Page 221 - Gear Technology Solutions

matcher such that the processed pressure waves are transferred into the coch-
lear fluid. The pressure waves are transmitted by the cochlear fluid to the tec-
torial membrane. The snail house shaped cochlear is lined with the tectorial
membrane which moves hair cells. Along the windings of the cochlear the hair
cells are sensitive to different frequencies, performing a spectral analysis of
the sound signal. The hair cell neurons convert the mechanical vibration into
digital electronic signals which are transmitted to the brain. The diagrams in
Figure 1 show the Fourier analysis of the noise emission from a gearset and
how the vibrations are converted to airborne sound which is received by the
order tracking neurons in the ear. The recognized frequencies and amplitudes
are similar but different than the one found in the Fourier transformation. One
reason for this is the residual error which is not captured by the Fourier analy-
sis.

Figure 1: Flow chart of ear function

An interesting phenomenon is that the pitch identification of ear and brain not
only uses the fundamental frequency but also employs the available (audible)
higher harmonic orders. The ear still identifies for example concert pitch A if
nd
th
only the 2 to 6 harmonics is received, and the fundamental frequency is ab-
sent in the received sound signal (ghost fundamental). The “A-signal” without
the fundamental sounds “smoother” than if the fundamental frequency was
present. The conclusion is that the ear as a pneumatic-mechanical-hydraulic-
electronic system has masses, springs and dampening and is created to rec-
ognize frequencies. It will mostly recognize harmonic air pressure changes
which are received in a periodic signal. However, an impulse will also be rec-

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