322 Lecture 17 
machiner y foundation distance 
O1 10 kHz O1 10 10kHzO01 10 OkHzO1 10 10kHz0 102 102m 
eS aT —we— § —— mf ——!sf ——e <x 
Fig. 17.1. Typical ranges for octave-band spectra measured during diesel engine operation. Left: velocity 
at engine footings; middle; velocity at substructure or foundation (ship's side of resilient mounts); right: 
velocity on shell plates, and pressure at 10 m from shell together with attenuation due to transmission in 
sea water. Reference values for db scale: velocity 13 pm/sec, pressure 20 4 N/m?. 
by means of hydrophones or microphones. There is no other simple means of 
detecting or measuring sound signals. Statements about sound pressure meas- 
urements, whether in air or in water, are easily made when use is made of a 
decibel scale; the latter needs a reference pressure. The contents and conclu- 
sions of a technical memorandum by Dr. R. W. Young offer sufficient information 
and illustration to establish 20-10~-° N/m? as the single reference pressure for 
both air-borne and water-borne sound measurements [2]. This reference pres- 
sure is in accordance with internationally standardized, albeit air-borne, prac- 
tacenlellr 
A sound pressure level (L,) may be mathematically defined thus: 
§ p°dt 
L,= 10 log | ® —__ (1) 
pol 
where po is the reference pressure mentioned, T is an integrating period, and 
p is the time-dependent sound pressure expressed in decibels (db). 
In the same way a particle velocity level L, can be defined for a time-de- 
pendent v: 
ue 2 
Sf v-dt 
Ee =A0iog a ———— (2) 
vit 
