HETA 96–0184–2663

Federal Aviation Administration — Bradley Airfield

Windsor Locks, Connecticut

Randy L. Tubbs, Ph.D.

John R. Franks, Ph.D.

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.

Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports
ii

PREFACE

The Hazard Evaluations and Technical Assistance Branch of NIOSH conducts field investigations of possible

health hazards in the workplace. These investigations are conducted under the authority of Section 20(a)(6)

of the Occupational Safety and Health Act of 1970, 29 U.S.C. 669(a)(6) which authorizes the Secretary of

Health and Human Services, following a written request from any employer or authorized representative of

employees, to determine whether any substance normally found in the place of employment has potentially

toxic effects in such concentrations as used or found.

The Hazard Evaluations and Technical Assistance Branch also provides, upon request, technical and

consultative assistance to Federal, State, and local agencies; labor; industry; and other groups or individuals

to control occupational health hazards and to prevent related trauma and disease. Mention of company names

or products does not constitute endorsement by the National Institute for Occupational Safety and Health.

ACKNOWLEDGMENTS AND AVAILABILITY OF REPORT

This report was prepared by Randy L. Tubbs of the Hazard Evaluations and Technical Assistance Branch,

Division of Surveillance, Hazard Evaluations and Field Studies (DSHEFS) and John R. Franks of the

Physical Agents Effects Branch, Division of Biomedical and Behavioral Science (DBBS). Desktop

publishing by Ellen Blythe.

Copies of this report have been sent to employee and management representatives at the FAA and the OSHA

Regional Office. This report is not copyrighted and may be freely reproduced. Single copies of this report

will be available for a period of three years from the date of this report. To expedite your request, include

a self–addressed mailing label along with your written request to:

NIOSH Publications Office

4676 Columbia Parkway

Cincinnati, Ohio 45226

800–356–4674

After this time, copies may be purchased from the National Technical Information Service (NTIS) at

5825 Port Royal Road, Springfield, Virginia 22161. Information regarding the NTIS stock number may be

obtained from the NIOSH Publications Office at the Cincinnati address.

For the purpose of informing affected employees, copies of this report shall be

posted by the employer in a prominent place accessible to the employees for a

period of 30 calendar days.
iii

Health Hazard Evaluation Report 96–0184–2663

Federal Aviation Administration — Bradley Airfield

Windsor Locks, Connecticut

November 1997

Randy L. Tubbs, Ph.D.

John R. Franks, Ph.D.

SUMMARY

The National Institute for Occupational Safety and Health (NIOSH) received a request from health and safety

management for the Federal Aviation Administration’s (FAA) New England Region to conduct a health hazard

evaluation (HHE) at the Bradley Airfield Air Traffic Approach Control Tower. The request followed an

Occupational Safety and Health Administration (OSHA) citation and notification of penalty to the Federal

Aviation Administration for failure to monitor workers’ noise exposures. There was concern that air traffic control

(ATC) specialists may be exposed to noise greater than 85 decibels on an A–weighted scale [dB(A)] over an

8–hour time–weighted average (TWA) period from their communication headsets. OSHA suggested that NIOSH

be contacted to make the measurements to determine the level of noise exposures.

On October 8–9, 1996, investigators from NIOSH visited Bradley Airfield to measure noise levels from the

communication systems used by the ATC specialists while a noise compression unit was inserted into the

communication line, as well as when the unit was removed and unattenuated signals were allowed to reach the

headset receiver. Also, ambient background noise measurements were made in the controllers’ work area.

Interviews were conducted with any ATC specialists from the day and afternoon shift who wished to speak to a

NIOSH investigator. Finally, NIOSH obtained the annual audiometric tests for the ATC specialists assigned to

Bradley Airfield over the last three years, a copy of the OSHA Log of Federal Occupational Injuries and Illnesses,

and an unused compression unit that could be further tested in the NIOSH laboratory. Laboratory analysis of the

headset receiver and the compression unit showed the controllers could be exposed to equivalent free field noise

levels up to 104 dB but that the compression units functionally reduced the exposure to a safe listening level.

Analysis of the audiometric records did not reveal any systematic occupational hearing loss in the population of

controllers at Bradley Airfield, even though over 75 noise incidents had been recorded on the injuries and illnesses

log.

The results of the evaluation lead NIOSH investigators to conclude that a hazard to the hearing of the

ATC specialists does not exist. However, there were deficiencies noted in the manner in which the

compression unit was used and in the hearing tests given to the controllers. There was also a moderately

high level of background noise in the work area that could possibly interfere with speech intelligibility.

Recommendations to alleviate these deficiencies are given in the last section of this report.

Keywords: SIC 9621 (Regulation and Administration of Transportation Programs), air traffic control

operations – government; noise; radio headsets; compression circuits; noise–limiting circuits; audiometric

testing; hearing conservation program.
TABLE OF CONTENTS

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Acknowledgments and Availability of Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Introduction .................................................................. 2

Background .................................................................. 2

Methods .................................................................... 3

Noise Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Medical Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Evaluation Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Noise Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Medical Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Page 2 Health Hazard Evaluation Report No. 96–0184

INTRODUCTION

In May 1996, the National Institute for

Occupational Safety and Health (NIOSH)

received a request from the Federal Aviation

Administration’s (FAA) Office of Occupational

Health and Safety in Burlington,

Massachusetts. The request concerned reported

loud tone bursts and static that air traffic control

(ATC) specialists experience on an intermittent

basis through their radio headsets or telephone

landlines at the controller’s radar screen located

at Bradley Airfield in Windsor Locks,

Connecticut. These incidents had been reported

as early as 1994 at Bradley Tower. From

November 3, 1995, to March 20, 1996, the

Occupational Safety and Health Administration

(OSHA) conducted an inspection at the facility

and issued citations on April 8, 1996. One of

the citations was for lack of a noise exposure

monitoring program where information

indicated that the employees may be exposed to

noise exceeding an 8–hour time–weighted

average (TWA) of 85 dB(A). The OSHA area

office gave the FAA the option of requesting a

NIOSH HHE to measure the noise exposures

through the headsets.

A headset was sent to NIOSH in Cincinnati,

Ohio, for examination and analysis prior to the

health hazard evaluation (HHE) site visit.

Researchers in the Physical Agents Effects

Branch, Division of Biomedical and Behavior

Science determined input/output functions for

the headset over a wide frequency range and

also determined the equipment necessary to

collect the relevant noise data at Bradley

Airfield. On October 8–9, 1996, researchers

from NIOSH performed a site visit at Bradley

Airfield’s Air Traffic Control Tower

(TRACON) to collect noise data from the

headsets used by the controllers at their work

positions. On the first day of the survey, an

opening conference was held with

representatives from the FAA, the National Air

Traffic Controller’s Association (NATCA), and

the OSHA Hartford, Connecticut office. On the

second day, several recordings of radio and

telephone communications were made at an

unmanned radar position along with octave

band sound levels of background noise in the

control room. Additionally, interviews were

conducted with ATC specialists from the day

and afternoon shifts about their experiences

with loud tones or static that they encountered

in their job. Audio tapes of several tone

incidents recorded by the FAA from the master

tape of control tower communications were

given to NIOSH investigators to analyze in our

laboratory. Finally, an unused noise

compression unit that had been purchased by

the FAA to limit the sound levels transmitted to

the controller’s headset was given to NIOSH

researchers for further analysis and

characterization. Copies of the U. S.

Department of Labor Log of Federal

Occupational Injuries and Illnesses for Bradley

TRACON and the audiometric examinations for

the past three years for all controllers was

requested by NIOSH and received from the

FAA for analysis.

BACKGROUND

The TRACON facility at Bradley Airfield

handles air traffic at Bradley International

Airport (BDL) serving the Hartford,

Connecticut area. The controllers are

responsible for aircraft approaches to BDL and

for planes leaving the airport until they reach a

location out of the region where they are

handed off to other control centers. The

darkened room where the controllers work

contains four radar screens, a supervisor’s

station, and a computer printer that records

flight numbers and flight plans for both inbound

and outbound aircraft.

The communication system in the FAA tower

relies upon head–worn microphone/receiver

sets. The body of the ear piece is shaped to fit

over the ear, held in place by an ear hook. The
Health Hazard Evaluation Report No. 96–0184 Page 3

microphone is located in the body of the piece

and is coupled to the mouth by a rigid tube. The

receiver is coupled to the ear with flexible

plastic tubing that ends at an olive–shaped

universal tip. The tip is available in six sizes

that are attached to flexible tubing that is

inserted into the ear. Figure 1 shows a

schematic of the microphone/receiver module

along with a schematic of the Tygon® tubing

and the ear piece.

The concerns of the flight controllers were that

the signal levels they receive from the

communication system through the headset

receiver were of sufficient volume to cause

hearing loss from long–term use and that when

the communication system became unstable and

oscillated (feedback), the tone could cause

instantaneous hearing loss due to its extremely

high level. The FAA determined that feedback

occurs during three different scenarios: (1) if

two or more aircraft simultaneously transmit

communications on the same radio frequency,

(2) if ATC specialists from other locations

attempt to communicate with personnel at BDL

and are improperly using a headset in very close

proximity to a loud speaker, and, (3) the

accidental transmission of a telephone company

test tone over the telephone landlines. Tones

were described by employees as loud,

squealing, shrieking, piercing, hissing, or shrill,

and that they persisted from one second up to

five minutes. In an effort to prevent the

controllers from receiving high intensity speech

or feedback, electronic compression units were

purchased and put into the signal path. While

these compression units prevented extremely

high levels of signals, controllers complained

that they reduced the loudness of the speech and

made it more difficult to understand.

METHODS

Noise Evaluation

To capture speech and other signals from the

headset receiver, it was necessary to record the

signals delivered to the system as they would

under normal conditions. A junction box was

made that allowed signals going to the receiver

set to also be recorded on digital audio tape

(DAT; Panasonic Model SV–250). The input

impedance of the DAT recorder was high so

that the line signal remained at 600 ohms (Z).

The DAT recorder was calibrated so that a

system signal of 0 decibel volume units (dB

VU; 1.0 volt rms @ 600 Z) to the receiver

module was equal to –20 dB VU on the DAT.

The DAT recorder has a dynamic range of 90

dB, so that it could accurately record signals

ranging in levels from +20 dB VU to –70 dB

VU.

Recordings of normal air–to–ground and

ground–to–air communications were made for

two conditions: the electronic compression unit

out of the system with normal communication

traffic, and the electronic compression unit in

the system with normal communication traffic.

Recordings of “tones” were also made with the

compression unit in and out of the

communications circuit.

In the laboratory, the DAT recordings were

played through the sample headset sent to

NIOSH by the FAA, complete with ear tip to

the artificial ear of a head and torso simulator

(KEMAR). The signals were analyzed to

provide readings of integrated maximum output,

integrated minimum output, and integrated

average output with a real–time acoustic

analyzer.

 

Area noise samples in the controllers’ work

space were made with a Larson–Davis

Laboratories Model 800B Precision Integrating

Sound Level Meter. Octave band

measurements at consecutive center

frequencies of 31.5 Hertz (Hz) to 16 kilohertz

(kHz) were made at the supervisor’s counter,

located in the center of the room behind the

controllers’ radar screens. Octave
Page 4 Health Hazard Evaluation Report No. 96–0184

measurements were made with the sound level

meter integrating the sound energy over

1–minute periods.

Medical Evaluation

During the site visit, FAA air traffic controllers

from the day shift and afternoon shift were

given the opportunity to meet with a NIOSH

investigator to discuss any tone incidents which

they may have experienced. The unstructured

interviews were conducted in a private office at

Bradley Airfield. NIOSH investigators also

requested that the FAA furnish copies of OSHA

logs for tone incidents in 1993, 1994, and 1995.

Finally, the last three years of audiometric

examinations given in conjunction with their

annual medical exams for all of the air traffic

controllers assigned to Bradley Airfield were

requested so that analyses of their hearing

abilities could be conducted. These data were

forwarded to NIOSH in February 1997. 

EVALUATION CRITERIA

Noise–induced loss of hearing is an irreversible,

sensorineural condition that progresses with

exposure. Although hearing ability declines

with age (presbycusis) in all populations,

exposure to noise produces hearing loss greater

than that resulting from the natural aging

process. This noise–induced loss is caused by

damage to nerve cells of the inner ear (cochlea)

and, unlike some conductive hearing disorders,

cannot be treated medically.1

 While loss of

hearing may result from a single exposure to a

very brief impulse noise or explosion, such

traumatic losses are rare. In most cases,

noise–induced hearing loss is insidious.

Typically, it begins to develop at 4000 or 6000

Hz (the hearing range is 20 Hz to 20000 Hz)

and spreads to lower and higher frequencies.

Often, material impairment has occurred before

the condition is clearly recognized. Such

impairment is usually severe enough to

permanently affect a person's ability to hear and

understand speech under everyday conditions.

Although the primary frequencies of human

speech range from 200 Hz to 2000 Hz, research

has shown that the consonant sounds, which

enable people to distinguish words such as

"fish" from "fist," have still higher frequency

components.2

The A–weighted decibel [dB(A)] is the

preferred unit for measuring sound levels to

assess worker noise exposures. The dB(A)

scale is weighted to approximate the sensory

response of the human ear to sound frequencies

near the threshold of hearing. The decibel unit

is dimensionless, and represents the logarithmic

relationship of the measured sound pressure

level to an arbitrary reference sound pressure

(20 micropascals, the normal threshold of

human hearing at a frequency of 1000 Hz).

Decibel units are used because of the very large

range of sound pressure levels which are

audible to the human ear. Because the dB(A)

scale is logarithmic, increases of 3 dB(A), 10

dB(A), and 20 dB(A) represent a doubling,

tenfold increase, and 100–fold increase of

sound energy, respectively. It should be noted

that noise exposures expressed in decibels

cannot be averaged by taking the simple

arithmetic mean.

The OSHA standard for occupational exposure

to noise (29 CFR 1910.95)3

 specifies a

maximum permissible exposure limit (PEL) of

90 dB(A) for a duration of 8 hours per day. The

regulation, in calculating the PEL, uses a 5 dB

time/intensity trading relationship, or exchange

rate. This means that a person may be exposed

to noise levels of 95 dB(A) for no more than 4

hours, to 100 dB(A) for 2 hours, etc.

Conversely, up to 16 hours exposure to 85

dB(A) is allowed by this exchange rate.

NIOSH, in its Criteria for a Recommended

Standard,4

 proposed a recommended exposure

limit (REL) of 85 dB(A) for 8 hours, 5 dB less

than the OSHA standard. The NIOSH 1972

criteria document also used a 5 dB

time/intensity trading relationship in
Health Hazard Evaluation Report No. 96–0184 Page 5

calculating exposure limits. However, in 1995,

NIOSH changed its official recommendation for

an exchange rate of 5 dB to 3 dB.5

 The

American Conference of Governmental

Industrial Hygienists (ACGIH®) also changed

its Threshold Limit Value (TLV®) in 1994 to a

more protective 85 dB(A) for an 8–hour

exposure, with the stipulation that a 3 dB

exchange rate be used to calculate time–varying

noise exposures.6

 Thus, a worker can be

exposed to 85 dB(A) for 8 hours, but to no more

than 88 dB(A) for 4 hours or 91 dB(A) for

2 hours. 

The duration and sound level intensities can be

combined in order to calculate a worker's daily

noise dose according to the formula:

Dose = 100 X (C1/T1 + C2/T2 + ... + Cn/Tn ),

where Cn indicates the total time of exposure at

a specific noise level and Tn indicates the

reference duration for that level as given in

Table G–16a of the OSHA noise regulation.4

During any 24–hour period, a worker is allowed

up to 100% of his daily noise dose. Doses

greater than 100% are in excess of the OSHA

PEL.

The OSHA regulation has an additional action

level (AL) of 85 dB(A); an employer shall

administer a continuing, effective hearing

conservation program when the TWA value

exceeds the AL. The program must include

monitoring, employee notification, observation,

audiometric testing, hearing protectors,

training, and record keeping. All of these

requirements are included in 29 CFR 1910.95,

paragraphs (c) through (o).

Finally, the OSHA noise standard states that

when workers are exposed to noise levels in

excess of the OSHA PEL of 90 dB(A), feasible

engineering or administrative controls shall be

implemented to reduce the workers' exposure

levels. However, in 1983, a compliance

memorandum (CPL 2–2.35) directed OSHA

compliance officers not to cite employers for

lack of engineering controls until workers’

TWA levels exceed 100 dB(A), so long as the

company has an effective hearing conservation

program in place. Even in TWA levels in

excess of 100 dB(A), compliance officers are to

use their discretion in issuing fines for lack of

engineering controls.

The OSHA regulation is designed to prevent

hearing losses from exposures to intense noise

levels. However, lower intensity noise can be

disruptive in the workplace. Interference with

speech and disruption of office activities are

possible results of unwanted noise. The noise

can interfere with the efficiency and

productivity of the office staff and can be

detrimental to the occupants' comfort and sense

of well–being. Noise criteria for occupied

interior spaces (NCB curves) have been devised

to limit noise to levels where satisfactory speech

intelligibility is obtained.7

 These criteria were

devised through the use of extensive interviews

with personnel in offices, factories, and public

places along with simultaneously measured

octave–band sound levels. The interviews

consistently showed that people rate noise as

troublesome when its speech interference level

is high enough to make voice communication

difficult. The recommended space

classification and suggested noise criteria range

for steady background noise heard in various

indoor occupied activity areas are shown in

Table 1.

The FAA has hearing requirements for its ATC

specialists which are verified annually by

pure–tone, air conduction audiometric

examinations. The controller must have hearing

levels at 500, 1000, and 2000 Hz that do not

exceed 25 dB in their worse ear or 20 dB in

their better ear. If the employee is unable to

meet the requirements for at least one ear, then

they are reviewed by the FAA medical staff on

a case–by–case basis.
Page 6 Health Hazard Evaluation Report No. 96–0184

RESULTS

Noise Evaluation

The headset receiver is made by Plantronics for

AT&T and is sold as model KS22915–L7. A

belt–worn module has an on/off switch and

cabling that is plugged into the controller’s

console. A wire runs from the belt–worn

module to a microphone/receiver module that

fits over the ear. The module is symmetrical

and may be worn over the right or left ear. A

microphone tube extends from the module so

that its opening may be placed just to the side of

the mouth. There is a nubbin on the bottom of

the module that will accommodate a length of

#13 Tygon® tubing attached to an ear tip that is

placed in the ear canal. The module is worn

over the ear. There are six sizes of ear tips

which are labeled "1" as the smallest and "6" as

the largest. The olive–shaped ear tips resemble

a premolded, no flange earplug with a hole in

the center through which the tubing passes. 

There is no volume control for this system. The

signal level reaching the ear is regulated by the

controller’s console. The unit has an operating

impedance of 600 Z. This makes calibrating

and describing the unit’s input signal levels

simpler because 1 volt at 600 Z equals 0 dB

VU. As seen in Figure 2, the unit has a very

wide frequency response from 100 to 5000 Hz.

Its acoustic output at –20 dB VU peaks at 96 dB

SPL at 3500 Hz. The curve is smooth, showing

only the resonance characteristics of the tubing

connecting the receiver to the ear tip. At 0 dB

VU (1 volt rms @ 600 Z), the unit produces a

peak sound level of 106 dB SPL at 3500 Hz.

The unit is capable of handling input voltages

beyond 25 dB VU, but its output is limited to

116 dB SPL. The free–field equivalent

A–weighted sound level to 116 dB SPL

measured in the ear simulator is 104 dB(A).

Thus, it is possible for the AT&T KS22915–L7

to produce sound levels that are hazardous to

hearing. Current NIOSH recommendations are

that exposure to sound levels of 104 dB(A) be

limited to 6 minutes or less over an entire

8–hour work period.

In order to limit the output of the receiver

module so that it would not produce high–level

sound, an electronic compression system was

introduced. The unit, the Personal Hearing

Protector model 1 (PHP unit), was

manufactured for the FAA to be used

specifically at the Bradley facility. The PHP

unit has balanced 600–Z input and output

impedances and is described as providing

output limiting so that signals cannot exceed a

set amount. The PHP units observed by NIOSH

investigators were set to limit the output to –14

dB VU or equivalent to a diffuse field level of

80 dB(A). The output levels captured on tape

the day of the sampling were generally low

enough not to be considered as hearing hazards

with the PHP unit in or out of the system.

Recordings with the PHP unit out of the system

provided maximum equivalent diffuse sound

field levels of 84 dB(A), while with the PHP

unit in operation, the maximum equivalent

diffuse sound field level was 80 dB(A),

consistent with the PHP setting.

The signal for which there were the most

complaints was referred to as “tones” by the

FAA personnel. These tones are the

consequence of feedback caused by

phase–locking the communication system and

the subsequent oscillation at the frequency of

highest output. The tones were reported as

occurring most often when a microphone switch

was left open by a pilot who was also receiving

a message from the flight controller. The tones

could also be generated when the controllers

were talking via telephone lines with controllers

in other facilities. Acoustic phase–locking must

occur for acoustic feedback to be generated.

None of the simulated tones generated during

the site visit were intense enough to be

hazardous.

The PHP unit is described as a compressor. If

such were the case, upon testing it would show
Health Hazard Evaluation Report No. 96–0184 Page 7

unity gain (output equals input) until it reached

the compression point after which there would

be no increase in output level from further

increase in input level. With the unit set to –14

dB VU, the PHP unit should have shown unity

gain up to –14 dB VU and then should have

shown no more increased output as the input

signal was increased from –14 dB VU to +20

dB VU. Figure 3 shows input/output curves for

the PHP unit with no compression (0 dB VU),

with moderate compression set to –14 dB VU,

and with maximum compression (–20 dB VU).

The I/O curves depict a device that certainly is

a compressor; above the compression knee it

appears to have a 10:1 compression ratio.

Below the knee the unit shows unity gain.

However, the PHP unit tested had a noise floor

of about –33 to –35 dB VU depending upon the

compression setting. The result of the noise

floor is to restrict the usable dynamic range of

the PHP unit.

 

The octave–band sound levels in the

controllers’ work area were made during the

morning of October 9, 1996. The sound energy

at each octave band center frequency was

integrated over a 1–minute period during

normal air traffic control activities. The results

are shown in Figure 4. The average (Leq) octave

band levels ranged from 52 to 71 dB, with the

greatest energy measured at 125 Hz. When the

sound levels in the work area are compared to

the balanced noise criteria, the controllers’

sound environment is near the NCB–60 criteria

which has been designated as meeting the

sound requirements for work shops and garages

(Table 1).

 

Medical Evaluation

Both the day shift and afternoon shift at the

Bradley TRACON facility were staffed by eight

ATC specialists and one supervisor. NIOSH

investigators were able to interview nine of a

possible 16 controllers who volunteered to

discuss their experiences with any tone

incidents. All interviewed employees had at

least one occurrence with a tone incident; most

had been severe enough to warrant a

completion of a Federal Employee’s Notice of

Traumatic Injury and Claim for Continuation of

Pay/Compensation (Form CA–1). Several of

the reported exposures caused pain and ringing

(tinnitus) in the employees’ affected ear for

hours in some cases and up to three days in

other cases. All but one employee reported that

they were informed by their physician that no

hearing damage had occurred. However, one

ATC specialist notes that he wears a hearing aid

as a result of a tone incident.

The interviewed employees were not convinced

that the noise compression circuits (black

boxes) were an optimal fix for the tone problem.

They reported that the black box would lower

the intensity of the communications, but that

sometimes the noise compression was too

much. They would be unable to hear radios

clearly and would have to request pilots to

repeat their radio traffic. In some instances, the

ATC specialists reported that the black box

would be by–passed to get around these poor

listening conditions. Of the controllers who

responded to a question about the plastic,

olive–shaped earpiece of the radio headset, over

80% said that it was uncomfortable or only

tolerable. However, when the plastic earpiece

was replaced with a foam earpiece (ACS

Contour Lx Ear Tiplet, Model 0008–LX–00),

the ATC specialists found it unbearable and

they went back to the olive–shaped earpiece.

The management at Bradley Airfield supplied

NIOSH investigators with data which they had

collected that documented any loud tone

exposures for the calendar years 1993–1995. A

total of 76 incidents were included in the

information. Over one–half of the incidents

included a CA–1 form that the employee

completed in conjunction with the exposure.

However, the OSHA logs for these same years

had a total of 35 reported occurrences for loud

tones in one of the ATC specialists’ ears. Three
Page 8 Health Hazard Evaluation Report No. 96–0184

of the 35 notations resulted in a lost time case

injury.

During the interviews, the employees reported

that they were required to receive an annual

physical examination near their birthday to

maintain their eligibility for employment as an

ATC specialist. Audiometric testing is included

in the examination and the ATC specialists

must meet FAA hearing requirements. Many of

the employees reported that even though they

had been regularly tested, they were not given

copies of the hearing tests or a detailed

explanation of their hearing ability and how it

had possibly changed over the years of testing.

NIOSH investigators requested audiometric

data from the FAA for the ATC specialists at

BDL. A total of 57 records were forwarded to

NIOSH for analysis. These records included

the last three audiometric examinations that the

employees had received during the years

1993–1996. Only four individuals who were

noted on the OSHA Log of Federal

Occupational Injuries and Illnesses did not have

audiometric data in the medical records.

Because the headset used by the ATC

specialists had an earpiece in only one ear, it

was speculated that if the tone incidents were a

permanent hazard to hearing, then the damage

would be seen more in one ear when compared

to the individual’s other ear. Thus, the hearing

data for the last recorded examination were

classified as better ear or worse ear before they

were analyzed. During this classification, six

records were removed from the analysis

because of irregularities in the test results. One

physician who administered audiometric

examinations for the FAA recorded hearing data

down to single numbers (e.g., 1, 7, 16) rather

than the routine practice of recording zeros and

fives (e.g., 5, 20, 25) as is practiced in

audiometry. These irregularities brought the

validity of the data into question and they were

therefore removed from the group that was

statistically analyzed.

Two ATC specialists had audiometric

examinations where their worse ear exceeded

the American Medical Association’s low fence

average of 25 dB at the test frequencies 500,

1000, 2000, and 3000 Hz that is calculated to

determine hearing impairment.8 In order to

determine if this finding was a trend for the

population of controllers at Bradley Airfield, the

last audiometric examinations for the 51 ATC

specialists were separated into the better and

worse ear simply by adding the total hearing

level (HL) values for the left and right ears. The

ear with the highest total was classified as the

worse ear. The mean HL values for the

employees’ better and worse ears are plotted in

Figure 5 for the pure–tone frequencies of 500,

1000, 2000, 3000, and 4000 Hz. The mean

values are within 5 dB of each other when

comparing better and worse. The values are

also less than the lower fence of hearing

impairment (25 dB). For an additional analysis,

the hearing data were further limited to male

employees because only four female ATC

specialists were included in the audiometric

testing. The 47 males had a mean age of 39.4

years (s.d. = 4.2 yrs.). Thus, the average

hearing levels for these employees were directly

compared to the age–effect data in the

American National Standard S3.44–1996 for

40 year old males from an unscreened

population in an industrialized society (Annex

B).9

 The 10th, 50th, and 90th fractiles for the

population and the mean FAA data are graphed

in Figure 6. The hearing ability of the ATC

specialists is very similar to the 50th fractile

(median) comparison population that has no

occupational noise exposure.

The audiometric data were also reviewed from

a hearing conservation program effectiveness

perspective using the ANSI S12.13 percent

better or worse sequential (%BW).10 This

metric uses the percent of the population which

shows a 15–dB shift either toward the better

hearing or the worse hearing at any test

frequency in either ear between two sequential

audiograms. In the audiograms that covered
Health Hazard Evaluation Report No. 96–0184 Page 9

1993–1994, 7.7% exceeded the %BW; for

1994–1995, 15.7% exceeded the %BW; and for

1995–1996, 18.4% of the tests exceeded the

%BW. Even though this metric is increasing

instead of decreasing over the three years that

were examined, all three of the comparisons fall

within the acceptable criterion range of 25% or

less. An indication that the audiograms may not

have been as accurate as possible is the number

of audiograms that had the same HL value for

all of the tested frequencies in both ears. Over

the 153 audiometric examinations reviewed by

NIOSH investigators, a total of 30 tests (19.6%)

had identical hearing levels at the 10 test

frequencies.

DISCUSSION

The results of the noise analyses show that it is

very possible that a feedback signal (tone) could

drive a receiver module to its maximum output,

giving the wearer a short blast at 116 dB SPL

[equivalent field level of 104 dB (A)]. In order

for this to happen, the console would need to be

set at full volume, there could be no

compression unit in the line, and the feedback

signal would have to originate from an

environment where both a microphone and a

loudspeaker were close enough to each other to

start the feedback oscillations. In this case, the

receiver module would not be in the feedback

loop, rather it would deliver the monitored

feedback signal to the wearer. The Personal

Hearing Protector (PHP) unit that the FAA has

installed at the workstations, however, is

effective at controlling the high intensity

feedback signal down to a safe listening level.

Once the signal is above the noise floor of the

unit, the PHP provides unity gain up to the

compression set level and then a 10:1

compression ratio over the remainder of the

dynamic range of the communication systems.

The present setting of the PHP unit to –14 dB

VU provides too much compression. At –14 dB

VU, the PHP unit is limiting output sounds to

80 dB(A) or less. The background noise level

of the PHP unit is around –32 dBVU; the noise

is always present and is audible. The speech

that the controllers need to hear must have an

equivalent diffuse sound field level of between

62 and 80 dB(A). They complain that when the

PHP unit is used, the level of signal they need to

hear is too soft and that there is too much

background noise.

The audiometric test data for the group of ATC

specialists assigned to Bradley Airfield indicate

that they have not been exposed to noise of a

sufficient level and duration to cause

occupational hearing loss. The difference

between the better and worse ear for the 51

controllers is negligible and the group

compares to an unscreened population who

have not been exposed to occupational noise.

For the two individual ATC specialists who

exceeded the AMA lower fence of 25 dB, one

appears to have a loss more indicative of

conductive hearing problems rather than a

sensorineural loss. The other individual does

exhibit a hearing loss pattern that is consistent

with noise exposure. However, it is impossible

to ascertain the exact cause of the loss from the

limited data obtained in this evaluation.

The audiometric tests do point to a problem

with the consistency and validity of the data

which can impact the usefulness of the program.

The increase in the percent of people who have

excessive variability in their annual hearing

tests, the hearing tests that show no differences

in HL over all test frequencies, and the

recording of data in a manner that is not

consistent with good audiometric practices are

examples of a medical test program that needs

re–evaluation.

The ambient noise levels in the controllers’

work space are high enough to interfere with

communications in the area. The NCB–60
Page 10 Health Hazard Evaluation Report No. 96–0184

criterion is the maximum level recommended in

areas where speech or telephone

communication is necessary. This less than

optimum listening environment is coupled with

the ATC specialist’s headset that also limits the

communication signal. Although the headset

receiver has a frequency range from 100 to

5000 Hz, the signals provided to it by the radio

and telephone communication systems are

limited to a narrower range of 300 to 3000 Hz.

This frequency range was determined to be the

optimum range for speech understanding in the

late 1920's when band widths were being set for

telephone systems. Speech passed through this

narrow frequency response range does not

sound natural, is of low fidelity, and is difficult

to understand in the presence of background

noise.

CONCLUSIONS

Feedback signals, or tones, generated by the

communications system at the Bradley Airfield

are capable of reaching levels of 116 dB SPL at

the ear of the ATC specialist which equates to

a free–field noise level of 104 dB. The NIOSH

REL limits worker exposure to this noise level

to 6 minutes or less during the workshift. The

Model 1 Personal Hearing Protector that has

been developed for this facility is capable of

reducing the feedback signal to a safe listening

level. However, too much compression has

been set on the PHP units by the FAA which

causes the communication signals to be too soft

to be heard over the background noise of the

units and the work area. Several controllers

reported that the PHP units are bypassed

because of this.

Analysis of the hearing examinations of the

controllers assigned to Bradley Airfield does

not indicate that permanent hearing damage has

been inflicted upon this group of employees as

a result of occupational noise exposure. The

analysis of the output function of the headset

receivers used by the ATC specialists does

show a low fidelity characteristic that, coupled

with the moderately high ambient background

noise measurements made in the work space,

leads to problems in understanding speech

signals fed through them. Finally, the review of

the audiometric data revealed some deficiencies

in the testing program that reflect on the validity

of the hearing tests given to the controllers.

RECOMMENDATIONS

The results of the evaluation of the ATC

specialists assigned to Bradley Airfield show

that a health hazard to controllers’ hearing does

not exist for the current employees. There were,

however, some situations discovered during the

evaluation that can be changed to improve the

working conditions and the medical testing

program. The following recommendations are

offered to the FAA to alleviate the problems

uncovered during the NIOSH evaluation at this

facility.

1. At –14 dB VU, the PHP unit is limiting

output sounds to 80 dB(A) or less. The speech

that the controllers need to hear must have an

equivalent diffuse sound field level of between

62 and 80 dB(A). They complain that when the

PHP unit is used, the level of signal they need to

hear is too soft and that there is too much

background noise. Acceptance of the PHP units

could be improved by raising the setting to –9

dB VU. This will still provide a safe setting,

would make the signal sound louder, and would

increase the speech to noise ratio so that speech

understanding could be enhanced. 

2. The PHP unit is only one type of limiting

circuitry. A second type of limiter is the zenier

diode. The zenier diode can be placed in the

receiver module in the line going to the receiver

and can be selected to peak clip any line voltage

above a selected level. A zenier diode is

immediate in response, costs little, does not

require power, and does not raise the noise floor

of the system. Zenier diodes are used in other
Health Hazard Evaluation Report No. 96–0184 Page 11

communication systems sold by AT&T and

Plantonics and the FAA specifications could be

written to incorporate them.

3. The narrow band width of the speech signal

is also part of the problem. In order to

overcome the narrow band width, the

controllers increase the intensity of the signal.

When they are protected from high signal levels

by the PHP unit, they complain because they

can’t make the signal loud enough to be clearly

heard. If controllers were provided a system

that employs the full spectrum of speech, from

at least 100 to 6000 Hz, they would not be so

concerned with making the speech louder. As

changes in the communication systems in use at

Bradley Airfield are made, equipment that

meets this wider bandwidth specification should

be sought by the FAA.

4. Controls to reduce the ambient noise levels

in the controllers’ work area should be pursued.

The octave–band noise data collected at the

facility seem to show that voices add a great

deal to the background noise. The use of

barriers or partitions between work stations may

reduce the amount of background conversations

that interfere with the controllers’ ability to hear

the radio and telephone signals. Also, the

addition of acoustical materials on hard surfaces

in the room should reduce the noise reflecting

off of these surfaces which would lower the

overall background noise.

5. The present headset receiver unit is coupled

to the controller’s ear canal by an ear tip that

comes in six sizes. Most of these sizes do not

exactly fit the controllers’ ears and so they must

use the best of the selection. A custom earmold

can be coupled to the receiver unit as well. The

custom earmold would provide the advantages

of sealing the listening ear from outside noises,

such as speech from other controllers, and

delivering a signal that is clearer and more

stable than is now possible.

6. The audiometric tests furnished to NIOSH

for analysis indicate that the hearing test

program lacks consistency between the

providers of the audiometric examination

service to the FAA. Also, many of the hearing

test results were of questionable accuracy

because the same hearing levels were reported

at all test frequencies or hearing level values

were not recorded according to standard

audiometric procedures. The FAA should

follow professional guidelines established to

ensure that accurate and valid hearing tests are

obtained during the annual medical examination

given to the ATC specialists.11

7. The FAA should continue the practice of

logging all feedback (tone) incidents that the

controllers experience. This generates a written

record of the problem that can be forwarded to

management personnel responsible for the

communication system and show how any

corrections to the system and equipment affect

future noise exposures to the employees.

REFERENCES

1. Ward WD [1986]. Anatomy &

physiology of the ear: normal and damaged

hearing. Chapter 5. In: Berger EH, Ward WD,

Morrill JC, Royster LH, eds. Noise & hearing

conservation manual. 4th ed. Akron, OH:

American Industrial Hygiene Association, pp

177–195.

2. Suter AH [1978]. The ability of mildly

hearing–impaired individuals to discriminate

speech in noise. Washington, D.C.: U.S.

Environmental Protection Agency, Joint

EPA/USAF study, EPA 550/9–78–100,

AMRL–TR–78–4.

3. Code of Federal Regulations [1992].

OSHA. 29 CFR 1910.95. Washington, DC:

U.S. Government Printing Office, Federal

Register.
Page 12 Health Hazard Evaluation Report No. 96–0184

4. NIOSH [1972]. Criteria for a

recommended standard: occupational exposure

to noise. Cincinnati, OH: U.S. Department of

Health, Education, and Welfare, Health

Services and Mental Health Administration,

National Institute for Occupational Safety and

Health, DHEW (NIOSH) Publication No.

73–11001.

5. Niemeier RW [1995]. Memorandum of

April 13, 1995, from R.W. Niemeier, Division

of Standards Development and Technology

Transfer, to NIOSH Division Directors,

National Institute for Occupational Safety and

Health, Centers for Disease Control and

Prevention, Public Health Service, U.S.

Department of Health and Human Services.

6. ACGIH [1994]. 1994–1995 threshold limit

values for chemical substances and physical

agents and biological exposure indices.

Cincinnati, OH: American Conference of

Governmental Industrial Hygienists.

7. Beranek LL [1988]. Criteria for noise and

vibration in communities, buildings, and

vehicles. Chapter 18. In: Beranek LL, ed.

Noise and vibration control. Revised edition.

Cambridge, MA: Institute of Noise Control

Engineering, pp 554–623.

8. Gates GA [1981]. Guide for the

evaluation of hearing handicap. Chapter 5. In:

Gates GA, ed. Clinical auditory evaluation.

Rochester, MN: American Academy of

Otolaryngology – Head and Neck Surgery, pp

130–137.

9. ANSI [1996]. American national

standard: determination of occupational noise

exposure and estimation of noise–induced

hearing impairment. New York, NY: American

National Standards Institute, ANSI S3.44–1996.

10. ANSI [1991]. Draft American national

standard: evaluating the effectiveness of hearing

conservation programs. New York, NY:

American National Standards Institute, Draft

ANSI S12.13–1991.

11. Suter AH [1993]. Hearing conservation

manual. Third Edition. Milwaukee, WI:

Council for Accreditation in Occupational

Hearing Conservation.
Health Hazard Evaluation Report No. 96–0184 Page 13

TABLE 1

Recommended Space Usage for Balance Noise Criteria Range

in Occupied Indoor Areas

Bradley Airfield

Windsor Locks, Connecticut

HETA 96–0184

Type of Space and Acoustical Requirements NCB Curve

Concert halls, opera houses, and recital halls 10 – 15

Large auditoriums, large drama theaters, and large

churches Not to exceed 20

Small auditoriums, small theaters, small churches,

music rehearsal rooms, large meeting and

conference rooms, or executive offices Not to exceed 30

Bedrooms, hospitals, residences, apartments, hotels 25 – 40

Private or semiprivate offices, small conference rooms,

classrooms, libraries 30 – 40

Large offices, reception areas, retail shops and stores,

cafeterias, restaurants 35 – 45

Lobbies, laboratory work spaces, drafting and

engineering rooms, general secretarial areas 40 – 50

Light maintenance shops, industrial–plant control

rooms, office and computer equipment rooms,

kitchens, and laundries 45 – 55

Shops, garages 50 – 60*

Work spaces where speech or telephone

communication is not required 55 – 70

* Levels above NCB–60 are not recommended for any office or communication situation.
Page 14 Health Hazard Evaluation Report No. 96–0184

AT&T KS22915-L7

R eceiver

Earhook

M icrophone

tube

Ear tip

tube connect

nubbin

#13 Tygon

tubing

Ear tip

To belt-worn

m odule

Figure 1

Diagram of AT&T microphone/receiver ear piece and tubing/ear tip assembly.

Bradley Airfield

Windsor Locks, Connecticut

HETA 96–0184
Health Hazard Evaluation Report No. 96–0184 Page 15

FAA Headset Measured on KEMAR

Swept-Sine Response #3 Eartip

Frequency (Hz)

100 1000 10000

Swept-Sine Response (dB SPL)

0

20

40

60

80

100

120

140

Input Level

dBV re 1.0 V

Input Level

Line VRMS

-20.0

-16.5

-6.9

+28.0

0.10

0.15

0.45

25.0

Figure 2

Output characteristics of the AT&T microphone/receiver module coupled to the artificial ear.

input voltages at 600 Z, sound pressure levels shown in dB SPL at plane of artificial eardrum. All 

Bradley Airfield

Windsor Locks, Connecticut

HETA 96–0184
Page 16 Health Hazard Evaluation Report No. 96–0184

Input Level (dB VU)

Output Level (dB VU)

-50

-40

-30

-20

-10

0

10

-20 dB VU

-14 dB VU

0 dB VU

Compression

Point

-40 -30 -20 -10 0 10 20

No Compression

Figure 3

Input/Output (I/O) functions for AT&T receiver. Shown are curves for

PHP compressor unit settings of –20, –14 (present setting) and 0 dB VU.

Also shown is function for a non–compressed system with linear gain.

Bradley Airfield

Windsor Locks, Connecticut

HETA 96–0184
Delivering on the Nation’s promise:

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