Audio Cues to Assist Visual Search in Robotic 

System Operator Control Unit Displays 

by Ellen C. Haas, Ramakrishna S. Pillalamarri, 

Christopher C. Stachowiak, and Michael A. Lattin 

ARL-TR-3632 December 2005 

Approved for public release; distribution is unlimited.
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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5425 

ARL-TR-3632 December 2005 

Audio Cues to Assist Visual Search in Robotic 

System Operator Control Unit Displays 

Ellen C. Haas, Ramakrishna S. Pillalamarri, Christopher C. Stachowiak, 

and Michael A. Lattin 

Human Research and Engineering Directorate, ARL 

Approved for public release; distribution is unlimited. 
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 Audio Cues to Assist Visual Search in Robotic System Operator Control 

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14. ABSTRACT 

Mission demands have made the robotics collaboration operator control unit (OCU) into a relatively dynamic, demanding, cognitively 

complex system where Soldiers must perform multiple tasks such as controlling multiple robots and processing large amounts of 

information in environments that sometimes contain high levels of noise. Research and modeling data indicate that audio display 

technologies would be very useful in OCU applications such as guiding visual display search. The purpose of this study was to examine 

the effectiveness of the integration of auditory display technologies in visual search tasks such as those that occur in robotic OCUs. 

Independent variables were audio signal mapping scheme, type of verbal positional cue, and visual target azimuth. Dependent variables 

were visual target search time and the National Aeronautics and Space Administration Task Load Index workload rating of the target 

search task. Participants were 36 students (15 males and 21 females) from Harford Community College. The results indicated that the 

use of auditory signal mapping and verbal positional cues significantly reduced visual display search time and workload and that 

positional cues mixed with specific audio mappings were the most efficient means of reducing search time. Specific design 

recommendations are made regarding the use of auditory signals in environments with narrow field-of-view visual displays. 

15. SUBJECT TERMS

 auditory assisted visual search; auditory displays 

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 Ellen C. Haas 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Background 1

1.1 Audio Mapping ...............................................................................................................2 

1.2 Verbal Positional Cues ....................................................................................................3

2. Method 4

2.1 Participants ......................................................................................................................4 

2.2 Apparatus.........................................................................................................................5 

2.3 Stimuli .............................................................................................................................6 

2.3.1 Visual Stimuli......................................................................................................6 

2.3.2 Auditory Stimuli..................................................................................................6 

2.4 Procedure.........................................................................................................................7 

2.5 Experimental Design .......................................................................................................8

2.6 Data Analysis .................................................................................................................9 

3. Results 10

3.1 Visual Target Search Time............................................................................................10 

4. Discussion 13

4.1 Target Search Times......................................................................................................13

4.2 Workload.......................................................................................................................15 

5. Conclusions 16

6. References 17

Appendix A. Williams Square Designs Used in This Study 19

Distribution List 20 
iv 

List of Figures 

Figure 1. Participant with visual and auditory task apparatus. .......................................................5 

Figure 2. Mean target search times for audio mapping x positional information, visual 

signals at 0 degrees absolute azimuth (+ 1 standard error of the mean)..................................11 

Figure 3. Mean target search times for audio mapping x positional information, visual 

signals at 7.74 degrees absolute azimuth (+ 1 standard error of the mean).............................11 

Figure 4. Mean target search times for audio mapping x positional information, visual 

signals at 15 degrees absolute azimuth (+ 1 standard error of the mean)................................12 

Figure 5. Mean workload ratings for Audio Mapping x Positional Information interaction 

(+ 1 standard error of the mean). .............................................................................................13 

List of Tables 

Table A-1. Williams square design, order of presentation of auditory mapping scheme to 

participants...............................................................................................................................19 

Table A-2. Williams square design, order of presentation of visual target stimuli to 

participants...............................................................................................................................19 
1 

1. Background 

The U.S. Army is exploring the insertion of multiple advanced technologies, such as spatial 

audio displays, into a robotics collaboration operator control unit (OCU). Mission demands have 

made the OCU into a relatively dynamic, cognitively complex system where Soldiers must 

perform multiple tasks such as controlling multiple robots and processing large amounts of 

information in environments that sometimes contain high levels of noise. These demanding, 

cognitively complex tasks may result in Soldier information overload as well as decrements in 

situational awareness and mission performance. In addition, a human factors analysis of current 

robotic OCU systems (Bodenhamer, 2004) reveals that OCU visual display design and 

accompanying system warnings are less than adequate, with display design hindering search and 

poor use of cues. In the MATILDA1

 small unmanned robotic ground vehicle, OCU display 

quality was found to be poor, and the displays lacked system status or robot orientation 

indicators for the operator. The Buster unmanned aerial vehicle OCU lacked sufficient visual 

contrast to allow indication and interpretation of needed information and lacked appropriate 

warning indicators. Both MATILDA and Buster would benefit from auditory displays that 

supplement visual displays by providing information regarding system status, orientation, speed, 

or system warnings. 

In the proposed robotics collaboration Army technology objective (ATO) systems and the Future 

Combat System (FCS), Soldiers will search visual displays for friendly and enemy objects and 

targets. Although there are no robotics collaboration platforms defined as of yet, it is possible 

that variations might use a standard 18-inch diagonal screen that produces a field of view (FOV) 

of approximately 30 degrees when viewed from a distance of approximately 2 feet (within the 

range of recommended viewing distances recommended in MIL-STD-1472F; U.S. Department 

of Defense, 1999). However, searching for and finding visual information within a 30-degree 

FOV might prove to be difficult, especially when there is a large number of visual objects on the 

display. Perrott, Sadralodabai, Saberi, and Strybel (1991) found that response time for target 

identification without audio cues doubled from 700 ms to 1400 ms when the number of visual 

objects increased from 2 to 64. Visual search may be very difficult in situations when visual 

targets include revisions of an already complex display, such as the change of military symbols 

on a map already populated by a large number of military symbols. 

Research has shown that spatial audio cues can enhance visual search. Providing spatial auditory 

cues as guides (also known as aurally guided visual search) can facilitate detection of a visual 

target (Perrott et al. 1991; Elias, 1995, 1996; Perrott, Cisneros, McKinley, & D’Angelo, 1995, 

 1Mesa Associates’ Tactical Integrated Light Force Deployment Assembly
2 

1996; Strybel, Boucher, Fujawa, & Volp, 1995; Fujawa & Strybel, 1997). Begault (1993) found 

that visual search times of commercial pilots were reduced by several (2.2) seconds when 

spatially correlated sounds were present. Elias (1996) found that aural cues enhanced visual 

search, particularly when 12 visual distractors were present. In two experiments, the slope of the 

function relating search time to number of distractors in an auditory spatial cueing condition was 

half that obtained with no auditory cue when targets are the same color as other items on the 

screen (Perrott et al., 1991; Strybel & Perrott, 1993). These data indicate that spatial audio 

displays may have considerable utility in guiding search of map displays. 

Although audio cues have been found to be effective in guiding visual search, there has been 

little research to determine effective design characteristics of these cues. Two approaches to 

providing effective audio cues for visual search are mapping spatial audio signals to visual 

displays and adding verbal azimuth information to audio cues. 

1.1 Audio Mapping 

Mapping spatial audio information to visual displays with a 30-degree FOV is a challenging 

problem that few researchers have explored. Begault (1993, 1998) used a mapping scheme for a 

display with a 70-degree visual FOV in which spatial audio cues were presented in seven 

positions from 9 to 3 o’clock (a total of 180 degrees), with these positions exaggerated in 

relationship to the visual stimuli by a factor of two (i.e., visual targets at 15 degrees’ azimuth 

would be signaled by an audio warning heard at 30 degrees’ azimuth). Note that this 

exaggerated relationship would result in a more than 2-1 visual-to-audio-cue ratio when a visual 

cue at a 70-degree FOV would be heard at 180 degrees. Begault (1993) used discrete spatial 

audio positions rather than a continuous spatial mapping because he believed that discrete 

positions gave a more convincing suggestion of what direction to search. He also suggested that 

discrete spatial audio signal positions that exaggerate visual positions may be advantageous for 

several reasons: (a) exaggerated audio positions preclude the entire visual space from being 

mapped to only a limited spatial audio arc or angle; (b) discrimination error (as opposed to 

localization error) would be reduced with the use of exaggerated positions; and (c) the use of a 

greater range of azimuths would employ the full range of interaural time delays that the auditory 

system interprets in terms of left-right displacement. Although Begault (1998) ran a second 

study using another mapping scheme with a more limited audio range (audio alarms at eight 

positions from ±60 degrees), he did not formally compare these two mapping schemes to each 

other, nor did he provide a suggestion of which was the more efficient. 

A search of the literature indicates that neither Begault (1993, 1998) nor any other researcher 

suggested mapping schemes for displays with a FOV of less than 70 degrees. The purpose of this 

study was to determine whether efficient mapping schemes for displays with a narrower visual 

azimuth, such as displays with a 30-degree FOV (such as those found on potential robotic OCUs), 

could be developed. Five different strategies, three of which are based on Begault’s 

recommendations regarding the use of discrete spatial audio positions and exaggerated audio 
3 

range, were compared in this study. The first strategy employed an audio display with three 

spatial audio positions at –15, 0, and +15 degrees’ azimuth (a range of audio angles corresponding 

to the azimuth of the 30-degree FOV visual display). A second mapping strategy used an audio 

display with three spatial audio positions at –60, 0, and +60 degrees azimuth (a relatively wide 

range of audio angles). A third strategy used three spatial audio positions at –37, 0, and +37 

degrees azimuth (approximately midway between ±15 and ±60 degrees). A fourth strategy 

employed a monaural presentation of audio cues, in which all cues would reach both ears at once, 

resulting in a nonspatial presentation in which all audio signals are heard as if coming from the 

center of the head (one audio position with no range of audio angles). Monaural audio was to be 

used because it is currently the most common mapping scheme used in audio displays. A fifth 

strategy was to use no sound at all (silence) as a baseline comparison condition and as a basis of 

comparison with displays that do not employ audio signals, such as displays with visual 

information only. It was hypothesized that because the first three mapping schemes use spatial 

audio, they would produce shorter visual search times than the monaural or the visual only 

schemes. 

Comparing different spatial mapping schemes with monaural and visual only presentation will 

help define an effective means for mapping audio cues to visual targets within a relatively 

narrow range, such as a 30-degree FOV display. There is a possibility that the spatial mapping 

scheme that uses the widest range of audio angles (±60 degrees) may produce significantly 

shorter response times than monaural displays and conditions with visual only but longer 

response times than displays with a narrower range of audio angles (±37 or ±15 degrees) because 

of a greater disparity between audio and visual cue. For example, an audio cue for a 15-degree 

visual target will be heard at 60 degrees for the ±60-degree audio display and at 37 degrees for 

the ±37-degree audio display. It is possible that for displays with a wider audio range, the larger 

disparity between audio and visual cue may confuse the listener, possibly increasing participant 

search time. However, the audio-visual cue disparity in Begault (1993, 1998) did not seem to 

interfere with participant performance. 

1.2 Verbal Positional Cues 

Adding verbal positional information to audio cues could be another means of providing 

effective audio cues. As suggested by Patterson (1982), speech cues can efficiently provide 

direct positional information that enables the listener to quickly evaluate message content. 

Speech cues using the words “target, target,” could provide the listener with the information that 

the object of interest is a target and to search for that target on the visual display. Adding verbal 

positional information to the speech cue (i.e., “target 15 degrees”) might further reduce search 

time by providing a more precise positional cue. In addition, spatializing the verbal cues (i.e., 

having “target 15 degrees” mapped to a representative location in space) could potentially further 

reduce the search time because spatialization provides further positional cueing, as per Perrott, 

Sadralodabai, Saberi, and Strybel (1991). However, no research has been conducted to 
4 

determine whether spatialized verbal positional cues would make efficient guides for visual 

search. In addition, no research has been conducted to determine how spatialized verbal cues 

would interact with different mapping schemes, especially when there is greater disparity 

between auditory and visual cues, such as found in the display using a ±60-degree auditory 

range. Research is needed to explore the use of verbal positional cues and the possible 

interaction of auditory signal mapping and verbal cues (positional or not) on aurally guided 

visual search. 

Improved Performance Research Integration Tool (IMPRINT) Mounted Combat System (MCS) 

modeling data suggest that Soldiers will experience high levels of workload when performing 

multiple tasks, including searching visual displays. Integrating auditory display technology into 

the robotics collaboration OCU has the potential of reducing Soldier motor, visual, and cognitive 

workload by permitting him or her to more efficiently search visual displays, such as the map 

display. Little research has been done to explore the design of an efficient and effective audio 

display, even though data obtained from IMPRINT models, which indicate that audio control and 

display technologies would be very useful in OCU applications for tasks such as guiding visual 

display search (Mitchell, Samms, Glumm, Krausmann, Brelsford, & Garrett, 2004). 

The purpose of this study is to examine the effects of the integration of spatial auditory display 

technologies in the use of visual display search tasks. Two objectives are explored within this 

study: (a) to determine what types of spatial audio signal mapping schemes and verbal positional 

cues are best for aurally guided visual display search tasks; and (b) to validate three-dimensional 

(3-D) audio IMPRINT models previously developed for multi-modal displays. This study will 

serve as the “test” phase of the “model-test-model” approach. The data used in the previous 

“model” phase came from journal articles and previous laboratory studies. The time and 

workload data obtained in this study will be compared with those previously entered into the 

IMPRINT MCS model using spatial audio resources related to the use of visual map displays. 

In this study, it is hypothesized that the use of auditory signal mapping and verbal positional cues 

will reduce visual display search time. This will be useful in future experiments exploring the 

use of 3-D audio in demanding environments such as moving vehicles (i.e., the Army high 

mobility multipurpose wheeled vehicle or HMMWV), which contain relatively high levels of 

vibration and jolt. 

2. Method 

2.1 Participants 

Thirty-six participants (15 males and 21 females) participated in this study. Participants had a 

hearing threshold level (HTL), that is, the decibel level over threshold at which the subject hears 
5 

a test stimulus, corresponding to Army physical profile H2; an average of no more than 30 dB 

HTL, no individual level greater than 35 HTL at 500, 1000, and 2000 Hz, and no level greater 

than 55 HTL at 4000 Hz, (U.S. Army, 1991). Participants had normal visual functioning (20:40 

or better) and otoscopically normal ears (no blockage or infection). Participants reported no 

history of otologic pathology (hearing problems). Participants also had 30 hours or more 

experience using a computer mouse, the level at which Whisenand and Emurian (1995) 

considered mouse users to be “experienced.” 

Participants were students attending Harford Community College (HCC), Bel Air, Maryland. 

They were paid $40 for their participation in the study. The experiment was conducted in a 

conference room at the HCC library building. 

2.2 Apparatus 

Hearing tests were conducted with an Earscan2 Microprocessor pure tone audiometer. Vision 

tests were conducted with a Snellen eye chart. Audio signals used in this experiment were 

synthesized into sound files by a Veridian Engineering 3-DVALS3 System II spatial audio 

engine, communicating with an Intel-based computer. Sound files were played on a second 

Intel-based computer, and participants listening to the sounds wore Sennheiser 580 headphones. 

Participants viewed the visual task and listened to sound files on an Intel-based computer. The 

computer monitor display had an 18-inch diagonal screen. Figure 1 shows a participant with the 

visual and auditory task apparatus. 

Figure 1. Participant with visual and auditory task apparatus. 

 2

Earscan is a registered trademark of Micro Audiometrics Corporation. 

3

3-DVALS, which is a registered trademark of the Veridian Corporation, stands for 3-D virtual auditory 

localization system. 
6 

2.3 Stimuli 

2.3.1 Visual Stimuli 

The visual stimuli (visual display) consisted of U.S. Army military map symbols displayed on a 

color Russian topographical map, shown on the computer screen directly in front of the 

participant. The placement of the screen afforded the participant a 30-degree horizontal and a 

26-degree vertical FOV of the visual display. On the map, Russian location names were used to 

discourage participant attempts to read and understand place names (English place names might 

potentially distract the participants from the search task). Fifty-one military map symbols were 

used in this study (25 U.S. Army aviation symbols, 25 Army armored cavalry symbols, and one 

infantry symbol to serve as the target). Fifty of these symbols appeared on the map at one time 

(the target symbol randomly replacing one of the other map symbols), all placed at random 

locations. The symbols were 0.25 inch high by 0.5 inch wide, subtending 0.6 degree of vertical 

visual arc and 1.2 degrees of horizontal visual arc, as per MIL-STD-1472F (1999). The location 

of the target was randomly changed without replacement in each trial by the software to appear 

at one of 31 locations between ±15 degrees’ azimuth. The center of the target was randomly 

located vertically between ±13 degrees elevation. Between trials, the map symbols disappeared 

and a red box appeared at the center of the screen to focus the participant’s gaze on the center of 

the screen before each trial. 

2.3.2 Auditory Stimuli 

The sounds heard in the experiment were pre-recorded verbal alerts played as sound files on a 

personal computer (PC). There were two types of audio alerts. The first was an alert without 

positional information, containing information regarding target occurrence with no additional 

azimuth location information. This alert consisted of the words, “target, target” spoken by a 

female voice. The second type of alert had positional information, consisting of the words, 

“target x degrees” spoken by the same female voice, in which “x” was the azimuth location of 

the visual target in degrees’ azimuth. 

All audio alerts had a total duration of approximately 2 seconds. The alerts were digitally prerecorded sound files of a female talker made in a sound-treated room. These sound files were 

played through a Veridian Engineering 3-DVALS audio sound engine to make them correspond 

to the different mapping conditions. Spatialized signals used generic head-related transfer 

functions developed by the U.S. Air Force. All sound files were transferred to an Intel-based 

desktop computer for play during the experiment. 

During the experiment, the participant heard the alerts through the Sennheiser 580 headphones. 

The root mean square (rms) amplitude of each alert, as measured under the earcup of the 

participant’s headphones, was 78 decibels A weighted (dBA) sound pressure level (SPL). At the 

same time, the participant heard continuous tank noise played through the headphones. The tank 

noise, recorded at the commander’s position in an M1A2 tank, was 65 dBA SPL measured under 
7 

the earcup of the participant’s headphones. These levels were chosen because they were fairly 

representative SPLs for tracked vehicle noise (a noise that may be commonly found in battlefield 

environments), which is still within the limits of human exposure for this study. All sound and 

noise levels were broadband to present minimal problem to participants with hearing loss. 

Sound and noise were within human use hearing conservation guidelines (U.S. Army, 1991). 

2.4 Procedure 

Only one participant was evaluated at a time. The participant was asked to answer the volunteer 

agreement affidavit, undergo a vision and hearing screening (an audiogram), and receive training 

in all experimental tasks. After this, the experimenter read the instructions for the experimental 

session. Then the participant was trained to perform the visual search task. 

During training and the visual search task, the participant donned the stereo headphones and sat 

in front of the computer monitor. Participants were seated in a chair placed 24 inches in front of 

a standard 18-inch diagonal computer monitor, eyes in line with the center of the screen, and 

were advised to keep their heads as still as possible during experimental trials. The computer 

screen directly in front of the participant showed the Russian topographic map with the red box 

in the center. No military symbols appeared on the screen. The experimenter instructed the 

participant to direct his or her attention to the red box at the center of the screen before each 

search trial. After 1.5 seconds, the red box disappeared and the computer screen showed the 

topographic map along with the 49 U.S. Army map symbols and one target symbol. At the same 

instant, the participant heard the 2-second auditory alert that described the location of the visual 

target. The participant was instructed to use the auditory alert as a guide to visually locate the 

target symbol. The participant was advised that when s/he knew where the target symbol was 

located, to use a computer mouse to move a cursor shown on the computer screen as quickly as 

possible to the location of the symbol s/he thought was the target. When the participant moved 

the cursor on top of the map symbol s/he had chosen, s/he was instructed to click the left-hand 

mouse button to indicate that s/he located the target. When the participant clicked the left-hand 

mouse button, the symbology disappeared and the screen with the red box reappeared to refocus 

the participant’s gaze upon the center of the screen. Then, 1.5 seconds later, the red box 

disappeared and the next trial began. The cursor was automatically moved to the center of the 

screen at the beginning of each trial. 

Before each session, the participant was trained to use the auditory mapping scheme assigned to 

that session and had 31 search trials as training, each trial preceeded by the screen with the red 

box. The target symbol appeared at all 31 azimuth angle locations (±15 degrees azimuth, 

including 0 degrees) without repetition. The targets also appeared at random vertical locations 

within 26 degrees’ elevation, so that half the targets appeared between 0 and 13 degrees’ 

elevation, and half appeared between -13 and 0 degrees’ elevation. After training, the participant 

performed 31 visual search trials in a data run session. For training and data run sessions, the 

location of the target symbols was changed for each trial by the software, and the order of 
8 

appearance was random. After the participant finished all trials in the data run session, s/he 

doffed the headset and completed the National Aeronautics and Space Administration Task Load 

Index (NASA-TLX) workload measure (Hart & Staveland, 1987). After this, the subject had a 

rest break of at least 5 to 20 minutes (based on participant preference) before the next session. 

During the experiment, each participant participated in a total of five sessions, each lasting 

approximately 15 minutes. Each session involved a new auditory display-mapping scheme with 

the same verbal positional information used in the previous session. After the participant 

completed the NASA-TLX for the last experimental session, the participant had an exit 

audiogram, then an exit interview conducted by the experimenter to determine the mapping 

strategy that the participant preferred most. After this, the experiment ended and the participant 

was compensated. The entire experimental session, from initial orientation to final 

questionnaire, lasted approximately 3 hours. 

2.5 Experimental Design 

The treatment structure for this experiment was a mixed model factorial structure. The 

independent variables were as follow: 

1. Auditory mapping scheme was a within-subject variable. The five categories of mapping 

scheme were 

 a. Monaural (the same auditory signal was heard in both ears); 

 b. Three spatial audio alerts, sounding at –60, 0, and +60 degrees (an audio span of 

±60°degrees); 

 c. Three spatial audio alerts, sounding at –37, 0, and +37 degrees (an audio span of 

±37°degrees); 

 d. Three spatial audio alerts, sounding at –15, 0, and +15 degrees (an audio span of 

±15°degrees); 

 e. Visual only (no audio signal was heard). 

2. Verbal positional information was a between-subject variable that consisted of 

 a. Audio alerts without positional information, and 

 b. Audio alerts with positional information 

3. Visual target azimuth was a within-subject variable that was the azimuth of the visual 

target. Visual targets were presented at all 31 azimuth positions from –15 to +15 degrees, 

including 0 degrees. 

The order of presentation of auditory mapping scheme was assigned to participants by means of 

a Williams Square design, which counterbalanced order of treatments to ensure that all 
9 

treatments were administered to participants without repetition in order of presentation. The 

order of presentation of visual target stimuli was assigned into five different random order 

presentations, each of which was assigned to participants in a Williams Square design. Both 

Williams Square designs are presented in appendix A. 

Type of verbal positional information was assigned to participants at random, with the levels of 

this condition equally divided within the male and the female group. Each participant received 

only one level of this variable, and both levels of this variable were distributed equally between 

all participants. 

The dependent variables were as follows 

1. Visual target search time, which was defined as the time interval between the simultaneous 

triggering of the visual symbology and the auditory mapping stimulus, and the click of the 

mouse button indicating participant identification of the visual target; and 

2. The workload rating of the target location task supplied in the NASA-TLX questionnaire. 

2.6 Data Analysis 

Visual target search time was analyzed in a 10 x 5 x 5 x 5 x 2 x 2 analysis of covariance 

(ANCOVA), with visual target azimuth as the covariate. Variables were Williams Square order 

of presentation of mapping schemes x auditory mapping scheme x sequential order of 

presentation of mapping scheme x order of presentation of visual target symbols x verbal 

positional information x gender. Interactions between the random and fixed effect factors were 

used as error terms for testing hypotheses about the fixed effect factors, as prescribed by a mixed 

model analysis. 

Workload ratings were analyzed in a 10 x 5 x 5 x 5 x 2 x 2 analysis of variance (ANOVA). 

Variables were Williams Square order of presentation of mapping schemes x auditory mapping 

scheme x sequential order of presentation of mapping scheme x order of presentation of visual 

target symbols x verbal positional information x gender. 

For target search time and workload data, effects showing a probability value p less than 0.05 

were considered statistically significant. The least significant difference (LSD) post hoc test was 

performed at p < 0.05 but only if the corresponding mixed model tests on the fixed effect were 

statistically significant at the 0.05 level. 
10 

3. Results 

3.1 Visual Target Search Time 

The ANCOVA revealed significant effects for the interaction of auditory mapping scheme and 

verbal positional information, F(4, 127) = 19.70, p < 0.0001, and for the main effect of auditory 

mapping scheme, F(4, 682) = 10.82, p < 0.0001. There were also significant effects for the 

interaction of visual target azimuth and auditory mapping scheme, F(4, 5365) = 5.13, p = 0.0004, 

and for the main effect of visual target azimuth, F(1, 5365) = 63.42, p < 0.0001. There were no 

other significant effects. Data were for correct and incorrect target recognitions, but all subjects 

correctly identified almost all (99.99%) targets. 

The post hoc analysis of the auditory mapping scheme x verbal positional information interaction 

is illustrated in figures 2 through 4 and is reported for absolute visual target azimuths of 0, 7.5, 

and 15 degrees, respectively. An absolute azimuth value of 0 degrees represents a target location 

at the center of the computer monitor, while absolute values of 7.5 and 15 degrees represent 

locations halfway across the screen and at the edge of the computer monitor, respectively, 

regardless of direction. 

Regardless of positional information, visual target search time with spatial audio signals was 

significantly shorter than search times with visual only, at all azimuths. All spatial audio target 

search times were significantly shorter than those for monaural signals with no positional 

information at all azimuths. Monaural signals with positional information showed target search 

times no different than those with spatial audio, with or without positional information at all 

azimuths. Monaural signals without positional information had significantly greater target 

search times than monaural signals with positional information at all azimuths. 

At all azimuths, the longest target search times were found for conditions in which visual data 

only were presented, when positional information was otherwise given in all other conditions. 

For conditions in which no positional information was given, visual only had significantly 

greater search times than any spatial audio signals, and significantly greater search times than 

monaural signals with positional information. 

For all audio mapping and types of positional information, visual search time was shortest for 

targets at the center of the screen and increased as the target approached either edge of the 

screen. The increase with azimuth was not significant for all spatial audio signals and for 

monaural signals with positional information. The increase was significant for all conditions 

with visual only and for conditions with monaural signals containing no positional information. 

The audio mapping x positional information interaction at different degrees azimuth was 

analyzed to determine if it precluded the interpretation of the main effect of mapping condition. 
11 

As can be seen in figures 2 through 4, at each azimuth, the monaural condition was not ordinal 

with respect to the other mapping conditions. Monaural audio without positional information 

had a greater mean than monaural audio with positional information, which is a reverse of the 

relationship of all the other variables. Therefore, this interaction precluded the interpretation of 

the mapping main effect. 

2.94 2.85 2.86

4.20

3.46

2.58 2.76 2.75 2.60

4.90

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

15° 37° 60° Monaural Visual

Only

Mapping Condition

Mean Target Search Time (Seconds)

No Positional Info

Positional Info

Figure 2. Mean target search times for audio mapping x positional information, visual signals at 

0 degrees absolute azimuth (+ 1 standard error of the mean). 

3.31 3.10 3.15

4.73 4.48

2.95 3.01 3.05 3.13

5.92

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

15° 37° 60° Monaural Visual

Only

Mapping Condition

Mean Target Search Time (Seconds)

No Positional Info

Positional Info

Figure 3. Mean target search times for audio mapping x positional information, visual signals at 

7.74 degrees absolute azimuth (+ 1 standard error of the mean). 
12 

3.68 3.35 3.44

5.26 5.49

3.32 3.26 3.34 3.66

6.93

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

15° 37° 60° Monaural Visual Only

Mapping Condition

Mean Target Search Time (Seconds)

No Positional Info

Positional Info

Figure 4. Mean target search times for audio mapping x positional information, visual signals at 

15 degrees absolute azimuth (+ 1 standard error of the mean). 

The ANOVA of workload data indicated significant effects for the interaction of auditory 

mapping scheme and verbal positional information, F(4,127) = 4.08, p = 0.004, and for the main 

effect of auditory mapping scheme, F(4, 127) = 8.97, p < 0.0001. Post hoc analysis of the 

auditory mapping scheme x verbal positional information interaction indicates that participant 

workload ratings for mapping conditions with visual only for participants assigned to the 

positional information condition were significantly greater than workload ratings for any other 

variable, regardless of positional information and mapping scheme, p < 0.01. This effect is 

depicted in figure 5, where it can be seen that visual only in the positional information produced 

workload ratings almost twice as high as those in other conditions, including visual only in 

positional information conditions. (The maximum possible workload score was 100.) 

There were no other significant differences between means. This indicates that there were no 

statistically significant differences between spatial audio and monaural signals with and without 

positional information. There were no significant differences between all spatial and monaural 

signals and visual only signals in the positional information condition. 

The Audio Mapping x Positional Information interaction was analyzed to determine if it 

precludes the interpretation of the main effect of mapping condition. As can be seen in figure 5, 

the monaural condition was not ordinal with respect to the other mapping conditions, meaning 

that the relationship between monaural with and without positional information differed in 

respect to the other variables. Monaural audio without positional information had a greater mean 

than monaural audio with positional information, which is a reverse of the relationship of all the 
13 

other variables. Therefore, this interaction precluded the interpretation of the mapping main 

effect. 

26.8 26.7 27.2

31.6 32.5

27.2 30.9 31.9 29.6

50.9

0

10

20

30

40

50

60

70

80

90

100

15° 37° 60° Monaural Visual Only

Mapping Condition

Mean NASA TLX Rating 

No Positional Info

Positional Info

Figure 5. Mean workload ratings for Audio Mapping x Positional Information interaction (+ 1 standard 

error of the mean). 

4. Discussion 

4.1 Target Search Times 

The data indicated that it was possible to determine efficient mapping and positional information 

schemes for displays with a narrow FOV of 31 degrees. Target search times for audio signals 

were found to be significantly shorter than those with visual only. These findings are in 

accordance with the previous findings by other researchers that auditory cues can facilitate 

detection of a visual target (Perrott et al., 1991; Elias, 1993, 1996; Perrott et al., 1995, 1996). 

These findings also agree with Begault (1993), who found that search times can be reduced when 

spatially correlated sounds were present. Note that spatial audio cues without positional 

information had search times not significantly different than those of spatial audio signals with 

positional information, which implies that spatial audio information alone provides efficient 

audio cues. However, the data also showed that target search times for monaural signals with 

positional information were not significantly different than those of spatial audio signals. This 

failed to support the hypothesis that the spatial audio schemes would produce shorter visual 

search times than the monaural mapping. The experimental data imply that audio cues do not 
14 

have to be spatialized to provide effective target search cues and that spoken positional 

information reduced search time to the same extent as monaural signals. In terms of design 

standards, this implies that spatial audio signals and monaural audio with positional information 

are best for enhancing target search. This also implies that if it is not possible to use spatialized 

audio signals, monaural signals with positional information can be used just as effectively. If 

coding audio signals with spoken positional information is not possible, the signals without 

positional information should be spatialized. 

One hypothesis stated in this study concerned the possible effect of spatial audio cue disparity on 

target search time. It was hypothesized that the mapping scheme that uses the widest range of 

audio angles (±60 degrees) may produce significantly shorter response times than monaural 

displays but longer response time than displays with a narrower range of audio angles (±37 or 

±15 degrees) because of the greater disparity between audio and visual cues. The data indicated 

no such difference; target search times for the widest range of audio angles had no significantly 

different search times than those for the narrow range of angles. These findings are in 

accordance with Begault (1993, 1998). In terms of design standards, this indicates that either 

mapping scheme would be as effective. Future research should be conducted to determine 

whether more extensive ranges, such as angles of ±90 degrees (directly to the left and to the right 

of the listener), could cause cue disparity that might adversely affect target search time. 

Results also indicated that monaural signals without positional information were not as effective 

as monaural cues with positional information. In most locations on the screen, monaural signals 

without positional information were as ineffective as cues with visual only information, when no 

positional information was given for other conditions. This may have occurred because 

monaural signals with no positional information did not have any meaningful signal content 

because they did not convey any information to the participant. If the screen sometimes did not 

contain a target, then the monaural nonpositional signal would have been meaningful when heard 

(signaling the occurrence of a target event whenever it occurred), and response time to the signal 

might have been shorter. This implies that monaural signals are most efficient if coded with 

positional information and least efficient when they communicate no positional information. 

At all azimuths, the longest target search times were found for conditions in which visual stimuli 

only were seen when positional information was given in all other conditions. Visual signals 

only had target search times almost twice as large as those with spatialized audio or monaural 

signals with positional information. Conditions in which visual only was seen and in which no 

positional information was otherwise given had significantly shorter target search times but were 

still significantly greater than those of spatial audio and monaural signals with positional 

information. Why did participants respond differently to visual signals only when the audio 

signals they heard in other conditions are heard with or without positional information? Why did 

participants respond differently to visual only signals with and without positional information? 

One could surmise that if one were exposed to audio signals with positional information, then 

having to find a target without that information would be more cognitively demanding because 
15 

the amount of information unavailable is greater for conditions with no positional information. 

However, if this were true, one would expect the results to show a significant interaction for 

mapping scheme x order of presentation of mapping scheme x positional information, in which 

participants who received the visual only condition first would have significantly shorter target 

search times because they would not have been exposed to the other audio conditions first. 

However, this was not the case; this interaction was not significant. Future research is needed to 

explain this finding. The results imply that visual only is the least efficient in enhancing visual 

search. 

One might ask whether the observed performance benefits provided by auditory displays might 

have been the result of simply providing more information to the listener in the form of location 

or positional information. If information regarding the likely target location is available and can 

be encoded in auditory form, why could it not be encoded in some other channel (such as visual 

highlights or tactile cues), and would this not lead to the same performance benefit? Perhaps it 

could, but the main point of this study was to demonstrate how enlisting other channels such as 

auditory might provide value, especially when visual channels are overloaded or visual displays 

are over-subscribed. Future research should examine human performance in the presence of 

other display modalities (such as tactile displays or a combination of auditory and tactile 

displays) in the presence of better visual cueing in conditions in which visual channels have 

heavy demands placed upon them. 

4.2 Workload 

The finding that workload ratings for signals with visual only in the positional information 

condition were significantly greater than all other signals indicates that visual only is perceived 

as a high workload condition. The workload data are similar to those for the target search times 

in that signals with visual only in the positional information condition had scores almost twice as 

large as those for spatialized audio and monaural signals of any kind. As with the target search 

time data, one could guess that if the participant is exposed to audio signals with positional 

information and then has to search for a target in the visual only condition (without auditory 

cues), performing the task would be cognitively demanding because the amount of information 

unavailable is greater for conditions with no positional information. This is indeed reflected in 

the workload data. However, as with target search time data, if this were true, one would expect 

the results to show a significant interaction for mapping scheme x order of presentation of 

mapping scheme x positional information. However, this was not the case; there was no such 

significant interaction. Future research is needed to explain this finding. In terms of design 

standards, this implies that the presentation of visual only in conditions in which positional 

information is otherwise available is the least efficient in enhancing workload associated with 

visual search. 

As with the target search data, the nonsignificant difference in workload ratings between spatial 

audio and monaural signals of any kind implies that audio cues do not have to be spatialized to 
16 

provide effective target search cues and that monaural signals with positional information 

reduced search time to the same extent as those without positional information. Unlike the target 

search data, the workload data indicated there was no significant difference between visual only 

in the no-positional information condition and spatial and monaural signals of any kind. This 

implies that the presence of visual only in no-positional information conditions has a perceived 

workload at the same level as those conditions with spatial and monaural mappings, with and 

without positional information. Future research is needed to explain this finding. 

5. Conclusions 

The purpose of this study was to examine the effects of the integration of auditory display 

technologies in visual search tasks such as those in robotic OCUs by determining what types of 

audio signal mapping schemes and verbal positional cues are best. The results indicate that the 

use of auditory signal mapping and verbal positional cues significantly reduced visual display 

search time and that positional cues mixed with specific audio mappings were the most efficient 

means of reducing visual display search time. The results of this study led to specific design 

recommendations regarding the use of auditory signals in environments with narrow FOV visual 

displays, such as OCUs for robotic systems such as MATILDA and Buster. Future research 

should explore why target search time is longer in visual only conditions, especially in 

conditions in which audio positional information is otherwise provided. Future research should 

also examine human performance in the presence of other display modalities (such as tactile 

displays) and better visual cueing in conditions in which visual channels have heavy demands 

placed upon them. 
17 

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the Situational Understanding as an Enabler for Unit of Action Maneuver Team Soldiers 

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Patterson, Perrott, D. R.; Cisneros, J.; McKinley, R. L.; D’Angelo, W. R. Aurally aided 

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Perrott, D. R.; Sadralodabai, T.; Saberi, K.; Strybel, T. Z. Aurally aided visual search in the 

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CHI95 Proceedings, 1995. 
19 

Appendix A. Williams Square Designs Used in This Study 

Table A-1. Williams square design, order of presentation of auditory mapping scheme to participants. 

Participants Session 1 Session 2 Session 3 Session 4 Session 5 

1,11,21,31 37 15 60 V M 

2,12,22,32 15 V 37 M 60 

3,13,23,33 V M 15 60 37 

4,14,24,34 M 60 V 37 15 

5,15,25,35 60 37 M 15 V 

6,16,26,36 M V 60 15 37 

7,17,27 60 M 37 V 15 

8,18,28 37 60 15 M V 

9,19,29 15 37 V 60 M 

10,20,30 V 15 M 37 60 

M = Monaural (the same auditory signal is heard in both ears) 

60 = Spatial audio alerts given from 3 positions within a range of +60 degrees 

37 = Spatial audio alerts given from 3 positions within a range of +37 degrees 

15 = Spatial audio alerts given from 3 positions within a range of +15 degrees 

V = Visual Stimuli Only (no audio cues) 

Table A-2. Williams square design, order of presentation of visual target stimuli to participants. 

Participants Session 1 Session 2 Session 3 Session 4 Session 5 

1,11,21,31 C D B E A 

2,12,22,32 D E C A B 

3,13,23,33 E A D B C 

4,14,24,34 A B E C D 

5,15,25,35 B C A D E 

6,16,26,36 A E B D C 

7,17,27 B A C E D 

8,18,28 C B D A E 

9,19,29 D C E B A 

10,20,30 E D A C B 

A through E are five different randomization schemes 
20 

NO. OF 

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 (PDF INFORMATION CTR 

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 8725 JOHN J KINGMAN RD 

 STE 0944 

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 1 US ARMY RSRCH DEV & ENGRG CMD 

 SYSTEMS OF SYSTEMS 

 INTEGRATION 

 AMSRD SS T 

 6000 6TH ST STE 100 

 FORT BELVOIR VA 22060-5608 

 1 INST FOR ADVNCD TCHNLGY 

 THE UNIV OF TEXAS AT AUSTIN 

 3925 W BRAKER LN STE 400 

 AUSTIN TX 78759-5316 

 1 DIRECTOR 

 US ARMY RESEARCH LAB 

 IMNE ALC IMS 

 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

 1 DIRECTOR 

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 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

 2 DIRECTOR 

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 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

 3 US ARMY NATICK SOLDIER CTR 

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 ATTN J B SAMPSON 

 100 KANSAS ST 

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