Page 74 - Spring 2019
P. 74

Student Ghnllnnga PI-nblsm
of the received signal, which is recorded in the file Time
vs. Frequency Observations. This file can be can be down-
loaded at acousticstodayorg/iscpasp2019. The first record at
Emfiscm WM time -1196 s and frequency 73.81 Hz indicates that the air-
- craft is inbound, and for the last record at time 1.176 s and
“'  rrenx~s frequency 53.19 Hz. it is outbound.
1;’ Thak ‘I
w Given that a turboprop aircraft is in level flight at a speed of
:5; 239 knots (123 mls) and an altitude of496 feet (151 In); that
St‘; the depth of the hydrophone is 20 m below the (flat) sea sur-
   3-‘;:,_;-;;7_;;;;:.  ,»‘._.-Lg face; that the isospeed of sound propagation in air is 340 mls;
i‘»'‘-‘7‘''i-'”' ‘' .':5-:L='-L'.":"’ ’""' and that in seawater, it is 1,520 in/s, the students are invited
Figure 1.Contributiuns to the underwater sound field from an to prediddhe with time of the mstmtaifieous fir?-
_ _ quency using Uricks two isospeed sound propagation media
mflmme mung" Aft" Unck (1 972)" approach and comment on its goodness of fit to the measure-
ments in the file.
of the aircraft. The base of the cone subtends an apex angle, Eek 2
which 15 twice the cnflcal angle‘ “_‘d the height of the mm Figure 2 is a surface plot showing the beamforined output
corresponds ‘O the altitude onhe aircraft’ of a line array of hydrophones as a function of frequency (0
'Die first activity of the Student Challenge Problem is to test to 100 Hz) and apparent bearing (0 to 180"). This plot shows
the validity of Uricl(s model for the propagation of a tone the characteristic track of an aircraft flying directly over the
(constant-frequency signal emitted by the rotating propeller array in a direction coinciding with the longitudinal axis of
of the aircraft) from one isospeed sound propagation medi- the array. The aircraft approaches from the forward end-fire
um (air) to another isospeed sound propagation (seawater), direction (bearing 0“; maximum positive Doppler shift in the
where it is received by a hydrophone. Rather than measuring blade rate), flies overhead (bearing 90"; zero Doppler shift),
the variation with of the received acoustic intensity as and then recedes in the aft end-fire direction (1S0°; maxi-
the acoustic footprint sweeps past the sensor (as Urick did), mum negative Doppler shift). For this case, the bearing cor-
it is the observed variation with time of the instantaneous responds to the elevation angle (E), which is shown in Figure
frequency of the propeller blade rate of the aircraft that is 3, along with the depression angle (y) of the incident ray in
used to test the model. This is a more rigorous test of the air. The (frequency, bearing) coordinates of 32 points along
model. "fine frequency of the tone (68 Hz) corresponds to the the aircraft track shown in Figure 2 are recorded in the file
propeller blade rate (or blade-passing frequency), which is Frequency vs. Bearing Observations. which can be down-
equal to the product of the number of blades on the propeller loaded at the above URL. Each coordinate pair defines an
(4) and the propeller shaft rotation rate (17 Hz). For a turbo- acoustic ray, Similar to the previous activity, for Task 2, the
prop aircraft, the propeller blade rate (or source irequency) students are invited to predict the variation with the eleva-
is constant, but for a stationary observer, the received fre- tion angle of the instantaneous frequency ofthe source signal
quency is higher (commonly referred to as the “up Doppler”) using Urick’s two isospeed media approach and to comment
when the aircraft is inbound and lower ("down Doppler”) on its goodness of fit to the actual data measurements. The
when it is outbound. it is only when the aircraft is directly aircraft speed is 125 m/s, the source frequency is 68.3 Hz,
over the receiver that the source (or rest) frequency is ob- and the sound speed in sea water is 1,520 m/s.
served (allowing for the propagation delay). The Doppler ef-
fect for the transit of a turboprop aircraft over a hydrophone 153k 3
53-11 be °i"5ei'V¢i‘i in the Vafiaiion Wii-ii ii-315 (iii i-imf SMPS To replicate Urick’s field experiment. a hydrophone is placed
of 0.024 s) of the instantaneous frequency measurements at a depth of 90 m in the ocean and its output is sampled
-7a 1 AI:uulI:II:- Tbdly 1 Spring znis

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