Download Pitch and rhythmic pattern discrimination of percussion

Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
PROCEEDINGS of the 22nd International Congress on Acoustics
Psychological and Physiological Acoustics (others):
Paper ICA2016-279
Pitch and rhythmic pattern discrimination of
percussion instruments for cochlear implant users
Federico Nahuel Cacavelos(a), Ricardo L. Marengo(b) Shin-ichi Sato(c) Florent Masson(d)
(a)
Universidad Nacional de Tres de Febrero, Argentina/Universidad de San Buenaventura Medellín, Colombia, [email protected]
(b)
Grupo CIAC, Argentina, [email protected]
(c)
Universidad Nacional de Tres de Febrero, Argentina, [email protected]
(d)
Universidad Nacional de Tres de Febrero, Argentina, [email protected]
Abstract
Cochlear implants are mostly designed for speech understanding of people with hearing
impairments. The appreciation of music by cochlear implant (CI) users is still under study. This
work investigates the capability of CI users to recognize impulsive pitch sounds. It focuses on
the discrimination of rhythmic patterns using percussion instruments samples from a kick, a
snare and a ride cymbal. These signals were tuned, without changing the natural sound, into an
easily recognizable pitch for CI users and were presented with different rhythmic patterns. An
ABX test was carried out with two groups: 30 normal hearing (NH) subjects and 7 people with
CI. The test was divided into two sessions. First, the same instrument was used for the whole
pattern of each stimulus and the three instruments were compared to each other. Second, the
samples of the three instruments were combined in each stimulus and the instrument of only
one sample was varied and compared. Results were compared with previous studies which
used the continuous tones as test signals. They showed that both groups can distinguish
rhythmic differences. However NH subjects can easily recognize patterns with different sample
while the CI users have much more difficulties than NH users to discriminate pitch difference in
combined samples composed by different percussion instruments sounds.
Keywords: Cochlear implant, subjective test, pitch, impulsive, discrimination.
22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
Pitch and rhythmic pattern discrimination of
percussion instruments for cochlear implant users
1 Introduction
Music perception of Cochlear Implant users (CI) has been under study for more than 25 years. Most of
the studies investigated the ability of melodic contour identification and timbre recognition, as well as
emotional communication, rhythm and meter recognition [1].
The central auditory processing in human being has two possible mechanism to perceive pitch. The
temporal theory considers that the perception of time periods smaller than 1 ms enabling people to
perceive frequencies up to 1 kHz. This is essential to recognize the fine structure of signals. For CI users
this mechanism can be hampered by the low stimulation rates used in some coding strategies. The other
mechanism used by the central auditory processing is the place theory. It describes how resonances of
the basilar membrane produce a spatial arrangement of frequencies, commonly called the tonotopic
distribution [2]. The CI cannot stimulate two sectors of the cochlea simultaneously due to electrical
interference between electrodes. Therefore the codification strategies keep just the signal’s peaks of a
period, discarding the weak intensity. This is why CI users have difficulty to detect fine structure of pitch
information and impulsive sounds.
Cognitive factors may also disturb the identification process. Disturbance Auditory Processing is a
hearing impairment in analyzing and/or interpreting sound patterns. In this sense it is necessary to attend
the central auditory processing [2, 3]. Several studies clarified that people with hearing impairments,
including CI users, perceive rhythm approximately as well as those with normal hearing [1, 4-6]. Gfeller
and Lansing performed a test called PMMA (Primary Measures of Music Audition developed by Gordon)
to 18 postlingual deafened CI subjects [8]. Mean identification scores on the rhythm subtest (88%) were
higher than on the tonal subtest (78%). Leal et al. conducted rhythm discrimination tasks with twenty-nine
postlingual deafened adults and showed their score was 95% [9]. Kong et al. performed an identification
task with four tempo conditions (60, 80, 100, and 120 beats per minute) and found no significant
difference between normal hearing subjects and cochlear implant users [10]. Their participants were
asked to read and chose the musical notation displayed on the screen that corresponded to the rhythmic
pattern presented.
There are few works addressing the perception of impulsive signal sounds and possible identification of
CI users. Therefore this study investigates the identification of impulsive signals by using percussion
instruments that were tuned in easily distinguishable tones for CI users. The hypothesis is that CI users
should perform worse pitch discrimination of impulsive signals than tone signals due to their codification
strategies.
After describing the methods and procedures for this research, the results of the tests are analyzed and
compared with previous studies.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
2 Procedure
2.1
Stimuli preparation
This work combines the methodologies of previous studies on pitch and rhythm discriminations [5, 11].
Source signal consist of rhythmic patterns made with three percussion instruments as the kick, snare and
ride cymbal. In order to get clear pitch identification, each percussion instrument was carefully processed
and transposed to have a 6 semitone distances between each instrument. The kick sound was adjusted
to a F3 (170 Hz), the snare to a C4 (261 Hz) and the ride cymbal to a G4 (390 Hz). This distance was
based on the minimum change condition found by Kim et al. [6]. Their spectrum is shown in Figure 1. The
three instrument signals were normalized in peak amplitude because of peak detection method used in
the common coding strategies. These signals were then used to create different rhythmic patterns as test
stimuli.
Figure 1: The three pitched instrument spectrums used in the patterns.
The test was divided in two different sessions. In Session 1, the three instruments are compared to each
other using the same pattern. This attempts to discern if CI users really perceive the difference in
impulsive pitch. An example of the MIDI sheets to the first session is shown in Figure 2a. In this figure
each frame corresponds to a sixteenth note.
In Session 2, the three instruments were combined using just a single pattern (Figure 2b). Between two
different stimuli just one note for one instrument was changed. This aims to analyze the discrimination in
a combined pattern of different instruments, providing more complex information for the recognition. In
both sessions, the number of notes for all rhythmic configurations was fixed to six, showing good
appreciation by Kim et al. [6] after testing with different quantities of tone intervals.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
(a)
(b)
Figure 2: Rhythmic patterns consisting of the three instruments for Session 1 (a) and Session 2
(b).
All sounds were made in Albeton Live 7.0 using an Electric Drum Roland DR-909 and DR-707, and
transpose of drum rack of native software. The tempo condition was fixed to 120 beats per minute (bpm)
following the results of Kong [10] since he did not find significant differences in discrimination skills when
the bpm was changed.
2.2
Subjective Test
An ABX test was conducted to examine the discrimination of the stimuli. In order to increase the number
of trials and improve the confidence interval of the responses, Session 1 was repeated twice while
Session 2 was repeated three times since more variation was expected in the results of Session 2. Thus,
in total 15 combinations were used in the ABX test. A self-developed algorithm was used in order to
present the stimuli randomly. For each repetition the sequence of the stimuli was changed to avoid the
subject to remember the judgments of the previous pattern. The subjects were asked to identify if the last
signal (X signal) is equal to A (X = A) or B (X = B). Each ABX combination was played only once.
The test was carried out with two groups: 30 normal hearing (NH) subjects of 20-55 years old and 7
postlingual CI user of 20-52 years old (more than 2 years of experience with CI). Implants with electroacoustic stimulation (EAS) were not considered in this study. Test environments were different for both
groups. The NH subjects conducted the tests using a headphone Audio Technica ATH-D49 in a quiet
room (NC 25) with acoustic treatment For CI users the line out of the processor was used to avoid the
effects of the acoustic field. In all cases a Notebook computer with M-Audio Fast Track was used to
perform the test. In both cases, the stimuli were presented to only one ear. The subjects were asked to
adjust their own conformable sound level. Total duration of the test was of approximately 5 minutes for
each subject depending on the time spent for the judgments.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
3 Results
The percentage of correct answers in the ABX test was calculated for all subjects of both groups. Figure
3a shows the results of Session 1 where the different instruments using the same pattern are compared.
Figure 3b refers to Session 2 where patterns are composed by different instrument combinations.
By the assumption that the statically analysis of an ABX test follows a binomial distribution, [12]. It was
able to obtain a 95% of confident interval of each result.
(a)
(b)
Figure 3. Percentage of correct answers for Normal Hearing subjects (NH) and Cochlear Implant
users (CI) for Session 1 (a) and Session 2 (b). The error bars show a 95% confident interval.
4 Discussion
The percentage of correct answers for NH subjects was more than 89 % in the combined patterns
(Session 2) while almost 100% for the patterns with the same instrument (Session 1). As shown in Table
1, these results are consistent according to the previous studies where rhythm discrimination was
performed.
Although the numbers of responses by CI users were not sufficient to permit a reliable data analysis, it is
possible to see that CI user performed worse than NH users. The present results for the CI users
revealed lower performance than the previous studies for a rhythm discrimination.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
Table 1: Comparison of the percentage of correct answers with previous studies
Present study
(Impulsive sounds)
Previous studies
(Continuous notes)
Session 1
Session 2
Sucher and McDermott [5]
Gfeller and Lansing [8]
Schulz and Kerber [11]
NH subjects
95%
89%
89%
95%
95%
CI users
86%
45%
60%
78%
68-84%
Both groups were able to distinguish pitch differences in accordance with previous studies. However CI
users have much more difficulties in discriminate pitch in rhythms composed by different percussion
instruments sounds than NH subjects. This result is consistent with the frequency tuning distance of Kim
et al. [6].
It is interesting to point out that the previous studies used a continuous note while this study used
impulsive sounds. As expected, impulsive sounds composed by different instrument combinations leaded
to lower performance in rhythm discrimination for IC users.
In the ABX test, some CI users showed a difficulty in the discrimination tasks, and the forced choice
obliged them to respond randomly. Furthermore, the subjects expressed that they were not able to
identify certain sounds but recognized the difference by the loudness rather than the pitch components.
One of the reasons for this is because CI users may have different performance of their calibration.
Sounds coincide with specific channels having different impedance and thus generate loudness
differences, easily recognizable by the CI users.
Some parameters could not be controlled in both groups. The most important factor is the degree of
subjects’ attention during the test because loss of attention may cause a decrease of the test
performance. This can be due to limited memory capacities and slower processing speed since IC users
normally presents disorders of the central auditory processing according to Oscar Cañete [4].
Furthermore different models, settings and codification strategy of the CI devices may add wide variation
to the identification process of the subject. Several authors also pointed out that there is a great variation
between the perceptual skills of users because of the resilience of the subjects to electrical stimulation of
their particular physiological characteristics [1, 6].
5 Conclusions
As expected in the hypothesis, the CI users have much more difficulties to discriminate the impulsive
pitch rhythms than the NH subjects. Particularly, it is much more difficult for them when the signal is
composed by a combination of different impulsive instruments. Comparison with previous studies show
that for the CI users, discrimination of impulsive pitch is more complex than that of continuous tones. This
effect has not been observed for the NH subjects.
A further development of this study could reveal valuable data for understanding the phenomenon
involving a substantial advance of the hearing aid mechanisms. To achieve more significant results the
number of CI users will be increased.
Possible future studies can include others factors by using more frequencies in order to achieve greater
accuracy in the cochlear implant identification although the previous studies using continuous tone
showed no variation in frequency scale. Some cochlear implant users can identify a difference between
sounds but it does not mean that they can perceive the sounds correctly. For this reason, future works
may consider more numbers of instrument sounds with different harmonic compositions.
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22nd International Congress on Acoustics, ICA 2016
Buenos Aires – 5 to 9 September, 2016
Acoustics for the 21st Century…
Acknowledgments
The authors would like to acknowledge the Board of the International Commission for Acoustics and ICAASA Young Scientist Conference Attendance Grants Programme for their financial support to present this
work at the congress. The authors also express special gratitude to Nilda Vechiatti for her support in the
application for this grant. The deepest gratitude is delivered to the CIAC Argentina institution to provide
test environment and the participants for the test as well as the assistance and dedication of ST.
Mastroianni S. Also a special thanks to the people who participated in the test without financial
remuneration.
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