Cochlear travelling waves. ("reinifrosch@xxxxxxxx" )


Subject: Cochlear travelling waves.
From:    "reinifrosch@xxxxxxxx"  <reinifrosch@xxxxxxxx>
Date:    Thu, 29 Dec 2011 17:34:45 +0000
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

------=_Part_2527_20775727.1325180085906 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Dear colleagues,=20 Since the List is very quiet now, I permit myself to post a second comment = on the paper "MoH 101: Basic concepts in the mechanics of hearing", by Berg= evin, Epp, and Meenderink, in the recently published proceedings volume "Wh= at Fire Is in Mine Ears: Progress in Auditory Biomechanics". In their secti= on "What are the requirements for traveling waves in the ear", the authors = have written: "As the membrane moves, it changes the volume on both sides o= f its given radial location. If the fluid is incompressible [...], the flui= d must move longitudinally along the tube (conservation of mass)." My comment: The motion of the liquid in the cochlear scalae during the perc= eption of a tone differs strongly from the motion of a liquid streaming thr= ough a tube as usually treated in hydrodynamics courses. Near the best plac= e [place of maximal excitation at low sound-pressure level], the liquid mot= ion is restricted to two narrow regions just above and below the cochlear p= artition [basilar membrane (BM) and attached organ-of-Corti cells]. If the = BM is in the xy-plane and the x-axis points towards the cochlear apex, then= the liquid particles just above and below the BM near the best place move = on closed circular trajectories in planes approximately parallel to the xz-= plane. Measurements of Recio et al. (1998) in chinchilla at x =3D 3.5 mm fr= om base, at a level of 104 dB (SPL), imply that at the tone frequency yield= ing the maximal excitation (7 kHz), the radius of the mentioned circular li= quid-particle trajectories amounts to about 1.4 micrometer only. [For compa= rison: The distance between the centers of two adjacent inner hair cells is= about 10 micrometers].=20 As mentioned yesterday, this liquid motion is similar to that in gravity su= rface waves on lakes, where circular liquid-particle trajectories in vertic= al planes are observed if the wavelength is short compared to [(2pi) times = (water depth)]. At the lake surface, the trajectory diameter is equal to th= e wave height. Below the surface, that diameter decreases. At one wavelengt= h below the lake surface, the trajectory diameter is shorter than that at t= he surface by a factor of exp(-2pi) =3D 0.0019. =20 Reinhart Frosch, CH-5200 Brugg. reinifrosch@xxxxxxxx . ------=_Part_2527_20775727.1325180085906 Content-Type: text/html;charset="UTF-8" Content-Transfer-Encoding: quoted-printable <html><head><style type=3D'text/css'> <!-- div.bwmail { background-color:#ffffff; font-family: Trebuchet MS,Arial,Helv= etica, sans-serif; font-size: small; margin:0; padding:0;} div.bwmail p { margin:0; padding:0; } div.bwmail table { font-family: Trebuchet MS,Arial,Helvetica, sans-serif; f= ont-size: small; } div.bwmail li { margin:0; padding:0; } --> </style> </head><body><div class=3D'bwmail'><P><FONT size=3D2>Dear colleagues, </FON= T></P> <P><FONT size=3D2>Since the List is very quiet now, I permit myself to post= a second comment on the paper "MoH 101: Basic concepts in the mechanics of= hearing", by Bergevin, Epp, and Meenderink, in the recently published proc= eedings volume "What Fire Is in Mine Ears: Progress in Auditory Biomechanic= s". In their section "What are the requirements for traveling waves in the = ear", the authors have written: "As the membrane moves, it changes the volu= me on both sides of its given radial location. If the fluid is incompressib= le [...], the fluid must move longitudinally along the tube&nbsp;(conservat= ion of mass)."</FONT></P> <P><FONT size=3D2>My comment: The motion of the liquid in the cochlear scal= ae during the perception of a tone differs strongly from the motion&nbsp;of= a liquid streaming&nbsp;through a tube&nbsp;as </FONT><FONT size=3D2>usual= ly treated in hydrodynamics courses. Near the best place [place of maximal = excitation at low sound-pressure level], the liquid motion is restricted to= two narrow regions just above and below the cochlear partition [basilar me= mbrane (BM) and attached organ-of-Corti cells]. If the BM is in the xy-plan= e and the x-axis points towards the cochlear apex, then the liquid particle= s just above and below the BM near the best place move on closed circular t= rajectories in planes approximately parallel to the xz-plane. Measurements = of Recio et al. (1998) in chinchilla at x =3D 3.5 mm from base, at a&nbsp;l= evel of 104 dB (SPL), imply that at the&nbsp;tone frequency yielding the ma= ximal excitation (7 kHz), the radius of the mentioned circular liquid-parti= cle trajectories amounts to about 1.4 micrometer only. [For comparison: The= distance between the centers of two adjacent inner hair cells is about 10 = micrometers].&nbsp;</FONT></P> <P><FONT size=3D2>As mentioned yesterday, this liquid motion is similar to = that&nbsp;in&nbsp;gravity surface waves on lakes, where circular liquid-par= ticle trajectories in&nbsp;vertical planes are observed&nbsp;if the wavelen= gth is short compared to [(2pi) times (water depth)]. At the lake surface, = the trajectory&nbsp;diameter is equal to the wave height. Below the surface= , that diameter decreases. At one wavelength below the lake surface, the tr= ajectory diameter is shorter than that at the surface by a factor of exp(-2= pi) =3D 0.0019.&nbsp;&nbsp;&nbsp;&nbsp;<BR></FONT><FONT size=3D2>Reinhart F= rosch,<BR>CH-5200 Brugg.<BR>reinifrosch@xxxxxxxx . </FONT></P></div></bod= y></html> ------=_Part_2527_20775727.1325180085906--


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