In real life people as patients, the hard and fast "rules" acoustics offers seldom gives simple test results.  I was taught as a student that conductive hearing losses gives greater hearing loss in the lower freqs than highs.  Spent the next thirty years seeing how that simplistic rule wasn't followed by people...  reasons: so many different conductive hearing losses, so much individual variation, so many additional ear conditions.  Search acoustic models of human ears to see what modelling gives then compare to a epr from an audiology dept to see real life test results.  Jonathan

Physics, really.  Clinically, it depends on the etiology and how it impacts conduction efficiency/mechanics of the middle ear.  Added mass impacts mostly high frequency conduction/transformation; increased stiffness impacts mostly low frequency conduction.  Some conditions impact both, especially as the advance.

The mass and stiffness of the middle ear space (including any obstruction that might be there, i.e., OME), as well as the frequency sensitivity of various portions of the TM.

I'd refer you to Katz (5th ed) Chapter 11, Handbook of Clinical Audiology, for review.

Hi,

Actually, transfer function in middle ear is affected by three components:

1- Stiffness

2- Mass

3- Resistance 

In the first stage of most of the middle ear impairments, stifness component is affected and low frequencies are stiff controlled. So we can see low frequency hearing loss in most of the middle ear impairments. Of course, in some of the middle ear impairments gradually the high frequencies are affected that it shows the mass of the system has increased. 

Its  true for Otosclerosis as the anterior focus hinders the piston like movement of the stapes due to involvement of the anterior part of the annular ligament. hence early on low frequencies are affected first.

agree with Dr. Jalaei's explanation. Most of my patients with conductive loss affect the lows at first. If the surgery could not help the hearing, the patients need more power if you want to fit hearing aids. 

Low frequencies have greater wavelength hence they are interfered or affected easily by obstructions. This may be due to increased mass or (and) stiffness . Therefore, we see low frequency losses in majority of conductive pathologies where mass and stiffness are affected.

Cheri, mass dominance vs. stiffness dominance with respect to frequency..... it’s tricky maths for non-mathematicians. We mortals stick to the above and buy mathematician friends a coffee while they explain the difficult sums!

iirc, transmission of low frequencies are most affected by the mass of the system they are trying to transmit through, transmission of high frequencies are most affected by the mechanical stiffness of the system...... or is it the other way around???

Cheri, im not a doctor, but thank you for your compliment!

Nice Question!

Cherizar Winke

Imagine thay you have two stones connected by a spring. Besides, the second stone is smaller and it is connected to another spring which is thick heavy and difficult to move.

The first stone is an analogy to the tympanic membrane, the second stone with the thick spring is an analogy of the stapes and the cochlea. Since the cochlea is filled with liquid and this is incompressible, the impedance is way greater.

If the spring is "elastic" (not so stiff) the movement of the first stone will find a low impedance (let´s say resistance), this produces an "impedance matching" that provides a mechanical advantage. If the first stone vibrates slowly (low frequency), the vibration will be transmitted by the spring to the second stone, if it is faster, this effect will be lower. This would be equivalent to a normal middle ear, which transmits the small vibrations delivered to the ear canal to the cochlea.

However, if the spring is stiff, the two stones will be connected and move together so the system looses the "mechanical advantage" and the lever effect. Therefore, the vibration induced to the first stone will find a stiff system that will be difficult to move and a higher force will be needed.

That is what happends in the case of otoesclerosis.

Other example, imagine that the spring is broken. Then, the fist stone (tympanic membrane) would move freely because of the low impedance.

Now, if we know that the compliance is "the opposite" of the impedance (at least at low frequencies), that would explain tympanometric results such as "lower or flat compliancias in otoesclerosis" and "abnormally high compliancia in case of ossicles disruption".

If one wants to be more accurate, I must say that the stones are actually membranes and the cavities also play a role. However, that makes everything more complex. I hope this example is useful.

I suggest these readings:

http://www.cochlea.eu/en/ear/middle-ear

Article Analysis of the Middle-Ear Function. Part I: Input Impedance

PS: Jonathan David Binnington, regarding your post, I think it is the other way around.

HEHE nice! Sorry if the stones were confusing, I wanted to avoid terms like, "piston" or "plate" or technical terms and make it work in your mind. Happy to hear that the question is solved (or at least clearer) than before.

Cheers

There are several examples that cause unique phenomena and hearing loss by changing in middle ear impedance, in relation to mass and stiffness. First, an acoustic reflex that happens by the contraction of the middle ear muscle represents the stiffness-controlled phenomenon, showing more reliable responses below 2 kHz. The stiffness limits the transmission of the sounds of low frequency. When it is augmented by a stiffening of the ossicular chain following contraction of the muscle, the low frequency responses are reduced sill further. In contrast, the responses are much less affected at high frequencies, above 1-2 kHz, where transmission is not stiffness-controlled [5].

Secondly, otitis media is associated with abnormal increase in mass and stiffness of the middle ear and its progress may physically interrupt the transmission of sound. Early onset of otitis media begins with reduced pressure in the middle ear cavity and increased stiffness of the middle ear structure. The decrease in pressure is commonly a result of dysfunction of the Eustachian tube. Due to dysfunction of the Eustachian tube, reduced pressure appears and stiffens the middle ear structure. As low frequency sound transmission is further disturbed, this early onset of otitis media is expected to cause low-frequency hearing loss and changed tympanogram from a type A to a type C. As otitis media progresses, otorrhea or runny ears may include abnormal increase in mass causing high frequency transmission disturbance. Other than low-frequency hearing loss, this stage of otitis media involves high-frequency hearing loss, thus, worse air-conduction hearing thresholds appeared not only at low but also at high frequencies and finally result in type B tympanogram.

Thirdly, otosclerosis is a disease of the bone of otic capsule, specifically at the footplate of the stapes. The stapes becomes "fixed" in the oval window as the disease progresses. Fixation causes stiffness enhancement of the ossicular chain. The audiogram with worse air-conduction thresholds at low frequencies, below 1-2 kHz, is typical for otosclerosis

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4491943/