How does pressure transfer from eardrum to stapes?

February 5, 2010 5:19 PM

Ear anatomy filter: How come vibration pressure increased by 22 time as sound travels from tympanic membrane to the base of the stapes while tympanic membrane is only 17 time larger than the base of the stapes?

I was reading Augur & Moore Clinical Anatomy and I came across this peculiar passage about the auditory apparatus. It makes a rather confusing comparison when it says that since the area of tympanic membrane is 17 times larger than the base of the stapes, its vibration will cause a pressure increase of 22 to 1 as the vibration travels from the tympanic membrane to the stapes base and through the perilymph.

"This doesn't sound right at all", I thought. So I took a pencil and start calculating. This is what I've come up with

P1 = P2 (the pressure doesn't change)
F1/A1 = F2/A2

Given that A1 = 17A2, A2 = A1/17

F1/A1 = 17F2/A1

F1 = 17F2

So, it makes sense to say that the force is increase by 17 times as proportional to the decrease in the area but to say that the pressure is increased 22x at the base of the stapes is baffling.

What am I missing?

posted by Carius to Science & Nature (1 answer total)

I suspect the factor you are missing is that the ossicles function as a lever in transducing the force/pressure from the tympanic membrane to the oval window via the stapes. The lever action of the ossicles can range from a factor of 1.3-3.1.

Also, incidentally, I found a source that suggested that the TM is actually up to 27 times larger than the stapes.
posted by drpynchon at 10:42 PM on February 5, 2010