What Are Otoacoustic Emissions?

Otoacoustic emissions are audio energy produced in the cochlea and released into the external ear canal via the ossicular chain and the eardrum. Acoustic emission is a transient elastic wave generated by the rapid release of energy inside a material, which is derived from acoustics. Otoacoustic emission refers to this kind of elastic wave energy recorded from the external ear canal from the cochlea.

Otoacoustic emission

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Otoacoustic emissions are classified into two categories based on the presence or absence of external stimulus acoustic signals and the types of acoustic stimuli:

1. Spontaneous Otoacoustic Emission (SOAE)

The cochlea does not require any external stimulation, and continuously emits machinery

One argument is that otoacoustic emissions originate from the cochlea for the following reasons:

(1) Otoacoustic emission response threshold can be lower than subjective

So far, the detailed mechanism of otoacoustic emissions is not very clear. The following only introduces some theories of otoacoustic emission generation theory on some phenomena. Although there are certain foundations, further research and verification are needed.

1. Active basement membrane structure

1. Test environment and subject requirements

Otoacoustic emission is a sound signal generated by air vibrations in the external ear canal, which is easily confused with or masked by noise in the ear canal; its intensity is very low, mostly between -5-20dBSPL, and excessive environmental noise will affect the ear Acoustic Emission Record. In order to minimize the effects of noise, the following requirements are required when recording otoacoustic emissions:

(1) Control of environmental noise

The ambient noise when recording otoacoustic emissions should be controlled below 40dB (A) as much as possible. Generally speaking, the test is best performed in a sound insulation room.

(2) Subject status

Participants take a comfortable posture, try to keep quiet and calm breathing, avoid movement and swallowing. Sedative hypnotics can be used for uncooperative children, which will not affect the test results.

(3) Prevent friction noise

Pay attention to the cable connecting the probe to avoid noise caused by friction with the subject's body or other objects.

(4) Eliminate electrical and acoustic interference

First, pay attention to removing electrical interference, pay attention to the electrical shielding of the instrument and the grounding of the cabinet. Secondly, techniques such as band-pass filtering, average superposition and phase-locked amplification are used to further process the signal.

(5) Place the probe correctly

During the test, the probe should be placed tightly on the external ear canal, with a small hole at the tip facing the eardrum. Be careful not to block the microphone or speaker channels. Conventional otoacoustic emission recording equipment generally has a probe inspection procedure, which should be run before the inspection begins to ensure that the probe is properly coupled in the ear canal. The procedure should also be repeated intermittently in the middle of the inspection test to check whether the position of the probe has changed to prevent the accuracy of the recorded results due to probe displacement.

2. Test equipment

Although otoacoustic emissions are of different types and various forms, the test methods have many similarities. The test hardware consists of miniature speakers,

Otoacoustic emission tester

High sensitivity microphone, digital processing board and computer system. In the test, the sound is given by the speaker in different ways, and the otoacoustic emission signal is picked up by the high-sensitivity microphone. After a series of processing, the signal-to-noise ratio is improved. Finally, it is displayed or recorded in the frequency or time domain form to complete the test. . The only difference is that the stimulus characteristics and corresponding signal processing methods used by various types of otoacoustic emissions are different. It is precisely that they determine that different otoacoustic emissions have different characteristics.

The above content is transferred from China Audiology Network