Occupational noise exposure is regulated by law, and protective measures are applied if the noise level exceeds 90 dB(A). The inferior exposure levels can cause an auditory fatigue which is defined as temporary and reversible hearing loss. Its mechanisms, however, remain largely underexplored with recent animal data suggesting that the long-term exposure (over several weeks) to noise above 85 dB Sound Pressure Level (SPL) may provoke a permanent damage to hearing. Furthermore, there is also a lack of research on cognitive and attentional repercussions of this phenomenon. Yet, this information could be important to prevent work-related errors and accidents. Lastly, although silent breaks (noise-free resting periods) throughout a day of work are preconized to limit the noise exposure, their effect on auditory fatigue has never been demonstrated.
Being a part of a larger project involving fundamental researchers (animal experimentation) and occupational health specialists (investigations at the workplace), this study aims:
* to evaluate how auditory fatigue caused by acute noise exposure in doses tolerated by law impacts auditory and cognitive performances in humans * to determine the effect of silent breaks proposed by occupational legislation on cognitive functioning * to issue new recommendations for the protection of workers exposed to noise
Healthy subjects aged between 18 and 35
With normal pure-tone audiometry data (air and bone conductions) on both ears: hearing thresholds ≤20 dB HL for each frequency tested - 125, 250, 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz
With normal Speech-in-noise test (the speech reception threshold (SRT) at which 50% recognition of the speech stimuli presented is achieved must be 3 dB higher than the standard test value established in subjects with normal hearing)
Without history of excessive noise exposure (1-MINUTE NOISE SCREEN ≤4)
History of sudden deafness or tinnitus lasting more than 48 hours
Conditions likely to affect hearing (tumours of the ear or base of the skull, etc.)
History of unresolved chronic otitis
Work in a noisy environment requiring workplace soundproofing or the use of personal protective equipment
Parallel
2h30 of noise exposure at 85 dB(A)
Other intervention2h30 of uninterrupted noise exposure at 85 dB(A)
2 noise exposures of 66 minutes
Other interventionExposure to 85.5 dB(A) noise for 66 minutes, followed by an 18-minute rest period, then exposure to noise again for 66 minutes
10 noise exposure cycles
Other interventionExposure to 10 cycles of (13 minutes of noise at 85.6 dB(A) / 2 minutes of rest)
No noise exposure
Other interventionNo exposure to noise: subjects spend 2H30 in a soundproof cabin.
Pure-tone audiometry thresholds
Variations in perceptive auditory performance related to the hearing threshold and the hearing threshold at each frequency will be compared between the experimental groups exposed to noise (1, 2 and 3) and a control group not exposed to noise (4). The exam will allow to determine the participants hearing levels at different frequences line according to Hughson-Westlake's procedure: 125, 250, 500, 1000, 2000, 3000, 4000, 5000, 6000, 8000, 10000, 12500Hz
Signal-to-noise ratio (SNR) at speech intelligibility threshold in noise
Variations in perceptive auditory performance related to the signal-to-noise ratio of the speech intelligibility threshold in speech audiometry in noise will be compared between the experimental groups exposed to noise (1, 2 and 3) and a control group not exposed to noise (4). The Speech-in-Noise (SIN) test assesses speech intelligibility in noise. It is conducted in a soundproof booth using headphones, with background noise set at 73 dB. After a brief training session, the participant repeats sentences presented at different levels of difficulty (signal-to-noise ratios ranging from +24 to 0 dB). Only key words are taken into account. The result corresponds to the noise level at which 50% of the words are correctly repeated.
Acoustic distortion products amplitude measured at acoustic reflex threshold
Variations in perceptive auditory performance related to acoustic distortion products amplitude measured at acoustic reflex threshold will be compared between the experimental groups exposed to noise (1, 2 and 3) and a control group not exposed to noise (4). PDA measurement combined with acoustic stimulation can therefore be used to assess the activity of various acoustic reflexes.The ECHOSCAN device automatically determines the reflex threshold by presenting contralateral acoustic stimuli of increasing intensity, each lasting 2 seconds. The stimulus intensity is limited to 100 dB HL.
Stroop test to assess cognitive load
The Stroop test will be used. In the first part of this test, a participant will hear 2 types of pure tones:187 Hz tone or 448 Hz tone, each lasting for 690ms. The choice of a tone perceived ("low"or "high")should be made rapidly by clicking an appropriate category on an electronic support. . 24 stimuli will be randomly presented at a ratio 1:1. In the second part, the participant will be exposed to the words "Grave" (Low) or "Aigüe" (High) synthetically generated with a low voice (187Hz) or a high voice (448Hz). 4 combinations are possible: "Grave" with a low voice, "Grave" with a high voice, "Aigüe" with a low voice and "Aigüe" with a high voice. 48 stimuli will be presented at a ration 1:1:1:1 and reaction time will be measured. The Stroop test evaluates modifications in cognitive charge in the groups exposed to noise (1, 2, 3) compared to the control group (4) through the error rate.
Baseline tone audiometry
This exam will be allow to determine the participants hearing levels at different frequences line according to Hughson-Westlake's procedure: 125, 250, 500, 1000, 2000, 3000, 4000, 5000, 6000, 8000, 10000, 12500Hz. It aims to evaluate the perceptive hearing performance of participants in group 1 (exposed to uninterrupted noise) compared to groups 2 and 3.
Assessment of speech perception in noise
This measure will be done with Speech-in-Noise (SIN) test. The test is conducted in a soundproof booth using TDH-39 headphones, in diotic binaural listening mode. The noise level is set at 73 dB Sound Pressure Level (SPL). To familiarize participants with the task while minimizing learning effects, the session begins with a single practice block of 10 sentences. The sentences are then presented in lists of 10. For each sentence repetition, only key words are scored. Each list tests 9 signal-to-noise ratios (SNRs) ranging from +24 dB to 0 dB in 3 dB steps. The total administration time for the three lists is under 4 minutes. The signal-to-noise ratio at which 50% of the words are correctly repeated is determined.
Assessment of Acoustic Distortion Products (ADP)
The EchoScan test will be used. Participants will receive auditory stimuli to measure the acoustic distortion product (ADP) generated by inner ear. ADP can be diminished by the middle ear reflex when sounds are delivered at high intensity. Thus, the stimulation intensity is limited to 100dB HL. This test allows to evaluate cochlear and middle ear functions and determine the acoustic reflex threshold activation which can be affected by auditory fatigue.
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University Hospital, Montpellier
Lead sponsor
National Research Agency, France
Collaborator
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