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