FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

sors or a pressure gradient sensor. Sensor arrays will
be considered in Chapter 6. Pressure gradient hydrophores
sense the pressure at two closely spaced points.
The distance between sensors, S, iS typically much less
than the wavelength of sound, k, in the propagation
medium, namely water in the calculations that follow.
A pressure gradient measurement can be accomplished by
means of either two distinct sensors, one at each point,
or by a single senaor apanning the distance between the
two points. Both types of sensora will be considered
here.
Since the output signal from a pressure-gradient
hydrophore is proportional to the preasure gradient,
ita reaponse ia proportional to the particle velocity.
Such sensors are therefore often called particle
velocity hydrophores. This is an advantage when operating
near a pressure releaae aurface where the particle
velocity almost doublea and the pressure itself goes to
zero. The tendency to reapond to particle velocity
renders them more aensitive to flow noiae than omnidirectional
hydrophores. Thia follows because particle
velocity fluctuations associated with flow are often
much greater than the particle velocity oscillations
associated with the acoustic signal being measured.
Consider the sine wave shown in Fig. 5.8 where the
Fig. 5.9
100
90
80
70
60
50
40
30
20
\
1
3 5 ID 2 5 100 2 5 1,000 2 5 10,000
FREOUENCY (Hz)
HEAVY
SS2 WIND
SPEED
10 KTS
RAIN
V a r i a t i o n of the acouatic energy spectrum
level aa a function of frequency for a fiberoptic
presaure-gradient hydrophore with
other noise levels in a sea subsurface environment.
P
PA- -
I
The calculated reaults shown in Fig. 5.9 is
for the case of the aound wave propagating parallel to
the line joining the two sensors. The sensitivity of
a Preasure gradient sensor to a sound wave propagating
perpendicular to this direction ia zero because both
sensors are then aubjected to the aame pressure. The
directivity is dipole-like aa shown in Fig. 5.10(a).
The cardioid directional responae shown in Fig. 5.10(b)
can be obtained by combining the dipole output with
that of an omnidirectional hydrophore with sensitivity
equal to that exhibited by the dipole at e = OO. The
Fig. 5.8
The pressure distribution aa a function of
distance from the zero-preasure point of a
single pressure wave.
instantaneous acoustic preasure, P, is given by:
p = pAsiI’i (2~/ka)x (5.10)
where pA is the acoustic amplitude, as is the sound
wavelength, and x is distance in the same units as ~.
The pressure amplitude at x = O and x = S (S