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Journal Information
Vol. 66. Issue 1.
Pages 36-42 (January - February 2015)
Vol. 66. Issue 1.
Pages 36-42 (January - February 2015)
Original article
DOI: 10.1016/j.otoeng.2014.05.020
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Auditory Fatigue
Fatiga auditiva
Julio Sanjuán Juaristi, Mar Sanjuán Martínez-Conde
Corresponding author

Corresponding author.
Unidad de Neurofisiología Experimental, Hospital Ramón y Cajal, Madrid, Spain
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Introduction and objectives

Given the relevance of possible hearing losses due to sound overloads and the short list of references of objective procedures for their study, we provide a technique that gives precise data about the audiometric profile and recruitment factor.

Our objectives were to determine the peripheral fatigue, through the cochlear microphonic response to sound pressure overload stimuli, as well as to measure recovery time, establishing parameters for differentiation with regard to current psychoacoustic and clinical studies.

Material and method

We used specific instruments for the study of cochlear microphonic response, plus a function generator that provided us with stimuli of different intensities and harmonic components. In Wistar rats, we first measured the normal microphonic response and then the effect of auditory fatigue on it.


Using a 60dB pure tone acoustic stimulation, we obtained a microphonic response at 20dB. We then caused fatigue with 100dB of the same frequency, reaching a loss of approximately 11dB after 15min; after that, the deterioration slowed and did not exceed 15dB. By means of complex random tone maskers or white noise, no fatigue was caused to the sensory receptors, not even at levels of 100dB and over an hour of overstimulation.


No fatigue was observed in terms of sensory receptors. Deterioration of peripheral perception through intense overstimulation may be due to biochemical changes of desensitisation due to exhaustion. Auditory fatigue in subjective clinical trials presumably affects supracochlear sections. The auditory fatigue tests found are not in line with those obtained subjectively in clinical and psychoacoustic trials.

Auditory fatigue
Acoustic trauma
Introducción y objetivos

Ante posibles pérdidas de audición a causa de sobrecargas sonoras y la escasa referencia de procedimientos objetivos para su estudio, aportamos una técnica que suministra datos precisos sobre el perfil audiométrico y el factor reclutamiento.

El objetivo del estudio es la determinación de la fatiga auditiva temporal a través de la respuesta microfónica coclear ante estímulos de sobrecarga de presión sonora y medida del tiempo de recuperación.

Material y método

Instrumentación específica para el estudio de microfónicos cocleares, más un generador que nos proporciona estímulos sonoros de diversa intensidad y componente armónico. Utilizamos ratas Wistar. Medimos la respuesta microfónica normal y después el efecto que sobre ella ha ejercido la aportación de sobrecarga acústica.


Utilizando un tono puro a 60dB obtenemos una respuesta microfónica. Fatigando de inmediato con 100dB en la misma frecuencia, a los 15min obtenemos una pérdida de 11dB, a partir de los cuales el deterioro se lentifica y no supera los 15dB. Mediante sonidos de banda compleja aleatoria o ruido blanco no se produce fatiga ni a niveles de 100dB durante una hora de sobreestímulo.


No existe fatiga a nivel de los receptores sensoriales. El deterioro de la respuesta mediante intenso sobreestímulo posiblemente se deba a alteraciones bioquímicas de desensibilización por agotamiento.

La fatiga auditiva en pruebas clínicas subjetivas afecta a tramos supracocleares. Las pruebas de fatiga auditiva encontradas no coinciden con las obtenidas subjetivamente en clínica ni en psicoacústica.

Palabras clave:
Fatiga auditiva
Trauma acústico
Full Text
Introduction and Objectives

The process of auditory deterioration can be quickened by diverse causes, including acoustic environmental factors.

Specific areas are protected with insulating panels designed for acoustic insulation. Companies with a high level of noise have to take precautions and adopt the laws in force.1

In discotheques and similar places, we have found noise levels of 100dB, with peaks at 115dB. Likewise, listening to music recorded at high sound levels is damaging.

Human beings can bear a sound level of 60dB without deterioration for 24h. At 85dB the exposure should not go over 4h; at 95dB the bearable limit is 1h; with 100dB exposure should not exceed 15min; and at 110dB it should last no longer than 1min. These considerations are an approximation to noise tolerance.

Fatigue and trauma2 are two interconnected states with special differentiation. There is a recovery period between both states that can be total or can leave minimum sequelae that are subjectively undetectable in the short term.

The reduction in the threshold of hearing has been studied using various techniques, almost all of them subjective. Enlistment or recruitment becomes present in neurosensory hypoacusis and especially in subjects predisposed to auditory fatigue.

Several authors have proposed techniques for studying recruitment. Fowler3 presented his method of binaural balance testing for unilateral hypoacusis.

The Lüscher and Zwislocki4 test (1949), performed on only one ear, consists in asking the patient to differentiate between the sensation of continuous tone and that of the same tone modulated in amplitude.

A technique that somewhat parallels our study is that of Peyser5 on post-stimulus fatigue.

According to Metz,6,7 there is positive recruitment if the stapedial reflex presents at less than 60dB over the threshold.

The Denes and Nauton test in 1950.8

Jerger's9,10 differential test.

The Theilgaard test, based on the fact that the frequency that is most affected is immediately above that of the traumatic stimulus.

Hinalaf11 studied auditory fatigue in adolescents based on analysing the oto-emissions related to recruitment.

Habermann, Russi, Larsen and many others have carried out studies on the subject.

In our article we provide data on fatigue and traumatism using an objective procedure, which also allows us to study with simulators and non-cooperative subjects through the complete audiometric tracing, plus the recruitment factor.12

Material and Methods

The equipment used for the functional study of the outer hair cells is of very restricted diffusion, limited to the academic environment in research and clinical practice. The instrument used is the model MC 99, designed at the Hospital Ramón y Cajal (Madrid, Spain) and structured by EGESA (Fig. 1). At present, the Centre of Biomedical Research at the Universidad Politécnica in Madrid is in the process of expanding the applications. The equipment has been described in numerous articles.13–15

Figure 1.

(1) Stimulator. (2) Selective amplifier. (3) Computer with the software for the CM. (4) Printer. (5) Audiometric profile and recruitment results. (6) Union of the previous to the amplifier. (7) Stimulus control microphone. (8) Coupling part to the CAE. (9) Previous differential. (10) Mastoid electrodes, mastoid, forehead.

Basic Working Principles

Cochlear microphones (CMs), after their discovery by Weber E.G. Bray, were practically abandoned because of their extremely limited derived potential, which prevented obtaining through non-traumatic contact electrodes.

Study, especially in early diagnosis, fell to supra-cochlear potentials of greater magnitude and easy access. Technical evolution in electronics and information technology have made it possible to study signals as weak as those of the CMs (10−6 volts, 10−9V) with total clarity, as long as they comply with certain requisites:

  • 1.

    They have to be pure sine way frequencies. Exact copy of the stimulus.

  • 2.

    They have to maintain wave phase with respect to the stimulus that provokes them.

  • 3.

    Their amplitude will be kept proportional to the stimulus that generates them.

These biological characteristics only occur in CMs.

Instrument Bases

  • (a)

    As this is a study on pure sine wave waves, we designed amplifiers in tune with the frequency of the stimulus generator. Consequently, the perturbations of a frequency different from that of the CM are eliminated. Random perturbations coinciding in frequency with the stimulus are left to be suppressed.

  • (b)

    The stimulus generator has the audiometric frequencies available and also generates a trigger signal for each wave, which marks its phase.

  • (c)

    The computer program, based on the trigger, eliminates by averaging all the waves that do not agree in phase with that of the CM.

  • (d)

    Thanks to the fact that the CM amplitude is proportional to that of the stimulus, the audiometric profile is attained using an appropriate algorithm.

It is important to bear in mind that the system studies key cochlear points, precisely those that coincide in frequency with the stimulus given and it completely ignores any external or biological perturbations. If some factor alters the cochlear point under study in the slightest, the CM changes proportionally.

We provide the sound stimulus through an acoustic tube, for reasons of quantitative precision and to put distance between the transducer and the patient, preventing electromagnetic induction on the electrodes.

The tube ends in the CAE, where we have a coupling part with a microphone connected to a digital sonometer, which guarantees in situ calibration (Fig. 1).

The equipment is rounded out with a Brüel & Kjaer model 1027 function generator to produce different sounds: pure tones, white noise and noise bands of a specific width.

We used Wistar rats, complying with European and American regulations, as indicated in Spanish law R.D. 53/2013, published on 8 February 2013. The field work was performed in the animal facility at the Hospital Universitario Ramón y Cajal (registry no. ES 280790002001).


After anaesthesia with 8°/0 chloral hydrate by intraperitoneal injection, two conical parts that fit in the stimulus-providing tube were placed (Fig. 1). These parts were attached to the CAE using a minimum of adhesive, to keep them in position without moving.

Three thin electrodes were placed on the forehead and on both mastoids, with a 3-mm subcutaneous penetration, the electrodes having a stop to limit penetration. The study could have been done with only contact electrodes, shaving the skin, but that would have complicated the technique and lengthened study time.

A response of some 20dB was obtained with a sine wave stimulus of 4000Hz at 60dB. This was the figure that we took as 0 for reference in all the cases.

We chose the frequency, 4000Hz, to reach a quick averaging, 4000 complete waves per second. In addition, this frequency is more centred in the rat within its hearing range.

Applying the overload stimulus, we produced fatigue during a controlled time period and then measured again, immediately noting the CM value, evoking at 60dB. We repeated the sequence of overstimulation and measurement of the reference tone as many times as necessary to trace the decline that was being produced.

With the data obtained, we traced the curve of decline from “fatigue” over time. We immediately went to the resting phase, where we likewise tested, in each time period, the CM response found (always at 60dB), noting the recovery.

The characteristics of the fatigue stimulus, pure tone or random noise, as well as the measurement intervals, are shown in the corresponding figures.

We used three procedures (a, b and c) on the ipsilateral ear, plus a check on the contralateral ear.

  • (a)

    We applied the same tone of 4000Hz at 100dB during different time periods (Figs. 2–4), consequently obtaining a specific effect on a very narrow cochlear area. This procedure was only for the study; we would never find a constant pure tone at such a level in normal acoustic surroundings.

    Figure 2.

    Results in the left ear of six rats. Fatigue at 4000Hz for an hour.

    Figure 3.

    Mean of the results of Fig. 2. The dashed line represents the mean of recovery of 4 of the 6 rats. Two rats came out of the anaesthesia before we were able to study the recovery.

    Figure 4.

    Results over six rats with 15min of overload and later recovery of 30min. This graph is divided into two parts: stimulus y recovery; after the first 15min, the fatigue is eliminated and the measurements of recovery are initiated for 30min.


    Fig. 2 corresponds to the left ear (LE) in six rats, submitted to a stimulus of 100dB and 4000Hz. We took a reading of hearing loss every 5min for an hour. Only one ear was used, so as not to prolong the anaesthesia and to check the possible contralateral effect later.

    The starting point “0” dB corresponded to the CM response obtained at 60dB in the same frequency and in each rat. We equated the small differences obtained in each specimen to “0” dB to have a common starting point.

    Fig. 3 presents the mean of Fig. 2 and that of recovery in an hour.

    Fig. 4 corresponds to data from six rats. Fatigue time of 15min followed by 30min of recovery. Stimulus at 100dB and 4000Hz.

  • (b)

    We used white noise and narrow-band random noises at 100dB. This overstimulation was more in accord with respect to the spectral composition of the surroundings.

    We did not use greater intensities and time so as not to involve the possibility of acoustic trauma. This test was used with four rats, for 45min.

    The Brüel & Kajaer 1027 function generator made it possible for us to work with noise bands of 3.16, 10, 31.6, 100 and 1000Hz, which we centred over 4000Hz. Lastly, we used white noise.

  • (c)

    We completed the study with the CM audiometric trace, without fatigue overstimulation and later made another tracing after an overstimulation at 2000Hz and 100dB for 30min (Fig. 5).

    Figure 5.

    Solid line, cochlear microphone audiometric profile without fatigue. Dashed line, audiometric profile after 30min of fatigue.


The trial finished by using white noise instead of fatiguing with the tone of 2000Hz.

Contralateral Ear Study

Apart from the procedures used, we traced the CM audiometric profile of the contralateral ear, not exposed to acoustic stimuli.

ResultsBased on the Method Used

  • (a)

    Application of 4000Hz a 100dB during different time periods. In this extreme experience, we observed an initial drop in the reference CM, which was quicker and more accentuated before the first 20min. This passed to almost horizontal between 20 and 60min of the study. This result was more obvious in the trace in Fig. 3, which corresponded to the mean.

    Recovery from this deterioration process was also initiated more quickly and then became slower afterwards, without reaching the level 0 of normality. After an hour of rest, there was still a residue of −5dB (Fig. 3) represented by the dashed line.

    When the study of the CM response was repeated at 24h, the CM level at 60dB of stimulus became normal again.

    By repeating the previous experience with only 15min of overstimulation (Fig. 4), we corroborated the data from Fig. 2 of CM deterioration in the first 15min. From minute 15, we initiated the recovery period for 30min; after this, a mean loss of only 3dB was left with respect to the normal level.

  • (b)

    Experience with white noise and narrow-band noises. It was demonstrated that “fatigue” only appeared when the noise band got close to the pure tone, with band widths of only 3.16 and 10Hz.

    On the day after performing the test, we carried out a new threshold check. The recovery coincided with normality. The differences found of ±2dB could be attributed to the new placement of the electrodes and, especially, to the coupling parts of the stimulus to the CAE.

  • (c)

    Alteration of the audiometric profile. Fig. 5 shows the CM audiometric profile, the mean of seven rats.

There was only a drop of 5dB over the overload frequency.

Contralateral Response

No alterations in the response of the sensory receptors of the contralateral ear were produced in any of the tests described.


The most commonly used tests on auditory fatigue are, in general, subjective. Furthermore, the results disagree with respect to those of this study. With the common tests, loses of sensation close to 50dB are reached, while objectively –with stimuli over 100dB–they do not exceed 12dB and the mean is only 8.

Auditory results with subjective techniques, including liminal tonal audiometry, are determined by the total behaviour of the auditory system. In our study procedure, we analyse only the response of the outer hair cells in a key cochlear area. The results indicate that the sensory receptors are only susceptible to “fatigue” with extreme stimulation.

The sound levels used, with respect to time, frequency and strength, do not occur in normal acoustic settings. We are unable to objectively detect the “fatigue” of the outer hair cells, if it does indeed exist, using the sound levels normally present.

Wide-range stimuli affect the cochlea, sharing their energy over multiple receptors. This makes them less damaging than pure tones or those that have a highly coded fundamental. However, subjective tests, in a clinical setting, carried out with white noise, show signs of fatigue, a circumstance that brings up the issue of its supra-cochlear location.

Elevated stimuli on an ear can sometimes cause a loss in the contralateral one, possibly of central origin. Philippides and Geiner16 observed a contralateral hypoacusis of 25dB after the removal of the left temporal lobe for a brain tumour. Galambos17 describes a drop in the threshold when the contralateral ear is exposed to an intense stimulus.

In our study, we never found an elevated CM loss, or contralateral effect. This leads us to consider that both the magnitude of the loss from fatigue and the effect on the opposite ear are not of cochlear origin.

The form of fatigue curves of a pure tone for an hour, in the face of extreme exposure, suggests the possibility that the sodium–potassium pump initiates an ion interchange with greater speed at the beginning, during the first 15min, and that it then slows down and becomes stable later on, in a desensitising process from exhaustion.

The magnitude of the CMs depends on the difference in potential established between the body of the hair cell and the endolymphatic fluid, in which the cilia are found, to some 160mV, a potential that is altered as the ion relation changes.

The intracellular concentration of sodium is less than the extracellular, while that of potassium is greater, which means a strong electrochemical gradient. As the membrane is impermeable to these solutes, controlling the entrance and exit of these substances, the cell generates changes in ion concentration on both sides of the membrane; given that ions have an electrical charge, potential is also modified through them.

An alteration of ion exchange from the unusual overload stimulus of the pure tone that we use in the study could be the abnormal cause of the decrease in threshold and later stabilisation of CM potential that we find.


In this study, extreme levels of sound pressure and even pure tones have been used to seek evident results of fatigue. We did this because of the impossibility of fatiguing the sensory receptors with lower stimulus levels. With this procedure, we analysed the lability of the auditory receptors, which remain immune to the sound pressure overload within important limits of intensity and time.

Auditory fatigue that is studied using subjective or psychoacoustic procedures does not agree with the fatigue found objectively on the sensory receptors.

With this contribution, we only want to show that the effect of fatigue evidenced in clinical practice is not produced at all at the level of the outer hair cells.

The effect of fatigue exists, but its origin and interpretation will have to be revised.

In fact, the term “fatigue” does not seem appropriate, in our opinion. The drop in the threshold of hearing, within settings of elevated stimulus level, seems to be a central reaction designed to reduce auditory sensation and allow simultaneous perception of other sensory receptors, without these being masked. This behaviour is repeated in other external receptors, and possibly in all of them. An example is olfactory fatigue. After a certain time in the presence of a strong odour, the sensation lessens. Olfactory fatigue is also a process of sensory adaptation in which the brain diminishes the sensitivity of the stimuli to prevent overloads at the level of the nervous system.

The effect of taste fatigue seems similar. In the first few moments, we perceive the flavour of a substance intensely; however, persistence produces a decrease in appreciating it.

This concept of physiological “fatigue” corresponds only to processes of discrete incidence without pathological repercussions. In spite of our conclusion on the impossibility of fatiguing sensory receptors during trials of limited duration, it has been demonstrated that the persistence of high levels of sound pressure can produce, in an undefined period of time, selective degeneration of the outer hair cells.

Conflict of Interests

The authors have no conflicts of interest to declare.

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Please cite this article as: Sanjuán Juaristi J, Sanjuán Martínez-Conde M. Fatiga auditiva. Acta Otorrinolaringol Esp. 2015;66:36–42.

Copyright © 2013. Elsevier España, S.L.U. and Sociedad Española de Otorrinolaringología y Patología Cérvico-Facial
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