A speech intelligibility test conforming to the Modified Rhyme Test of ANSI S3.2 "Method for Measuring the Intelligibility of Speech Over Communication Systems" was conducted using a prototype 12-channel acoustic beamformer system. The target speech material (signal) was identified against speech babble (noise), with calculated signal-noise ratios of 0, 5 and 10 dB. The signal was delivered at a fixed beam orientation of 135 deg (re 90 deg as the frontal direction of the array) and the noise at 135 deg (co-located) and 0 deg (separated). A significant improvement in intelligibility from 57% to 73% was found for spatial separation for the same signal-noise ratio (0 dB). Significant effects for improved intelligibility due to spatial separation were also found for higher signal-noise ratios (5 and 10 dB)

Begault, Durand R.

Godfroy, Martine

Otto, Peter

Sunder, Kaushik

NASA Technical Reports Server

Audio Engineering Society 
Convention e-Brief 
Presented at the 139th Convention 
2015 October 29–November 1 New York, USA 
This Engineering Brief was selected on the basis of a submitted synopsis. The author is solely responsible for its presentation, 
and the AES takes no responsibility for the contents. All rights reserved. Reproduction of this paper, or any portion thereof, is not 
permitted without direct permission from the Audio Engineering Society. 
 Speech Intelligibility Advantages using an 
Acoustic Beamformer Display 
 
Durand R. Begault1, Kaushik Sunder,1,2, Martine Godfroy1,2 and Peter Otto3  
1 NASA Ames Research Center, Moffett Field, CA 94035 
Durand.R.Begault@NASA.gov 
2 San Jose State University Foundation  
3 Cal IT2, University of California San Diego 
ABSTRACT 
A speech intelligibility test conforming to the Modified Rhyme Test of ANSI S3.2 “Method for Measuring the 
Intelligibility of Speech Over Communication Systems” was conducted using a prototype 12-channel acoustic 
beamformer system. The target speech material (signal) was identified against speech babble (noise), with calculated 
signal-noise ratios of 0, 5 and 10 dB. The signal was delivered at a fixed beam orientation of 135° (re 90° as the 
frontal direction of the array) and the noise at 135° (co-located) and 0° (separated). A significant improvement in 
intelligibility from 57% to 73% was found for spatial separation for the same signal-noise ratio (0 dB). Significant 
effects for improved intelligibility due to spatial separation were also found for higher signal-noise ratios (5 and 10 
dB).  
 
1. BACKGROUND 
The intelligibility advantage gained from spatial 
separation of multiple speech channels or “streams” is 
explained by the well-known “cocktail party effect” [1] 
and has been implemented in spatial auditory displays 
for teleconferencing or for radio communications for 
some time (e.g. [2]). The current study focuses on the 
use of a 12-channel phased array “beamformer” system 
to improve the intelligibility of a target communication 
stream heard simultaneously with an undesired 
communication stream. By strategically manipulating 
the positions of incoming communication streams in an 
auditory display, an intelligibility advantage should be 
gained. 
 
2. SUBJECTS 
Ten volunteer participants (ages 18-30) were recruited 
from the NASA Ames Human Systems Integration 
Division. Experimental procedures were compliant with 
Human Research Institutional Review Board protocols. 
All had normal hearing based on a screening 
questionnaire. 
AES 
 
https://ntrs.nasa.gov/search.jsp?R=20150020454 2019-08-31T05:39:44+00:00Z
Begault et al. SI Advantage using Beamforming Type shortened paper title here> 
 
AES 139th Convention, New York, USA, 2015 October 29–November 1 
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3. STIMULI GENERATION 
Participants were seated at a distance of 1 m from a 12-
channel acoustic beamform display comprised of a 
horizontal array of isolated 1 in drivers (modified 
Innovox MLA-ST-12 loudspeaker and custom amplifier 
hardware, “SoundBender” software from UC San Diego 
Cal-IT2, written within Max 7 software from Cycling 
‘74). The height of the array was adjusted to ear level. 
The speech playback level was set to a level of 62 dB. 
Figure 1 shows the level of pink noise as a function of 
beam azimuth, measured at 1 m in front (90°). Figure 2 
shows a diagram indicating the two beam azimuth 
directions (0° and 135°) used in the experiment. The 
experiment used a 15 s LAeq of speech material to 
calibrate levels relative to a 62 dB(A) playback level at 
the 135° position where the subject was seated. 
4. EXPERIMENT TASK, CONDITIONS 
The methodology used conformed to the Modified 
Rhyme Test (MRT) from ANSI S3.2 “Method for 
Measuring the Intelligibility of Speech Over 
Communication Systems” [3]. The test is a six-
alternative forced choice of target words that differ in 
either their initial or final consonant. Within each of five 
experimental blocks, 50 trials of the six words 
corresponding to the 50 word sets of the MRT were 
presented. Blocks were repeated twice in random order. 
The stimuli presented during each trial consisted of two 
simultaneous speech signals, as follows. A speech 
signal stimulus, i.e. a target word from the MRT list, 
duration ~1.0–1.2 s, was played at a random start time 
following the start of the block or after the completion 
of a trial. In addition, a multilayered stream of speech 
babble was played continuously during each block. The 
speech babble was randomly mixed from three male and 
three female talker recordings. Three of the talkers’ 
words were uttered in reverse. 
For each MRT word, six different talkers (three male, 
three female) were randomized between trials. Signals 
were always presented in the same direction towards the 
listener (135°) while the position of the babble was 
varied between 135° and 0° (ref. Figure 2). 
Subjects indicated the target word that was heard from 
the six alternatives, which were presented on a touch 
screen surface (Apple iPad) immediately after the 
presentation of the target audio stimulus. Each answer 
was recorded by the experimental software, which then 
triggered the next stimulus.  
A total of ten experimental blocks were presented to 
each subject in randomized order, to test spatial location 
and playback level as independent variables, in a 
repeated measures within-subjects design. Table I 
shows the spatial locations of signal and noise and their 
relative levels; a “block ID #” is provided as a shorthand 
reference to the condition. Each of these five blocks 
were evaluated twice by each subject.  Each test resulted 
collectively in a total of 1000 responses per block type. 
Subjects took an average of 7-8 minutes to complete a 
block. 
Figure 1 Level of Pink noise as a function of 
beam azimuth, measured at 1 m in front (90°).  
 
Figure 2 Configuration of listener, beamformer and 
stimuli angles used in the experiment. 
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Block 
type # 
Signal 
position 
Noise 
position 
Noise  
level 
S/N 
ratio 
1 135 135 62 0 
2 135 0 52 10 
3 135 0 62 0 
4 135 135 57 5 
5 135 0 57 5 
Table I. Experimental conditions. The playback level of 
the signal was held constant at 62 dB(A). 
Selected pairs of blocks were chosen to evaluate if there 
was a significant improvement in speech intelligibility 
as a function of spatial location and signal-noise (S/N) 
ratio. There are two spatially co-located condition 
blocks with the sound beam projecting both signal and 
noise to 135° and three non co-located conditions with 
the signal beam at 135° and noise at 0° (ref. Figure 2). 
-Block 1 compared to block 2: the advantage, if any, 
of spatial separation of the speech babble to a 
different location. The spatial separation causes a net 
S/N increase due to a decrease in level of the babble 
by 10 decibels at the listener position (ref. Figure 1). 
-Block 1 compared to block 3: if any, of spatial 
separation of the speech babble to a different location, 
with S/N adjusted to 0 decibels (by increasing the 
level of the speech babble by 10 dB). Unlike block 2, 
the S/N ratio in block 3 was the same as for block 1.  
-Block 4 compared to block 5; the advantage, if any, 
of spatial separation of the speech babble to a 
different location, with the S/N ratio held constant at 
5 dB. 
We also checked the significance of S/N ratio with 
spatial location fixed (blocks 1 & 4, co-located spatial 
position; blocks 2, 3 and 5, non co-located spatial 
positions). Finally, we investigated differences in fixed 
S/N ratio and varied spatial position, and fixed spatial 
position and varied S/N ratio. 
Subjects also rated their confidence in their responses 
after completing each block. A ten-point scale was used, 
anchored by the words “minimally confident” (0) and 
“maximally confident” (10). This allowed evaluation of 
the relationship between objective and subjective 
performance. 
5. RESULTS 
5.1. Effect of Spatial Separation  
With the S/N ratio of 10 dB (comparison of block 1 to 
block 2), spatial separation of the signal from speech 
babble resulted in an increased rate of correct responses 
from 64.2% to 91.4%, a difference that was statistically 
significant 𝑋!! = 214.55, 𝑝 < .0001 . 
With the same S/N ratio 0 dB (comparison of block 1 to 
block 3), spatial separation of the signal from speech 
babble resulted in a significant increase in the 
percentage of correct responses, i.e. 64.2% to 
77.1%   𝑋!! = 40.31, 𝑝 < .0001 . 
With the same S/N ratio of 5 dB (comparison of block 4 
to block 5), spatial separation of the signal from speech 
babble resulted in a significant increase in the 
percentage of correct responses from 82.8% to 
86.3%   𝑋!! = 4.68, 𝑝 = .03 . 
5.2. Effect of S/N ratio 
With spatial location fixed at 135° and noise at 135° 
(comparison of block 1 to block 4), the increase in S/N 
ratio from 0 to 5 dB led to significantly greater 
percentage of correct responses, 64.2% to 82.8% 𝑋!! = 89.08, 𝑝 < .0001 . 
With spatial location of signal at 135° and noise at 0°, 
the increase in S/N ratio from 0 dB (block 3) to 5dB 
(block 5) to 10 dB (block 2) increased significantly the 
percentage of correct responses from 77.1% to 86.3% to 
91.4% 0, 5  dB:  𝑋!! = 28.30, 𝑝 < .0001;   5, 10 ∶   𝑋!! =13.12, 𝑝 < .0001 . 
Intelligibility is scored in terms of the percentage of 
adjusted correct answers out of the total number of 
responses. All data were corrected for the possibility of 
guessing per [3] by implementing	  the formula	  (adjusted 
correct answers  = correct answers – (wrong answers / 
5)). The corrected values are shown in Table II. 
Begault et al. SI Advantage using Beamforming Type shortened paper title here> 
 
AES 139th Convention, New York, USA, 2015 October 29–November 1 
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Block type # Raw Score % Adjusted Score % 
1 64.2 56.9 
2 91.4 89.6 
3 77.1 72.5 
4 82.8 79.3 
5 86.3 83.5 
Table II. Adjusted percentages for intelligibility by 
block type. 
A Kruskal-Wallis H test was conducted to determine if 
there were significant differences in the confidence 
scores rated by the subjects.  
The mean rankings increased 1824–2065–3068, 
respectively, for S/N ratios of 0, 5, and 10 dB, all 
significantly different (0, 5: 𝑋!! = 581.66, 𝑝 < .0001; 
5,10:   𝑋!! = 18.14, 𝑝 < .0001). 
Significant differences were also observed on 
comparing the mean confidence scores across the 
different spatial locations. Indeed, the mean ranking was 
significantly higher for spatially separated positions (0°: 
mean rank = 2779, 135°: mean rank = 2080,   𝑋!! =309.99, 𝑝 < .0001). 
We were interested in whether or not accuracy of 
responses was affected by the initial or final consonant 
sounds in the six alternative forced choice. The six 
words in each MRT ensemble differ only in the initial or 
final consonant. The results showed that the rate of 
correct responses was significantly higher by 6% when 
the initial consonant was different (“thaw” vs. “jaw”) 
than when the final consonant was different (“sum” vs. 
“sun”) (initial: 83.3%, final: 77.3%;   𝑋!! = 29.10, 𝑝 <.0001).  
Further analyses of initial versus final consonant 
advantage showed no significant difference for S/N of 0 
dB for the S135, N0 condition. At S/N 5 and 10 dB, 
there were significant differences (𝑝 < .05 or better) 
with MRT ensembles with the initial consonant 
difference having greater intelligibility. For all S135, 
N135 conditions, the S/N comparisons were significant 
favoring the initial consonant difference  (𝑝 < .05  or 
better). 
6. DISCUSSION 
The increase in intelligibility as a function of S/N is not 
surprising. However, we found that with a fixed S/N 
ratio, spatial separation contributes a ~16% increase in 
intelligibility. This indicates an inherent advantage to a 
spatial auditory display using beamforming independent 
of gain adjustment. The spatial separation with its 
inherent S/N advantage represents an additive effect, 
yielding a ~33% intelligibility advantage. 
Subjective ratings of confidence in intelligibility were 
found to parallel objective increases in intelligibility. 
We also found that intelligibility was increased when 
the initial consonant of the MRT word ensemble was 
different, compared to those ensembles where the final 
consonant was different. Similar results have been 
reported in the literature [4].The only exception to this 
was when the S/N ratio was 0 and the spatial locations 
were separated.  
7. ACKNOWLEDGMENTS 
This work was supported by NASA’s System-wide 
Safety Assurance Technologies (SSAT) Project; and a 
NASA Space Act Agreement between CalIT2 and UC 
San Diego. We appreciate the assistance of Mark R. 
Anderson and our colleagues in the Advanced Controls 
and Displays Laboratory at NASA Ames’ Human 
Systems Integration Division.  
8. REFERENCES 
[1] E. C. Cherry, “Some experiments on the 
recognition of speech with one and two ears,” J. 
Acoustical Soc. Am., vol. 25,  pp. 975-979. 
[2] D. R. Begault, 3-D Sound for Virtual Reality and 
Multimedia (Academic Press Professional, 
Cambridge, MA, 1994). 
[3] ANSI S3.2-2009, “Method For Measuring The 
Intelligibility Of Speech Over Communication 
Systems”, Melville, N.Y.: Acoustical Soc. of 
America. 
[4] M. A. Redford and R. L. Diehl, “The relative 
perceptual distinctiveness of initial and final 
consonants in CVC syllables” J. Acoustical Soc. 
Am., vol. 106,  pp. 1555-1565 (1999). 
 


Speech Intelligibility Advantages using an Acoustic Beamformer Display

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