Hearing Research, 13 (1984) 83-88

Elsevier

HRR 00451

Notes on the comparative mechanics of hearing. III. On Shrapnell’s membrane

L.U.E. Kohll~ffel

lnstimt fir Physiologic und Biokybemetik, Universitl Eriangen - Niirnberg, Universit&sstr. I?, B_VO Erlangen, F.R.G.

(Received 11 July 1983; accepted 21 November 1983)

Shrapnell’s membrane is very large in many mammals. A large membrane can affect the coupling between sound field and ossicular vibration. Some effects on sound transmission are suggested.

Shrapnelt’s membrane, aural sound transmission, eardrum function

In many mammals the eardrum consists of two distinct parts: the taut pars tensa and the limp pars flaccida, The pars flaccida is also called Shrapnell’s membrane. Hyrtl [9] named it mem- brana tympani accessoria. The eardrum is the in- terface between the acoustic field in the earcanal and mechanical ossicular vibration. Mechanical power is transmitted to the malleus through the stiff, tightly coupled pars tensa [4,6,7,15,25]. The slack, loosely coupled pars flaccida transfers little power since its deformation elicits negligible ossic- ular reaction force. Various modes of pars flaccida operation have been suggested. It may improve ossicular mobility 1221. It can reduce quasi-static pressure build up across pars tensa during middle ear ventilation through the Eustachian tube [19,21]. It can have an effect similar to raising the acoustic stiffness of the bullar air space [5]. In ears with a pars flaccida/pars tensa area ratio equal to or even greater than 0.5 pars flaccida appears much oversized for any of these functions. Therefore other mechanisms are sought and the possible role of a large Shrapnell’s membrane in interface cou- pling is considered.

Methods

Ears from 21 mammalian species were collected and fixed in 10% formalin solution. The eardrum was exposed and the pars flaccida/pars tensa area ratio /3 was measured from drawings made with the stereomicroscope. The area ratio fl varies widely across mammals, those ancient (insectivores) and those more recent (e.g. ungulates) (see also [3,8,10,13]):

Insectivores: shrew - Sotex QM~~US *; mole - Ta@z europaea 0; hedgehog - Er~~~c~e~ e~opae~ 0.68;

Lagomorphs: rabbit - Oryctolirg~ ctmiculus 0.13; hare - &pus ewopaeus 0.27;

Rodents: mouse - MU.S muscu/u.r 0.52; rat - Rams rattus 0.25; golden hamster - Mes0cricets.s auratw 0; mongolian gerbil - ~erio~es #n~icu~ar~ 0.11; guinea pig - Cavia cubaya 0; chinchilla - Chinchilln /aniger 0; red squirrel - Sciurm vulgaris 0;

Carnivores: polecat - Mwtelu put&w 0.06; cat - Fe&s cam 0.02;

* fi could not be measured in shrew because of the peculiar ossification pattern. The tympanic ring of the eardrum is

nearly closed. However, the ring is not supported by bone but held in position by tendinous tissue and the bulla is only

partly ossified.

0378-5955/84/$03.~ 0 1984 Elsevier Science Publishers B.V.

84

Ungulates: horse - Equus cabahs 0.12; cow - Bos

taurus 0.042; deer - Capreolus capreolus capreolus 0.45; sheep - Oois aries 0.48; goat - Capra

aegagrus hircus 1.1; pig - Sus scrofa 1.3 ;

Man 0.027.

The isthmus tympanicus was exposed to ex- amine the acoustic shunt conditions between the

tympanic and epitympanic cavity. In three cases (hare, goat, pig) the sound pressure in the meatal recess, the epitympanic and tympanic (bullar) cav-

ity was measured in a closed sound system to check for the isthmic acoustic volume flow at low frequencies ( < 1.5 kHz). At shunt resonance, fs,

the acoustic mass reactance of pars flaccida and isthmus tympanicus approximately equals the

stiffness reactance of the bulla. Then the bullar sound pressure exceeds meatal pressure in ampli- tude and lags in phase by 90’. In goat, fs = 300 Hz; in hare, fs = 400-600 Hz, but in hare, fs was

seen to increase up to 1 kHz as Shrapnell’s mem- brane became dry. In pig there was no evidence

’ lmm ’ Fig. 1. (a, b) Examples of dehiscence in eardrum frequently

occuriing in hares. (c) Rare c&se of double dehiscence in hare

eardrum.

for shunt flow through the isthmus tympanicus. Zuckerkandl [24] noticed a fenestra per-

tympanica occurring frequently in hares. Of 35 hares 12 had a bilateral, and 5 a unilateral dehis-

cence in the eardrum, an opening of variable size close to the pars tensa-pars flaccida boundary. Zuckerkandl’s finding was followed up and 30

hare heads were collected from local dealers in the hunting season of 1981. In 5 cases there was a bilateral, in 8 cases a unilateral dehiscence. Fig. 1 shows examples of the fenestra pertympanica whose aetiology is unknown. The effect of this ‘naturally’ occurring opening on sound transmis-

sion may be mimicked by an eardrum perforation. It causes a transmission loss that’increases at the rate of 12 dB per octave towards low frequencies

[1,12,14]. However, if the perforation has the right size it can cause a transmission gain below the

resonance frequency of pars tensa. Namely at the frequency where the mass reactance through the perforation approximately equals the stiffness re- actance of the air space behind the eardrum.

Drawings are by the author.

Results

Fig. 2 shows the ears of deer, goat and pig *. In all three cases Shrapnell’s membrane is very large

and can be expected to allow substantial acoustic volume flow from the meatus into the epitympanic cavity. However, ears differ at the isthmus

tympanicus, the constriction between the epi-

tympanic and tympanic cavity. In the deer the isthmus is wide (as for instance in hedgehog, mouse and hare). In the goat (and in sheep) it is merely a narrow passage through the recessus stapedialis bounded by a stiff membrane and cochlear bone. In the pig the isthmus is acoustically blocked by a bony shield over the round window and mucous membranes enwrapping the ossicles. Thus, the acoustic coupling between the middle ear cavities varies and the extent differs in which acoustic volume flow through Shrapnell’s membrane can

* Features discussed are not due to domestication. The ears of

the domestic pig (SW scrofa domestica) and the wild pig (Sur

scrofa) are found alike except for the pinna. The middle ear

in goat (Capra aegagrw hircus) appears very similar to that

in alpine ibex (Capm ibex ibex).

85

5mm

c 1

Fig. 2. Exposure of ear in deer (a), goat (b) and pig (c) to show large Shrapnell’s membrane and isthmus tympanicus. 1, right ear,

lateral view; 2, left ear, posterior view. In deer the isthmus is wide, in goat it is narrow, in pig it is blocked. M, external auditory

meatus; PF, pars flaccida; PT, pars tensa; E, epitympanic cavity; B, bullar cavity; Vrr, volume flow through isthmus tympanicus; T,

tympanic cavity (the large, spongy bulla in pig is not shown).

enter into the air space behind pars tensa. Pars tensa is coupled twice to the meatal sound

field when pars flaccida and isthmus tympanicus together allow shunt flow around pars tensa (Fig.

‘3a). The effect on sound transmission can be read from the equivalent circuit in Fig. 3b. Z,, Z, and

Z, are the acoustic impedances of pars tensa, pars flaccida and isthmus tympanicus, respectively. Z, can be approximated by an acoustic compliance in the frequency range below pars tensa resonance

fr:

Z 1 _-

T - j2?rfC, forf<f,

C, and C, are the acoustic compliances of the epitympanic and bullar cavities. P, is the sound pressure in front of the eardrum. A Ps is the sound

pressure across pars tensa driving the ossicular chain and the inner ear. If we take A P as the tensa

pressure in the absence of Shrapnell’s membrane,

Fig. 3. The acoustic shunt around pars tensa - through pars

flaccida (Z,) and isthmus tympanicus (Z,) - influences tensa

pressure AP,. Effect on sound transmission in c follows from

equivalent circuit in b.

u

-!iz!!l J- b

Fig. 4. For negligible isthmic flow, pars flaccida operates in

front of pars tensa. It can be part of an acoustic filter - a

Helmholtz resonator H in b - that controls the power flow N

into the meatus and influences meatal sound pressure PM (c).

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we can define an insertion factor F:

lAPSI F=20log ,Ap,dB

By insertion of pars flaccida sound transmission is increased (transmission gain) if F > 0 (IA PSI >

)A PJ) and decreased (transmission loss) if F < 0

(IA PSI -c IdPI). The frequency dependence F =

F(f) is shown qualitatively in Fig. 3c. At low sound frequencies there is a transmission loss since AP,cAP:

Here the effect of Shrapnell’s membrane may be likened to an increase in the acoustic stiffness of

the middle ear cavity as suggested by Fleischer [S]. At f0 the shunt impedance 2, + Z, is a small

resistance:

At f0 there can be maximum transmission loss provided

As the frequency is raised (f > fO) the acoustic

shunt impedance is mass controlled:

Transmission gain is possible through resonance at

fs (fs <f.r):

fs = 1

27+,+Ca)W,+~,) Hz

If pars tensa is much stiffer than the bullar cavity:

fs = 1

29GEXJ Hz, if C, e C,

Then the resonance at fs is due to interaction between shunt mass and bullar cavity stiffness.

The acoustic shunt circuit around pars tensa -

through Shrapnell’s membrane and isthmus tympanicus - causes a transmission loss at very low frequencies. However, it can cause a transmis- sion gain in a frequency band below pars tensa resonance. Technical acoustics offers striking anal- ogies. In microphones and loudspeakers (bass re- flex cabinet) a shunt path around the active di- aphragm is frequently employed to boost interface coupling in the frequency range below diaphragm resonance. This is at the expense of a sharper low

frequency roll-off [11,17,20]. In the pig intercavity flow is blocked and in the

goat and sheep flow is cut off at rather low fre- quencies since the mass reactance of the narrow isthmus tympanicus becomes rapidly much greater

than the stiffness reactance of pars tensa and bullar cavity *. Then Shrapnell’s membrane oper- ates entirely in front of pars tensa. It affects interface coupling by changing sound pressure in front of the eardrum. As shown in Fig. 4 the situation is similar to introducing an acoustic filter _ a Helmholtz resonator - as side branch. At the frequency f, one expects a minimum in meatal sound pressure P,:

fH = ’ _ Hz 2+?,

* The mass reactance of the isthmus tympanicus and the

stiffness reactance of the bulla are in resonance at /,a:

/,a = 1/(2rm) Hz. In goat /,a = 450 Hz.

At fn the transducing part of the middle ear is

shunted by the acoustic resistance of pars flaccida

RF since

J2flf &E =R,

It depends on the relation between RF and the specific acoustic impedance of the earcanal whether

the filter operates in a reactive or dissipative mode, whether it prevents acoustic power from reaching

pars tensa by reflecting or absorbing power. In either case sound transmission is decreased.

As the frequency is raised further the wave- length of sound approaches the dimensions of pars flaccida and pars tensa and simple lumped circuit

models lose applicability. The high-frequency,

wave-mechanical behaviour of pars flaccida must be treated in connection with earcanal and pinna. The acoustic antenna system, pinna-earcanal-

eardrum, is complex and capable of multiple reso- nances [2,18]. Shrapnell’s membrane may play a part in enhancing and damping antenna reso- nances. The pig’s earcanal reminds one at once of sound ducts treated for anechoic performance [16]. That is to say, the unechoic tube and its lining are so designed “that the attenuation per unit length increases slowly (to avoid partial reflection) but steadily from a small value near the source end to a large value at the far end” (quoted from [16], p. 115). In a sound-treated, unechoic earcanal multi- ple signal transits between eardrum and pinna are

prevented. This shortens acoustic transients and may improve temporal resolution.

Discussion

It is suggested that in the evolution of mam- malian hearing Shrapnell’s membrane is one of the parameters available for shaping peripheral sound transmission. Shrapnell’s membrane can be part of an acoustic shunt circuit around pars tensa, it can be part of an acoustic filter preceding pars tensa and it can be part of the complex antenna system of the external ear.

From existing behavioural audiograms it ap- pears that a lack of low frequency sensitivity goes with the presence of a large Shrapnell’s membrane (e.g. hedgehog, mouse, rat, sheep). The sheep

87

audiogram [23] shows a much reduced sensitivity

(20-30 dB) up to 10 kHz as compared to the cat. Whether the predicted transmission gains find ex-

pression in sound conduction cannot be learned from the existing literature since reported experi-

ments were not aimed at studying the role of Shrapnell’s membrane.

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