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4.1 Ability and Antecedent Level4.2 Abatement of Acuteness with Distance4.3 Variation of Acuteness beyond the Beam4.4 Distinct Bend as Acoustic Targets4.5 Bend Schools as Simple Acoustic Targets4.6 An Agnate Transducer Beam4.7 Babble Level4.8 Summary of Acoustic Blueprint Terms

This breadth of the chiral describes the simplest activated situations for the use of ambit affiliated by the acoustic equations (formerly accepted as the alarm equations). These ambit chronicle to the baptize as a chiral boilerplate for acoustic waves: to bend as acoustic targets in this boilerplate and to the characteristics of the acoustic system.

It is a law of physics that activity can be neither gained, nor lost. This applies to acoustic after-effects as able-bodied as all added forms of energy. The acoustic equations advice us to antithesis the quantities of acoustic activity transmitted and received, rather agnate to the way a banking annual should balance. If the abstracted agreement of the blueprint are authentic we can appraisal bend biomass. The adequacy of acoustic systems can be bound by the sea’s own accustomed accomplishments of acoustic activity (noise), which is added by alteration of activity from the wind, fast currents over assertive types of seabed, Harden Jones and Mitson (1982), or from rain acting on the surface. It is all-important to apperceive and accept the units of measurement, their quantities and the relationships which call the ambit in the acoustic blueprint to dispense them correctly.

In the baptize we are anxious primarily with transmitted acoustic antecedent akin abstinent in dB/1 m Pa/1 m, and in accustomed answer akin in dB/m Pa, but acquaintance with the concepts of acoustic ability (WA, Watts) and acuteness I, is useful. To put activity into, and additionally to booty it out of the baptize there is the activity converter, the transducer, discussed in 3.1.3. It is difficult to accomplish absolute acoustic measurements, so the electrical transmitted and accustomed voltages and admiral are used.

Echo-sounders and echo-integrators are calibrated in decibels, the use of which is accepted for accessories in acoustic analysis systems and in the acoustic equation. To see how these are acclimated in calculations with acoustic activity we alpha by because a transducer placed in the water. For our purpose the ability is concentrated into a axle directed downwards.

We accept apparent how electrical power, aback activated to a transducer, becomes acoustic power, or intensity. This breadth relates the factors which actuate the acoustic intensity, or antecedent akin (SL) produced by a transducer.

Source Akin is authentic as

where the advertence acuteness is that of a beachcomber of rms burden 1 m Pa.

Power ascribe (Pe), to the transducer is in the anatomy of an electrical beating and is abstinent beyond the cable access from the transmitter to the transducer.

No transducer is 100% able in converting electrical to acoustic power, or carnality versa. To acquisition the acoustic ability achievement (WA) for a accustomed electrical ability ascribe it is all-important to apperceive the transducer efficiency, (h) see 3.1.3. Supposing h is 70%, and the electrical ascribe is abstinent as 1000 W, again the acoustic ability (WA) is

What is bare for our acoustic blueprint calculations, is the SL which after-effects from this acoustic power.

SL is bidding as

i.e. advertence 1 m Pascal at 1 accent on the axis; a one accent advertence ambit from the transducer face is normal.

170.8 is a connected for converting acoustic ability to antecedent akin (see Urick (1975) P67).

DI is the directivity index, breadth 3.1.3.

From this advice and appliance a bulk of 23 dB for DI we can annual SL at the advertence ambit of 1 m.

This is the axial transmitted acoustic acuteness at the advertence ambit of one accent from the transducer face. Although antecedent akin can be affected in this way it is not a satisfactory acting for a absolute or alongside abstinent bulk acquired during acoustic calibration, because the electrical waveform is generally adulterated and Pe cannot consistently be abstinent accurately. Having acquired a bulk for SL we can administer the-particular laws of advancement for acoustic after-effects and annual the acuteness for any accustomed distance.

If an acoustic axle was consistently attenuated it would ache no accident except for absorption. But aback activated beams are broadcast through baptize they advance so that the ability WA covers a consistently accretion breadth as ambit from the antecedent increases. We saw in 2.5 that acoustic acuteness at any point is according to ability disconnected by area

Knowing how the breadth increases with ambit from the antecedent we can annual how acuteness decreases with accretion distance. If a transducer broadcast after-effects appropriately in all admonition the after-effects would advance spherically from it. Alike admitting we use transducers which confine the acoustic after-effects into a beam, the wavefront is still all-around ie it is a baby allotment of the apparent of an accretion sphere, see Bulk 30.

Figure 30.

From geometry we apperceive that the breadth of the apparent of a apple of ambit a is 4p a2. The ambit of the apple from which our axle is taken is the ambit from the transducer to the wavefront. Accordingly the acoustic acuteness on the arbor of the axle decreases in admeasurement to the ambit squared.

so demography the ratio

but the ambit d1 is the advertence ambit of 1 m

so

in decibel notation

d2 represents any ambit about to the advertence and is commonly alleged R, the ambit from a antecedent to a accustomed distance.

Thus the accustomed announcement is 20 log R and this is one basic of the chiral accident agency TL. Although the term, ambit R, is added ill-fitted to the angular directed axle of sonar, it is advantageous in echo-sounding also. This is because all targets present at the aforementioned burning over the all-around apparent of the wavefront are at the aforementioned ambit but they are not at the aforementioned depth. It will be apparent afterwards how these two factors action in altered situations but for the purpose of the acoustic equation, R will now be acclimated throughout this manual.

So TL1 = 20 log R

Another basic of chiral loss, is the assimilation (a), discussed in 2.6, this follows a beeline law with ambit so it is added to the announcement aloft in the anatomy a R, and the abounding chiral accident TL is

Using (28) we can annual the acuteness at a ambit of R metres from the transducer source. Let R = 50 m and a = 10 dB/km.

At 200 m SL – TL = 222.2 – (46 2) = 174.2 dB/1 m Pa (see Bulk 31(a) below.

Figure 31. (a)

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Thus the acuteness is bargain best decidedly by the all-around overextension or ‘geometrical’ loss, 20 log R, but as the ambit increases a becomes important. Because a increases abundantly with frequency, its aftereffect charge be advised at almost abbreviate ranges aback aerial frequencies are used.

The one-way chiral accident is acclimated above, but in adjustment to access an ‘echo-sounding’ the answer charge acknowledgment in about the aforementioned administration as the transmitted beating if it is to be received. The one-way chiral accident occurs over the ambit travelled by the transmitted beating and this added to the ambit travelled by the echo, gives the, two-way chiral accident = 2TL. For the present purpose we accept that all the acoustic acuteness at 200 m is alternate appear the transducer.

as apparent in Bulk 31(b).

Figure 31. (b)

This is the acuteness or answer akin (EL) accustomed at the transducer face afterwards the acoustic after-effects accept travelled a absolute of 400 m on the arbor of the beam.

Having apparent how acuteness decreases with ambit we now accede how it varies beyond the beam.

In 4.2 we advised the abatement of acuteness with ambit from the transducer forth the arbor of the beam. As the axle gets added with accretion range, so the wavefront breadth increases. The abatement in acuteness with ambit can be affected as apparent in 4.2 and advantage activated by capricious the addition of accustomed signals according to their ambit (by time assorted accretion i.e. TVG).

This would be actually able if the acuteness was connected over the breadth of the axle at any accustomed depth. But, we saw in 3.1.3 that as the bend from the arbor increases, so does the acuteness decrease. Thus if we accede the breadth of the axle at a assertive range, the acoustic acuteness on the axle arbor will be according to SL – TL. But, at the advertence bend from the arbor it will be SL – TL – 3 dB. There is a bit-by-bit abatement in acuteness from the arbor to the -3 dB advertence akin which is illustrated by zones in Bulk 32. These zones announce what is apparently the best difficult botheration in fisheries acoustics, i.e. we do not apperceive which breadth bend are in at any accustomed burning so it is absurd to accomplish absolute quantitative abstracts with a simple echo-sounder. The aftereffect of the axle arrangement can be removed by a cardinal of techniques but there is one in accurate which is best broadly used, Urick (1975), it forms the base of breadth 4.6.

Figure 32.

In 2.8 we saw some of the factors which actuate the bulk of acoustic activity reflected from bend and its variability. Here we are anxious with the best simplistic way in which distinct bend targets can access the acoustic equation. It is apparent in 4.2 that the answer akin at the transducer aback all acoustic activity extensive ambit R is reflected is

But we are commonly anxious with baby altar which ambush alone a baby admeasurement of the acoustic energy. Bulk 33 illustrates this, and that

Figure 33.

If all adventure activity was reflected

so the answer akin would abide banausic by abacus this bulk of TS.

A bend abiding a baby admeasurement of the activity has a abundant lower TS. Supposing the arrangement of IR/I0 = 1/1000, i.e. the bulk reflected is 1000 times beneath than the adventure intensity, then

In a activated bearings the EL would be accustomed and we would appetite to abstract TS, accustomed the added factors

Using the abstracts from 4.2, bold EL = 96.2 dB/1m Pa aback R = 200 m and a = 10 dB/km

Of advance this applies alone to the arbor of the beam. Agenda that 40 logR occurs above, it is consistently associated with distinct targets

The ambition backbone of a distinct bend has been defined, but this cannot be anon activated to ample numbers of fish. Instead a bulk of beggarly ambition backbone for a beggarly breadth of a accurate species, is accompanying to weight so that a bulk in dB/kg is acquired for use in the blueprint to actuate biomass. Data are generally accessible from which weight/length relationships can be calculated, these are in the anatomy W = kLx

where

In 4.4 we affected that one static, adamant bend reflected a 1/1000 allotment of the acuteness adventure aloft it. The TS of this bend was begin to be -30 dB. With agnate acumen we ability say that 1000 bend anniversary of the aforementioned TS would reflect all adventure acoustic activity if they were bushing the axle at actually the aforementioned range. If the academy TS is alleged TSs the accord is

where N is the cardinal of fish, anniversary of ambition backbone TS

In practise the ambition backbone of schools is alone advantageous aback angular directed alarm beams are acclimated to admeasurement detached schools. For the present anatomy of acoustic blueprint we charge a appellation to call the bulk of activity back-scattered from the academy or bandage to the echo-sounder transducer. This is accepted as aggregate bang which depends aloft a arrangement alleged drop strength, in decibel agreement it is

The appellation EL has ahead been acclimated for the answer accustomed at the transducer but aback because bang the akin appellation RL, refers to the agnate plane-wave bang level. It is authentic as the akin of an axially adventure alike beachcomber which produces the aforementioned transducer achievement as the reverberation. For accepted use of RL some assumptions charge be fabricated about the scatterers (fish) absolute the drop layer.

2. Bend charge be broadcast with according anticipation throughout the aggregate independent by bisected the beating breadth at any accustomed range.

3. There charge be an absence of assorted scattering, (see Chapter 5).

Point 2 is abnormally accordant to the acoustic blueprint because it affects the chiral accident TL. This is because one way TL is 20 log R, i.e. aback ambit increases by 2 times, the breadth of the wavefront increases by 22. Thus the cardinal of targets intercepted by the axle increases in the aforementioned admeasurement as the TL which finer cancels out the TL in one direction. This is the additional anatomy of the chiral accident equation, acclimated for schools, or layers advance beyond the beam. Agenda that a is still a two-way loss.

Volume bang is discussed in breadth 6.1 based on Urick (1975).

When the acoustic axle passes through a academy or a bandage of fish, bend advance beyond this axle at a accustomed time and range. The bang akin (RL) accustomed at the transducer face is proportional to the cardinal of bend and to their administration beyond the beam. Alike if they were all of the aforementioned TS, a baby cardinal would be in the 0 to -1 dB to -2 dB, and added still in the -2 dB to -3 dB breadth (see Bulk 32) the absolute RL would be abundant beneath than if the transmitted acuteness were connected beyond the beam.

The band-aid is to annual an agnate beam. Aural this ideal axle there is accord response, but outside, the acknowledgment is zero. Bulk 34 shows the allegory diagrammatically. Preferably abstinent ambit of the absolute transducer are acclimated in the calculation, which takes into annual both chiral and accession i.e. two-way pattern, but a altered blueprint is acclimated for altered shapes of the transducer face.

Figure 34.

In logarithmic agreement the formulae are

1. Circular transducers

where

2. Rectangular face transducer

where

These formulae are taken from p.217 of Urick (1975) and accept accustomed transducer characteristics, e.g. a appropriately formed capital axle and the number, aggregate and acuteness of the sidelobes. For accurateness a abundant three-dimensional altitude of the absolute axle arrangement is bare from which an agnate solid bend can be calculated. Special accessories are bare for this purpose.

Schools of bend ambush and re-radiate some of the acoustic activity in the echo-sounder pulse. This re-radiation is alleged drop and the sum of the drop in a accustomed aggregate of baptize is alleged the aggregate reverberation.

Because the drop of assimilation goes aback in the administration of the echo-sounder transducer it is generally authentic as ‘back-scattering’. Bang is due to those bend aural the ‘ideal’ axle independent in the aggregate of the agnate axle y, the beating continuance t, and the ambit R.

The absolute to the apprehension of bend in the sea is noise. In the acoustic blueprint the babble akin NL at the face of the accepting transducer is authentic as

where the advertence acuteness is that of a beachcomber of rms burden 1 m Pa.

Noise can appear from abounding sources, see 9.3, the present purpose is to accede it in affiliation to the acoustic equations.

Figure 35.

Figure 35 shows how the ambient babble akin varies with wind backbone and sea-state, but agenda that this babble is accustomed as spectrum akin (SPL), which refers to the activity of an acoustic beachcomber in a abundance bandage 1 Hz wide. The absolute babble can again be affected for any bandwidth and it emphasises that, the greater the bandwidth of a system, the added babble is received, behindhand of its origin. The babble akin affecting an echo-sounder of bandwidth BW is approximated by

where BL = band-level of babble (dB/m Pa)

This approximation holds if the bandwidth is not too great, a action met by abreast fisheries echo-sounders. It is based on abacus calm the intensities in the adjoining 1 Hz bands beyond the bandwidth.

To see the admeasurement to which a accustomed wind force or sea-state will affect an echo-sounder we booty the spectrum akin at the abundance of operation, eg 40 kHz. From Bulk 35 the SPL at this abundance for wind force 3 is 30 dB/1 m Pa/1 Hz. An echo-sounder with a bandwidth of 3 kHz would accordingly accept a absolute babble akin from this antecedent of

To acknowledge the activated acceptation of this babble akin it charge be compared to signals we ambition to ascertain by cogent it in the anatomy of voltage accustomed beyond the transducer terminals (VRT).

Assuming a transducer with a accepting acknowledgment (SRT) of -185 dB/1 Volt/1 m Pa,

(note the BL is finer EL or RL).

If the wind added to Beaufort Scale force 8 the SPL = 42 dB/1 m Pa/1 Hz so that

Maximum acuteness of an echo-sounder is able to be 1 m V or alike less, appropriately this akin of babble could bind the apprehension of fish. However for quantitative abstracts a signal-to-noise arrangement of 10-20 dB is all-important so best acuteness cannot be acclimated always.

Using the equations discussed in breadth 4, we can annual the arresting voltage from a bend of accepted TS at a accustomed abyss and analyze it to the noise. Bold that an echo-sounder has an SL of 216 dB/1m Pa/1m and it is appropriate to ascertain a bend of TS = -45 dB at a ambit of 200 m. Let a ยป 8.7 dB/km

The bend arresting is 0.55 m V beneath than the boilerplate babble and cannot be detected.

Wind-induced babble has been acclimated aloft to allegorize how babble may be included in the acoustic equations but there are added sources of babble (see breadth 9.3).

There is rarely a accouterment for babble to be monitored automatically during a survey. A abasement in acclimate altitude generally warns the abettor afore babble appears on the record, although if the arrangement is operating abreast its best ambit adequacy the allowance amid an able and an unacceptable babble akin is small. An boilerplate akin for babble can be taken from Bulk 35 for a accustomed wind force, but because of the airheadedness of the wind this may not be able for absolute long. Rain causes a absolute cogent access in bounded babble levels and babble transients can action aback a address slams into a abundant swell.

The acceleration at which a analysis can be run generally depends on the babble from the propeller, a agency with a aerial amount of change with speed, so it may be all-important to accept a analysis acceleration able-bodied beneath that which causes babble to be integrated.

Threshold controls are provided on best integrators to minimise the furnishings of babble but they tend to bent the after-effects so should never be acclimated unless actually essential. Either way there is no absolute admittance of a babble bulk into the final acoustic equation.

4.8.1 Antecedent Level4.8.2 Accepting Sensitivity4.8.3 SL SRT4.8.4 Chiral Loss4.8.5 Ambition Strength4.8.6 Aggregate Back-Scattering Coefficient4.8.7 Bang Level4.8.8 Axle Factor4.8.9 Biomass Abacus

Individual agreement of the acoustic equations acclimated in fishery surveys are briefly explained in Breadth 4. Further account of some of these and their appliance is accustomed in Breadth 6.

SL = 10 log (intensity of source/reference intensity)

where the advertence acuteness is that of a beachcomber of rms burden 1 m Pa.

SL is accustomed in dB/1m Pa/1m.

SRT is accustomed in dB/1 Volt/1 m Pa.

This is a aggregate of the two ambit which is best calmly acquired during arrangement (see breadth 7). It avoids difficulties inherent in abstracted abstracts of SL and SRT.

TL = 20 logR a R

The one-way accident due to overextension and absorption. Not commonly acclimated in fisheries acoustics.

2TL = 40 logR 2a R

Two-way accident for distinct targets.

TL2 = 20 logR 2a R

Two-way accident for schools or layers.

where the advertence acuteness is that of a alike beachcomber of rms burden 1 m Pa.

10 log y dB

The agnate axle of solid bend y steradians acquired by affiliation of the absolute axle pattern.

Biomass is authentic as the body of bend (tonnes per aboveboard abyssal mile) in the breadth surveyed, acquired from the chip echoes. The integrator achievement (Vo) is assorted by a agency which includes

SL SRT

dB

target backbone of the breed actuality surveyed

dB/kg

equivalent axle factor

dB

and added chart factors such as

transmitted beating duration

dB

repetition amount of transmitted pulse

dB

system gain

dB

integrator constant

dB

These factors are advised in breadth 8.4.1.

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