06.11.2015
Since newborns and infants only react to relatively loud sounds, hereby demonstrating auditory perception, objective audiometric procedures are used when testing their hearing (e.g. measuring the brain waves evoked by acoustic stimuli). The acoustic stimuli are hereby transmitted via insert earphones. The reference hearing threshold levels necessary for this can, however, only be determined as equivalent reference threshold sound pressure levels of young adults with healthy hearing.
A conventional simulator for the occluded ear canal (standardized in IEC 60318-4) is used for this; it simulates an adult's ear. Calibrating the stimuli for newborn/infant audiometry with this conventional simulator therefore represents a contradiction, since the hearing threshold levels refer to the ear canal of an adult whose geometry, however, considerably differs from that of a newborn or of an infant. For example, the effective volume of the ear canal of a 3-month-old child is on the order of 0.5 cm3 whereas it is approximately 1.2 cm3 in adults. The consequence is that, depending on frequency and on the signal shape, sound pressure levels occur in the infant's ear canal which are approx. 2 dB to 10 dB higher than in the conventional ear simulator according to IEC 60318-4 and in the average adult's ear.
Within the scope of the EMRP project "EARS", a prototype of a simulator of the occluded ear canal was developed which is based on the ear canal geometry of newborns. This prototype is shown in Figure 1, together with the standardized adult ear simulator. An insert earphone ("Bio-logic SINSER"), as is frequently used in audiometry, is connected to each of the simulators.
Figure 1: EARS newborn ear simulator prototype (A) and simulators for adult ears (B), each of them connected to a Bio-logic SINSER insert earphone system (C).
In a first step, reference hearing thresholds were determined for this earphone on 25 young adults with healthy hearing for the standardized short-term signals (reference impulse and reference short-term signals to trigger auditory evoked potentials) as well as for pure tones, and defined on the adult ear simulator. The EARS newborn and infant ear simulator prototype is then used to calibrate the stimuli for newborns and infants, and after this, the reference levels measured in the adult ear simulator are set. Figure 2 shows, as a function of the frequency and of the type of stimulus, how much lower the level in the child's ear can be set with this procedure.
Figure 2: Differences of the equivalent hearing threshold levels for the insert earphone "Bio-logic SINSER" between the EARS newborn ear simulator (prototype) and the adult ear simulator (IEC 60318-4).
When using the newborn ear simulator to calibrate the stimulus level to perform audiometry of newborns and infants, the stimulus is thus no longer adjusted much too loud, but is adequate. Firstly, this prevents wrong conclusions when estimating the hearing ability of infants; secondly, this avoids unnecessary sound exposure of infants' ears during hearing tests.
Contact person:
Thomas Fedtke, FB 1. 6, AG 1. 61, E-Mail:
Thomas.Fedtke(at)ptb.de
Johannes Hensel, FB 1. 6, AG 1. 61, E-Mail: Johannes.Hensel(at)ptb.de