**2. Material and method**

We conducted a descriptive observational study of a set of cases of children aged 6–24 months from our ENT clinic at the University Hospital Santa Lucía, Cartagena (Murcia, Spain) in the period between April 2016 and January 2017 who underwent ABR and ASSR-MF using insertion earphones and ABR and ASSR-MF using freefield stimulation.

The selected patients fulfilled the criteria of normality with insertion earphones, that is with latencies and amplitudes within normality in ABR with V-wave threshold at 20 dB HL and with stabilization of responses in ASSR before 6 minutes and threshold of 20 dB HL at the four frequencies of 500, 1000, 2000, and 4000 Hz. These patients, after testing with insertion headphones, were tested again with a free-field stimulus. Children outside the age range and children with some degree of hearing impairment were excluded.

Following these criteria, the children were selected and the ABR and ASSR-MF with free-field stimulus were recorded after the conventional tests with insertion headphones, in the same exploratory act, under the same conditions, using the same sound stimulus, unilateral clicks for the ABR and amplitude-modulated tones in ASSR-MF, and taking advantage of the child's sedation. All cases were performed and recorded in the same environmental conditions, same acoustic booth, same equipment, and same explorer.

To carry out free-field stimulation, new software and hardware had to be incorporated. These modifications were carried out by the company Audiología, S.L. (Gijón, Spain), Interacoustic's technical service and distributors, and with the brand's permission. The modification of the software consisted of the possibility of choosing the use of loudspeakers in the stimulation menu, in this case using Phonestra © preamplified free-field loudspeakers whose potentiometer was mechanically fixed to avoid changing the gain of the tests.

The calibration of hearing thresholds with the correction coefficients of insert earphones is governed by IEC-60645-7 "Instruments for the measurement of auditory brainstem responses" [29]. We are not aware of any specific standards and correction coefficients for the realization of ABR and ASSR tests in the free field. For the calculation of correction coefficients, we based ourselves on the ISO-389 standard for the "zero" reference calculation in the calibration of audiometric equipment. Free-field pure-tone control audiometry was performed on 25 healthy individuals aged 14–38 years. An ASSR measurement was made in each subject, stimulating in a free field, with 0 dB correction. New correction coefficients for ASSR in the free field were obtained by the difference of the values recorded with the free-field tonal tests and the ASSR without correction, calculating a correction coefficient of 5 dB HL (±1.5 dB HL) in the four frequencies with respect to the correction rates with insertion headphones. The theoretical calculation was made following the "law of spherical divergence" by means of the behavior of sound in the free field which allows us to define the attenuation or variation of level between two previously defined points, r1 and r2 (RE: 1, 2). With these modifications and with the sound source (loudspeaker) 70 cm away from the ear to be tested, we performed the ABR and ASSR with free-field stimulus in the same environmental conditions, same booth, same equipment, and same explorer as with insertion headphones to control non-differential errors.

The tests were carried out in the rather quiet outpatient room, inside a booth with an acoustic attenuation of 38 dB SPL on average, which also houses the laptop and the explorer who operates the equipment.

The patient should be relaxed to reduce electrical noise as much as possible [30], with physiological sleep or, as in most of our cases, with mild sedation which we achieve with the oral administration of Chloral Hydrate at a dose of 75 mg/kg/weight which allows 2–3 hours of sedation. Chloral hydrate has very few adverse reactions and although it has a bad taste, it is well tolerated by children. The maximum dose of 2 g should not be exceeded and it cannot be used to maintain prolonged sedation due to the sedative effect of its metabolites [31].

After careful cleaning of the skin with alcohol, we use an abrasive cream to peel off light desquamation to reduce the resistance of the skin in the location of the electrodes that are placed once the child is asleep, placing the active electrode in the vertex, the reference electrode in the mastoid (right and left) and zygomatic region. After placing the electrodes, the ABR3A type earphones are inserted into the external auditory canal, held in place by a silicone cushion to hold them in place and to stagnate them in the size best suited to the canal orifice.

#### **2.1 Recording the ABR with insert earphones**

We stimulated with alternating clicks at a rate of 44/s with contralateral white noise masking with −30 dB HL of the stimulus intensity, using a 100 Hz high-pass filter and a 1500 Hz low-pass filter, a maximum of 4000 stimuli, a 12 ms screen window, admitting a curve quality response of 99% and a residual noise of 40 nV. The procedure is programmed using 70, 60, 40, and 20 dB HL in descending order, and the intensity can be changed manually. When it ends in one ear, it automatically starts the stimulus in the contralateral ear and we can stop the test when we consider it convenient when we have reached the threshold of wave V. In case of absence of response or poor quality of the response at 70 dB HL, we will continue with stimuli at 80, 90 or 100 dB HL until we find a graph of sufficient quality to observe amplitudes and latencies.

Once the graphs are obtained, latencies and interlatencies are measured and the results are stored for later evaluation.

#### **2.2 Recording ASSR with insert earphones**

We performed multi-frequency stimuli allowing us to stimulate both ears simultaneously at frequencies of 500, 1000, 2000, and 4000 with CE-Chirp© [17], the stimulation rate at 90 Hz, and a rejection level of 40 nV. The maximum stabilization time of the response was set at 6 minutes. We also stored the results.

#### **2.3 Recording of the ABR with free-field stimulus**

The procedure is similar to the ABR recording with insertion earphones and is carried out after performing the tests with insertion earphones and without modifying the conditions, in the same clinical act and with the patient being selected following the inclusion and exclusion criteria, as described above. Using a stimulus through a loudspeaker, selecting in the transducer menu, placed 70 cm from the ear to be tested, with masking of contralateral white noise and storing results.
