a ccanplex 

sound. 

In order that sound level meters made by different manufacturers will agree 

adequately when measuring various sources, their characteristics are specified 

by the International Standards Organization (ISO) and Amrican National 

Standards Institute (ANSI). This-includes the characteristics of the weighting 

networks and the mter darrping, as ml1 as the cwerall accuracy. Soltnd level 

mters are divided by ANSI standards into several groups: m,,: 

or "Precision" meters; Type 2, or "General Purpose," Tyneq, or *'Survey,*' and 

Type S or "Special Purpose." Type 1 meters meet the rigid tolerances for 

Precision mters and provide filtering and impulse waswing opticms. 

mter is reccmrmded for evaluating cockpit noise. 

A Type 1 

A high quality FM tape recorder should be used to record the noise in 

the cockpit. Good results can be obtained from a portable battery-p-red 

system. Several manufacturers now advertise high quality cassette recorders for 

instrumntation use. 

The sound level mzter or the recording system, if recordings are made should 

be calibrated using a PISTCN-PHcbJE, crother calibration instnanents, before and 

after the test data is recorded. These calibration devices are available fran 

manufacturers of sound level meters and measurement microphones. It is designed 

to fit tightly on the microphone, with adapters for various micrwhone sizes, 

and it produces a tone of accurately knck~n sound pressure at the micrqha?e 

diaphragm at one or more standard frequencies, A set-screw is usually provided 

in the sound level mter to standardize its output. 
AC 20433 

mpndix 3 

3/22/89 

A calibration signal is particularly necessary when the microphone is used with 

mnplifiers otkr than a standard sound level meter or when a recorder is used. 

This "end-to-end" calibration should be made both at the beginning and end of a 

test run, and at any other ti.mz where there is a possibility that the system 

gainmayhave been changed. 

It is inportant in all test qera tions to maintain an accurate log of all 

conditions: microphone placement I weather conditions if outdoors, system 

channel connections (if more than one channel), all attenuator and calibra :ted 

amplifier gain settings, time of day and date, source and distance from source 

to micrOphone, etc. Wkn a tape recorder is used, the log information should be 

recorded vocally on the tape. 

While the PISTON-PHONE calibrator is an essential part of any acoustic 

.masurexIEnt program, it does not give an adequate check of microphone, amplifier 

and recorder frequency characteristics. Tk instrmntation and procedures 

required for full calibration are beyond the SC- of this discussion, but se 

provision should be made for periodic recalibration of system ccxnponents by the 

manufacturer or by a reliable and well-equip@ standardization laboratory. 

CALIEEWTICN 

A preflight sensitivity check should be used to adjust the gain of the sound 

level meter to match the output of the acoustic calibrator as adjusted for 

atmospheric pressure. A Warm-up” time of at least 1 minute should be allomd 

before checking the sensitivity of the sound level meter. If a tape recorder is 

used, the sensitivity checks shall also be recorded. 

If an in-flight acoustic sensitivity check is used, it should be taken when the 

aircraft has reached the desired cruise altitude and the aircraft's internal 

pressure is at the desired value. The indicated sound pressure level of the 

output of the acoustic calibrator should be noted; tk gain of the sound level 

aster should not be adjusted in flight if the indicated level is not the same as 

the acoustic calibration level obtained before takeoff. Tf necessary, cabin 

pressure should be noted so that adjustxxznts for differences in air pressure may 

be made. 

2 
3/22/89 

APPENDIX4 

AC 20-133 

Appendix 4 

EXAMPLE-TICN OFPSIL 

T.O. Pwer Nom& Cruise Pcwer Approach Puwer 

Octave Band Avg. Meas. Avg. Meas. Avg.Meas. 

Cntr. Frea. Data Data Data 

63 

125 

250 

500 

1000 

2000 

4000 

8000 

db(C) 

db(A) 

PSIL 

106.2 

114.5 

110.0 

99.1 

84.6 

81.2 

76.9 

76.1 

116.2 

104.3 

88.3 

103.0 

111.6 

109.2 

95.8 

80.1 

78.4 

73.8 

74.1 

113.8 

102.7 

84.7 

102.8 

110.0 

100.5 

86.5 

73.9 

73.2 

74.8 

73.7 

110.9 

96.6 

77.9 

q . . From the above it can be seen that the takeoff and normaL cruise power noise 

levels exceed a PSIL of 78 and speech interference can be expected in the 

c&pit in those flight regimes.- The db (A) in all three flight regimzs also 

exeed the recmmznded level of 88. 

* L is the noise level (flat) at the specified octave band center 

frequency. 
3122189 

APPmDIX 5 

MODIFICATICNS CF SIGNAL-IWNOISERATIOS 

AC 26133 

Appendix 5 

An easy speech intelligibility ccslcept to grasp is that the louder the speech is 

in comparison to the background noise, the easier it is to understand. 

Obviously, there are practical limits to the concept, ht through most of the 

range of audible sound pressuresr this statement about the speech-to-noise or 

the signal-to-noise ratio (S/N) is true. Where both speech and noise are 

extremely quiet or extremely intense, nonlinearities arise. For the 

cockpit-noise situation, cne may confront a degree of high-intensity 

nonlinearity.) 

& improvement in S/N, then, will serve to improve the intelligibility of 

speech. 

The most direct approaches call for an increase in absolute signal level or a 

decrease in absolute noise level. One may also try to create relative 

differences between the signal and the noise levels. 

The difficulty with trying to decrease cockpit noise levels at the source has 

already been discussed. However, it should be noted that noise attentuaticn 

materials are available for light aircraft. The use of inflatable door seals 

and acoustic blankets can reduce interior noise levels. Nevertheless, the most 

effective option may be to increase signal levels a modify the relationship 

between signal and noise. 

Signal levels can be increased by increasing the gain of an intervening 

amplifier (for electronically transmitted comnunications II or by moving the 

talker and listener closer together. Research has shown a deterioration of 

intelligibility with an extrezly weak or strong vocal force. 

Hearing protectors for aviators can provide protection against hearing 

loss that results froan noise elcposure and improves speech intelligibility. They 

perform the intelligibility @rovertent task in two ways. The lesser of these 

is that they I= the overall intensity of the sound that enters the human 

auditory system into a middle range of sound pressures where the system operates 

optimally. (Note that hearing protectors do not remove sound; they only 

decrease its intensity). The other way is selective filtering which can be 

effect&? in some noise environmnts. 

Sane precautions are necessary, though, before oa?e elects to use hearing 

protectors for the purpose of improving voice carmunication. First, a 

wzll-sealed, wAl-fitted protector is necessary. Second, sm auditory 

functions are changed by the introduction of hearing protectors into the 

transmission system. For example, scm pewle report a decrease in the ability 

to make fine pitch discriminations, many people report a decrease in the ability 

to judge the azimuth of a sound source, Hover, the human auditory system 

rapidly accanmodates itself to environmental change of all sorts, so one can 

ass= that with a bit of practice these functions can be brought back into the 

nom1 range. Third, because one adjusts orlle's vocal effort to overccme the 
AC 20433 

Appendix 5 

3122189 

noise one hears, baring-protector wearers (since they hear less noise) usually 

don't speak loudly enough. Persons who ear hearing-protectors must train 

themselves to speak more locldly. 

In most cockpits where noise is a problem, the noise spectrum tends to have the 

sam shape as the average speech spectrum. As a result, one cannot count on 

selective filtering to improve speech intelligibility. Whatever changes are 

made in one spectrum will be made similarly in the other. The S/N stays about 

the same. Thus, in cockpits with similar noise and speech spectr-urns, 

the improvement in speech intelligibility for pilots and copilots who -wear 

hearing protectors is probably limited to the small munt that arises from 

bringing signal intensities into the linear, middle frequency range where the 

auditory system works better. 

A microphone may help SCE, because if it is held close to the muth, it is 

s-what like reducing the distance to the ear. Considerably more irnprovzment 

in S/N can be obtained by using noise cancelling micrqhones in cmunication 

systems. The noise-cancelling micrqhone is built to accept sound from the 

front, the back, or the top. In a fairly hqeneous sound field, 

approximately the same ambient-noise wave form enters from both sides, serving 

to cancel much of the effect of the noise on the microphone diaphragm. .A 

talker, though, directs his or her speech to one side only, so the cancellation 

effect for speech is far less than for noise--if the user understands the 

proper way to use the microphone. Covering the rear vents with the hand 

diminishes the cancellation effect. Holding the front of the microphone more ,' 

than a few inches froan the lips of the talker permits the speech to enter the 

back with nearly as much intensity as en&rs the front, thus cancelling speech 

as ~11 as noise. Another potential loss of S/N iqroment results from the 

reflex that leads a talker to speak with enough effort to be heard above the 

noise: if the talker expects to be heard (by the microphone) at a distance of 

3 inches rather than 3 feet, h or she is likely to reduce vocal effort 

accordingly. 

Miniature headsets have cm into use mng pilots in recent years. 

The headsets, which are worn over the ear, conduct sound to the microphone 

diaphragm via a hard, plastic tube that is hinged so that it can be roved about 

at will. Although these headsets are not noise-cancelling devices in the usual 

sense, the tip of the plastic tube can be moved so close to the talker% lips as 

to make a significant ir?proverclent in SD over face-to-face camnunications in the 

same noise environment. Again, the likelihood of improvement is a direct 

function of how much vocal effort is exerted and of haw close the tube is to the 

mouth; if the tube has been moved out of the way (as it needs to be for eating 

or drinking), any S/N irrrprovement will be markedly diminished. 

I 

Sarrvs headsets are equipped with circurrraural muffs which attenuate the cockpit 

noise and enhance the S/N for electronic cosrmunicaticgls. This type of ear muff 

furnishes S~IIE hearing protection and acts s-what like an ear plug in normal 

cockpit voice c~unications. Headsets eqipped with the better designed 

circumaural muffs may attenuate cockpit noise TIy>re that 20 dI3. These headsets 

used with noise cancelling or bean microphaes and an intercom system can 

substantially enhance the S/N and markedly improve crew corrmw?ications. 

2 
3/22/89 AC 20433 

Appendix 5 

Proper use requires holding the noise-cancelling microphone so that the vents 

are not blocked, holding it close to the mouth, and speaking as loudly as if the 

listener were a few feet away. When themicrophone is used properly, it can 

make a significant difference in S/N. 

It should be noted that increasing the gain of an amplifier or trying to do 

selective electronic filtering will make no useful change in the S/N; it will 

stay the sm as it was at the face of the microphone whose sounds are being 

amplified or filtered. If the S/kJ is poor to begin with, wlifying both the 

speech and the noise cannot make the situation any better. Also, electronic 

filtering is no different in its effect than the acoustic filtering that a 

hearing protector does: if the spectrum of the noise and the spectrum of the 

speech are similar, selective filtering will not help. 

Adlditional information m aircraft atiio system characteristics and standards 

can be found in Radio Technical Carrmission for Aeronaut&s (RYEA), Document 

No. Do-170, "Atiio Systems Characteristics and Minimum Performance Standards, 

Aircraft Microphones (Except Carbon), Aircraft Headsets and Speakers, Aircraft 

Audio Selector Panels and kplifiers," January, 1980.