By Ted Venema
I believe there is a huge gap between the science of middle ear transmission and the clinical application of tympanometry. This is especially apparent for the clinician who attempts to make the leap from typical tympanometry to multi-frequency tympanometry. It concerns the vertical axis of the tympanogram. Obfuscation (look it up) offers lots of Impedance but little Admittance to an understanding of Tympanometry.
The horizontal axis on the tympanogram is always “friendly.” It shows positive, neutral, and negative air pressure, in units of mm H2O or dekaPascals (these units are essentially the same in value).
The vertical axis, in my opinion, is a source of audiometric consternation. Tympanometry measures the amount of sound bouncing back off the TM (tympanic membrane) and being picked up by the probe microphone as a function of changes in air pressure (Figure 1). Why then, does the vertical axis not simply read in “dB SPL that bounced back”?
Let me tell you why. The vertical axis does not read in dB SPL that is picked up by the probe microphone because, if it did, the resultant values would vary hugely across individuals, and so would the size of their tympanograms! This huge variation results from things like different probe insertion depths that would change the ear canal volume in any one person. In addition, ear canals themselves vary in size across individuals.
An alternative, then, is to measure the physical properties of the middle ear. The middle ear is a “stiffness-dominated system,” and so it makes sense that we would want to quantify its stiffness (or its inverse, compliance) per se. This actually does allow for a fairly standard range of tympanogram sizes and shapes to be used as normative. It also renders similar-sized tympanograms independently from the depth of probe insertion or ear canal size. In tympanometry then, the less sound picked up by the probe microphone, the more compliance you have.
The confusion to me, however, stems from the fact that while we actually measure dB SPL picked up by a probe microphone, we do this in order to quantify something else, namely, compliance!
COMING TO TERMS WITH SOME TERMS
To better understand compliance and the vertical axis of a tympanogram, we had best look at some things that impede the transmission of sound through a system like the middle ear:
- Stiffness: opposes transmission of low Hz and passes high Hz; the chief factor in middle ear impedance
- Mass: opposes transmission of high Hz and passes low Hz
- Resistance: like simple friction; in any system it is the same for all Hz
- Impedance: combination of the above
Since the middle ear is stiffness dominated, mass and resistance do not play much of a role in its overall impedance; the ossicles are tiny, and the ligaments holding the ossicular chain in place don’t give much friction. Today’s tympanometry, however, measures what the middle ear admits, rather than what it impedes. Here are some Admittance terms:
- Inverse of stiffness is compliance; audiology textbooks often call it “compliance susceptance”
- Inverse of mass is not given a short name (since the middle ear is stiffness dominated); audiology textbooks often call it “mass susceptance”
- Inverse of resistance is called “conductance”
- Inverse of impedance is called “admittance”; it’s a combination of the above
For admittance then, the “camel” in the room is Compliance, along with two “mice” called Mass Susceptance and Conductance. The ohm is a unit used to describe Impedance; for admittance, the word “ohm” is simply flipped around to read “mho.” Since the ear is small, it is more practical to use thousandths of a mho or millimhos (mmho’s) to indicate units for compliance on the vertical axis of the tympanogram.
MULLING OVER MULTI-Hz TYMPANOMETRY
The fun really begins when we attempt to move to multi-frequency tympanometry. The chasm here is filled with opaque concepts and, in my opinion, the bridge to take you across the chasm is hard to find. But let me give it a try:
In multi-frequency tympanometry, not only is the probe tone Hz manipulated, but three different tympanograms can be measured for each Hz! The combined contributions of the “camel of compliance” and a “mouse of mass” are called “susceptance.” These are plotted as what is called a “B” tympanogram. The other “mouse,” called “Conductance,” is plotted as a ‘G” tympanogram. The sum total of these is admittance, which is plotted as “Y” tympanogram.
Note: For the normal middle ear, the Y tympanogram will be quite similar to the B tympanogram, because the main component of admittance is compliance. With multi-frequency tympanometry, however, the course of various middle ear pathologies can be tracked by the interactions among these three tympanograms at any one probe Hz.
The interaction of middle ear susceptance and conductance is often shown as vectors, much like those that would be drawn to show how a blowing wind might affect the passage of a boat floating along with the water current (Figure 2). Regarding the middle ear, the vector normally radiates upward, showing that it is mainly controlled by its stiffness; if the vector radiates downward it is mainly controlled by its mass. The vector length would show the overall strength by which any sound transmission system is controlled by all of these three interacting elements.
The resonance of a sound transmission system like the middle ear is found when its admittance due to compliance (compliance susceptance) is equal to its admittance due to mass (mass susceptance). When these cancel each other out to give an admittance of 0 mmho, the only player in overall admittance is conductance (Figure 3). Multi-frequency tympanometry shows that the normal middle ear has an overall resonance to Hz just above 1000 Hz.
Various types of middle ear pathology affect middle ear resonance. Negative middle ear pressure or otosclerosis will stiffen the system, resulting in a higher resonating Hz. A monomeric TM, PE tubes, or disarticulated ossicles will decrease the stiffness and accordingly, lower the resonating Hz.
Why doesn’t typical tympanometry separate these three elements of admittance? He reason it doesn’t is that the effects of most middle ear pathology, such as otitis media, are so gross (not at all subtle) that the three contributing components of admittance do not need to be analyzed separately. Typical tympanometry with its 226-Hz probe-tone frequency sticks to the overall Y (admittance) tympanogram, which we know to be governed mainly by the camel named “Compliance,” along with its two mice friends named “Mass Susceptance” and “Conductance.”
Typical tympanometry, even by itself as a test of middle ear function, is still a fascinating story. It assumes that in order for the middle ear to be most efficient at passing incoming sounds through it, air pressure must be even on both sides of the TM. If the least amount of sound bounces back off the TM when the air pressure in the outer ear canal is at regular room air pressure, this means the air pressure behind the TM is the same. In this indirect way, tympanometry measures conducted in the outer ear canal can actually tell us about the middle ear air pressure behind the TM!
Ted Venema, PhD, teaches at Conestoga College in Kitchener, Ontario, and is the founder and director of its program for Hearing Instrument Specialists. He has a PhD in audiology from the University of Oklahoma. He frequently gives presentations on hearing, hearing loss and hearing aids and is author of the textbook Compression for Clinicians, published by Thompson Delmar Learning and now in its second edition.