Post on 16-Feb-2021
transcript
Broadband Acoustics
John Horne University of Washington
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)
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
Acoustic Pulses
Ideal Pulse
Narrowband
Broadband
stable amplitude
Broadband Pulse Types CHIRP pulses: energy distributed across frequencies
up-sweep
down-sweep
Exponential up-sweep
Linear
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
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)
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 >>
Signal Processing Continuous wave or Tone burst
Broadband
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
Example Broadband Systems EdgeTech Simrad EK80
Herring Survey: Gulf of Maine
120 kHz narrow band
Improved Target Resolution 120 kHz narrow band
Herring Survey: Fish Density and ID
Sparse patch
Dense patch
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
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