The acoustic immittance is measured by putting a piece into the ear called a probe top. This is place in the ear enough to create a hermetcic seal. This tip includes several things. First, a receiver/tone generator, which is a speaker that will play a tone into the ear. The tone generator creates a certain frequency at a set intensity, and the speaker transduces the output of the tone generator to form a sound wave that then is sent to the ear canal. Second is a microphone and sound level meter that will monitor the sound within the ear canal. Thirdly, a pressure pump and manometer, the pressure pump directs changes in air pressure to the ear canal, and the manometer shows the amount of air pressure conveyed to the ear canal.

Immitance in measured in compliance, compliance is the movement of the tympanic membrane. This is done by stimulating the ear by a pure tone and a consistent intensity. Then the sound pressure level is measured. This measurement is then used to determine the impedance (how well the energy flows though the system) of the middle ear and the tympanic membrane and everything that is attached to it. The immittance of the ear is derived from a few sources of mechanical and acoustical stiffness, mass and resistance. The stiffness component comes from the volumes of air in the outer ear and middle ear spaces, the tympanic membrane, the tendons and ligaments of the ossicles. The mass come from the ossicles, the ear drum and the perilymph. The resistance is introduced by the perilymph. The impedance of an object is dependent of frequency. The formula for determining impedance is the square root of R2 + (2p f M – S / 2p f )2 when R= Resistance, M = Mass, S = Stiffness, f = frequency.

A few things to keep in mind are that mass is an important factor for high frequencies and stiffness in the important factor in low frequencies for the response of the system. Resistance is mainly determined by the ligaments that attach to the ossicles and the mass is determined by the weight of the ossicles and the tympanic membrane. Stiffness is determined primarily by the pressure the fluid from the cochlea on the footplate of the stapes.

Tympanometry and Acostic Reflex fall under the category of immittance audiometry. Tympanometry is the term for evaluating the movement of the tympanic membrane. Typically this is a graphical display of the change in compliance of the tympanic membrane as the ear canal pressure is varied from negative to positive. As pressure adjust from zero to its maximum negative or its most positive position impedance increases. The point in the graph where the pressure in the ear canal is equal to the pressure in the middle ear cavity impedance is at its minimum value, in other words, compliance is at its highest value. The graphical display is called a Tympanogram can have several types. In clinical use these graphs are divided into different Jerger types in order to diagnose. A Type A tympanogram is characterized by pressure that is + 50mm H20. This is classified as normal. The Type B tympanogram is typified by no peak and seams flat. This is frequently in serous or chronic otitis media. The Type C tympanogram is distinguished by a peak indicating negative pressure in the middle ear. This is usually due to Eustachian tube dysfunction. An abnormal typanogram can be determined if it has too many peaks or if it is too wide.

An acoustic reflex is what happens when a sufficiently intense sound (70 dB HL) is presented to either ear and it results in the contraction of the stapedius muscle in both ears. This reflexive muscle contraction stiffens the conductive mechanism via the stapedius tendon, and changes the ear’s immitance. The acoustic reflux is easily measure because the immitance change is picked up by the probe top and displayed on the immitance device meter. How this works is that the afferent nerve from an ear goes to the ipsilateral ventral cochlear nucleus. Neurons then go to the superior olivary complexes on both sides of the brainstem. Both superior olivary complexes on send signals to the facial nerve nuclei on their own sides. And then finally the efferent motor legs of the acoustic reflex involve the right and left facial nerves, which direct the stapedius muscles to contract in both ears.

The results of the acoustic reflux are complicated but once understood become simple. A pathological ear is defined as the ear with a problem in it. This could be a dead cochlea or a conductive or sensory-neural hearing loss. If an ear is normal the stapedius muscle will contract in both ears. It the stimulus is presented to the pathological ear and the ear just had a conductive hearing loss the reflux will show up after the conductive hearing loss has been overcome and the ear has received 70 dB HL. Then the reflux will show in both ears. In a dead cochlea, the stimulus will never cause the reflux to occur. In a sensory hearing loss that is profound the reflex will not be found in the pathological ear. Likewise in residual hearing, the reflux will be absent in the pathological ear. These results are better seen in the slides. It is very difficult to explain them in words.

It is also good to not that in reporting the results of Acoustic Reflux testing, the term ipsilateral and contralateral should only be used with direct reference to the probe and stimulus ear.

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