ACOUSTICAL NOISE MEASURING BUOY WITH DIGITAL DATA RECORDING 
by DR. T. F. HUETER, D.M. BAKER, and J.T. SHAW 
Minneapolis-Honeywell Regulator Company 
Ordnance Division, Seattle Development Laboratory 
Seattle, Washington 
ABSTRACT 
An ambient noise measuring system is being 
developed for use in a deep-water moored buoy. 
The acoustic spectrum between 50 and 400 cps is 
measured at four discrete frequencies and 
sampled at regular intervals. The analog signal 
is converted to a binary digital code and photo- 
graphically recorded for later processing. 
A qualitative measurement of sea state is 
made during each measurement cycle, permitting 
a correlation between wave height and ambient 
noise to be accomplished. 
Auxiliary devices are provided which effect 
buoy recovery upon reception of a coded acoustic 
command signal. The buoy is designed for 
Operation at depths to 1000 feet with a dura- 
tion on station of three months. Solid-state 
devices are used where possible to keep power 
consumption to a relatively low value. 
INTRODUCTION 
Generally, the variations in the ambient 
noise spectra at frequencies above 500 cps 
can be identified with one of several sources 
which happen to predominate at a given time 
and location. Examples of these sources are 
marine life, precipitation, ship traffic and 
man-made noise, seismic sources, and the wind- 
generated agitation of the sea surface itself. 
The well-known Knudsen curves demonstrate the 
dependence of acoustic noise levels on sea 
state over the range of frequencies for which 
the curves have been calculated. 
In the case of frequencies below 500 cps, 
however, investigations have shown that the 
dependency of noise levels upon surface con- 
ditions decreases as the frequency is lowered 
and that below 100 cps the variations may be 
independent of that source. 
The development of a remotely operated 
buoy system for the measurement of ambient 
noise in the ocean could be expected to provide 
a useful tool in furthering the investigation 
of mechanisms involved in the low-frequency 
acoustic phenomena. The use of a submerged 
buoy system for such a purpose offers several 
important advantages as a "listening platform" 
from which to make low-level measurements. A 
submerged buoy can be an inherently quiet plat- 
form, free of unnatural disturbances and noise 
21 
produced by extraneous sources. The mooring 
problem is simplified and the probability of buoy 
loss reduced due to the absence of strain on the 
anchoring system created by storms and high sea 
states. A degree of freedom from possible 
damage to the system from encounters with sur- 
face traffic is also realized. 
Electrical power derived from storage bat- 
teries or other low voltage de power sources 
results in an electrically quiet environment in 
which to operate low-level, high-gain amplifier 
circuitry. The absence of interfering electro- 
static and magnetic fields associated with 
systems operating from high voltage ac power 
sources or with systems which employ long lengths 
of electrical cable is a distinct advantage in 
reducing extraneous electrical noise. This re- 
sults in better signal-to-noise ratios and in- 
creased dynamic ranges in measurement circuitry. 
The Office of Naval Research has sponsored 
the development of such a buoy by the Minneapolis- 
Honeywell Regulator Company. The buoy system to 
be described is intended for use in the open 
ocean for the measurement and recording of 
ambient noise levels at specific frequencies in 
the range between 50 and 400 eps. 
ACOUSTIC MEASUREMENT BUOY SYSTEM 
The acoustic measurement buoy is designed 
for continuous remote operation for periods up 
to three months. Simultaneous measurements of 
wave height and the acoustic spectrum level in 
discrete bands at center frequencies of 50, 
100, 200, and 400 cps are made at predetermined 
intervals and photographically recorded on 35- 
mm film. The acoustic levels are recorded in 
digital form by use of a 6-bit binary coding 
system. 
The acoustic measurement buoy is composed 
of seven basic subsystems: (1) buoy hull and 
mooring system, (2) timing and programming 
system, (3) acoustic measurement system, (4) 
wave-height measurement system, (5) data con- 
version and recording system, (6) recovery 
system, and (7) the electrical power system. 
