PRESSURIZED GAS DRIVEN SOUND EMITTER

The invention relates to a sound emitter for generating sound of a predetermined frequency, which is driven by pressurized gas and has a resonator horn and means for controlling the emission of pressurized gas through the resonator horn while providing sound oscillations substantially at said predetermined frequency.

The most common sound emitter of this type for use as an alarm siren, a signal apparatus at sea and in industry, e.g. for fluidizing slow- moving material and for sonic cleaning of furnaces and the like, is the well-known sound emitter of the diaphragm valve type wherein a diaphragm in co-operation with a seat controls the emission of the pressurized gas through the resonator horn. A sound emitter of this type is shown e.g. in US-A-3942 468, Though this type of sound emitter is of a reliable and well developed construction, there is no getting away from the fact that the diaphragm as a movable element is a relatively damageable element and is the component most exposed to wear and break downs in this type of sound emitter.

The object of the invention is to provide a sound emitter of the type referred to above which completely in lacks a diaphragm or other movable component for controlling the emission of the pressurized gas through the resonator horn, and this object is achieved by a sound emitter as characterised in claim 1.

US-A-3237 712 discloses a fluid-operated acoustic device_' but this device has no resonator horn with a substantially equal impedance member.

US-A-3 111 931 relates to a sonic generator having horns which are closed with diaphragms and hence have a fluid return.

US-A-4 194 095 describes a loudspeaker with horns which are controlled by valves.

The sound emitter of the invention can be made of a small number of elements (three or four pieces) and as a consequence thereof will be cheap in manufacture and requires low maintenance and limited storage of spare parts, because the fluidistor has no moving elements and is practically impossible to wear out.

In order to illustrate the invention a number of exemplifying embodiments thereof will be described in more detail below, reference being made to the accompanying drawings in which

• FIG. 1 is a diagrammatic longitudinal sectional view of the simplest embodiment of the sound emitter according to the invention,
• FIG. 2 is a diagrammatic longitudinal sectional view of the sound emitter according to the inveri- tion controlled by means of a separate control fluidistor,
• FIG. 3 is a diagrammatic longitudinal sectional view of the sound emitter according to the invention provided with a self-oscillating power fluidistor,
• FIG. 4 is a diagrammatic longitudinal sectional view of a modified embodiment of the sound emitter in FIG. 3,
• FIG. 5 is a diagrammatic longitudinal sectional view of a further modified embodiment of the sound emitter in FIG. 3,
• FIG. 6 is a diagrammatic longitudinal sectional view of a still further modified embodiment of the sound emitter in FIG. 3,
• FIG. 7 is a diagrammatic longitudinal sectional view of a further development of the sound emitter in FIG. 6,
• FIG. 8 is a diagrammatic longitudinal sectional view of the sound emitter in FIG. 3 provided with two resonator horns, and
• FIG. 9 is a diagrammatic longitudinal sectional view of the sound emitter in FIG. 8 with a phase shift between the two resonator horns.

The sound emitter in FIG. 1 comprises a power fluidistor 10 having a straight inlet passage 11 to be connected to a pressure gas source, including not only a source of pressurized gas but also a . source of pressurized gas mixture (such as air) or steam. The fluidistor has two outlet branches 12 and 13 forming a certain angle with each other and separated by a sharp edge 14 in the conventional manner. A resonator horn 15 communicating with the surrounding air is connected to one of said outlet branches while the other outlet branch is connected to a tubular member 16 communicating with the surrounding air and forming a tuned restriction such that said member has substantially the same acoustic input impedance as the resonator horn. The power fluidistor operates according to the wall effect (Coanda effect) and when pressurized gas is supplied to the inlet passage 11 there is formed at the transition from the inlet passage to the two outlet branches 12 and 13 at the sharp edge 14 between said branches a vortex providing a subatmospheric pressure. The fluidistor in itself .is bistable but due to the connection of the resonator horn 15 the air will be directed alternately to one and the other of the two outlet branches at a frequency determined by the volume of the passage (the resonator horn) connected to the outlet branch 12, and thus the fluidistor in this case operates as a self-oscillating fluidistor controlled by the acoustic impedance of the resonator horn 15, the oscillations generated being amplified in the horn.

Instead of allowing the resonator horn to control the oscillation proper of the power fluidistor 10 the fluidistor may be arranged for positive control by means of a separate self-oscillating control fluidistor of a conventional type. This is shown in FIG. 2 wherein the fluidistor 10 is provided with two control nozzles 17 and 18 connected to the outlet branches of the control fluidistor 19. For optimum output power from the sound emitter the resonator horn in this case should have a resonance frequency which substantially equals the frequency at which the power fluidistor 10 oscillates under the control of the self-oscillating control fluidistor 19. One and the same control fluidistor can be provided to control several sound emitters.

FIG. 3 also discloses a self-oscillating power fluidistor but in this case the fluidistor is rendered self-oscillating by a control passage or "control resonator" 20 being provided between the two control nozzles 17 and 18. The volume of the control passage in co-operation with the resonator horn 15 determines the oscillation frequency of the fluidistor.

Another manner in which the power fluidistor 10 can be made self-oscillating is disclosed in FIG. 4 wherein the two control nozzles 17 and 18 are connected by passages or conduits 21 and 22 to the outlet branch 13 and the outlet branch 12, respectively. By ejector action in the outlet branches there is produces a pressure change in the control nozzles, by which the air flow through the fluidistor is switched and the fluidistor accordingly will be self-oscillating at a frequency determined by the dimensioning of the passages or conduits 21 and 22.

FIG. 5 discloses a further manner for controlling the power fluidistor 10. In this case one control nozzle 17 is replaced by a cavity 23 and control air is supplied to said cavity through the other control nozzle 18. At a proper relationship between the pressure of the control air and the volume of the cavity 23 the fluidistor will start self-oscillating.

The embodiment according to FIG. 6 is a further modification of the means for controlling the fluidistor. In this case the member 16 is connected to the neck of the horn 15 and thus opens into the horn to communicate with the surrounding atmosphere through the horn. It is constructed with such a length that there is obtained a phase shift between the air flow through the horn and the air flow through the member 16 in order to double the pressure in the resonator horn.

In the embodiment according to FIG. 7 the member 16 is also connected to the neck of the resonator horn 15 opposite to an outlet 24 from the neck of the resonator horn in order to provide thereby a pressure decrease in the resonator horn by ejector action, a self-oscillation of the fluidistor 10 also being obtained thereby.

In the embodiments according to FIGS. 1 to 5 the member 16 can be replaced by a further resonator horn 15' as is shown in FIG. 8. In this case the resonator horns are identical. The drawback of the arrangement of two horns in this way is that the sound wave from one horn will be in opposition to the phase of the sound wave from the other horn. By extending one of the horns in order to provide a phase shift of 180° as shown in FIG. 9 wherein the horn 15' is provided with an extension 25 for this purpose, the sound waves may be brought into phase with each other.

In the diagrammatic figures, the power fluidistor has been shown to be of the flat type but it can advantageously be constructed also as a cylindric fluidistor and generally be of the several embodiments existing in the general fluidistor technique because the fluidistor as such forms no part of the invention.