Analysis of Nonstandard Noise Dosimeter Microphone 

Positions 

David C. Byrne1 and Efrem R. Reeves2 

1Robert A. Taft Laboratories, National Institute for Occupational Safety and Health, Cincinnati, Ohio 

2United States Army Aeromedical Research Laboratory, Fort Rucker, Alabama 

This study was conducted as part of a project involving 

the evaluation of a new type of noise exposure monitoring 

paradigm. Laboratory tests were conducted to assess how 

“nonstandard” dosimeter microphones and microphone positions measured noise levels under different acoustical conditions (i.e., diffuse field and direct field). The data presented in 

this article reflect measurement differences due to microphone 

position and mounting/supporting structure only and are not 

an evaluation of any particular complete dosimeter system. 

To varying degrees, the results obtained with the dosimeter 

microphones used in this study differed from the reference 

results obtained in the unperturbed (subject absent) sound field 

with a precision (suitable for use in an ANSI Type 1 sound level 

meter) 1

/2 -inch (12.7 mm) measurement microphone. Effects of 

dosimeter microphone placement in a diffuse field were found 

to be minor for most of the test microphones/locations, while 

direct field microphone placement effects were found to be quite 

large depending on the microphone position and supporting 

structure, sound source location, and noise spectrum. 

Keywords dosimeter, microphone, noise 

Address correspondence to: David C. Byrne, Robert A. Taft 

Laboratories, NIOSH, 4676 Columbia Parkway, Mail Stop C-27, 

Cincinnati, OH 45226; e-mail: dbyrne@cdc.gov. 

INTRODUCTION 

The technique for measuring a worker’s daily noise 

dose with a person-worn device was developed in the 

1960s. Until this time most noise exposure measurements 

were obtained with a hand-held sound level meter, using 

a stopwatch to quantify the exposure duration. Microphone 

placement was one issue to resolve when determining the 

accuracy of newly developed personal noise dosimeters. An 

obvious disadvantage of personal noise dosimetry is that the 

measurement microphone may be shielded from a directional 

noise source by the head/body of the wearer. This “shadow” or 

“baffle” effect may cause a measurement inaccuracy of several 

decibels under certain conditions, for example, where there are 

few reflective surfaces. 

A government report written by Muldoon(1) in 1973 described the results of an investigation to quantify the magnitudes of response variations for four microphone placements 

(center of the chest, left breast pocket, between the shirt-collar 

tabs, and on top of the left shoulder) and two microphone orientations (pointing vertically upward and pointing horizontally 

forward at the sound source) on an anthropometric dummy 

in a free field. Muldoon’s general conclusions stated that for 

azimuths where the body is not acting like a barrier between 

the sound source and the microphone, there is an increase in 

the lower octave bands and a decrease in the higher bands. 

All octave bands showed a decrease when the head or body 

acts as a barrier, with the greatest reduction occurring at 4000 

Hz (which was the highest octave band measured). Muldoon 

went on to suggest that locating the dosimeter microphone on 

top of the shoulder (oriented parallel to the body) would yield 

data most comparable with that obtained with a sound level 

meter. 

The U.S. Mine Safety and Health Administration (MSHA) 

was actively involved in the early use of personal noise 

dosimeters and in 1978 issued revised health regulations (30 

CFR Parts 70 and 71) to allow the use of dosimeters in 

underground and surface coal mines.(2) Based on previous 

research and internal reports, the revised regulation stated 

that noise exposure measurements shall be made with the 

microphone located at the top of the shoulder, midway between 

the neck and the end of the shoulder with the microphone 

pointing in a vertical upward direction. This description was 

supplemented with a line drawing illustrating the proper 

microphone placement on a miner. 

The two most recent research studies involving dosimeter microphone placement corroborated the selection of the 

MSHA top-of-shoulder position.(3,4) Part of the work reported 

in 1996 was based on microphone placement data collected 

14 years earlier by MSHA.(5) The 1978 MSHA regulation 

was based on research with noise dosimeters having 1-inch 

(25 mm) microphones, and essentially, the same work was 

replicated by MSHA in 1982 using newly available 1

/2­

inch (12.7 mm) microphones. The original shoulder-mounted 

microphone placement data were collected in 45◦ increments, 
and similar data were collected at 10◦ azimuthal angles several 

years later.(6) In general, better resolution was observed with 

the smaller angular step size, while differences between the 

1

/2-inch and 1-inch microphone diameters did not appear to 

be significant. MSHA’s 1995 study was a field comparison 

of sound level meters and noise dosimeters with shouldermounted microphones.(3) They determined that there was no 

practical difference between levels measured with sound level 

meters and dosimeters as long as instruments meeting ANSI 

S1.4 Type 2 tolerances were used. 

The Occupational Safety and Health Administration 

(OSHA) Occupational Noise Exposure Standard and Hearing 

Conservation Amendment (29 CFR 1910.95) does not specify 

a mandatory microphone location.(7) Industrial hygiene professionals have generally adopted the MSHA top-of-shoulder 

position, and its use has been recommended by hearing 

conservation experts for more than 20 years.(8−11) A common 

caveat regarding the top-of-shoulder position is that the 

microphone should be placed on the side anticipated to receive 

the most noise exposure. The American National Standard 

Measurement of Occupational Noise Exposure (ANSI S12.19­

1996, R2001) also prescribes the top-of-shoulder location (or 

as near as is feasible) and indicates that when an employee 

is consistently exposed to noise from one side, that shoulder 

should be chosen to wear the microphone.(12) 

The current MSHA Health Standard for Occupational 

Noise Exposure was published on September 13, 1999 (30 

CFR Parts 56, 57, 62, 70, and 71), and enforcement of 

the new rule began in October 2000.(13) This rule closely 

resembles the 1983 OSHA regulation and replaced the different 

standards for occupational noise exposure in coal mines and in 

metal/nonmetal mines with a single, new standard applicable to 

all mines. Section 62.110 in the 1999 MSHA regulation states 

that mine operators must establish a system of monitoring 

to assess and evaluate miners’ noise exposures; however, the 

new rule is flexible to the extent that it does not prescribe 

precisely how the mine operator must accomplish these goals. 

As such, the exact monitoring methodology is not specified, 

and innovation and improvements in technology would be 

considered. 

A start-up company (doseBusters USA, State College, 

Pa.) developed a monitoring protocol that purportedly solves 

many of the longstanding noise monitoring and hearing 

protection problems encountered in typical hearing conservation programs.(14,15) Their methodology/concept includes a 

patented device referred to as an Exposure Smart Protector 

(ESP). The ESP device consists of traditional hearing protection (i.e., earplugs or earmuffs) integrated with a personal 

noise dosimeter. The dosimeter’s microphone is positioned to 

measure sound levels on the “protected” side of the hearing 

protector. With this type of arrangement, the sound level 

reaching the microphone at any given time will depend on 

exactly how the device is being worn. A user has the option of 

either primary (ears protected) or secondary (ears unprotected) 

wearing positions for the ESP. The measurement methodology 

is based on the concept of using both protected and unprotected 

noise exposures to define a miner’s daily noise dose; therefore, 

the ESP is designed to measure and document a worker’s actual 

at-the-ear daily noise exposure. 

The ESP manufacturer approached MSHA and requested 

that the use of the ESP be permitted under the current noise 

regulation. While MSHA wants to promote practices to 

prevent noise-induced hearing loss, it recognizes that ESP 

takes a different approach to determining a miner’s noise 

dose. Accordingly, MSHA requested that NIOSH conduct a 

scientific study to address the relevant research questions. 

The study described herein used human subjects to examine 

simultaneously eight microphone positions: the “standard” 

left/right top-of-shoulder location and three left/right pairs of 

“nonstandard” positions as defined by the ESP manufacturer. 

Results of the second part of this project, which was an 

evaluation of the same microphones to accurately measure the 

“protected” noise exposure (i.e., as measured in the ear canal) 

for the same human subjects, will be published separately. 

METHODS 

Subjects 

Subjects (11 male, 2 female; age range 30–59) were recruited from employees within the NIOSH Pittsburgh campus. 

The test protocol was reviewed and approved by the NIOSH 

Human Subjects Review Board. Participants were provided 

with the required assurances of confidentiality, and a standard 

Informed Consent form was signed prior to conducting any 

testing. The subjects were outfitted with typical miners’ 

coveralls and a hard hat. 

Acoustical Test Environments 

A room designed for measuring the real-ear attenuation of 

hearing protectors was used for diffuse sound field testing. 

The walls and ceiling are lined with an acoustically reflective 

surface laminated to 3/4-inch (19 mm) high-density particle 

board. This room meets the requirements for sound field 

uniformity, directionality, reverberation time, and ambient 

noise levels as specified in ANSI S12.6-1997 (R2002).(16) The 

center-of-head location (i.e., midpoint of an imaginary line 

drawn through the subject’s ear canals) was the geometric 

center of the enclosure and was defined as the reference 

position for measurements of the undisturbed sound field (with 

no experimenter or subject present); all subsequent testing was 

referred to measurements at this point. 

A small anechoic chamber was used for direct field testing. 

The construction of this facility allows for testing down to 

approximately 100 Hz, with a usable area between wedge tips 

of 8.5 ft (2.6 m) long by 4.3 ft (1.3 m) wide, and a height of 7.5 ft 

(2.3 m) from the suspended floor to the overhead wedges. The 

subject was located in the far-field of the noise source, which 

was defined as a distance of at least one-quarter wavelength of 

the center frequency of the lowest frequency band of interest. 

With the subject centered along the width of the room, there 

wasa distance of 1.8 ft (0.55 m) from the test subject’s ear 
to the closest wedge tips. Therefore, the lowest frequency that 

can be accurately measured with a subject present would have 

a wavelength equal to 4 × 1.8 = 7.2 ft (4 × 0.55 = 2.2 m), 

which corresponds to a frequency of 157 Hz. 

Test Microphones 

A 1

/2-inch measurement microphone (Type 4189; Bruel & 

Kjaer, Naerum, Denmark) was used as the reference microphone for the center-of-head measurements. This microphone 

is classified for use with a Precision or Type 1 measurement 

system according to the American National Standard Specification for Sound Level Meters (ANSI S1.4-1983, R2001).(17) 

The microphone was connected to a Bruel & Kjaer ZC 0026 

Input Stage and suspended via the microphone cable from the 

ceiling of the test chamber, such that the microphone was 

oriented vertically with the diaphragm pointing downward. 

Another identical 1

/2-inch measurement microphone was used 

as a continuous monitor of the sound field during testing. 

This microphone was also suspended by its cable from the 

ceiling and similarly oriented “upside down” (the purpose of 

this second microphone is described in the Instrumentation 

section below). 

Two noise dosimeter microphones (No. 056-963; Quest 

Technologies, Oconomowoc, Wis.) were used for the “standard” personal noise exposure measurement positions, enabling both left and right top-of-shoulder measurements to be 

obtained simultaneously. 

The “nonstandard” microphone positions were selected 

according to potential wearing positions specified by doseBusters USA for the ESP device. Two different versions of 

the ESP exist—an in-the-ear (earplug) and an over-the-ear 

(earmuff) system—both of which use a miniature electret 

condenser microphone cartridge (No. WM-60AY; Panasonic, 

Secaucus, N.J.). For the in-the-ear device, a small threaded 

earpiece encapsulates the microphone and serves as a holder for 

an insert-type eartip. Sound reaches the microphone through 

the hollow core of the threaded earpiece and certain styles 

of compatible eartips. Similar eartips are used in clinical 

audiometric testing as a combination sound delivery device 

and ambient noise attenuator when used with insert earphones. 

In the ESP application, the eartips provide hearing protection 

when the ESP earpiece is adequately inserted into the ear canal. 

In the earmuff version, the microphones are mounted inside the 

hard plastic shell of a typical noise-attenuating earmuff. 

Measurements reported in this article were obtained with 

the ESP microphones located in three possible “secondary” 

positions where the intent is to measure a worker’s unprotected 

noise exposure. Three pairs (a total of six) of ESP microphones were used, representing simultaneous left and right 

measurements of (1) the ESP microphones/earplugs draped 

around a subject’s neck (resting on the person’s chest), (2) the 

ESP microphones/earplugs hanging from a hard hat (near the 

wearer’s ears), and (3) hard hat-mounted ESP earmuffs with 

the muffs rotated up and away from the subject’s ears. Figure 

1 shows all microphone locations. 

FIGURE 1. Placement of test microphones as worn on the body. 

(a) Quest Technologies dosimeter microphones at conventional 

shoulder position. (b) ESP devices resting on subject’s chest— 

threaded earpiece microphone holders only—no noise-attenuating 

eartips attached. (c) ESP threaded earpiece microphone holders 

dangling or hanging from hard hat, without noise-attenuating 

eartips. (d) Identical ESP threaded earpieces mounted inside the 

shells of earmuffs (not visible in photo) 

Instrumentation 

An audio editing software program (Adobe Audition 1.0; 

Adobe Systems Inc.) was used to generate pink noise, as well 

as representative “surface,” “drill,” and “underground” noises; 

the last three noises are of the same spectrum types used in 

previous work done by MSHA.(4) Test signals were saved as 

WAV files (Windows PCM, 44,100 Hz sampling rate, 16-bit 

quantization, monaural) and played back through a computer 

sound card (Aardvark Direct Pro 24/96; Aardvark Computer 

Systems). For the diffuse field measurements, the output of the 

sound card was connected to a splitter/mixer (Rane SM26B; 

Mukilteo, Wash.) and routed to three loudspeakers (Electro 

Voice T251+; Burnsville, Minn.) in the reverberant enclosure 

(hearing protector test room). 

Each loudspeaker was oriented in a different plane for 

maximum sound dispersion throughout the enclosure. Separate 

channels of a 1500-watt power amplifier (Sherbourn; North 

Billerica, Mass.) were used to drive each of the three loudspeakers. In the anechoic test chamber, the four test signals 

were routed from the output of the computer sound card to the 

external input on a Bruel & Kjaer Type 4224 Sound Source. The 

sound source was placed on a stand and vertically positioned 

to be level with the center-of-head location. 

To avoid subjecting the human participants to very high 

noise levels, each noise signal was adjusted to produce an 

overall level of 90 dB SPL at the center-of-head position when 
FIGURE 2. Measurements of the test signals at the center-of-head location (no subject or experimenter present). The same spectrums were 

used for both the diffuse and direct sound fields 

played back in both of the test environments. When these 

adjustments are made, the pink and drill noise spectra are 

quite similar, and the surface and underground spectra closely 

approximate each other. Therefore, only the spectra for the 

pink noise and the underground noise (hereafter referred to 

as “machinery noise”) are used in this report. Representative 

measurement results of the test signals at the center-of-head 

location (no subject or experimenter present) for both sound 

fields are shown in Figure 2. 

Instead of using the actual dosimeter units for data acquisition, each of the test microphones was connected directly to 

a multichannel analyzer (LMS Pimento; LMS International; 

Leuven, Belgium) via an independent battery-operated power 

supply. The analyzer was controlled by a laptop computer, 

and was configured to measure 1/3-octave bands using a 15­

sec average time for each measurement. A separate 1

/2-inch 

microphone (Bruel & Kjaer Type 4189) was positioned a few 

feet up and away from the center-of-head location and was 

used to continuously monitor the sound fields to ensure that 

the noise generation system remained stable. A picture of 

the laptop computer screen while collecting data is shown in 

Figure 3. 

Data from each left/right pair of test microphones are 

contained in the top four windows, and readings from the 

center-of-head and monitor microphones are displayed in the 

lower left corner window. The overall level at the monitor 

microphone is also presented as a digital display in the lower 

right corner to allow the operator to quickly identify any signal 

generation problems during a test. The same instrumentation 

set-up was used for all data collection, regardless of whether a 

subject was being tested or center-of-head (i.e., subject absent) 

measurements were being conducted. Therefore, data from 

the center-of-head microphone channel was meaningless (and 

therefore discarded) while a subject was being tested, and data 

from all dosimeter microphone channels were not relevant 

when the center-of-head measurements were taken. 

Procedures 

A calibration check with a sound level calibrator (Type 

4231; Bruel & Kjaer) was performed on all 10 microphones 

before each test session. The manufacturer’s standard adapter 

(No. 56-989) was used to accommodate the Quest microphones, and a small machined aluminum adaptor was designed 

specifically to fit the ESP earpieces (microphone holders). 

A Quest microphone was mounted vertically on the midpoint of each subject’s shoulder, with the grid pointing upward. 

To replicate the three possible secondary wearing positions, 

the ESP microphones were positioned as follows: (1) the 

wiring harness for an earplug-type set of microphones was 

draped around the back of the subject’s neck, with the 

microphones/earplugs positioned to rest evenly across the 

chest; (2) a second set of earplug-type microphones were 

affixed to the hard hat, and positioned to hang down on each 

side approximately 2 inches above the shoulder; (3) hard hatmounted earmuffs with integral microphones were rotated up 

and back so that the cushions did not touch the hard hat. For the 

earplug versions, the chest level ESP microphones were fitted 

with expandable polyvinyl foam inserts; preformed four-flange 

earplugs were attached to the earpieces suspended from the 

hard hat. These eartips (earplugs) were chosen as representative 

devices that would typically be attached to the microphone 

holders when an ESP system is being used. 

In the reverberant room (diffuse field), the subjects were 

seated on a small stool, with the head at the room’s 

center. In turn, each of the four test signals was played 

while a measurement was taken with the multichannel analyzer. Initially, the first few subjects were rotated horizontally with the same four measurements repeated at 45◦ 
FIGURE 3. Screenshot of laptop computer screen while collecting soundfield data. 

increments; however, no significant spatial variability in the 

diffuse sound field was found, and the rotation scheme was 

discontinued. 

Subjects were seated on the same swivel stool during 

testing in the anechoic chamber. Measurements were taken 

at eight different azimuthal angles (0◦, 45◦, 90◦, 135◦, 180◦ , 

225◦, 270◦, 315◦, and repeated at 360◦), as the subject was 

rotated clockwise in a horizontal circle as viewed from above. 

The 0◦ position and the duplicate measurement at 360◦ were 

defined as directly facing the sound source. The sound source 

remained stationary throughout all testing, with the center of 

the loudspeaker cone at the same height as the subject’s centerof-head location. 

Finally, a set of measurements were taken to compare the 

frequency response of each test microphone with the precision 

measurement microphone. These measurements were taken at 

the center-of-head location in the reverberant test chamber, 

with the subject absent. This comparison is important, since 

the initial hearing loss damage risk criteria was based primarily 

on an assessment of workplace noise levels as measured in a 

diffuse field at the worker’s center-of-head location but with 

the worker absent. 

RESULTS 

Measurement errors for each left/right pair of test microphones are shown in Figure 4. These graphs were 

generated by subtracting the precision microphone reading 

from the individual test microphone reading when each of 

the microphones was placed (one at a time) at the centerof-head position in the diffuse sound field (no subject or 

experimenter was present). The Quest dosimeter microphones 

deviated slightly from the “true” center-of-head levels, mostly 

in the high frequencies (Figure 4a). A resonant peak centered 

around 4000 Hz was observed with the ESP earplug-type microphones (Figure 4b,c). The muff-mounted ESP microphones 

differed the most from the precision microphone reading 

(Figure 4d). 

Tables I and II contain the mean A-weighted errors and standard deviations measured with the various test microphones 

worn by the human subjects, as compared with the precision 

1

/2-inch measurement microphone undisturbed center-of-head 

readings. Negative values indicate that the test microphone produced a lower reading than the precision reference microphone, 

while positive values indicate an erroneously high reading 
FIGURE 4. Individual test microphone errors (test microphone reading minus the precision microphone reading at the center-of-head position 

in the diffuse sound field with no subject or experimenter present). (a) Quest dosimeter microphones. (b) ESP microphones contained inside 

threaded plastic earpiece, with foam eartip attached. (c) ESP microphones contained inside threaded plastic earpiece, with four-flanged eartip 

attached. (d) ESP microphones contained inside threaded plastic earpiece and mounted inside earmuffs attached to a hard hat 

from the test microphone. These results reflect the individual line inside the box indicates the median value. Vertical lines 

microphone frequency response errors, the diffraction and (whiskers) are drawn from the edges of the box to encomresonance/antiresonance effects of the microphone mounting pass the 95th percentile (upper line) and the 5th percentile 

and supporting structures, and the body-worn microphone (lower line). 

placement effects. Figures 5 through 13 graphically display Figure 5 shows that in the diffuse field, the shoulderthe results in which box plots are used to give a better mounted dosimeter microphones tended to slightly underreport 

visualization of the data distribution. The lower boundary the actual noise level. Larger measurement errors occurred 

of each box represents the 25th percentile, and the upper with the pink noise spectrum, which contained more highboundary represents the 75th percentile. The vertical length frequency energy than the machinery noise spectrum. The ESP 

of the box represents the interquartile range, which means microphones dangling from the hardhat produced similar rethat 50% of all data points are within the box. The horizontal sults. The ESP microphones draped around the neck and resting 

TABLE I. Mean A-Weighted Errors and Standard Deviations—Pink Noise Spectrum 

Shoulder-Left Shoulder-Right Left-Chest Right-Chest Hanging-Left Hanging-Right Muff-Left Muff-Right 

Error SD Error SD Error SD Error SD Error SD Error SD Error SD Error SD 

Diffuse −1.0 0.4 −0.7 0.3 1.2 0.4 1.1 0.9 −1.0 1.0 −0.5 0.8 2.1 0.3 2.7 0.4 

0◦ 0.3 0.5 0.3 1.1 3.4 0.8 4.0 0.9 2.2 0.8 2.9 0.5 −3.6 0.4 −3.9 0.7 

45◦ 1.3 0.4 −5.2 1.2 3.8 0.8 2.3 1.7 3.4 0.8 0.0 1.3 −0.8 1.4 −0.7 0.8 

90◦ 2.5 0.3 −5.5 0.9 1.5 0.9 −2.3 1.0 3.8 0.6 −4.2 0.6 4.1 0.9 2.1 1.0 

135◦ 2.4 0.4 0.0 0.9 −3.7 0.8 −6.9 1.4 2.6 0.8 −5.7 1.0 6.5 0.8 5.3 0.6 

180◦ 1.1 0.5 2.1 0.4 −7.0 0.8 −7.6 1.0 −0.1 1.1 −0.1 2.0 5.5 0.7 6.3 0.6 

225◦ −2.2 1.3 3.1 0.4 −6.9 0.8 −3.4 0.9 −6.0 0.7 3.3 1.5 5.3 0.7 5.5 0.6 

270◦ −6.4 0.6 2.4 0.3 −1.8 0.9 2.1 0.8 −4.7 0.5 4.2 0.9 3.1 0.6 4.6 0.7 

315◦ −4.5 1.4 1.6 0.5 2.1 1.2 3.9 1.0 −1.6 1.2 4.0 0.8 0.0 0.8 −0.2 0.9 
TABLE II. Mean A-Weighted Errors and Standard Deviations—Machinery Noise Spectrum 

Shoulder-Left Shoulder-Right Left-Chest Right-Chest Hanging-Left Hanging-Right Muff-Left Muff-Right 

Error SD Error SD Error SD Error SD Error SD Error SD Error SD Error SD 

Diffuse −0.2 0.4 −0.2 0.4 1.1 0.6 1.0 0.5 −0.1 1.2 −0.1 0.7 3.1 0.5 4.1 0.4 

0◦ 0.8 0.7 0.1 1.3 4.5 0.9 3.8 0.6 2.8 0.8 2.8 0.6 −1.2 0.5 −1.3 0.7 

45◦ 2.3 0.4 −3.6 0.9 4.8 0.7 2.3 1.5 4.8 0.9 0.0 1.1 0.8 1.4 1.4 0.7 

90◦ 3.3 0.3 −4.3 0.4 2.5 1.0 −1.3 1.0 5.2 0.7 −3.0 0.4 5.5 0.8 2.8 0.8 

135◦ 3.2 0.4 0.5 0.9 −1.6 0.6 −5.4 1.2 4.0 1.0 −4.4 0.7 7.2 0.5 5.2 0.9 

180◦ 2.1 0.7 3.0 0.5 −5.4 0.8 −6.2 0.8 0.4 1.4 −0.6 2.0 6.4 0.5 6.8 0.5 

225◦ −1.7 1.2 3.5 0.4 −4.9 1.0 −1.9 0.6 −4.5 0.5 3.6 1.2 5.5 0.7 6.9 0.5 

270◦ −4.4 0.4 3.2 0.4 −0.4 0.7 2.2 0.6 −2.7 0.4 4.8 0.9 3.4 0.6 6.1 0.5 

315◦ −3.5 1.1 2.3 0.6 3.4 0.8 3.8 0.4 −0.7 0.9 4.4 0.7 1.5 0.8 1.9 0.8 

on the subject’s chest averaged about 1 dB(A) high, and the 2–4 dB(A) high. Readings from the muff-mounted ESP 

earmuff-mounted ESP microphone readings were consistently microphones were approximately 1–4 dB(A) low. 

2–4 dB(A) high. The largest ESP earmuff measurement errors Figures 6–13 reveal distinct differences in the microphone 

occurred with the machinery noise spectrum, where there was errors depending on the angle of the incident sound. The range 

predominantly low-frequency energy. of average errors for the shoulder-mounted microphones was 

Figure 6 displays the direct field results with the sub- −6.4 dB(A) [pink noise, 270◦] to +3.5 dB(A) [machinery 

ject directly facing the sound source (0◦ azimuth). The noise, 225◦]; average errors for the microphones resting on 

shoulder-mounted microphones read less than 1 dB(A) high, the subject’s chest ranged from −7.6 dB(A) [pink noise, 180◦] 

while the earplug-type ESP microphones were approximately to +4.8 dB(A) [machinery noise, 45◦]; average errors for 

FIGURE 5. Results obtained for the test microphones in the diffuse sound field. Overall A-weighted errors are shown for each test microphone 

while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 
FIGURE 6. Results obtained for the test microphones in the direct sound field at 0◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 

FIGURE 7. Results obtained for the test microphones in the direct sound field at 45◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 
FIGURE 8. Results obtained for the test microphones in the direct sound field at 90◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 

FIGURE 9. Results obtained for the test microphones in the direct sound field at 135◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 
FIGURE 10. Results obtained for the test microphones in the direct sound field at 180◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 

FIGURE 11. Results obtained for the test microphones in the direct sound field at 225◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 
FIGURE 12. Results obtained for the test microphones in the direct sound field at 270◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 

FIGURE 13. Results obtained for the test microphones in the direct sound field at 315◦. Overall A-weighted errors are shown for each test 

microphone while measuring pink and machinery noise spectrums, as compared with the precision microphone center-of-head measurement 
the hanging (from hard hat) microphones ranged from −6.0 

dB(A) [pink noise, 225◦] to +5.2 dB(A) [machinery noise, 

90◦]; and average errors for the muff-mounted microphones 

ranged from −3.9 dB(A) [pink noise, 0◦] to +7.2 dB(A) 

[machinery noise, 135◦]. 

DISCUSSION 

The frequency response of the Quest Technologies dosimeter microphones deviated only slightly from the precision 

1/2-inch measurement microphone used in this study. Hollowcore eartips (the polyvinyl foam or four-flanged inserts) 

attached to the ESP microphones caused a resonant peak 

FIGURE 14. Summary of direct field results indicating high (+2 

dB) and low (–2 dB) differences between the true center-of-head 

level and the test microphone readings, assuming the worker and 

sound source are stationary. Left-shoulder reading represents a 

single measurement, while the higher of the left/right ESP readings 

was used 

in the frequency response centered around 4000 Hz. The 

muff-mounted ESP microphones differed the most from the 

precision microphone reading due to fact that the microphone 

was mounted inside the earmuff. 

As seen in Figures 6–13, the results for certain pairs of direct 

field measurements (0◦ and 180◦; 45◦ and 315◦; 90◦ and 270◦; 

135◦ and 225◦) exhibit opposite trends due to the symmetry 

of measurements. As expected, the magnitude of the errors 

varied depending on the angle of incidence and type of noise 

(i.e., low-frequency vs. high-frequency content). This stands to 

reason when the position of the sound source and the rotation 

scheme are considered. 

As a validity check for these data, the left-shoulder results 

were compared with those published by Giardino and Seiler(4) 

in 1996. The diffuse field errors and the direct field errors 

from 0◦ through 180◦ were generally within 1 dB of the results 

obtained in the present study. Although the error patterns were 

similar, somewhat larger errors (absolute values of 1–2 dB 

greater) were observed in the present study than in the earlier 

MSHA data at the 225◦, 270◦, and 315◦ azimuths. 

A method to minimize the errors associated with microphone placement is built into the ESP manufacturer’s new 

noise exposure monitoring paradigm. One aspect of this new 

approach to noise monitoring is to always use both a leftside and a right-side microphone instead of a single left 

or right shoulder-mounted microphone. According to the 

manufacturer, the dose computation algorithm programmed 

into the ESP dosimeter unit incorporates the higher sound level 

received by either microphone at any given time. The intent 

is to limit the range of potential errors for the nonstandard 

measurement locations to “positive” values, which would 

provide a “conservative” estimate of a worker’s actual noise 

exposure in a directional sound field. This means that the ESP 

dosimeter reading possibly could be higher than a traditional 

dosimeter measurement taken with a single shoulder-mounted 

microphone. A higher reading would be obtained by the ESP 

device since the sound level at both microphones is measured 

simultaneously, and the higher reading is used to calculate the 

dose. 

Any number of plausible and/or typical work scenarios 

could be envisioned that either accentuate or minimize the 

effects of dosimeter microphone measurement errors. Assuming that the subject/worker spent an entire workshift facing 

exactly the same direction (i.e., both the sound source and 

the worker remained stationary) and using greater than 2 

dB(A) as the criterion for high/low readings, the chart in 

Figure 14 summarizes the direct field results. It is important 

to note that these results are for only one shoulder-mounted 

microphone (i.e., left shoulder), since only one dosimeter is 

used traditionally to monitor a worker’s dose. On the other 

hand, the ESP measurement paradigm calls for the higher of 

the two independent microphone readings to be used in the 

dose calculation. Therefore, the higher of the two microphone 

readings for each of the three secondary wearing positions was 

used in this analysis. Figure 14 highlights the possible high and 

low extremes that could be obtained. 
CONCLUSIONS 

Measurements obtained from the eight different test 

microphones at various body-worn positions were compared with measurements from the 1

/2-inch precision measurement microphone at the center-of-head position (subject absent). This demonstrated the frequency-dependent errors due to 

a combination of both frequency-dependent sensitivity differences between the test and reference microphones themselves, 

as well as body placement and microphone supporting structure (including diffraction and cavity resonance/antiresonance) 

effects, as compared with center-of-head readings obtained 

with the subject absent. In a diffuse field, overall dB(A) 

errors due to dosimeter microphone placement were found 

to be minor for most of the test microphones/locations. 

Conversely, direct-field microphone placement effects were 

found to be quite large depending on the microphone position 

and supporting structure, sound source location, and noise 

spectrum. 

Technological advancements such as the ESP dualmicrophone monitoring methodology may be able to minimize some of the potential problems currently encountered 

when conducting personal noise dosimetry measurements 

with body-worn microphones. Additional research in this 

area might include investigating the use of signal processing 

techniques to offset the inherent frequency response errors and 

placement effects when inexpensive (i.e., nonprecision) bodyworn microphones are used. 

REFERENCES 

1.	 U.S. Department of the Interior, U.S. Bureau of Mines: Response 

Variations of a Microphone Worn on the Human Body (RI-7810) by T.L. 

Muldoon. Washington, D.C.: United States Department of the Interior, 

U.S. Bureau of Mines, 1973. 

2. “Use of Personal Noise Dosimeters,”	 Federal Register 43:177 (12 

September 1978). pp. 40760–40764. 

3. U.S. Department of Labor, Mine Safety and Health Administration: 

Comparison of Noise Exposure Measurements Conducted with Sound 

Level Meters and Noise Dosimeters Under Field Conditions (Informational Report No. 1230) by M.P. Valoski, J.P. Seiler, M.A. Crivaro, and G. 

Durkt. Washington, D.C.: Department of Labor, Mine Safety and Health 

Administration, 1995. 

4.	 Giardino, D.A., and J.P. Seiler: Uncertainties associated with noise 

dosimeters in mining. J. Acoust. Soc. Am. 100:1571–1576 (1996). 

5.	 Seiler, J.P.: Microphone placement factors for one-half inch diameter 

microphones. J. Acoust. Soc. Am. Suppl. 1, 72:S81–82 (1982). 

6.	 Giardino, D.A., and J.P. Seiler: Interaction of body-worn measurement 

microphones with the undisturbed sound field. J. Acoust. Soc. Am. 

100:2672 (1996). 

7. “Occupational Noise Exposure: Hearing Conservation Amendment; Final 

Rule,” Code of Federal Regulations Title 29, Part 1910.95, 1983. 

8.	 Earshen, J.J.: Sound measurement: Instrumentation and noise descriptors. In Noise & Hearing Conservation Manual, 4th Edition, E.H. Berger, 

W.D. Ward, J.C. Morrill, and L.H. Royster (eds.). Akron, Ohio: American 

Industrial Hygiene Association, 1986. pp. 37–95. 

9.	 Royster, L.H., E.H. Berger, and J.D. Royster: Noise surveys and data 

analysis. In Noise & Hearing Conservation Manual, 4th Edition, Berger, 

E.H., Ward, W.D., Morrill, J.C., and Royster, L.H. (Eds.), pp. 97–176. 

Akron, Ohio: American Industrial Hygiene Association, 1986. 

10.	 Earshen, J.J.: Sound measurement: Instrumentation and noise descriptors. In The Noise Manual, 5th Edition, E.H. Berger, L.H. Royster, J.D. 

Royster, D.P. Driscoll, and M. Layne (eds.). Fairfax, Va.: American 

Industrial Hygiene Association, 2000. pp. 41–100. 

11.	 Royster, L.H., E.H. Berger, and J.D.Royster: Noise surveys and data 

analysis. In The Noise Manual, 5th Edition, E.H. Berger, L.H. Royster, 

J.D. Royster, D.P. Driscoll, and M. Layne (eds.). Fairfax, Va.: American 

Industrial Hygiene Association, 2000. pp. 165–244. 

12.	 American National Standards Institute (ANSI): American National 

Standard Measurement of Occupational Noise Exposure (ANSI S12.19­

1996, R 2001). [Standard] New York: ANSI, 2001. 

13. “Health Standards for Occupational Noise Exposure; Final Rule,” Federal 

Register 64:176 (13 September 1999). pp. 49548–49634. 

14.	 Burks, J.A., and K.L. Michael: A new best practice for hearing conservation: The Exposure Smart ProtectorTM (ESP). In INCE Conference 

Proceedings, June 23, 2003. NoiseCon03 112(1):723–732, 2003. Ames, 

Iowa: Institute of Noise Control Engineering of the USA, 2003. 

15.	 Michael, K.L: Continuous noise monitoring and reduction system and 

method. J. Acoust. Soc. Am. 114:2542 (2003). 

16.	 American National Standards Institute (ANSI): American National 

Standard Methods for Measuring the Real-Ear Attenuation of Hearing 

Protectors (ANSI S12.6-1997; R 2002). [Standard] New York, ANSI, 

2002. 

17.	 American National Standards Institute (ANSI): American National 

Standard Specification for Sound Level Meters (ANSI S1.4-1983, R 2001). 

[Standard] New York, 2001.