Page 43 - Singiresu S. Rao-Mechanical Vibrations in SI Units, Global Edition-Pearson (2017)

40 Chapter 1 Fundamentals oF Vibration

FiGure 1.7 Vibration testing of the space shuttle Enterprise. (NASA)

connection, the mechanical engineer tries to design the engine or machine so as to mini-
mize imbalance, while the structural engineer tries to design the supporting structure so as
to ensure that the effect of the imbalance will not be harmful [1.20].

1.3.1 Because humans experience vibrations in different forms such as entertainment
Conversion of (music), annoyance (noise), and something that needs to be eliminated or controlled
Vibrations to (vibration of machinery), it would be useful to understand how vibrations are con-
verted into sound by the human ear. In fact, in many cases, experienced engineers can
sound by the identify the nature of fault or damage sustained by a machine just by hearing a change
human ear in the nature of sound of the machine (produced by the vibrating machine). The human
ear consists of three parts – the outer, middle, and inner ear as shown in Fig. 1.9(a).
When someone talks or plays a musical instrument, for example, a pressure wave is
generated in the air, that is, a longitudinal wave in which the air molecules spread out
into regions of compression and rarefaction. The ear recognizes the wave as sound if
the frequency of the wave falls in the audible range of 20 to 15,000 Hz. In such a case,
the pressure wave will make the eardrum to vibrate. Because the hammer is attached
to the center of the eardrum at one end and is connected by ligaments to the anvil and
the oval window at the other end, the hammer and the anvil move in unison when the
eardrum vibrates. Thus the vibrations of the eardrum are transferred to the inner ear
through the components of the middle ear. The major component of the inner ear,
called cochlea, looks like a snail shell. It is composed of two chambers called scala
vestibuli and scala tympani that are separated by a strip of skin called basilar mem-
brane. The chambers contain a fluid known as perilymph. These parts are shown in
the stretched out form of cochlea in Fig. 1.9(b). The underside of basilar membrane
contains thousands of hair cilia that look like a small tuft of hair. The vibrations at the