**2.2.3 Otoacoustic emissions**

Otoacoustic Emissions (OAE) may occur spontaneously or may be evoked by acoustic stimulation, appearing to originate from within the cochlea and propagating through middle ear structures to the external auditory meatus. OAE are acoustic energy leakages from the biochemical reactions (echoes) of a healthy cochlea that possess a latency of 2-20 msec. If an emission by the cochlea is present, it is likely that hearing is normal at that frequency; those with a hearing loss greater than 25–30 dB usually do not produce these soft sounds. Auditory neuropathy may have OAE even though the hearing loss may be profound.

To measure OAE, the equipment produces a quiet, clicking sound that is emitted by the loudspeaker and OEA is measured with a small probe inserted into the ear canal; recording of the response signal is captured with the aid of a sensitive microphone (Choi, 2011). A number of response epochs must be averaged to improve the signal-to noise ratio (SNR) and produce a clear waveform. Mathematical methods are used for evaluation of the signal, such as Fast Fourier (FF) and Wigner-Ville transforms (WVT), Digital filtering, and Correlation analysis (CA) (Buller, 1997), or Hilbert–Huang transform (HT) for detection of OAE and Time–Frequency Mapping (Janušauskas et al., 2006). The measuring system employed to detect otoacoustic emissions consists of the following several parts, as illustrated in Figure 10: a measuring probe that includes a microphone and loudspeaker; a low-noise

Technology for Hearing Evaluation 15

(51-500 msec). Early potentials reflect electrical activity at the cochlea, eighth cranial nerve, and brain-stem levels, while later potentials reflect cortical activity. In order to separate evoked potentials from background noise, a system computer, as shown in Figure 11 (Nicolet™ EMG/PE), analyzes how well the ears respond to the sound by averaging auditory evoked responses at 1,000 to 2,000 clicks at least. Early evoked responses may be analyzed to estimate the magnitude of hearing loss and to differentiate among cochlear,

eighth nerve, and brainstem lesions.

Fig. 11. Evoked Potential/EMG measuring system

For purposes of neonatal screening, only limited auditory evoked potentials or limited evoked otoacoustic emissions are considered medically necessary. Neonates who fail this screening test are then referred for comprehensive auditory evoked response testing or

preamplifier and amplifier signal; an Analog-to-digital converter (ADC), and Digital Signal Processing (DSP) based on a central processing unit that provides ultra-fast calculation, such as filtering, averaging, FF, WVT, CA or HT in order to obtain otoacoustic emissions response. Additionally, the equipment must be accompanied by the printing and displaying module.

Because the subject being tested is not required to respond, this is an ideal test method for neonates and infants or for those who cannot be evaluated using conventional techniques (Buz & Bower, 2009). OAE are valuable in testing for ototoxicity, detecting blockage in the outer ear canal, as well as the presence of middle-ear fluid and damage to the cochlea outer hair cells.

Fig. 10. Block Diagram of the measuring system used to detect OAE
