Course and Learning Outcome(s)Course and Learning Outcome(s)

[CO1] Student is able to explain the concept of sound [ ] p punderwater, SONAR system, depth sounding, and bathymetric mapping.

[LO01] Explain generation of sound underwater, Sound Level, intensity unit acoustic wave properties pulse (length repetition)intensity, unit, acoustic wave properties, pulse (length, repetition), particle celerity, impedance.

[LO02] Explain propagation of sound underwater propagation [LO02] Explain propagation of sound underwater, propagation mechanism, Transmission Loss, role of water properties, active SONAR equation (NL/RL dominated).

Generation of soundGeneration of sound

S d h i ll d d di b i h Sounds are mechanically generated due to disturbance in the medium

Pressure differences

Disturbance in a medium is considered as pressureDisturbance in a medium is considered as pressure differences/gradient working at a unit area

It compresses medium particles to a side that will be followed byIt compresses medium particles to a side that will be followed by decompression in the other side

Sound or acoustic pressure (p)

E d N/ 2 P

Sound or acoustic pressure (p)

Expressed as N/m2 = Pap0

+p

p0

pp0+pp0–p p0

p = p – p0

compressiondilation

p p p0withp = pressure difference = disturbancep = instantaneous pressurep = instantaneous pressurep0 = hydrostatic pressure (pressure without disturbance)

In one-dimensional axisIn one-dimensional axisdilation

(decompression)

compression

+p

p0

–p

Natural soundNatural sound

1 = no sound2 = ‘audible’ sound3 = hydrostatic pressure3 = hydrostatic pressure4 = sound pressure

Particle displacementParticle displacement

D d ( ) di l f di i l ( ) i Due to sound pressure (p), displacement of medium particle () is expressed as*:

= /2f

with = particle velocitywith = particle velocity

*medium produces specific vibration due to a given sound pressure and frequency

Sound speed and particle velocitySound speed and particle velocity

S d d ( ) i h l i f i f i Sound speed (c) is the celerity of propagation of acoustic wave through medium

Particle velocity () is motion of molecules in the medium due to Particle velocity () is motion of molecules in the medium due to sound generation and depends on the applied pressure (p) and medium density ():

p = c

with = 2f, then:

p = c2fp = c2f

Acoustic impedance (Z )Acoustic impedance (Za)

A di d ifi ib i (i i l di l A medium produces specific vibration (i.e. particle displacement, particle velocity) due to applied pressure at a given frequency.

Za relates p and

Sound or acoustics intensity (I)Sound or acoustics intensity (I)

S d i ( ) ki i ( ) Sound or acoustic power (P) working on a unit area (A)

PI

P is directly proportional to p2

AI

P is directly proportional to p

Note:

Sound Pressure Level (SPL)Sound Pressure Level (SPL)

SPL i l d S d L l ( ) SPL is also termed as Sound Level (L)

pref = 1Pa

2

2

10log10Lfp

p

refp

commonly written as just 20log(p) dB

Use of dBUse of dB

A i f i l l h Acoustic power ranges from a very tiny level to a very huge one Use of large numbers are impractical Instead of using power level, dB (deciBel) unit is usedg p , ( )

dB is a relative power of a given intensity with respect to a reference (in water p = 1Pa)reference (in water pref = 1Pa)

PropagationPropagation

P i Propagation Since the medium is elastic, the side experiencing compression balances it and transfers the compressing force to the other side p gand so on and sofort. Consecutive compression and dilation propagates energy to all directions.

Speed of propagation (c)The speed of acoustic wave propagation is mainly governed byThe speed of acoustic wave propagation is mainly governed by medium density.

P i d ( ) i h di i d b Propagation speed (c) in the medium is governed by

Medium density ()Medium density ()In practical sense, higher density of medium facilitate a more efficient transfer of energy

Elasticity modulus (E)This relates to the so-called medium compressibility

c = (E/)0.5c (E/)p = EV/V

Sound experiences lossSound experiences loss

S d i i l i h i i di f Sound intensity losses with increasing distance from sources Geometrical spreading and attenuation are the primary

mechanism of loss acoustic energy

ensonified

gy

e so edsurface20logR

source

increasing distance from source

20logR

SLincreasing distance from sourceR

Loss mechanismLoss mechanism

G i l di Geometrical spreadingAcoustic energy experiences loss since the energy is spread to all directions

AttenuationThrough out the medium, the propagated acoustic energy also g p p g gyexperiences loss. This is due to inter-particle collision and relaxation of molecules in the water that converts acoustic energy into other forms (e g heat)energy into other forms (e.g. heat)

Attenuation versusfrequency

Hydro-acoustic systemHydro-acoustic system

H d i li i i f d h h Hydro-acoustic system relies on transmission of sound through water

i.e. SONAR = SOund Navigation And Ranging active SONAR

Notes: Passive system = listening for soundPassive system listening for sound Active system = making sound and listening to the echo

SONAR equation SLSONAR equation

S

SL

Sensor Source Level (SL) Detection Threshold (DT)

Medium

DT DI

Medium Noise Level (NL) Transmission Loss (TL) Reverberation Level (RL) NLReverberation Level (RL)

Source Target Strength (TS)

TL NL

RL

TS

Passive SONAR equationPassive SONAR equation

Reverberation and noiseReverberation and noise

Si l b d d if i i h h b k d d Signal can be detected if it is stronger than the background due to reverberation and noise

noise is due to wind, wave, biological activity, and shipping

reverberation is backcattered sound from source decayingreverberation is backcattered sound from source, decaying source’s replicate

SIGNAL > BACKGROUND

Ambient noiseAmbient noise

Target propertiesTarget properties

T S h (TS) Target Strength (TS)

Active SONAR systemActive SONAR system

NL NL DT

DITRLSLNL

TLTLNL

TLTL

TS NLDI

EL drops with increasing distance due to TL

Rr is range (R) when RL ELRL domination above NL

Rn is range (R) when NL ELNL domination above RL

Level

RL

Level

ELRL

SL + DIT + TS = 2TL + RL

NLNL

RangePerform Does Not Perform

Level

ELRL

Level

RL

NLSL + DIT + TS = 2TL + (NL−DI)

NL

RangePerform Does Not Perform

Active SONAR equationActive SONAR equation

NL d i d NL dominated

SL + DIT + TS − 2TL − (NL−DI) = DT T ( )

RL dominated

SL DI TS 2TL RL DTSL + DIT + TS − 2TL − RL = DT

Note that in RL dominated equation, NL−DI is no longer appropriate and replaced by RL