324 Lecture 17 
Thus, mechanical noise impedance Z,, is introduced as 
I a 
Lav =. st (7) 
Veff 
where F2, and v2, are taken from Eqs. (5) and (6), respectively. For measure- 
ment, computation, or graphical representation, moreover, it is convenient to 
define an impedance level L; as 
20 lo y (8) 
where Z equals Za in Eq. (7) or the modulus of a complex impedance Z, and 
where Zo is taken as 1 kg/sec for simplicity. Besides the levels for pressure 
already defined in Eq. (1), velocity in (2), and impedance in (8), a power level 
Lpis used and defined as 
lio = 10 log (9) 
where P is the sound power (in watts) to be expressed in db and P, equals 10-!2 w, 
the convenient reference power for air-borne sound measurements. 
Having defined some symbols, units and levels we proceed now to follow the 
sound path from a source into the water. Some of the steps in the path will be 
considered in more detail. 
17.3. RESILIENT MOUNTS 
A simplified classical system involving a resilient element or spring is 
shown in Fig. 17.2. An (electrodynamic) exciter or shaker causes one side of 
the spring to vibrate sinusoidally in a "normal" direction; normal refers to the 
direction normal to the two parallel contact planes at each side of the spring. 
exciter 
Fig. 17.2. Classical example of vibration isolation; 
the electromechanical analog shows clearly the pur- 
Vi pose of the spring (condenser): acoustical uncoupling 
or release. The mass is hanging from a spring in- 
stead of resting on it; the two are not fundamentally 
different. 
spring Vi Vm 
(s) 
exciter s m 
Mi =) — vm =)1- £2 
ata) cae 
