Transcript Light aircraft, propeller noise and Döppler shifts: Michael J. Buckingham

Light aircraft, propeller noise and Döppler shifts:
tools for underwater acoustics experiments
Michael J. Buckingham
Marine Physical Laboratory
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, CA 92093-0238, USA
First Informal Miniworkshop on
Acoustic Cosmic Ray and Neutrino
Detection
Physics Department
Stanford University
13-14 September 2003
Research supported by the
Office of Naval Research
BAC 111, Keflavik, Iceland
East Greenland
Marginal Ice Zone, Greenland Sea
Icebergs, Scoresby Sund, East Greenland
Low-frequency sound speed in sediments
1.
Aircraft sound sources (50 - 1000 Hz)
2.
Measured propeller noise spectra in air
with aircraft stationary on ground
3.
Measured (in flight) propeller noise spectra
a) in air (microphone)
b) in seawater (hydrophone)
c) in sediment (buried hydrophone)
4.
Döppler measurements of low-frequency (< 1 kHz)
sound speed in air/seawater/sediment
5.
Wave types in shallow water channels
6.
Looking to the future
Aerospatiale Socata Tobago TB10: Lycoming 180 h.p. 4-cylinder,
4–stroke engine; variable pitch (constant speed) two-blade propeller.
Typical flight parameters for experiments:
airspeed, 53 m/s (106 knots)
engine speed, 2500 rpm
altitude between 33 m and 330 m
Navigation
Latitude, longitude and altitude, plus time, continuously recorded
on GPS with built-in (pre-flight calibrated) pressure sensor.
Communication:
Two-way radio contact with Boston whaler near sensor station on
sea surface.
Tobago TB10 overflying sensor station
at an altitude of 66m on 2 July 2002
Time series (left) and spectrogram (right) of sound from stationary
Tobago on the ground with engine operating at 2000 rpm, 26 May 2002.
Note the harmonics. (Microphone near port wingtip about 1 m above
the ground and 20Þbehind the plane of the propeller.)
Spectogram of airborne sound as Tobago flew over microphone
10 m above sea surface. Note Doppler shifted propeller harmonics
and Lloyd’s mirror effect.
(Aircraft speed, 53 m/s, 2500 rpm; 4 March 2002).
Aircraft track flown on
2 July 2002. The small
red circle marks the
sensor location.
GPS altitude and ground speed data for flight of 2 July 2002.
microphone
sea
surface
hydrophone
vertical array
sea bed
bender
hydrophone
Instrumentation for measurement of low-frequency (50 - 1000 Hz)
sound speed and shear speed in a marine sediment. (Not to scale).
The vertical array incorporates a 2-axis tilt sensor with compass.
A SeaBird temperature profiler yields the sound speed profile in
the channel. GPS navigation is used in the aircraft to record its 3D track as a function of time.
11-element vertical array
4 sub-arrays of 5 elements
octave spacings
total bandwith 0.25 - 2 kHz
Tilt/compass
sensor
0.325 m
12 m
ITC 6050C
Total cable lengh = 50 m
Common power and ground
Coax cables for signal and shield on
6050's
Twisted pair for compass/tilt
Total of 26 wire leads
Tilt/compass unit is 6.35cm x 5.08cm x
3.175cm
100 lbs max weight
(b) Hydrophone
normalized power
spectral density
(a) Microphone
frequency, Hz
frequency, Hz
Spectral lines (propeller harmonics) in the acoustic
signature 1.06 s before CPA of a Tobago TB10, speed
53 m/s and altitude 62 m (8 May 2002). a)ΚAirborne
spectrum 1Κm above the sea surface. b)ΚUnderwater
spectrum 13 m beneath the sea surface. Note the higher Doppler shifts in air compared with water.
v
1
sea
surface
microphone
2
hydrophone
2
3
sea bed
hydrophone
The Doppler shift on the red ray is the same
in the air, the water and the sediment:
f
[1 - (v/ci) sin( i)]
where f is the unshifted frequency, c is sound speed
and i = 1, 2, 3.
fD =
v
1
sea
surface
microphone
2
hydrophone
2
3
sea bed
hydrophone
The maximum difference between Doppler shifted frequencies on approach and departure scales inversely
with the local sound speed:
fD = 2fv/ci
where f is the unshifted frequency, c is sound speed
and i = 1, 2, 3.
Data from July 2, 2002
• Temperature and pressure
profile (Sea-Bird T&P
Profiler SBE 39)
• Microphone 1 m above the
air/sea interface
• 7 Hydrophones spanning
much of the 15 m water
column
• Buried hydrophone
Some Preliminary Results
for Sediment Sound Speeds
• Used minimization
technique with a cost
function that maximized
power along Doppler shift
curve
• Started with microphone
data – get v, h, ca, t0 and f0
• Proceeded to water
column and sediment to
get cw,cs
Calibrated spectrogram of sound in air as Tobago
flies over the sensor station. Note large Doppler shift.
(Aircraft speed, 53 m/s; altitude 66 m; 2500 rpm; 2 July 2002)
(Color bar: dB re 1 Pa2/Hz)
Application of Minimization
Technique to Air Data
• Microphone data
– Predicts average sound speed
in air (342.3 m/s) consistent
with temperature conditions
(343.5m/s)
– Showed a flight direction bias
of about 5 m/s, consistent with
a wind effect (verified in GPS
data)
– Average aircraft velocity (54.5
m/s) in good agreement with
average velocity from GPS
data (54.8 m/s)
Calibrated spectrogram of sound in sea water (at depth of 10 m)
as Tobago flies over the sensor station. Note smaller Doppler
shift than in air. (Aircraft speed, 53 m/s; altitude 66 m; 2500
rpm; 2 July 2002)
(Color bar: dB re 1 Pa2/Hz)
Calibrated spectrogram of sound in sediment (very fine sand)
as Tobago flies over the sensor station. Doppler shift is slightly
less than in sea water. (Aircraft speed, 53 m/s; altitude 66 m;
2500 rpm; 2 July 2002)
(Color bar: dB re 1 Pa2/Hz)
Water and Sediment Data
• Water data
– Acoustic data
• ĉ = 1529.5 m/s
•  = 23.4 m/s
– Sea-Bird data
• ĉ = 1512.4 m/s
• Sediment data
– Acoustic data
• ĉ = 1649 m/s
•  = 23.6 m/s
direct ray
reflected ray
head wave
modal "eigenrays"
sea
surface
refracted ray
evanescent wave
fast sea bed
Wave types in a shallow-water channel
N.B. A water-column sensor could detect the Doppler difference-frequency
on the head wave from the sea bed to determine
the sediment sound speed
Future measurements using an aircraft sound source
Sediment sensors
a) in-situ sediment sound speed (Döppler difference frequency)
b) in-situ sediment attenuation (one or more buried phones)
Water column sensors
a) sediment sound speed (Döppler difference
frequency of head wave)
b) sediment density & porosity (near normal
incidence reflection coefficient)
Looking ahead ………..
Diamond Star DA40, Palomar, CA
Super Decathlon (aerobatic)
“James Bond” SeaBee (amphibian)
DHC Otter and Cessna 206, Honolulu
Cessna 206, Honolulu
Honolulu
“Jurassic Park”, Oahu
Casa Saeta, Spanish jet (sonic boom research)
Cessna 172
A light aircraft as a source of sound
for measuring sediment dispersion and attenuation
at low frequency (0.1 - 1 kHz)
Michael J. Buckingham & Eric M. Giddens
Marine Physical Laboratory, Scripps Institution of Oceanography
University of California, San Diego, La Jolla, CA 92093-0238, USA
SAX'04 Workshop, Nashville, Tennessee
Sunday, 27 April 2003
Research supported by the Office of Naval Research