MultiFit4: An Improved System for Insert-Type
 

By: August J. Kwitowski, adk6@cdc.gov, Civil Engineer-NIOSH, Angela M. Carilli, Information Technology Specialist-NIOSH and Robert F. Randolph, Research Psychologist-NIOSH 

ABSTRACT 

Earplugs are a widely-used and inexpensive means of preventing 


noise-induced hearing loss, but low field attenuation performance 


puts many users at risk. The National Institute for Occupational 


Safety and Health is addressing this issue with hearing protector 


training and fit interventions that are best evaluated at worksites 


with miners and other workers from the target audience. To evalu

ate these interventions, an improved system has been developed. 


Called Multifit4, the system is designed to test as many as four 


subjects simultaneously using the real-ear attenuation at threshold 


(REAT) method. To allow multi-subject mobile testing, the 


subjects are seated together in a closed sound-isolated chamber, 


and the stimuli are presented through circumaural headphones 


(rather than the loudspeakers typically employed in diffuse-field 


REAT testing). Subjects signal their responses using a standard 


hand switch as they perform a Bekesy threshold task with the 


ears open and then with earplugs inserted (ears occluded) to 


determine insertion loss. Mutifit4 consists of custom software 


controlling Tucker Davis Technologies (TDT) Series 3 modules in 


a Windows® environment. The software directs stimulus presenta

tion and attenuation by the TDT modules. A setup screen is used 


by the operator to configure the order and type of testing and is 


flexible enough to accommodate multi-trial randomized experi

ments. Setup configurations can be saved for fast setup and to 


develop testing templates. The results of tests and the actual data 


can be saved in real time automatically to a database. Preliminary 


testing conducted on mining students has shown that the Multifit4 


system is reliable, accurate, and easy to use.
 

INTROduCTION 

Earplugs are commonly used hearing protection devices that can 


significantly reduce the risk of noise-induced hearing loss when 


worn properly. However, their effectiveness is often limited by 


poor fit, as shown by real-world noise attenuation that is far below 


optimized laboratory attenuation values such as the labeled Noise 


Reduction Rating (NRR) (Berger, Franks, & Lindgren, 1996). 


Typical interventions to help earplug users attain higher prevention involve instruction and training (Park & Casali, 1991). Less 


common are interventions that modify the earplug itself, such as 


the use of a lubricant (Randolph & Kissell, 2008). Whatever the 

intervention, the goal of increased protection can be evaluated 

by methods that measure improvements in attenuation, typically 

through a real ear attenuation at threshold (REAT) or microphone 

in real ear (MIRE) method. However, existing REAT and MIRE 

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systems can only evaluate one subject at a time and some, including 

REAT testing according to the ANSI S12.6, require the use of a 

static laboratory. The MultiFit4 system was developed to improve 

the efficiency of field testing insert-type hearing protectors. Up 

to four subjects can be tested simultaneously using a largely 

automated system that provides high accuracy and flexibility and 

is user friendly. The system was developed to replace and expand 

on a prior system that Michael & Associates, Inc. developed for the 

National Institute for Occupational Safety and Health (NIOSH) 

based on four of the company’s FitCheck systems (Michael, 1998) 

and custom software to test four subjects at once (Michael & 

Byrne, 2002). Although the FitCheck-based four-person system 

was functional, it did place several limitations on the types of 

experiments that could be conducted and how the stimuli could be 

presented. For future experiments, a system with greater control 

over stimuli and experimental procedure was needed, so NIOSH 

developed the Multifit4 system. 

Figure 1 shows the Multifit4 system work area. 

Figure 1 

The black instrument rack contains four independent sound production and 

attenuation modules, a computer for interacting with and controlling the sound hardware, and a large uninterruptable power supply 

(UPS). The rack is mounted with a special suspension to minimize 

FIGURE 1, Multifit4 system in trailer 
shock transmission. This is necessary because the Multifit4 system 

is housed within the NIOSH Hearing Loss Prevention Unit, a 

32-ft mobile audiometric laboratory, which is towed to worksites 

to enable researchers to test the target audience of miners and 

other workers. The Multifit4 system operator sits to the side of the 

hardware and interacts with the computer through the ubiquitous 

monitor, mouse and keyboard combination. The entry to the sound 

isolation testing chamber is shown on the right edge of Figure 1 

(see page 17). The chamber can seat and sound-isolate up to four 

subjects and provides each subject with a test switch and a pair of 

headphones. 

T h e h a r d w a r e f o r e a c h c h a n 

nel consists of : one Tucke r Dav is 

Technology (TDT) RP2.1 Enhanced 

R e a l-Ti me Pr o c e s s o r t o p r o d u c e 

sound; two TDT PA5 Programmable 

A t t e n u a t i o n u n i t s t o a t t e n u a t e 

t h e s t e r e o s o u n d s ; a T DT H B7 

Headphone Driver to drive the head

phones; and a set of Sennheiser HD 

280 Pro Headphones (Fig ure 2) to 

present the sounds to the subjects. 

FIGURE Figure 2 2, Circumaural headphones 

A 

standard desktop computer with the 

Windows XP operating system runs 

the software and communicates with 

the hardware over a TDT FD5 fiber 

optic zBus Gigabit Ethernet interface. 

The hardware generates test stimuli 

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by playing prepared sound files in the Microsoft Audio for 

Windows (WAV) format. The WAV files contain one-third 

octave band pulses of noise with specific center frequencies. 

Although the RP2.1 devices can be internally programmed 

to produce sounds, WAV files were employed for consis

tency with the earlier FitCheck system (Michael & Byrne, 

20 02), g reater control over the stimuli, and because it 

simplified the development process. 

Multifit4 was written in Microsoft Visual Basic .NET® 

using Microsoft Visual Studio 2008®. The software refer

ences and communicates with the hardware units as ActiveX® 

objects. Multifit4 is intended to be used by an experienced 

operator who controls the system while interacting with the 

test subjects. When the program is launched, some basic 

diagnostics are run to determine the presence and status of 

the TDT hardware. If the hardware is missing, not turned 

on, or misconfigured, the operator is directed to correct the 

problem(s). After a successful program launch, the operator 

is presented with a setup screen to configure the testing, 

Figure 3. 

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Figure 3 

Here, the operator sets the number of test subjects, 

adjusts the initial sound level for each subject, and chooses 

the test configuration. Default values are provided so that 

the operator may quickly bypass the setup screen, if desired. 

Configuration files allow favorite system configurations to be 

saved and retrieved. 

FIGURE 3, Setup screen
Actual testing is conducted with the operator using the Testing 

screen, shown in Figure 4. 

Figure 4 

FIGURE 3, Setup screen

Here, the operator and subjects are 

stepped through hearing tests with the selected conditions. Test 

grids are presented for each active subject and contain impor

tant test parameters. A status bar at the bottom of the form 

includes information about the testing order, with the current 

test being highlighted. Also included on this bar are the program 

status, such as “Awaiting Operator Action” or “Playing Sound” 

and a button to display and select whether raw data is writ

ten to files in real-time. The operator can use the “Play Sample 

Sound” button to play the 1 kHz sound over the headphones 

at the starting volume level determined on the Setup screen. 

Multifit4 uses headphones to deliver audio stimuli to subjects, 

whereas traditional diffuse-field REAT testing uses loudspeak

ers. Headphones offer several advantages over loudspeakers for 

evaluating insert-type hearing protectors, such as being able to 

test multiple subjects simultaneously using the same audiometric 

booth and allowing independent testing of the left and right ears. 

Past research with the FitCheck and other systems has shown that 

using headphones for field REAT testing is feasible and reliable 

(Michael, 1998) (Michael & Byrne 2002). After evaluating several 

candidate headphones for repeatability and linearity, Sennheiser 

HD 280 Pro Headphones were selected for use with the system as 

they are circumaural, large enough to accommodate earplug wear, 

and readily available. 

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dESCRIPTION OF THE SOFTWARE 

Figure 5 shows a very simplified flow diagram for a single channel of 

the software. 

Start

Isswitch

pressed ?

Y es

Istesting

completed for this

frequency ?

Istesting

completed for this

subject ?

Istesting

completed for

thisoption ?

Y es

End

Increase SPL

@

1.0 Db/sec

No

Waitfor othersto

finish N o

Y es

Istesting

completed for

thisrun ?

Y es

No Waitfor othersto

finish

Decrease SPL

@

1.0 Db/sec

N o

Startnext

frequencysound

Yes ,

subject

hears

sound

No,

subject

can’thear

sound

Figure 5 

A primary function of the software is to determine the 

FIGURE 4, Testing screen

FIGURE 5, Flow diagram for singl
status of the switch held in the subject’s hand. When a subject 

hears a sound, he/she should have their switch pressed and closed. 

When the software detects that the subject’s switch is closed, the 

sound level is attenuated at the rate of 1.0 dB/sec by sending 

control signals to the TDT PA5 programmable attenuation 

units. The subject is instructed to release the switch when he/she 

can no longer hear the sound. When the software detects that the 

subject’s switch is open, corresponding to a subject not hearing 

a sound, the sound level is amplified at a rate of 1.0 dB/sec via 

control signals to the attenuation units. The software reads each 

active subject’s switch position and updates each subject’s PA5 

attenuation unit every 300 ms. The remainder of the flowchart 

is described in the “Testing Screen” section. 

Setup Screen 

On the Setup screen, Figure 3, (see page 18) the operator uses the 

“Active Subjects” checkboxes to select from one to four subjects 

for testing. The “Active Subjects” checkboxes are located in the 

“Subject Options” group of the Setup screen. 

MultiFit4 employs a descending presentation method that starts 

at a suprathreshold (perceptible) level. A reason for this is that 

the actual testing may be preceded by the presentation of about 1 

minute of aural instructions and it was desired that the subjects 

be able to listen to the instructions at a comfortable hearing 

level. Using the sliders and textboxes adjacent to the “Active 

Subjects” checkboxes, the operator may increase/decrease or 

type in the starting sound levels for each subject. The default 

level is 70 dB. Clicking the “Test Level” button plays a 1 kHz 

centered one-third-octave noise signal over the headphones at the 

selected/displayed level. The sound plays for a timed 15 seconds 

or until the operator toggles the sounds off by re-clicking the 

button. Additionally, subjects can individually set their own 

starting sound levels when the operator clicks the “Subjects 

Set Level” button. This causes instructions to be played over 

the headphones that direct the subjects on how to adjust and 

set the sound levels using their hand switch. The background 

colors of the level sliders are keyed according to how the levels 

were set, either by default, by the operator, or by the subjects. 

For example, when subjects are allowed to set their own levels, 

the background color becomes light blue. Clicking the “Reset” 

button below the “Subject Options” group resets all values to 

their default values. 

Using the “Testing Options” group, the operator can select 

from one to nine frequencies to be included in the testing. The 

frequencies correspond to ANSI S12.6 standards (ANSI/ASA, 

2008), ranging from 125 Hz to 8 kHz. The sounds are one-third 

octave bands of noise with specific center frequencies. The 

software gives priority to presenting sounds near the 1 kHz 

center frequency first, consistent with the ANSI S3.21 protocol 

for air-conduction audiometry (ANSI/ASA, 2009). If the 1 kHz 

center frequency signal is selected, it will always be presented 

first. Otherwise, the selected frequency closest to 1 kHz is 

presented first. The rest of the selected frequencies are then 

presented in ascending order. However, if the “Randomized” 

checkbox is checked, the remaining frequencies are instead 

presented in random order. 

If desired, the operator can have the software calculate a 

Reduction Rating (RR) using a standard formula. The “Calculate 

RR” checkbox is only enabled if the correct frequencies required 

to run the analysis are selected. 

There are several selectable options for when the presentation of 

a certain frequency to a subject concludes. If “Time, sec” is chosen 

using the radio buttons from the “Sound ends with” group, the 

sound terminates after a timed interval that can be adjusted from 

5 to 120 seconds. If “Reversals, RVs” is chosen, the sound ends 

when the subject has cycled through several reversals (instances 

of hearing the sound pass through the threshold of audible to 

inaudible). A reversal is counted when the sound attenuation 

changes direction because the subject pressed/released the test 

switch. The software allows selection of 3 to 12 reversals. 

The subjects control the sound pressure levels (SPLs) of the 

sounds by using their hand switches. When the button is pressed 

(the switch is closed), the sound level is attenuated at the rate of 

1 dB/sec. Following the task instructions, subjects release the 

switch when the sound becomes inaudible. After the button is 

released (the switch becomes open), the sound level is amplified 

at the rate of 1 dB/sec. Threshold levels should correspond 

closely to where subject switch actions reverse. 

If desired, the operator can add a short period of complete silence 

to the end of each individual sound presentation. The silence is 

accomplished by shutting off the output of the subject’s RP2.1 

device and setting the PA5 attenuation units to their lowest 

sound output levels of 0 dB. This silent period can be from 

1 to 20 seconds and could be used to detect a subject that is 

responding arbitrarily or simply guessing, as no switches should 

be pressed during this absence of sound. 
Testing Screen 

The Testing Screen is shown in Figure 4 (see page 19). Before 

starting the tests, the system plays instructions through the 

headphones that describe what the subjects can expect and how 

to perform the threshold task. If the “All Instructions” button 

is used to start the instructions, the subjects must each listen to 

the entire 90 second audio recording at least once before they are 

allowed to continue. Using the “Abbreviated Instructions” button 

allows each subject to interrupt the instructions by firmly press

ing his/her switch. After all subjects complete the instructions, 

the “Start” button becomes enabled and the testing is ready to 

begin. Clicking the “Start” button begins the testing. The current 

sound frequency being played is shown as a highlighted row in 

each subject’s grid, as shown in the figure. Additional informa

tion displayed on each row includes the current sound level, the 

time the sound has been playing, reversals counted, the average 

open threshold value, the average occluded threshold value, 

and the calculated earplug attenuation at that frequency. The 

highlighted row changes colors to indicate the subject pressed 

or released his/her test switch. The switch status, if pressed, is 

displayed at the bottom of each grid. The operator is reminded 

of what conditions were selected to end each sound presentation 

by the information displayed in the grid’s upper right corner. 

The software steps every subject through each sound frequency 

being tested. The sound frequencies are played in the order shown 

by the rows within the grids. A modified Bekesy method is used 

where the program selects the sound frequency being played and 

the subject controls the intensity of the signal by means of the 

pushbutton switch. Each subject is tested independently from the 

other subjects, with the software changing the test frequencies 

as appropriate to complete the subject’s testing. To occupy those 

subjects that complete the testing quickly, the software plays 

frequency sounds at random until all subjects are done. 

After all subjects have completed one condition of the testing, 

the software alerts the operator and goes into a paused mode that 

allows researchers to guide the subjects into the next phase of 

testing. For example, the initial testing may have been conducted 

open, with no earplugs, and the second phase may be an occluded 

test calling for the researchers to carefully instruct and guide the 

subjects on inserting a particular earplug. After all the scheduled 

phases of testing are completed for each subject, the software 

instructs the operator that the testing is over and then goes into 

another paused mode where the Testing screen is kept open. 

This allows the operator to access the databases or other tools 

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from the menus or to return to the Setup screen by using the 

“Back” button. 

Multifit4 allows subjects that started the testing to be paused, 

removed, or deactivated. Right-clicking a grid with an active subject 

causes a menu to appear where the operator can choose to “Pause”, 

“Stop”, or “Resume” the currently playing sound. This might allow 

researchers to interact with a particular subject without disturbing 

the other subjects. Clicking the “Active” checkbox above an active 

subject’s grid allows the operator to deactivate and remove the 

subject from the testing, also without interfering with the other 

subjects. For confirmation, several warning dialog levels will then 

appear with opportunities to cancel the action. 

dATA STORAgE 

The software automatically records real-time data to three database 

tables: “Sessions”, “Runs”, and “Details”. The tables are designed in 

three separate forms using the Microsoft Access® platform. The 

operator can access any of the tables while testing is ongoing. Most 

of this higher-level data is collected and recorded automatically. The 

operator can also start or stop the writing of low-level data files. 

Table 1 shows the data fields of the three database tables. 

Table 1 

The 

“Sessions” table is mainly for use by the operator to fill in useful demo

graphic information about the test subjects such as “Date of Birth”, 

“Subject ID”, and “Comments”. The software automatically gener

ates and writes the information for the other data fields. This table 

can be edited while a test is running. The “Runs” table is generated 

automatically, and the operator can view but not write to it. As testing 

progresses, average threshold values are calculated for each frequency 

and for each subject. This information is written to threshold data 

fields within the Runs table for each active frequency and subject. 

Threshold entries are made as text strings where the lead number 

is the average value and is followed by the individual data readings 

separated by comma delimiters. The “Runs” table is composed of 

an individual row for each subject/test combination. Therefore, 

rows can increase or decrease depending on the number of subjects. 

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FIGURE 6, Database display screen

Figure 6 shows the “Details” table as displayed by the Multifit4 

program. 

Figure 6

It records the important values from the testing. Only 

the “Subject” and “Comments” columns can be edited by the 

operator and the program automatically writes the other data 

fields (some of which are not shown). 

If the operator elects to record low-level data, a text file is 

updated every second of elapsed time for each active subject. 

Each line in the file contains information about the: ear (left, 

right, or both), current frequency being tested, switch condition 

(open or closed), present attenuation level, and elapsed time 

from the start of testing for the current frequency. In the event 

of problems with the databases, such as corruption, etc., the files 

contain sufficient information to reconstruct the testing. 

TESTINg 

An independent consultant evaluated the linearity of the 

Multifit4 system using a Larson Davis IEC 318 coupler (artificial 

ear for measurement of circumaural headphones) and a System 

824 sound level meter with calibration traceable to the National 

Institute of Standards and Technology (NIST). The evaluation 

showed the system’s linearity to deviate a maximum of 0.3 dB, 

well within the ANSI S12.6 maximum of 1 dB. 

On April 29, 2008, a shake-down test of the Multifit4 system 

was conducted at the Fayette County Branch of the Pennsylvania 

State University. The test objective was to determine how the 

system would perform in a typical research data-gathering 

session. The test session was planned and run by a researcher 

with experience in conducting field evaluations of earplugs and 

who served as the operator of the Multifit4 system. This was a 

test of the full system, as the mobile trailer housing the system 

was towed to a site on campus and set up on the morning of the 

testing. 

There were 13 mining students available for testing. Testing 

was conducted for three groups of four subjects plus an extra 

subject. It was desired to test four subjects simultaneously, as 

this is the maximum capacity and as severe a test as possible for 

the Multifit4 system. 
To simulate a typical field data gathering session, the first 

group of four subjects was tested after detailed instructions 

were given on how to properly insert foam earplugs. The 

second group did not receive the instructions. These eight 

subjects were tested so that an individual sound file was 

ended when the subject had cycled through five threshold 

levels involving five reversals. A second set of subjects was 

tested using a different type of earplug and using a timed 

interval of 30 seconds to end each sound file presentation. 

The final single subject was also tested using the timed 

interval method. 

Figure 7 presents some results from the testing as written 

to the “Details” database table. 

Figure 7 

As shown, data rows are 

created and written for each subject/test combination. The 

unique identifier number, “Identifier” column, of each row 

associates the data with a test subject. The “Level ” column 

contains the starting SPL used. For each frequency tested, a 

data column and a standard deviation column were created. 

The data column is identified by the frequency tested and 

contains the average value and the individual threshold 

values recorded. The calculated standard deviation for that 

frequency’s data is written to the adjacent column. 

Prior to the testing, an otoscopic inspection was conducted 

on all of the participants to ensure a clear ear canal. One 

subject had an ear canal that was not clear. Therefore, the 

second set of testing with this subject was conducted without 

earplugs (open) and is the reason why the measured values 

show little attenuation. 

The low-level data files were written for all subjects during 

this testing. Subsequent evaluation of the data contained in 

these files confirmed that the low level values were consistent 

with the higher-level values written to the database tables. 

A weakness found for the software during the field trial was 

that the operator did not have full use of the database tables 

while testing was underway. Originally, the three tables were 

created inside the same database. With a test ongoing, the 

software continually writes to all tables and some sharing 

violation errors occurred when the operator also tried to 

manually update the tables. This was solved by creating each 

table within its own database. 

Overall, the Multifit4 system performed very satisfactorily 

in this testing. The database problem and some other small 

issues were identified and addressed in a subsequent revision 

of the software. 
SuMMARy 

Noise-induced hearing loss is nearly 100% preventable, but once 

acquired it is permanent and can be life-altering. Individual fit 

testing of hearing protectors is a recommended “best practice” 

by the National Hearing Conservation Association’s Alliance 

Program with NIOSH and the Occupational Safety and 

Health Administration (NIOSH, 2008). The Multifit4 system 

was designed to be used as a tool by qualified researchers to 

evaluate hearing protector training and fit interventions at 

worksites with workers from the target audience. Initial testing 

showed that the system delivers test stimuli with sufficient 

accuracy and linearity and functions well in an actual field 

research setting. Additional studies are planned to compare the 

Mutifit4 system with laboratory REAT testing conducted in 

the NIOSH Auditory Research Laboratory that is accredited 

for ANSI S12.6 testing by the NIST National Voluntary 

Laboratory Accreditation Program. The Mutifit4 system will 

then be used in upcoming field evaluations of the NIOSH 

QuickFit earplug test device (NIOSH, 2008) and other hearing 

protector training interventions. Subsequent improvements 

are also expected as the Multifit4 system is used on a more 

frequent basis and operators’ feedback is incorporated into the 

revised design. 

DISCLAIMER 

The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. 

REFERENCES 

1. Elliott H. Berger, John. R. Franks, and F. Lindgren [1996], “International 

reviewoffieldstudiesofhearingprotectorattenuation,”Chap.29inScientific 

Basis of Noise-Induced Hearing Loss, edited by Alf Axelsson, Hans M. 

Borchgrevink,RogerP.Hamernik,Per-AndersHellstrom,DonaldHenderson, 

Richard J. Salvi (Thieme Medical Publishers, New York, 1996). 

2. Park MY, Casali JG, [1991] A controlled investigation of in-field attenu

ation of selected insert, earmuff, and canal cap hearing protectors. Human 

Factors 33(6):693-714. 

3. Robert F. Randolph and Fred N. Kissell [2008], “The effect of an inser

tion lubricant on the noise attenuation of foam earplugs,” NOISE-CON 2008: 

Proceedings of the 2008 National Conference on Noise Control Engineering, 

Dearborn, Michigan, July 28-31, 2008 

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4. John R. Franks, William J Murphy, Dave A. Harris, Jennifer L Johnson, 

and Peter B. Shaw, [2004] “Alternative field methods for measuring hear

ing protector performance,” Am Ind Hyg Assoc J 64(4):501-509 

5. Michael KL, Byrne DC [2002]. Current State of Insert-Type Hearing 

Protector Fit-Testing: Follow-On Measurements in the Steel Industry and 

Fit-Testing in a Mobile Environment. Spectrum 19(Suppl 1):14-16 

6. Michael KL [1998]. Comprehensive use of hearing protectors: Integration 

of training, field monitoring, communication and documentation. Journal 

of Occupational Hearing Loss 1:67–74. 

7. ANSI/ASA [2008]. American National Standard Methods for Measuring 

the Real-Ear Attenuation of Hearing Protectors. ANSI/ASA S12.6-2008. 

8. ANSI/ASA [2009]. American National Standard Methods for Manual 

Pure-Tone Threshold. ANSI/ASA S3.21-2004 (R2009). 

9. NIOSH, Technology News [2008], “QuickFit Earplug Test Device”, No. 

534, December 2008