Acoustic impedance is measured by placing a piece in the ear called an upper probe. This is just enough room in the ear to create a watertight seal. This advice includes several things. First, a tone generator/receiver, 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 is then sent to the ear canal. The second is a microphone and a sound level meter that will monitor sound within the ear canal. Third, 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 delivered to the ear canal.

Immittance is measured in compliance, compliance is the movement of the tympanic membrane. This is achieved by stimulating the ear with a pure tone and a constant intensity. Then the sound pressure level is measured. This measurement is then used to determine the impedance (how well energy flows through the system) of the middle ear and the tympanic membrane and anything attached to it. The impedance of the ear is derived from a few sources of stiffness, mass, and mechanical and acoustic resistance. The stiffness component comes from the air volumes in the outer and middle ear spaces, the tympanic membrane, the tendons, and the ossicular ligaments. The mass comes from the ossicles, the eardrum and the perilymph. Resistance is introduced by the perilymph. The impedance of an object depends on the 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.

Some things to keep in mind are that mass is an important factor for high frequencies and stiffness is an important factor at low frequencies for system response. Strength is primarily determined by the ligaments that attach to the ossicles, and mass is determined by the weight of the ossicles and the tympanic membrane. Stiffness is primarily determined by the pressure of the fluid in the cochlea on the base of the stirrup.

Tympanometry and acoustic 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 tympanic membrane compliance as ear canal pressure varies from negative to positive. As the pressure is adjusted from zero to its maximum negative or most positive position the impedance increases. The point on the graph where the pressure in the ear canal equals the pressure in the middle ear cavity, the impedance is at its minimum value, in other words, the compliance is at its highest value. The graphic display is called a tympanogram and can have several types. In clinical use, these charts are divided into different types of Jerger to be able to diagnose. A type A tympanogram is characterized by a pressure that is + 50mmH20. This is classified as normal. The type B tympanogram is characterized by having no peak and flat seams. This frequently occurs in serous or chronic otitis media. The type C tympanogram is distinguished by a spike indicating negative pressure in the middle ear. This is usually due to Eustachian tube dysfunction. An abnormal font can be determined if it has too many peaks or is too wide.

An acoustic reflex is what happens when a loud enough sound (70 dB HL) is presented to either ear to cause the stapedius muscle to contract in both ears. This reflex muscle contraction stiffens the conductive mechanism through the stapes tendon and changes the immittance of hearing. Acoustic reflux is easily measured because the top of the probe senses the immittance change and displays it on the immittance device’s meter. How this works is that the afferent nerve from one ear goes to the ipsilateral ventral cochlear nucleus. The neurons then go to the superior olivary complexes on either side of the brainstem. Both superior olivary complexes 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 stapedial muscles to contract in both ears.

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

It is also good to note that when reporting acoustic reflux test results, the term ipsilateral and contralateral should only be used with direct reference to the probe and stimulus ear.