[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Answers, comments welcome.
Dear Randy and List,
Randy, in your message about dichotic stimulation of the basilar membrane
[BM] you formulated your remarks and asked for answers and/or comments on
the following topic:
** Do the BM's in a dichotic experiment using two harmonically related tones
(e.g. 200/300 hz) have the same vibration profile or are they different? **
And you apologized in the following way:
** I don't know if this is beyond the scope of this forum in which case I
apologize. However, if this topic is not too crazy, I would welcome any
answers, guesses or speculations. **
To my opinion your remarks are to the highest level relevant for everybody
who is involved in the research of our hearing sense, so also for members of
And in my view it is far from crazy.
At the risk of fluttering the dovecote I want to give you my answers and
comments you asked for.
However for a better understanding of my comments I can only do this in two
Please let me first reopen as shortly as possible that other topic issue,
because it is directly related with the setup of my present answer to you.
In November/December last year we have had the discussion whether a
traveling wave exists inside the cochlea or on the BM that transfers the
sound pressure stimulus of a pure tone to the point where, for the
corresponding frequency, the BM can resonate. Also the model that makes use
of the transmission line concept was discussed then.
I on my turn presented in that discussion session in a PDF the solution of
the non-stationary Bernoulli equation, that is perfectly well valid in the
case of the push-pull movements of the perilymph inside the scala tympani
[ST] and scala vestibuli [SV], while the in between embedded scala media
[SM], filled with endolymph at rest, has substantial ? and therefore not
negligible ? dimensions.
According to hydrodynamic rules these dimensional conditions make that the
hypothesis in which both the influence of the Reissner membrane and the
content of the SM can be ignored and the cochlear duct can be considered as
a folded tube with only the BM as an interface in between is definitely
At the end of that discussion Dick Lion stated that in his opinion the local
frequency dependent flexibility or compliance of the BM makes it possible
that this membrane is bending outwards ? a local movement of the BM towards
the SV ? and that this bending is the cause of evoking sound related stimuli
in the BM, organ of Corti and finally via the auditory nerve to the auditory
He therefore firmly disagreed with my point of view and my theoretical work
couldn?t convince him (and others on this List) that the functional
mechanism in the cochlear partition might be completely different from what
is assumed at the moment.
Well like the well-known promoter of physics, MIT professor Walter Lewin,
does in his magnificent physics courses, I have built my own demonstration
equipment for clearly showing what happens on the walls of a duct in which
an alternating flow in core direction is evoked.
The one experimental set-up is extremely simple, but therefore also highly
To mimic utmost compliance in the ?walls? in one of the experiments I have
hanged on thin wires in an open frame two sheets of paper that can move
freely. Between the two I can evoke with a spatula an alternating flow
parallel to the surfaces of the sheets of paper.
And I have constructed a closed loop with a tube and a bellow, the latter
centrally subdivided by a plate, with which I can create a push-pull flow in
the tube, while in the other branch of the tube locally a flexible membrane
is mounted in the wall, which registers what happens on the wall of the
The obtained results I found in both experiments?
The evoked motion patterns are exactly identical to what I could predict out
of the theory I have presented last year on this List.
The two sheets of paper are not at all moving in outward direction as was
suggested. They are moving in opposite direction, so towards the core line
of the alternating flow. And under a steady alternating stimulus (with
constant amplitude) they both do that with a stationary deflection on which
an alternating deflection is superposed with doubled frequency.
This indicates that both sheets experience the influence of an alternating
and in average lower pressure evoked in the space between the two sheets.
The tube experiment also shows that the membrane in the wall is always
moving inwards ? so towards the core line of the tube. And superposed on a
constant deflection inwards the membrane also deflects periodically with
double frequency related to the original stimulus frequency.
Without any doubt this is indicating that at least squaring of the input
stimulus plays a dominating role.
[Note: To make it even more convincing for everyone I will place a video
registration of these experiments fairly soon on internet, like Walter Lewin
does with his physics courses.]
For now the only clear and firm conclusion I can draw is that the
suggestions on this item of Dick Lion and others are wrong. The medium in
the tube is moving as a whole. And therefore these experimental results, in
combination with the theoretical solution of the non-stationary Bernoulli
equation, are one of the reasons that the transmission line concept cannot
play a role in it either.
The second reason for rejecting the traveling wave concept is the following:
I also have studied the different possibilities for ?traveling waves? in
literature. And then especially I have looked at the conditions, parameters
and geometrical dimensions under which such waves can exist.
In short (you don?t need expensive literature retrievals, because you can
read a summary of the possible wave forms in Wikipedia) we can state that
there are three forms to distinguish:
1. Rayleigh waves
Rayleigh waves are a type of surface acoustic waves which travel on solid
The typical speed of these waves is slightly less than that of so-called
shear waves. And it is by a factor (dependent on the elastic constants)
given by the bulk material. This speed is of the order of 2?5 km/s.
For a sound signal with a 1000 Hz frequency this means that the minimal
wavelength is approximately 2 meter. While the BM has a length of
approximately 35 millimeter, it is impossible to make a realistic
combination for application in the cochlea.
Besides that Rayleigh waves are surface waves where the thickness of the
material must be relatively high related to the concerned wavelength.
With a fraction of a millimeter thickness for the BM you can forget that
this type of wave can play a role in the BM vibrations.
2. Love waves
In the field of elastodynamics, Love waves, named after A. E. H. Love, are
described as horizontally polarized shear waves guided by an elastic layer,
which is "welded" to an elastic half space (so a very thick part of bulk
material) on one side while bordering a vacuum on the other side. In
literature can be found that the wavelength of these waves is relatively
longer than that of Rayleigh waves.
And also these conditions and parameters are nowhere found in the cochlear
3. Lamb waves
Lamb waves propagate in solid plates. They are elastic waves whose particle
motion lies in the plane that contains the direction of wave propagation and
the plate normal (the direction perpendicular to the plate). In 1917, the
English mathematician Horace Lamb published his classic analysis and
description of acoustic waves of this type.
The wave propagation velocities of the two possible modes in Lamb waves are
comparable with that of the Rayleigh wave. And therefore they also don?t
provide for a possible application in the traveling wave description inside
In other words: we also cannot make a realistic fit with Lamb waves inside
Of course everybody can persist in believing that until now registered
auditory experimental results justify the formulated hypothesis that such
types of waves can exist in the cochlea. Then however you are forced to
answer the following question:
On what underlying physics grounds is it possible that material quantities
and acoustic process parameters inside the cochlea can be altered in such a
way that as a result the wavelength of 1.5 meter for a 1000 Hz stimulus in
bulk perilymph fluid can be altered in less than 1.5 millimeter?
As can be seen from the Rayleigh, Love and Lamb waves the circumstances and
material properties cannot provide for a scaling factor better than 0.5 from
bulk material sound velocity to the concerned type of wave.
Be aware that inside the cochlea a scaling factor of 0.001 or even smaller
will have to be possible. This can be considered as completely impossible.
What remains is that just as I stated before:
The described non-stationary Bernoulli effect, that provides for the sound
energy stimulus everywhere in front of the BM, is driving the BM vibrations.
And dear Randy this last statement above is my answer to your following
** I have always wondered about what drives BM vibrations **
It is the everywhere present sound energy stimulus that drives the BM.
My following contribution will show the implication of all this for the rest
of your request.