Introduction
The ability to locate a sound source is present in human beings with normal hearing
from birth. It is a primitive capacity that is triggered at the level of the lower
brainstem.[1] It is directly related to the ability to obtain binaural and three-dimensional sound
information.[2]
Knowing the direction from which the sound comes is important for the individual to
orient himself in space, to prevent danger (such as traffic danger), and to turn his
attention to important daily sounds.
The brain compares the information received by the ears, such as the difference between
the sound receiving time and its intensity in the ears. These two indicators allow
determining the sound location and directional transfer.[3]
Even individuals with normal hearing tend to have difficulties locating a sound source
in unsuitable environmental conditions (that is, noisy environments), and this situation
is aggravated when the person has some type of hearing loss.[4]
[5]
Locating the sound source becomes a problem[6]
[7] to people with unilateral or asymmetric hearing loss. There are cases in which the
difficulties with listening in different daily situations generate psychological alterations.
These alterations may seem disproportionate to the level of acoustic hearing residual
loss.[8]
[9]
In these cases, a hearing aid has been a widely used resource. However, its use does
not always result in a specific improvement of the localization ability.[10] As examples, we cite the cochlear implants in unilateral hearing loss,[11] the use of prostheses anchored in the bone in unilateral losses,[12] the cross-system of hearing aids,[13] and even conventional prostheses adapted to severe asymmetric hearing losses.[14] In all cases, assessing the ability to locate a sound source can be a good resource
for regulating the device[15] and for checking the benefits of the device fitting, especially if there is a competitive
noise.
Thus, evaluating the capacity of locating the sound has been a constant demand in
speech therapy. Tests were developed for this purpose, such as the Spatial Sound Perception
Analyzer (ASPE, in the Portuguese acronym)[16] and the sound localization test,[17] both involving laboratories with several speakers for multidirectional sound evaluation.
Although they are effective in their objectives, the reproduction of many tests is
unfeasible in clinical settings, such as medical or speech-therapy clinics. That is
because they require a lot of equipment, which makes the procedure costly.
Based on these assumptions, as well as on the purpose of evaluating the benefits of
hearing aids on the locating ability, the sound localization test was developed (sound
localization test in presence of noise). It is an accessible and easy to apply test
that was validated in a pilot study. Subsequently, it was tested in individuals with
normal hearing. Therefore, the purpose of the present study is to describe, validate
and establish the normality curve of the sound localization test in subjects with
normal hearing.
Materials and Methods
This is a clinical study with experimental aspects. It is self-controlled to ascertain
the accuracy of the sound localization test. The study was approved by the Ethics
Committee of the institution under the number 0051/14, and data collection occurred
between February and April 2015.
A total of 103 subjects participated in the present study; 3 of them were excluded
from the study because they presented hearing loss in a tonal audiometry exam. Considering
the criteria proposed by the Federal Council of Speech Therapy (CFFa, in the Portuguese
acronym),[18] all of the 100 subjects that comprised the sample had normal hearing. Of the total
sample, 17 subjects were men and 83 were women, aged between 19 and 64 years old.
The average age was 34.1 years old, with a standard deviation (SD) of 10.8 years.
The participants were randomly selected from a waiting room at a clinic. After receiving
orientation about the study, they signed an informed consent form.
To perform the present research, the following device was specially developed: a booth
with a two-channel audiometer, a three-output free field device, as well as a noise
source attached to the audiometer, and a pure tone localization test. The pure tones
were randomly presented in the side speakers, sometimes to the left, other times to
the right of the subject being tested. All of the equipment was calibrated according
to the norms of the CFFa.[19]
Test Setting
The test setting consisted of an audiometric booth containing 2 speakers located at
45° (to the right and to the left of the subject being tested), which are routinely
used in the speech-therapy clinic for free field evaluation, and a third speaker,
or third channel, installed at 180°. The speakers were distributed in a way to allow
a better perception[5] and practical organization of the booth, as shown in [Fig. 1].
Fig. 1 Positioning of the acoustic speakers in the booth. (1) Speaker placed at 45° to the
right of the subject. (2) Speaker placed at 45° to the left of the subject. (3). Speaker
placed at 180°, with noise.
The free field (CL30-V, Oto Sonic, São Paulo, SP, Brazil) used was model without a
serial number, calibrated on October 17, 2014, with the certificate number 415–2013F,
according to ISO8253–3 and IEC645–2; 1993. The standards were met by using the following
devices for calibration: Sound Pressure Meter Larson Davis, Mod 824 (Larson Davis,
Depew, NY, USA), serial number 824A2867 (Certificate number 50 381/2014); Sound Calibrator
Larson Davis, model CAL 250 (Larson Davis, Depew, NY, USA), serial number 4128 (Certificate
No: 50 378/2014); and Microphone Larson Davis, Mod 2575 (Larson Davis, Depew, NY,
USA), serial number 1698 (certificate number 50 379/2014).
The third channel for the free field was developed to control and amplify a third
sound source used as a competitive signal inside the booth. It has a circuit consisting
of the following: an input preamplifier, a calibration circuit with gain adjustment
from 0 to 40 dB, a linear output attenuator with 5 dB steps and a total range from
0 to 100 dB SPL, a T-class digital power amplifier with 50 watts, a microchip micro
controller, model free field (PIC18F2550, Master Audiology, Curitiba, PR, Brazil),
a display of 2 lines by 20 characters, and a keyboard. The third channel equipment
was calibrated on July 31, 2014, with the certificate number 425a-2014-F, according
to ISO8253–3 and IEC645–2; 1993. The standards were met by using the following devices
for calibration: Brüel & Kjaer, Mod. 2250 (Brüel & Kjaer Sound & Vibration Measurement
A/S, Naerum, Denmark), serial number 3006245 (Certificate number CBR1400264/2014);
Acoustic Calibrator Brüel & Kjaer, mod. 4231 (Brüel & Kjaer Sound & Vibration Measurement
A/S, Naerum, Denmark), serial number 3007539 (Certificate number CBR1400268/2014);
and Larson Davis Microphone, Mod. 2575 (Larson Davis, Depew, NY, USA), serial number
2119 (Certificate number 60.381/2014).
The operation of the system allows the adjustment of the external sound source. In
the present project, the researchers used a Samsung cellular phone (Galaxy CS, Samsung,
Seul, South Korea) with the Sound Generator (SG) application. The application was
configured to generate white noise (broad band). The professional can calibrate the
correct level of the signal, used by the calibration mode of the equipment, by displaying
the signal and by adjusting it to 0 dB in the volume unit (VU). Once set, the signal
can be displayed at the level selected in the attenuator through the stimulus button
that turns the noise on or off.
Test
The sound localization test comprised 30 stimuli (pure sounds) that were presented
in the free field.
The synthesis and the recording of the test sounds were performed using the Sony Sound
Forge 11 sound editing software (Sony Corporation, Tokyo, Japan). Sounds at 1,000 Hz
at a level of 0 dB were generated in uncompressed .wav files to ensure fidelity and
to avoid distortions and cross-referencing data from one channel to another.
The tones used had a duration of 1 second, and they respect the rising and falling
times commonly used in the presentation of stimuli of audiometers (100 ms, according
to IEC 60645–1), and the silence breaks between the tones had a duration of 2 seconds.
Four different test files were created; one for training, and the others for assessment,
in which the stimulus sequence contains 10 presentations of the sounds. They are presented
to the right or to the left of the subject. In conjunction with the sound sequences,
follow-up guides were generated so that the examiner knows the sequence and the sides
of the presentation, and is able to validate the responses.
Data Collection
All of the subjects filled out an identification form and were submitted to an otoscopy
and to a threshold tonal audiometry to determine their auditory thresholds. After
the audiometry, those who had hearing within the normal standards were submitted to
the sound localization test.
The subject being tested was sitting in the center of the booth without headphones.
He listened to 30 stimuli (pure tones) that randomly came from the speakers located
at 45°. The individual was instructed to pay attention to the whistle sound and to
point to the right or to left side, according to where the sound came from. Background
noise should be ignored. Then the training was performed with a sequence of 10 stimuli.
The test was applied in 3 situations: a) 10 pure tone stimuli, alternated between
the right and left speakers, presented without competitive noise (control); b) 10
pure tone stimuli, alternated between the right and left speakers, presented in a
signal-to-noise ratio of 0 dB (study); c) 10 pure tone stimuli, alternated between
the right and left speakers, presented in a signal-to-noise ratio of - 10 dB (study).
The pure tone was presented at 40 dBNS, that is, 40 dB above the tri-toned average
obtained in the tone threshold audiometry. In the signal to noise ratio of 0 dB, the
whistle and the noise were set at the same intensity. At the signal-to-noise ratio
of - 10dB, the noise was 10 dB louder than the stimulus (pure tone), the stimulus
was presented in the field system (speakers at 45°), and the competitive noise was
presented in the third channel (at 180°).
Data Analysis
The data were noted in the protocol of self-register ([Table 1]) and analyzed statistically. For comparison purposes, the Student t-test was used at a significance level of 0.05.
Table 1
Sound localization test answer sheet
|
Tri-toned Average
|
|
Pure Tone of 1,000 Hz
|
|
_ dB
|
|
01
|
02
|
03
|
04
|
05
|
06
|
07
|
08
|
09
|
10
|
|
Absence of noise
|
R
|
R
|
L
|
R
|
L
|
L
|
L
|
R
|
R
|
L
|
|
Presence of noise
|
relation
0 dB
|
L
|
R
|
R
|
L
|
R
|
R
|
L
|
L
|
L
|
R
|
|
relation
−10 dB
|
L
|
R
|
L
|
L
|
R
|
R
|
L
|
R
|
L
|
R
|
Abbreviations: L, the sound comes from the speaker to the left of the subject; R,
the sound comes from the speaker to the right of the subject.
The following variables were analyzed and compared: noise-free test results (control)
with the results of the test with noise at 0 dB and at −10 dB (study). The test results
also took into consideration the interference of the age factor. Two age groups were
analyzed, considering the wide age range of the sample. Notice that G1 was formed
by subjects < 40 years old, and G2 by people aged ≥ 40.1 years old. Finally, the test
results were analyzed according to gender.
Results
A total of 17 men and 83 women participated in the present study. All of them had
normal hearing, considering the criteria proposed by the CFFa.[18] All of the participants easily understood the requested task, and the average duration
of the test was 99 seconds, considering the application of the 3 steps.
The use of sound stimuli at 40 dB SPL ensured that all subjects were listening to
both sounds, that is, to the stimuli and to the background noise.
The descriptive results considering the location of the sound source with and without
the presence of noise are described in [Table 2], where it is possible to verify that the presence of noise did not cause a reduction
in performance.
Table 2
General descriptive data acquired in the sound localization test (n = 100)
|
Listening conditions
|
Minimum number of correct answers
|
Maximum number of correct answers
|
Average correct answers
|
Median
|
Standard deviation
|
|
With no competitive noise
|
1
|
10
|
7.24
|
9
|
3.04
|
|
Competitive in relation to 0 dB
|
1
|
10
|
7.08
|
7.5
|
3.05
|
|
Competitive in relation to −10 dB
|
0
|
10
|
7.03
|
8
|
3.08
|
The results of the test considering the interference of the age factor are described
in [Table 3], and the test results evaluating the outcome by gender are presented in [Table 4].
Table 3
Sound localization test results according to age (n = 100)
|
Variable
|
< 40 years old
|
≥ 40.1 years old
|
p-value
|
|
Average correct answers
|
Standard curve
|
n
|
Mean
|
Standard
deviation
|
p
|
|
With no competitive noise
|
72
|
7.19
|
3.05
|
28
|
7.29
|
3.02
|
0.4415
|
|
Competitive in relation to 0 dB
|
72
|
7.02
|
3.06
|
28
|
7.09
|
3.07
|
0.4592
|
|
Competitive in relation to 10 dB
|
72
|
6.99
|
3.10
|
28
|
7.05
|
3.09
|
0.4654
|
Statistical analysis: Student t-test.
Level of significance: 0.05.
Table 4
Sound localization test results according to gender (n= 100)
|
Variable
|
Female
|
Male
|
p-value
|
|
n
|
Average correct answers
|
Standard curve
|
n
|
Mean
|
Standard
deviation
|
|
With no competitive noise
|
83
|
7.22
|
3.04
|
17
|
7.27
|
3.01
|
0.4754
|
|
Competitive in relation to 0 dB
|
83
|
7.05
|
3.06
|
17
|
7.08
|
3.06
|
0.4853
|
|
Competitive in relation to −10 dB
|
83
|
7.00
|
3.08
|
17
|
7.04
|
3.07
|
0.4806
|
Statistical analysis: Student t-test
Level of significance: 0.05.
No significant difference was observed (p > 0.05) in the Student t-test, at the significance level of 0.05 (5%), among the results obtained from both
groups in relation to the variables age and gender.
Discussion
The ability to locate a sound source connects an individual to the environment in
which they live.[20] It allows three-dimensional perception of sounds, which facilitates speech intelligibility.
Therefore, it is an area of study in audiology, since people with different hearing
losses, including unilateral loss, complain of comprehension difficulties, especially
in noisy environments.[21]
Evaluating this ability has been a challenge to speech therapists. The tests developed
in the academia[16]
[17] are often not feasible because they demand a sophisticated laboratory, consisting
of several speakers, all of them installed in a free field, which raises the costs
of these studies.
In daily life, noise can affect people from different angles. However, a test that
reproduces these conditions is not feasible in practical speech therapy due to the
necessary cost and time. Thus, the noise proposed by the test described here occurred
at an angle of 180°.
The test proposed presented low cost, easy setup and application, in addition to using
standardized material (recorded), which ensured the reliability of the results.[22]
[23]
When analyzing the mean of the correct answers of the subjects ([Table 1]) in the sound localization test, it was observed that there was no significant difference
when considering the conditions of listening without noise and with competitive noise.
The scores obtained were practically identical, ranging from 7.03 to 7.24. Thus, in
individuals with normal hearing, it is expected that the subjects correctly locate
70% of the presented stimuli.
In people with symmetric bilateral hearing, the ability to locate the sound source
develops naturally, resulting from the comparison between the arrival times of the
stimulus in the auditory pathways within the brainstem.[24]
When the age variable was tested, no significant difference between the subgroups
was found, even with the evidence that cochlear degeneration and structural changes
in the auditory nerve and central pathways in the brainstem are associated with the
aging process.[25]
The same results were observed when the gender was tested as the variable. Studies
indicate that men have better location abilities than women;[26]
[27] however this difference was not significant in the study.
It can be inferred, therefore, that the normality standard of the sound localization
test is 70% correct answers, for people with normal hearing, regardless of age or
gender.
The test described in the present study can be a valuable tool in the investigation
of auditory processing, in the selection and recommendation of hearing aids, as well
as in the performance assessment of patients who use sound amplification and implantable
prostheses. Currently, this type of investigation is based on the subjective perception
of the individual, taking into account questionnaires of performance verification.[28]
