broadband acoustics - university of...
TRANSCRIPT
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Broadband Acoustics
John Horne University of Washington
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What is Broadband?
Frequency
cf
BW3dB 3dB
broadband??
band narrow
3betweenin
10
3
<
>⇒=
dB
c
BWf
Q
Advantages of Broadband compared to Narrowband: - Higher signal to noise ratio (SNR) - Higher signal temporal resolution (target characterization) - Continuous frequency band (more information for target classification)
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Broadband Transducers
160
165
170
175
180
185
190
80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160
dB m
icrP
a/Vo
lt
Frequency in kHz
Voltage Response ES120-7 comp
165
170
175
180
185
190
195
150 165 180 195 210 225 240 255 270 285 300
dB m
icro
Pa/
Volt
Frequency kHz
Voltage response ES200-7
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Acoustic Pulses
Ideal Pulse
Narrowband
Broadband
stable amplitude
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Broadband Pulse Types CHIRP pulses: energy distributed across frequencies
up-sweep
down-sweep
Exponential up-sweep
Linear
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Improving Signal Resolution Continuous wave target resolution: ∆𝑟 =
𝑐𝑐2
Broadband target resolution:
Shorten pulse duration
BUT short broadband pulses have uneven distribution of frequencies
Broadband Solution: Matched Filter – a filter with bandwidth matching the pulse duration (aka Pulse Compression)
Continuous wave Broadband
𝑆𝑆𝑆 =𝐴𝑟𝑟𝑟2
𝐵𝐵𝐵𝑆0 𝑆𝑆𝑆 =
𝐴𝑟𝑟𝑟2 𝑇𝐵𝐵𝐵𝑆0
where: A=Amplitude, BW= bandwidth, N0= spectral density of noise, T = period of filter
So What? Enables decoupling of known signal from unknown noise, maximizes SNR
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Matched Filter: Pulse Compression
Input pulse echo envelope
Input pulse frequency envelope
Matched filter time delay
Output pulse echo envelope Gain in SNR:
𝐺𝑑𝐵 = 10 log10 𝐵𝐵 + 10 log10 𝑇 (f2 – f1)
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Adding a Matched Filter Time series of echo returns:
)()()( tntstx += noise : signal :)(
n(t)ts
Apply a filter a(t) to the time series:
)(*)()(*)()(*)()( tatntatstatxty +==
A time-shifted signal )( )( tskta −=
)( )( )( trktrkty nsss +=
where is the auto-correlation function of the signal and is the cross-correlation function of the noise and the signal, which approaches to zero for a white noise, i.e. .
)(trss )(trns
)( )( trtr nsss >>
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Signal Processing Continuous wave or Tone burst
Broadband
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Challenges When Using Broadband
1. Frequency-dependent beam width (affects integration volumes) 2. Non-linear effects: harmonics (avoided using up or down sweeps) 3. Large increases in volume of data (EK60 0.2 GB/hr; EK80 CW 1.67
GB/hr; EK80 FM 41.67 GB/hr) 4. Data processing time
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Example Broadband Systems EdgeTech Simrad EK80
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Herring Survey: Gulf of Maine
120 kHz narrow band
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Improved Target Resolution 120 kHz narrow band
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Herring Survey: Fish Density and ID
Sparse patch
Dense patch
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Herring Classification: Resonance
100 2
Dense Patch (0.3 m-3)
Sparse Patch
(0.05 m-3)
Resonance (3.7 kHz)
Difference in echo amplitude is due to difference in fish density, not size, orientation, or distribution of fish
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16 10/30/2018 Application - 24
EK80 70 kHz Broadband Echogram
Broadband AcousticsWhat is Broadband?Broadband TransducersAcoustic PulsesBroadband Pulse TypesImproving Signal ResolutionMatched Filter: Pulse CompressionAdding a Matched FilterSignal ProcessingChallenges When Using BroadbandExample Broadband SystemsHerring Survey: Gulf of MaineImproved Target ResolutionHerring Survey: Fish Density and IDHerring Classification: ResonanceEK80 70 kHz Broadband Echogram