Patent Description:
<CIT> discloses an exemplary device for generating an air bubble curtain for attenuating sound energy underwater.

More specifically the fluid is a gas. However, due to the nature of fluidic technology, the invention can equally be used with a liquid instead of a gas. Because of the prevalence of gas-related uses, the invention is described below with reference to a gas but is not limited to a gas.

From the technical field of fluidics, oscillators are known to enable switching of gases for generating two or more pulsating gas flows from a constant gas flow. A typical example of such an oscillator is the well known standard bistable two-loop fluidic oscillator, such as for instance shown in <FIG> of 'Taxonomic trees of fluidic oscillators. Tesař, Vaclav. EPJ Web of Conferences. <NUM>/epjconf/<NUM>.

Traditional oscillators however have the disadvantage that they are designed with certain geometric characteristics, which determine the oscillation frequency at which they operate. Usually, the oscillation frequency can be varied to a limited degree by varying the inlet pressure of gas into the oscillator, but this is usually only over a relatively small range and will also strongly influence the total flow rate of gas through the oscillator, which is often undesirable.

There is therefore a need for oscillators of which the oscillation frequency can be adjusted independently of the gas pressure or gas flow rate.

The invention therefore provides for an oscillator for generating two or more pulsating gas flows from a constant gas flow, whereby the oscillator comprises a first gas inlet for receiving a gas flow and a first gas outlet and a second gas outlet for each outputting said gas flow, whereby the oscillator comprises a bistable fluidic amplifier for amplifying a control signal, whereby the fluidic amplifier is placed between the first gas inlet and the gas outlets, whereby the oscillator comprises a piezo-electric actuator for generating said control signal, wherein the piezo-electric actuator is a bender actuator. The bender actuator actuates by bending in a direction perpendicular to a direction in which the bender actuator extends.

The term fluidic amplifier also comprises an amplifier with an amplification factor of <NUM>, which can also be considered a fluidic switch.

The control signal can be any type of mechanical energy based control signal.

It will be clear that the piezo-electric actuator needs to be connected to a voltage source to generate a displacement as a control signal, and to a source of alternating voltage to generate one or more periodic control signals. Such periodic control signals are in general a train of mechanical pulses, so in other words a wave, whereby the wave may have any known wave shape, such as sinusoidal, saw-tooth, or block.

The advantage of this is that the frequency of the control signals generated by a piezo-electric actuator can be very easily controlled and modulated, resulting in control of the oscillation frequency of the oscillator, so the pulsation frequency of the pulsating gas flows, independently of the inlet gas pressure or the gas flow rate.

As such piezo-electric actuators are widely available commercially and require only low voltages and currents to operate, this is an easy, cheap and safe way of generating such a control signal. The invention therefore allows reliably controlling and modulating large gas flows using small, industry-standard control circuits.

Due to the fact that the fluidic amplifier amplifies the control signal generated by the piezo-electric element, this can be achieved with a very low consumption of electrical energy.

The oscillator comprises one piezo-electric actuator or two or more piezo-electric actuators which are arranged to actuate at the same frequency, whereby the piezo-electric actuator is or the piezo-electric actuators are arranged to alternately act in two opposite directions and generate a said control signal during action in both opposite directions.

In case there are several piezo-electric actuators, they can actuate in phase with each other. This means that they actuate in the same direction at the same time. However, it is also conceivable that, depending on the response speed of the gas flow to a control signal that is required for specific applications, they are arranged to actuate out of phase with each other, but nevertheless in a coordinated fashion.

In a preferred embodiment, the piezo-electric actuator is, or the piezo-electric actuators are, arranged to generate a first said control signal by acting in a first of said directions and to generate a second said control signal by acting in the second of said directions.

Hereby, preferably, when the oscillator is in use, the first control signal causes a gas flow flowing from the first inlet to the first outlet to switch to the second outlet, and the second control signal causes a gas flow flowing from the first inlet to the second outlet to switch to the first outlet.

The oscillator comprises a source of alternating voltage, whereby the source of alternating voltage is electrically connected to the piezo-electric actuator.

In a preferred embodiment at least one of the gas outlets is provided with a perforated plate having a plurality of holes. Preferably all of the gas outlets are provided with such a plate. Preferably the plates have at least three, and more preferably at least six, holes each. Preferably the holes in the plate or plates have the same cross-sectional area. These measures ensure that the oscillator is particularly suited for generating relatively small gas bubbles in a liquid, whereby the gas bubbles that are produced have a narrow size distribution.

Within the general invention there are two main groups of preferred embodiments related to the configuration of the piezo-electric element in relation to the fluidic amplifier.

In the first main group of preferred embodiments the fluidic amplifier comprises a gas inlet channel which is connected to the first gas inlet, whereby the piezo-electric element is placed in or around the gas inlet channel of the fluidic amplifier.

In this first group, the oscillator can act alone to generate pulsating airflows that are used directly for an intended application. However, the outlet channels can also be connected to the control ports of a traditional fluidic amplifier second, so that the oscillator acts as a master, generating gas pressure waves in the outlet channel, whereby these gas pressure waves are then amplified by the other fluidic amplifier acting as slave.

Note that in such a configuration the outlet channels can also be split and connected to the control ports of several different traditional fluidic amplifiers, so that a single master-multiple slaves configuration is obtained, whereby the master is the oscillator according to the invention and the slaves are the traditional fluidic amplifiers.

Preferably, the oscillator comprises at least two of said bender actuators which are placed in a spaced-apart, parallel, side by side configuration. This ensures that the control signal is transferred more reliably to a gas flow fed into the gas inlet channel because the bender actuators provide a degree of containment of the air flow lines.

In a preferred embodiment in the first main group at least a part of the wall of the gas inlet channel is formed by a said piezo-electric actuator. This ensures that the control signal is fully and efficiently transferred to a gas flow fed into the gas inlet channel.

In a preferred embodiment in the first main group the piezo-electric actuator extends, when no voltage is applied to it, in the direction in which the gas inlet channel extends.

An alternative way of defining this is that the piezo-electric actuator extends in the flow direction of the gas in the gas inlet channel when the oscillator is in use.

In a preferred embodiment in the first main group the gas inlet channel has a smallest dimension, whereby those two bender actuators of the at least two said bender actuators that are the furthest apart have a maximum mutual distance, whereby this maximum mutual distance is at least <NUM>% of the smallest dimension of the gas inlet channel.

As a result, a major part of the gas flowing through the gas inlet channel will flow between the two bender actuators, so that a good transfer of energy is obtained from the bender actuators to the flowing gas, leading to efficient amplification.

In the second main group of preferred embodiments the fluidic amplifier comprises at least two control ports, whereby the oscillator comprises a second gas inlet which is connected to at least two control channels, whereby the control signal is a gas pressure wave control signal, whereby the piezo-electric actuator is arranged to generate a said gas pressure wave control signal in the at least two control channels, whereby a first of the control channels is connected to a first of the control ports and a second of the control channels is connected to a second of the control ports.

Generating said gas pressure wave control signals can clearly easily be done by applying an alternating voltage to the piezo-electric actuator, which can be modulated as required.

In this second group, the second gas inlet and the control channels, in combination with the piezo-electric element, act as a master, generating gas pressure waves in the control channels, whereby these gas pressure waves are then amplified by the fluidic amplifier acting as slave.

Note that in such a configuration the control channels can also be split and connected to the control ports of several different fluidic amplifiers, so that a single master-multiple slaves configuration is obtained.

In a preferred embodiment in the second main group the piezo-electric actuator or a valve member attached to the piezo-electric element is configured to partly or completely block, when the piezo-electric actuator is electrically activated, the connection between the second gas inlet and at least one of the control channels while keeping the connection between the second gas inlet and another of the control channels open.

Preferably the oscillator comprises exactly two control channels, whereby said piezo-electric actuator or said valve member is configured, when the piezo-electric actuator is electrically connected to a source of alternating voltage, to partly or completely block alternatively the connection between the second gas inlet and exactly one of the control channels or the connection between the second gas inlet and the other of the control channels.

By imposing an alternating voltage on the piezo-electric actuator, a pressure wave is thereby generated in one or more of the control channels.

Two separate control waves, phase shifted by half a period, are thereby obtained, one in each of the control channels.

The invention further concerns the use of an oscillator according to the invention for generating a pulsating flow of gas, whereby the piezo-electric element is supplied with a alternating voltage of a single, but preferably adjustable frequency, whereby preferably the pulsating flow of gas has a single, but preferably adjustable, pulsation frequency.

The invention concerns a method of generating gas bubbles in a liquid, whereby an oscillator according to the invention is used, whereby the piezo-electric actuator is supplied with an alternating voltage, whereby the first gas inlet is connected to a source of pressurized gas, whereby a pulsating gas flow from at least one of the gas outlets is used to generate gas bubbles, whereby the size of the bubbles is adjustable by adjusting the frequency of the alternating voltage.

These gas bubbles are air bubbles and are used to form a bubble screen under water to attenuate the sound energy of underwater sound from a source of undesirable underwater sound and thereby reduce the propagation of such sound. This is advantageous because the abortion and scattering of sound waves under water is dependent on the bubble size distribution, which can be controlled by controlling the oscillator frequency.

In order to illustrate the invention, exemplary embodiments are explained below, with reference to the following figures, wherein:.

In the explanation below the invention is described with reference to air and water. It should be noted that the invention can equally be used with other gasses instead of air, and even with liquids instead of air, and with other other liquids than water.

The oscillator 1a of <FIG> is a traditional fluidic oscillator, of which the air outlets <NUM>,<NUM> are provided with perforated plates <NUM> with fifty round holes of <NUM> diameter each. This oscillator 1a is further called the first oscillator.

The first oscillator 1a comprises a first air inlet <NUM> and an air inlet channel <NUM> leading away from the first air inlet <NUM>. The air inlet channel <NUM> widens and diverges into two air outlet channels, more specifically a first outlet channel <NUM> and a second outlet channel <NUM> which lead to the two aforementioned air outlets <NUM>,<NUM>, more specifically to a first air outlet <NUM> and to a second air outlet <NUM>, which are provided with said perforated plates <NUM>.

The two outlet channels <NUM>, <NUM> are separated by a splitter <NUM> with a concave nose <NUM>.

The splitter <NUM> and the air inlet channel <NUM> and the outlet channels <NUM>, <NUM> jointly constitute a bistable fluidic amplifier arranged to amplify control signals, whereby in this case the control signals are fed to the fluidic amplifier via a first control port <NUM> and a second control port <NUM>.

From each of the air outlets <NUM>,<NUM>, a feedback channel <NUM> leads back to the control ports at the point where the air inlet channel <NUM> widens.

The first oscillator 1a works as follows: A constant airflow is established at the first air inlet <NUM> and through the air inlet channel <NUM>. This airflow will either flow through the first outlet channel <NUM> or through the second outlet channel <NUM>, but not through both at the same time. If undisturbed, the air will continue to flow this way because of the Coanda-effect, which enhances the tendency for a fluid to follow a curved surface. The transition from the air inlet channel <NUM> to each of the outlet channels <NUM>, <NUM> is such a curved surface. The concave nose <NUM> of the splitter <NUM> helps to create an induced secondary airflow that further stabilises the airflow through that particular outlet channel <NUM>,<NUM>.

Most of the air flowing through this outlet channel <NUM>,<NUM> will then exit at the corresponding air outlet <NUM>,<NUM>. However, this airflow also generates a pressure pulse which is sent back via the corresponding feedback channel <NUM> to the corresponding control port <NUM>, <NUM>, and which cause the airflow to switch to the other outlet channel <NUM>,<NUM>.

If left undisturbed, a stable airflow through the other outlet channel <NUM>, <NUM> will now be established. However, also at the other air outlet <NUM>,<NUM>, a pressure wave is generated, which will be fed back via the feedback channel <NUM> to the corresponding control port <NUM>,<NUM>, so that the airflow switches to the other outlet channel <NUM>,<NUM> again.

This way, a sequence of pressure control signals, in other words a pressure control wave, is established at both control ports <NUM>, <NUM>, every time switching the airflow from the first outlet channel <NUM> to the second outlet channel <NUM> and back, thereby generating two pulsating airflows, one in each of the outlet channels <NUM>, <NUM>, each pulsating with the same oscillation frequency and phase shifted by half a wave period.

These sequences of control signals are thereby amplified by the fluidic amplifier.

The oscillation frequency of the first oscillator 1a is more or less fixed, depending on the exact design of the first oscillator 1a. A change in air pressure at the first air inlet <NUM>, resulting in a change in the total air flow rate through the first oscillator 1a, will influence the oscillation frequency to a relatively small degree, but the oscillation frequency can not be controlled independently of the air flow rate.

A first embodiment of an oscillator according to the invention, further to be called the second oscillator 1b, as shown in <FIG>, differs mainly from the traditional oscillator in that there are no feedback channels <NUM> and no control ports <NUM>, <NUM>. Consequently, the outer contours of the housing of the second oscillator 1b can be much smaller.

It is noted that, although they are no perforated plates <NUM> indicated in the <FIG> because this is not important for showing the difference with the first oscillator 1a, the air outlets <NUM>,<NUM> of the second oscillator 1b can easily be provided with such perforated plates <NUM>, and usually are provided with such perforated plates <NUM>.

In order to generate a control signal, the second oscillator 1b is instead provided with two piezo-electric bender actuators <NUM>, which are extending in the length direction of the air inlet channel <NUM> and which are fixedly attached at one of their extremes 17a, near the first air inlet <NUM>.

The actuators <NUM> are each connected to a source of alternating voltage, with a controllable frequency, via electrical wires which are not shown in the figures, but which run via wire channels <NUM> provided in the housing of the second oscillator 1b.

The actuators <NUM> can bend in two directions, depending on whether a positive or a negative voltage is applied to them. This results in a movement of the free extreme 17b of the actuators <NUM>, so that the actuators <NUM> can adopt two working positions, one of which is shown in <FIG>, and the other of which is shown in <FIG>.

It will be clear that in case no voltage is applied, the actuators <NUM> adopt a neutral position, intermediate between these two working positions.

In order to accommodate the actuators <NUM> in their working positions, the housing of the second oscillator 1b is provided with matching recesses <NUM>.

As can be seen in <FIG>, the actuators <NUM> constitute at least part of the wall of the air inlet channel <NUM>.

The second oscillator 1b works as follows:
Like for the first oscillator 1a, a constant airflow is established at the first air inlet <NUM> and through the air inlet channel <NUM>, with the actuators <NUM> in their neutral position. This airflow through the air inlet channel <NUM> establishes itself in a stable flow pattern into either the first outlet channel <NUM> or the second outlet channel <NUM>, and then onward towards the corresponding air outlet <NUM>,<NUM>.

Oscillation of the airflow through the second oscillator 1b is induced by applying an alternating voltage to the actuators <NUM>. These actuators <NUM> will then alternatingly switch between the two working positions. A movement of the actuators <NUM> in one direction, eg from the working position shown in <FIG> to the working position shown in <FIG>, will then cause the airflow to switch from the first outlet channel <NUM> to the second outlet channel <NUM>, and therefore constitutes a first mechanical control signal for such a switch.

A movement of the actuators <NUM> the other direction, so from the working position shown in <FIG> to the working position shown in <FIG>, will cause the airflow to switch from the second outlet channel <NUM> to the first outlet channel <NUM>, and therefore constitutes a second mechanical control signal for an opposite switch of the airflow.

A repeated movement of the actuators <NUM> thereby generates a control wave of a mechanical-energy signal, which is amplified by the fluidic amplifier, every time switching the airflow from the first outlet channel <NUM> to the second outlet channel <NUM> and back, thereby generating two pulsating airflows, one in each of the outlet channels <NUM>, <NUM>, each pulsating with the same oscillation frequency and phase-shifted by half a wave period.

Even though it may appear from <FIG> that the actuators <NUM> are directing the airflow from the first air inlet <NUM> channel towards one of the two outlet channels <NUM>, <NUM>, once the airflow is established in one of two outlet channels <NUM>, <NUM>, the actuators <NUM> are not needed anymore to maintain this airflow. Due to the Coanda-effect this airflow will remain stable even if the actuators <NUM> would be absent.

In order to obtain a reliable oscillation behaviour of the second oscillator 1b, both actuators <NUM> should be actuating at the same frequency. They do not necessarily need to operate exactly in phase, as, depending on the situation, a faster or slower switch from an airflow in one outlet channel <NUM>,<NUM> to an airflow in the other outlet channel <NUM>,<NUM> may be required and can be obtained by making one of the actuators <NUM> move slightly earlier than the other actuator <NUM>.

It will be clear that the oscillation frequency of the second oscillator 1b will be the same as the frequency of the alternating voltage. This oscillation frequency can therefore be easily controlled by electronically altering the frequency of the alternating voltage. This can be done independently of the actual airflow through the second oscillator 1b.

A second embodiment of an oscillator according to the invention, further to be called the third oscillator 1c, differs from the first oscillator 1a in that the feedback channels are absent. There are however, like in the first oscillator 1a, two control ports <NUM>, <NUM> present in the air inlet channel <NUM>. This part of the third oscillator 1c is essentially a traditional bistable fluidic amplifier with control ports <NUM>, <NUM>, and is not shown separately. Different to the first oscillator 1a, the third oscillator 1c comprises a pressure wave generator <NUM>, shown in <FIG>, for generating a control signal. This pressure wave generator <NUM> comprises a second air inlet <NUM>, which splits at a junction <NUM> into a first control channel <NUM> and a second control channel <NUM>.

On the opposite side of the junction <NUM>, compared to the second air inlet <NUM>, a cavity <NUM> is present. In this cavity <NUM> a single piezo-electric bender actuator <NUM> is fixedly attached at one of its extremes 17a. The other, free, extreme 17b of the actuator <NUM> extends into the junction <NUM>, and is provided with an approximately triangular valve member <NUM>.

The first control channel <NUM> is connected to the first control port <NUM> and the second control channel <NUM> is connected to the second control port <NUM> of the bistable fluidic amplifier.

The actuator <NUM> is connected to a source of alternating voltage with a controllable frequency, via electrical wires which are not shown in the figures, but which run via a wire channel <NUM> provided in the housing of the pressure wave generator <NUM>.

The actuator <NUM> can bend in two directions, depending on whether a positive or a negative voltage is applied to it, and can thereby adopt two working positions, one of which is shown in <FIG>, and the other of which is shown in <FIG>. It will be clear that in case no voltage is applied, the actuator <NUM> adopts a neutral position, intermediate between these two working positions.

The third oscillator 1c works as follows:
Like for the first oscillator 1a, a constant airflow is established at the first air inlet <NUM> and through the air inlet channel <NUM>. This airflow through the air inlet channel <NUM> then establishes itself, like in the first oscillator 1a, in a stable flow pattern into either the first outlet channel <NUM> or the second outlet channel <NUM> due to the Coanda-effect, and then onward towards the corresponding air outlet <NUM>,<NUM>.

A constant airflow is also established at the second air inlet <NUM>, which is the air inlet of the pressure wave generator <NUM>. This constant airflow is much smaller than the airflow through the air inlet channel <NUM> and is less than <NUM>% of the airflow through the air inlet channel <NUM>. This second airflow will be used to generate two control pressure wave signals which are fed to the control ports <NUM>, <NUM>.

In order to obtain this, an alternating voltage is applied to the actuator <NUM>.

This actuator <NUM> will then alternatingly switch between the two working positions. A movement of the actuator <NUM> in one direction, eg from the working position shown in <FIG> to the working position shown in <FIG>, will cause the second control channel <NUM> to become blocked at the junction <NUM>, so that the the airflow from the second air inlet <NUM> will exclusively flow into the first control channel <NUM> and thereby cause a pressure signal in the first control channel <NUM>.

A movement of the actuators <NUM> the opposite direction, so from the working position shown in <FIG> to the working position shown in <FIG>, will cause the first control channel <NUM> to become blocked at the junction <NUM>, so that the the airflow from the second air inlet <NUM> will exclusively flow into the second control channel <NUM> and thereby cause a pressure signal in the second control channel <NUM>.

It is noted that the control channels <NUM>, <NUM> do not necessarily need to become totally blocked. Partial blocking of the control channels <NUM>, <NUM>, so that the majority, preferably at least <NUM>%, of the air will flow into the other control channel <NUM>,<NUM>, is sufficient, although not optimal.

A repeated movement of the actuator <NUM> thereby generates two pressure wave control signals, one in the first control channel <NUM> and one in the second control channel <NUM>, whereby these pressure waves are phase-shifted by half a wave period.

Because the first control channel <NUM> is connected to the first control port <NUM> and the second control channel <NUM> is connected to the second control port <NUM>, a pressure signal in the first control channel <NUM> causes the airflow in the air inlet channel <NUM> to flow into the second outlet channel <NUM> and so towards the second air outlet <NUM>. a pressure signal in the second control channel <NUM> causes the airflow in the first air inlet <NUM> channel to flow into first outlet channel <NUM> and so towards the first air outlet <NUM>.

This means that the control waves of pressure signals in the control channels <NUM>, <NUM> are amplified by the fluidic amplifier in the third oscillator 1c, every time switching the airflow from the first outlet channel <NUM> to the second outlet channel <NUM> and back, thereby generating two pulsating airflows, one in each of the outlet channels <NUM>, <NUM>, each pulsating with the same oscillation frequency and phase-shifted by half a wave period.

It will be clear that the oscillation frequency of the third oscillator 1c will be the same as the frequency of the alternating voltage. This oscillation frequency can therefore be easily controlled by electronically altering the frequency of the alternating voltage. This can be done independently of the actual airflow through the third oscillator 1c.

An advantageous way of using the first, second and third oscillators is in a method of generating a bubble screen to reduce the propagation of underwater sound by attenuating the sound energy of this sound, which is desirable to limit the effects of construction works on animals in the water.

This will be described below for the second oscillator 1b, referring to <FIG>. Note that a third oscillator 1c can also be used instead of the second oscillator 1b without adaptations. Note that a first oscillator 1a can also be used with some limitations, as will be described below. Note that <FIG> are schematic representations and are not to scale.

In these figures a device <NUM> is shown for attenuation of sound energy, in other words for reducing underwater sound propagation, originating from a source <NUM> of underwater sound,eg offshore/inland water construction activities, such as pile driving for building constructions or platforms.

The device <NUM> comprises a flexible air hose <NUM>, to which bubble generation units <NUM> are connected. The air hose <NUM> is connected to a source <NUM> of pressurized air.

The bubble generation units <NUM> are arranged in a circle with a radius of circa <NUM> around a source <NUM> of underwater sound, for instance a pile-driving activity.

The bubble generation units <NUM> have a footprint of circa <NUM> by <NUM>, and contain two parallel rows 34a, 34b, on either side of the air hose <NUM>, of eight identical oscillators 1b per row 34a, 34b.

A cross-section of the bubble generation units <NUM> is shown in <FIG>. As can be seen, each of the bubble generation units <NUM> comprises a bubble generation unit body <NUM>, typically made of rubber, in which for each oscillator a channel <NUM>, running from close to the base of the bubble generation unit body <NUM> to the top of the bubble generation unit body <NUM>, is provided.

Inside the bubble generation unit body <NUM>, two rows 34a, 34b of second oscillators 1b are provided. These oscillators 1b are indicated schematically in <FIG>. The first air inlet <NUM>, and second air inlet <NUM> if present, of these oscillators 1b are connected to the air hose <NUM>, whereby the air outlets <NUM>,<NUM> of the oscillators 1b debouch in said channel <NUM>.

The device further comprises a control unit <NUM>. The control unit <NUM> will usually be mounted on a ship, which is not shown in the figures. The control unit <NUM> is connected to the piezo-electric actuators <NUM> of the oscillators and is arranged to supply an alternating voltage to these actuators <NUM>, via a cable channel <NUM>.

The device <NUM> is set up such that separate connections for applying an alternating voltage from the control unit <NUM> to the separate rows of oscillators are present, so that an alternating voltage with a different frequency can be supplied to the piezo-electric actuators <NUM> of the oscillators in the respective rows.

The device <NUM> further comprises a hydrophone <NUM> which is connected to the control unit <NUM> and which is positioned outside the circle formed by the air hose <NUM>. The control unit <NUM> is programmed to analyse a sound frequency spectrum captured by the hydrophone <NUM> and to determine the dominant frequency or frequencies in this sound frequency spectrum.

The use of the device <NUM> in a method for attenuating sound energy or reducing sound propagation under water is as follows.

When the source <NUM> of underwater sound, eg. a pile driving activity, is active, pressurized air is supplied to the air hose <NUM> and an alternating voltage with a certain starting frequency is supplied by the control unit <NUM> to the oscillators 1b.

As a results the oscillators 1b will oscillate at that same frequency, and generate air pulses at the air outlets <NUM>,<NUM>. At their air outlets <NUM>,<NUM>, in this case provided with perforated plates <NUM>, air bubbles <NUM> are thereby generated in the channels <NUM>. The air bubbles <NUM> will start to move up, acting as an air pump and establishing an upward water flow in the channels <NUM>, as indicated by the arrows A in <FIG>. This water flow will effectively remove newly forming air bubbles <NUM> at the holes in the perforated plates <NUM>, so that an equilibrium situation with a constant air bubble <NUM> size is quickly established. These air bubbles <NUM> are released from the channels <NUM> in the bubble generation unit body <NUM>, so that these channels <NUM> in effect become a bubble outlet channel <NUM>.

Due to the fact that many bubble generation units <NUM> are present, a circular air bubble screen is thereby formed. This bubble screen is effective in reflecting and absorbing sound, so that the long range effect of sound coming from the source <NUM> of underwater sound is limited.

At the same time, the hydrophone <NUM> captures the sound spectrum of the underwater sound outside the circle, so the sound spectrum of the underwater sound not absorbed or reflected by the air bubbles <NUM> in the bubble screen. This sound spectrum which is analysed by the control unit <NUM> by means of fast fourier transform so that the sound frequency or frequencies having the highest sound pressure can be established.

Accordingly, the control unit <NUM> can actively adapt the frequency of the alternating voltage supplied to the oscillators 1b, and thereby the oscillation frequencies of the oscillators 1b, and thereby the size of the air bubbles <NUM> generated, in order to achieve a maximum reduction of the propagation of the underwater sound.

It is particularly advantageous that two rows 34a, 34b of separately controllable oscillators 1b are provided in the same bubble generation units <NUM>, so that two dominant frequencies or frequency ranges can be efficiently absorbed and reflected.

Hereby, a higher oscillation frequency will lead to smaller air bubbles <NUM>, whereby smaller air bubbles <NUM> are effective in absorbing and reflecting sound of a higher frequency, compared to larger air bubbles <NUM>.

Note that also a first oscillator 1a is usable in the bubble generation units <NUM>. The air bubble size will then not be controllable and adjustable, but the advantage of obtaining a constant and stable flow of air bubbles <NUM> of a constant size out of such a bubble generation unit <NUM> is nevertheless present, compared to air bubbles created by random phenomena, such as occur when air is pressed out of a standard hole or nozzle, which will vary in size and in mutual distance.

A bubble generation unit <NUM> having a first oscillator 1a can firstly be better designed than traditional bubble generation units to generate air bubble sizes matching the sound frequency spectrum expected from specific underwater noise-generating activities. Secondly, such air bubbles <NUM> will coalesce less compared to air bubbles coming out of traditional bubble generation units, so that they remain active and effective longer, in other words over a greater vertical distance as they rise through the water.

Clearly, two, or even more, of such devices <NUM> can be used together, either or not supplied with pressurized air from the same source <NUM>.

In such a case, a second device <NUM> is placed in a circle around a first device <NUM>.

As a consequence, two concentric bubble screens are formed.

Since the sound frequency spectra of a first sound, generated by the source <NUM> of underwater sound, and a second sound on the outside of the inner bubble screen, which sound results from partial absorption and reflection of the first sound, will usually have different frequencies which have the highest sound pressure, the oscillators 1b in the bubble generation units <NUM> of the second second device <NUM> will usually work at a different oscillation frequency than the oscillators 1b in the bubble generation units <NUM> of the first device <NUM>, thereby generating different bubble sizes in the inner bubble screen than in the outer bubble screen.

It is noted that both devices <NUM> comprise a separate hydrophone located on the other side of the respective bubble screen compared to the source of underwater sound, so that the performance of both devices <NUM> can be optimized independently for greater overall performance.

Claim 1:
Method of generating fluid bubbles (<NUM>) in a liquid, wherein an oscillator (1b, 1c) for generating two or more pulsating fluid flows from a constant fluid flow is used, whereby the oscillator (1b, 1c) comprises a first fluid inlet (<NUM>) for receiving a fluid flow and a first fluid outlet (<NUM>) and a second fluid outlet (<NUM>) for each outputting a said pulsating fluid flow, whereby the oscillator (1b, 1c) comprises a bistable fluidic amplifier (<NUM>,<NUM>,<NUM>,<NUM>) for amplifying a control signal, whereby the fluidic amplifier (<NUM>,<NUM>,<NUM>,<NUM>) is placed between the first fluid inlet (<NUM>) and the fluid outlets (<NUM>,<NUM>), whereby the oscillator (1b, 1c) comprises a piezo-electric actuator (<NUM>) for generating said control signal, whereby the piezo-electric actuator (<NUM>) is connected to a source of alternating voltage, whereby the first fluid inlet (<NUM>) is connected to a source (<NUM>) of pressurized fluid, whereby a pulsating fluid flow from at least one of the fluid outlets (<NUM>,<NUM>) is used to generate fluid bubbles (<NUM>) in the liquid, characterized in that the fluid bubbles (<NUM>) are air bubbles (<NUM>) and are used to form a bubble screen under water to attenuate the sound energy of underwater sound coming from a source of undesirable underwater sound.