AN INVESTIGATION INTO AUDIOLOGICAL FINDINGS IN INDIVIDUALS EXPOSED TO NOISY ENVIRONMENTS

Summary

Materials and Methods: A total of 80 male participants, (40 noise-exposed [NE], 40 controls), aged 18-50 years were included in this study. Pure-tone and extended high-frequency (EHF) audiometry between 125 Hz and 20 kHz were performed in both groups. The Weinstein's Noise Sensitivity Scale (WNSS) was used to assess noise sensitivity. In the NE group, the Threshold Equalizing Noise (TEN) test was used to evaluate possible cochlear dead region findings between 1 and 8 kHz.

Results: EHF thresholds were significantly higher in the NE group particularly between 1 and 18 kHz (p < 0.01), compared to the control group. Characteristic threshold elevations around 4 kHz and 16 kHz were observed in both ears. WNSS scores did not differ significantly between groups (p>0.05). While no significant correlations were observed between WNSS scores and audiological parameters in the NE group, weak significant negative correlations were identified between WNSS scores and UCL values in the control group (right ear: r=-0.409, p=0.009; left ear: r=-0.368, p=0.019). TEN findings compatible with possible cochlear dead region involvement were observed at 1000, 3000, 4000, 6000, and 8000 Hz, whereas no TEN-defined findings were identified at 2000 Hz. The highest frequency of TEN-defined findings was observed bilaterally at 4000 Hz.

Conclusion: These findings highlight the importance of including EHF audiometry and dead region assessments in the audiological evaluation of individuals working in noisy environments.

Introduction

Noise, broadly defined as unwanted sound that interferes with the ability to detect or differentiate signals, represents a significant environmental and occupational health hazard. Beyond its auditory consequences, noise can induce various psychological, physiological, and behavioral effects[4]. Occupational noise is one of the primary causes of adult sensorineural hearing loss, particularly in industrialized or industrializing societies[5]. Prolonged exposure to high noise levels in such environments often leads to permanent and irreversible noise-induced hearing loss (NIHL) if preventive measures are not taken[6,7].

In response to this risk, many countries have established regulations to limit industrial workers" noise exposure. In Turkey, the Ministry of Labour and Social Security introduced a regulation in 2013 requiring employers to ensure that weekly noise exposure does not exceed an 86 dB(A) limit[8]. It is well established that prolonged exposure to noise levels above this threshold increases the likelihood of hearing loss, and research suggests that nearly one-third of all hearing loss cases are attributable to noise[5].

Individuals may vary in their susceptibility to noise; therefore, psychometric instruments such as the Weinstein Noise Sensitivity Scale (WNSS) are frequently used to assess individual differences in sensitivity to such environmental noise[9]. While the WNSS assesses subjective sensitivity, cochlear dead regions may be evaluated using the Threshold Equalizing Noise (TEN) test, which measures hearing thresholds in the presence of ipsilateral masking noise[10]. Although the TEN test is conventionally applied within the 500-4000 Hz range, it has been reported to allow assessment of cochlear dead regions at frequencies up to 15000 Hz[11,12]. Few studies have investigated the applicability of TEN testing beyond 4000 Hz. Therefore, in the present study, the TEN test was applied up to 8000 Hz to explore possible high-frequency cochlear dead regions in noise-exposed individuals.

Previous studies have assessed hearing thresholds in the conventional (250-8000 Hz) and extended high-frequency (EHF) ranges in noise-exposed individuals[13,14]. Among workers exposed to noise levels of 86 dB(A) and above, the risk of hearing loss has been reported to be approximately three times higher, whereas the use of hearing protection significantly reduces this risk[14].

Few studies have jointly evaluated EHF audiometry, WNSS, and the TEN test in individuals with occupational noise exposure. In addition, the potential auditory effects of chronic occupational noise exposure within currently accepted exposure limits remain incompletely understood. Therefore, this study aimed to compare auditory findings between noise-exposed industrial workers and normal-hearing controls using EHF audiometry, WNSS, and the TEN test, and to investigate whether these measures may provide complementary information regarding auditory characteristics associated with occupational noise exposure.

Methods

Ethic Approval
The study was approved by the Non-Pharmaceutical and Non-Medical Device Research Ethics Committee of KTO Karatay University (Decision No: 2021/015, Dated: 27.09.2021). Written informed consent was obtained from all participants, and the study was conducted in accordance with the ethical principles of the Declaration of Helsinki.

Participants
There were 80 male participants in the study, all between the ages of 18 and 50, divided into two groups: 40 males who worked in locations that met noise exposure limitations and 40 males who were not exposed to such conditions.

Participants in the noise-exposed (NE) group were male workers aged 18-50 years who had been employed for at least three years in environments where the measured noise exposure level did not exceed 86 dB(A) and who had normal otoscopic findings. The control group consisted of age-matched males without self-reported occupational noise exposure with normal otoscopic findings and normal hearing sensitivity confirmed by audiometric testing. Exclusion criteria for both groups included a history of otologic surgery, neurological disorders, chronic middle ear pathology, and congenital hearing loss, known ototoxic drug exposure, and systemic diseases that could affect auditory function.

Procedure
All participants underwent otoscopic examination. To evaluate hearing performance, pure-tone audiometry, speech audiometry (including SRT, SDS and UCL measured during speech testing), and extended high-frequency audiometry tests were administered. Subsequently, all participants completed the WNSS. In addition, the TEN test was administered specifically to the NE group. The NE group consisted of male individuals working in a facility manufacturing agricultural machinery. Environmental noise measurements were obtained in different working areas of the facility using a TES-1351 Sound Level Meter (TES Electrical Electronic Corp., Taiwan; IEC 651 Type 2, ANSI S1.4). Measurements were performed at ear level during active working hours at multiple time points and locations within the production environment. Noise levels ranged approximately between 64 and 98 dB(A), with most measurements falling within the 73-86 dB(A) range. Detailed environmental noise measurements obtained from different working areas are presented in Table 1. Participants reported routine use of hearing protection devices during occupational activities.

Table 1: Environmental Noise Measurements Obtained from Different Working Areas

Assessment Methods
Pure Tone and Extended High-Frequency Measurements
Pure-tone audiometry was performed using an Interacoustics AC-40 clinical audiometer in a sound-treated booth meeting Industrial Acoustics Company standards. Air-conduction hearing thresholds were measured using Sennheiser HDA300 high-frequency headphones, while bone-conduction thresholds were obtained using a B-71 bone vibrator. Conventional audiometric thresholds were assessed using the Modified Hughson-Westlake technique. Extended high-frequency audiometry measurements were performed up to 20 kHz. Hearing loss degrees were classified based on pure-tone averages (PTA). Speech audiometry tests were conducted using TDH-39 standard headphones.

Uncomfortable Loudness Level (UCL) Measurements
UCL was assessed using continuous speech stimuli during speech audiometry measurement. Starting from the participant's most comfortable listening level, the intensity was increased in 5-10 dB steps until the individual reported the sound as uncomfortably loud. This procedure was applied for both ears using the Interacoustics AC-40 clinical audiometer with TDH-39 headphones. The final UCL value was recorded as the highest intensity level tolerated without discomfort. This method aligns with clinical practice, where increasing in 5 dB steps is recommended unless the participant exhibits a limited dynamic range warranting finer resolution[15].

Weinstein Noise Sensitivity Scale
All participants completed the Turkish version of the WNSS, whose validity and reliability were established by Yıldız et al. The 21-item questionnaire assesses individual differences in subjective noise sensitivity. The questionnaire was administered after providing participants with standardized instructions, and the forms were completed independently by each participant without researcher interference. During the process, participants answered the questions without requiring any guidance. The Tr-WNSS consists of 21 items rated on a 6-point Likert scale ranging from 1 (strongly agree) to 6 (strongly disagree). Participants completed the questionnaire by marking the response that best reflected their own perception. Total scores were calculated to determine each individual's noise sensitivity Level[16,17].

TEN Test Measurements
The NE group participating in the study underwent the TEN test at frequencies of 1.000, 2.000, 3.000, 4.000, 6.000, and 8.000 Hz in the silent cabin of Industrial Acoustics Company standard using the TDH-39 standard headphones and Interacoustics AC-40 clinical audiometer. Participants" hearing threshold levels at the test frequency were used to calculate the TEN levels. Starting levels were adjusted as follows: For frequencies where the hearing threshold was better than 60 dB, the TEN level was adjusted to 70 dB; for frequencies where the hearing threshold was between 70 dB and 90 dB, it was adjusted to +10 dBSL; and for frequencies where the hearing threshold was worse than 90 dB, the TEN level was adjusted to be at the threshold level at that frequency. A cochlear dead region was verified when the TEN thresholds exceeded both the original threshold and the TEN level by at least 10 dB[18].

Statistical Analysis
Statistical analyses were performed using IBM SPSS Statistics v26 (IBM Corp., Armonk, NY, USA). Descriptive statistics were presented as frequency (n), percentage (%), mean ± standard deviation, median, minimum, and maximum values. The normality of numerical variables was assessed using the Shapiro-Wilk test, and variance homogeneity was evaluated using Levene's test. For comparisons between independent groups, the independent samples t-test was used for normally distributed data, whereas the Mann-Whitney U test was used for non-normally distributed variables. Correlation analyses were performed using Spearman's rho correlation coefficient. TEN test findings were evaluated descriptively based on frequency-specific distributions and threshold values. Statistical significance was accepted as p < 0.05.

Results

Table 2: Demographic and Clinical Characteristics of the Participants

In measurements performed between 125 Hz and 20 kHz, the NE group demonstrated significantly higher EHF hearing thresholds in both the right (RE) and left ears (LE) compared to the control group. Significant differences were observed between 1000 Hz and 18 kHz in the right ear and between 250 Hz and 18 kHz in the left ear (Figures 1 and 2). In the NE group, characteristic notches were observed at 4 kHz and 16 kHz in both ears, while hearing thresholds improved at 9 kHz and 20 kHz.

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Figure 1: Right Ear Extended High-Frequency Measurement Graphs for the Noise Exposed and Control Group

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Figure 2: Left Ear Extended High-Frequency Measurement Graphs for the Noise-Exposed and Control Group

The mean WNSS scores were 85.8 ± 15.24 for the NE group and 85.08 ± 12.56 for the control group, with no statistically significant difference between groups (Table 3). Additionally, Table 4 presents the correlations between WNSS scores and audiological parameters. In the NE group, no significant correlations were observed between WNSS scores and audiological parameters, including frequency-specific hearing thresholds (p > 0.05). In the control group, WNSS scores showed weak negative correlations with UCL measurements in both ears (RE: r = -0.404, p < 0.05; LE: r = -0.365, p < 0.05). Weak positive correlations were also identified for right-ear thresholds at 1000 Hz and left-ear thresholds at 20 kHz. No other statistically significant correlations were observed (Table 4).

Table 3: The WNSS Scores Compared by Groups of Studies

Table 4: Correlations Between WNSS Scores and Audiological Parameters

TEN findings compatible with possible cochlear dead region involvement were observed at 1000, 3000, 4000, 6000, and 8000 Hz in both ears of the NE group, whereas no TEN-defined findings were identified at 2000 Hz. Mean TEN thresholds demonstrated similar patterns between the right and left ears across frequencies, with higher threshold values observed particularly at higher frequencies (Figure 3). The highest frequency of TEN-defined findings was observed bilaterally at 4000 Hz. Frequency distributions and descriptive TEN threshold findings are presented in Table 5.

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Figure 3: Right and Left Ear TEN Thresholds for the Noise-Exposed Group

Table 5. Frequency Distribution and TEN Threshold Characteristics of Possible Cochlear Dead Region Findings in the Noise-Exposed Group

Discussion

Noise-induced hearing loss (NIHL) is one of the most common forms of acquired sensorineural hearing loss after presbycusis and results from prolonged exposure to excessive noise levels [6]. In this study, the NE group showed the distinctive NIHL notch at 4 kHz and 16 kHz, along with elevated hearing thresholds at 3, 4, 6, and 8 kHz in the left ear. These findings are consistent with the literature, where NIHL often appears at 3, 4, or 6 kHz as a notch that deepens with continued exposure and progressively affects lower and higher frequencies[19-21].

Jaafar et al. (2017) found a notch at 3-6 kHz in 82.6% of lawnmower workers, while Mehrparvar et al.(2014) emphasized the effects of noise on frequencies 4-6 kHz in standard audiometry and 14-16 kHz in EHF audiometry[20,21]. Similarly, Carrol et al. (2017) observed bilateral or unilateral notches at 3-6 kHz in one-third of noise-exposed workers, and Lopes et al. (2012) reported the lowest thresholds at 3, 4, and 6 kHz in drivers[19,22]. This pattern may be related to the selective vulnerability of the basal turn of the cochlea, where high-frequency-sensitive outer hair cells (OHCs) are located and are particularly susceptible to acoustic trauma. Cochlear biomechanics and ear canal resonance may further contribute to the increased vulnerability of the 3-6 kHz region and the characteristic 4 kHz notch observed in NIHL[23]. Additionally, cochlear hair cell loss progresses with age, with OHC being affected more severely than IHC, particularly in the cochlea's high-frequency regions[24].

When comparing noise-exposed individuals with controls, significant hearing threshold differences were seen across all frequencies from 250 Hz to 18 kHz. These results align with De Sá et al. (2007), who found significant threshold differences at all frequencies up to 18 kHz, and Porto et al. (2004), who reported greater hearing loss between 6 and 14 kHz in noise-exposed workers[25,26]. Previous studies have suggested that EHF audiometry may provide additional information regarding early high-frequency auditory changes associated with noise exposure. For example, Kumar et al. (2017), Sulaiman et al. (2014), and Korres et al. (2008) reported significant high-frequency hearing losses among individuals exposed to personal listening devices and occupational noise[27-29]. Despite the widespread use of hearing protection devices in the NE group, elevated EHF thresholds and TEN findings were still observed.

The WNSS scores were comparable between groups, with a mean of 85.8±15.24 in NE group and 85.08±12.56 in controls (Table 3). These results align with Alimohammadi et al. (2006), who reported similar scores in non-industrial workers[30]. As shown in Table 4, no significant correlations were observed between WNSS scores and most audiological parameters, including frequency-specific hearing thresholds, in the NE group. This finding may suggest that subjective noise sensitivity does not directly correspond to measurable peripheral auditory changes in individuals chronically exposed to occupational noise. In the control group, weak negative correlations were observed between WNSS scores and UCL values in both ears, suggesting that individuals with higher noise sensitivity tended to report lower loudness tolerance levels. Additionally, weak positive correlations were identified only for right-ear 1000 Hz and left-ear 20 kHz hearing thresholds. However, given the isolated nature and limited strength of these correlations, their clinical significance remains uncertain. The underlying mechanisms of the relationship between noise sensitivity and UCL remain unclear; however, differences in subjective sound tolerance and individual sensitivity to environmental sounds may contribute to these findings[31,32]. Belojevic et al. (2003) also noted that noise impacts non-exposed individuals by increasing annoyance and reducing performance in noisy environments[33].

The TEN test in this study identified findings compatible with possible cochlear dead region involvement. Table 5 demonstrated that TEN-defined findings were most frequently observed at 4000 Hz in both ears, followed by 3000 Hz. Similar high-frequency TEN findings have previously been reported in individuals with sensorineural hearing loss and cochlear dead regions. Moore and Vinay (2009), noted that high-frequency dead regions are associated with cortical plasticity and low-frequency information processing[18]. Furthermore, Pepler et al. (2008) reported that individuals with a dead region at 4 kHz showed a more pronounced hearing loss curve, highlighting the specific impact of dead regions on hearing thresholds[34]. Munro et al. (2005) reported dead regions at 5-10 kHz in 91% of cases, and Jacop et al. (2006) observed that dead regions are more frequent in sloping sensorineural hearing loss[35,36]. The absence of dead regions at 2 kHz in this study diverges from prior findings, which may reflect frequency-specific variability or functional contributions from neighbouring cochlear regions[37]. Previous studies have suggested that identification of cochlear dead regions may be considered in hearing aid amplification strategies and rehabilitation planning; however, the additional clinical contribution of these tests remains controversial[38]. From a clinical perspective, the identification of possible cochlear dead region involvement in noise-exposed individuals may also be relevant for audiological monitoring and hearing conservation strategies in individuals with ongoing occupational noise exposure.

The study's inclusion of only male participants limits the generalizability of the findings with respect to gender. Furthermore, the cross-sectional design of this study did not allow for the evaluation of the long-term effects of industrial noise exposure over time. Therefore, causal relationships between occupational noise exposure and the observed auditory findings could not be definitively established. In addition, TEN measurements were performed only in the NE group, limiting direct comparison of possible cochlear dead region findings between groups. Individual noise dosimetry and cumulative lifetime noise exposure data were also unavailable. Potential confounding factors such as smoking habits and individual susceptibility to noise exposure should also be considered when interpreting the findings. Therefore, the findings should be interpreted cautiously, particularly regarding causality and the specificity of TEN findings to occupational noise exposure. Accordingly, future research employing longitudinal designs is recommended to better understand the extended impact of noise exposure on auditory function.

Conclusion

Financial and technical support: No financial and/or technical support was received for this study.

Conflict of interest: There is no conflict of interest.

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