Abstract:
A device for regenerating powerful acoustic pressure comprises at least an arrangement for generating sound while utilizing the flextensional technique, i.e. having at least one surface element ( 2 ), the opposite ends ( 4, 5 ) of which are arranged to be influenced to oscillate away from and towards each other, and the surface element oscillating transversely thereto and generating sound.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to a device for generating very high sound pressure in air for example to prevent trespass, intrusion or unauthorized staying in an area indoors or outdoors so as to deliver a warning signal calling for attention as a siren. 
     There are several devices based on different techniques which however have disadvantages as, 
     areas with lower sound pressure 
     low sound pressure at frequencies important for public address 
     low efficiency 
     becomes very large and heavy when very powerful sound pressure is to be achieved 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a new device which to a large extent reduces the disadvantages above of already known devices and which also combine a very high sound pressure for a siren function at lower frequencies with a very high sound pressure for frequencies important for the public address function, and also very high sound pressure for even higher frequencies used for example to prevent intrusion or unauthorized staying. 
     This object is obtained by providing a device comprising at least an arrangement for generating sound while utilizing the flextensional technique, i.e., it has at least one surface element ( 2 ), the opposite ends ( 4 , 5 ) of which are arranged to be influenced to oscillate away from and towards each other and the surface element oscillating transversely and generating sound. 
     By using the flextensional technique in air and within totally new fields and adapting it in accordance with following disclosure can very high sound pressure be achieved within a broad frequency range with advantages as higher efficiency, lower weight, smaller size in comparison with earlier known techniques and devices for these applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in grater detail with reference to the accompanying drawings, in which 
     FIG. 1 is a simplified sectional view of a flextensional sound generator in accordance with the present invention; 
     FIG. 1 a  is a more detailed sectional view illustrating the mechanical transformer lever mechanism formed by the fulcrum in accordance with the present invention; 
     FIG. 2 is a sectional view similar to FIGS. 1 and 1 a  and illustrating the air transformer and/or pressure chamber in accordance with the present invention; 
     FIG. 3 is perspective view of FIG. 2; 
     FIG. 4 is sectional view illustrating positioning of horns; and 
     FIGS. 5 a ,  5   b  are respective sectional views, with FIG.  5 ( b ) illustrating a cross-section of a membrane formed by two first layers and an intermediate thicker layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Experience has shown that flextensional devices with light membranes adapted to air may give high resonance effects, up to 10 dB or more, and the technique is therefore suitable to be used for sirens. The problem is to achieve a high sound pressure for the siren function in combination with higher sound pressure for the public address function which combination is desired for many applications. 
     A membrane works for lower frequencies as one unit where the membrane area works in the same direction, called first mode, with a transmission ratio (defined below) depending on the constitution thereof as the thickness, length and bending form. Experience has shown that after this first mode there is a frequency range where the surface element works in a second mode where two areas of the membrane surface work out of phase with the third in (the areas at the ends is out of phase with the mid area). Further up in frequency the surface element totally collapses with a low transmission ratio. 
     A first main resonance in the first mode of the membranes can be achieved if the driving unit is not made stiff and/or the mass load is made high so that this system resonance occur between driving unit/membrane(s)/horn (FIG.  4 ). This resonance effect is normally about 4 times. (the stiffness of the driving unit can also be made so high so this resonance will not occur in the first mode). 
     A device where the stiffness of the driving unit is such that a first main resonance (f 1 ) occurs in the membranes first mode can be characterized by the transmission ratio and by the resonances. The sound pressure will show three specific main maxima when sweeping from lower to higher frequencies with the same current. In the area where the surface element work in the second mode is one maxima called f 2 . 
     Experience has shown that this first main resonance frequency for the system (driving unit-membranes) is lowered with an increase of the transmission ratio when the system has a first main resonance in the membranes first mode. This change in the resonance frequency can be described as the transmission factor contribute with the transmission ratio in square multiplied with a fixed mass, giving a total fictive mass (M). The calculated resonance frequency is a function of 1/{square root over (M)}. 
     Experience show that by using a high transmission ratio the resonance frequency can easily be lowered without using real masses at the end beams or heavy membranes and also at the same time accordingly achieve a higher acoustic power at a required siren frequency. Experience has also shown that the second mode occurs and a membrane collapses for lower frequencies for membranes with a high transmission ratio. 
     Preferred is use of a mechanical transformer (FIG. 1 and 1 a ) for a higher transmission ratio which has several advantages. Preferred is a total transmission ratio between 8 to 40 (mechanical transformer and membrane). 
     By introducing a mechanical transformer (FIGS. 1 and 1 a ) for the above applications, 
     the membrane transformation ratio can be substantially reduced and the membrane can then work at much higher frequencies in the first mode i.e. frequencies very important for the public address and frequencies important to prevent intrusion or unauthorized staying 
     a much higher sound pressure can be achieved at higher frequencies where the element has collapsed, 
     makes it possible to lower the resonance frequency without using a heavy membrane etc. (increase the mass load M) and achieve a first main resonance in the first mode and a high sound pressure for a siren at lower frequencies, 
     larger membranes can be used, which second mode does not occur/or are not collapsing at preferred higher frequencies, for public address etc. and accordingly a higher sound pressure, 
     gives a smoother frequency response. 
     The mechanical transformer can for example be a lever arm (FIG. 1 a ) and be a part of an end beam. The end beam can be split in two equal sections where each section work as a lever arm and bends over a fulcrum. Fulcrums can be made by introducing two plates, parallel with each membrane, connected to the lever arms (endbeams). 
     Preferred is also for example, as a siren, to combine a mechanical transformer, a membrane working in its first mode for frequencies essential for public address 800-1500 Hz with an air transformer and/or pressure chamber (FIGS. 2 and 3) with a horn (FIG.  4 ). The mechanical transformer make it possible to use a membrane that does not break up into a second mode which will dramatically reduce the acoustical output power essential for the siren and public address function especially when used together with above air chamber/horn loading. Preferred is also to have the first main resonance in the area essential for public address 800-1500 Hz. 
     Experience has also shown that the resonance effect (the end beam movement divided with the calculated driving unit movement) and sound pressure shows a maxima for frequencies above f 2  when the membrane has collapsed with a very low transmission factor. This resonance effect is about 6 to 8 times. 
     The transmission factor of the device is defined, when two opposite ends are driven, as the quotient of the amplitude of the maximum oscillation of the membrane and the amplitude of any end thereof and/or the total amplitude of the total driving unit movement divided by 2. The transmission ratio has shown at lower frequencies to be relatively constant and at a point increase at higher frequencies and with a high transformation ratio before entering into the second mode. 
     Another preferred embodiment of the invention is that said surface element has a high stiffness with respect to the average density thereof by incorporating material portions therein having a lower density than the rest of the surface element or by arranging cavities therein. It is by this possible to obtain a light but nevertheless stiff surface element. This may for instance be done in any of the ways described in PCT/SE/95/00571 or PCT SE/95/01113. A portion of the surface element may be formed by at least two first layers and an intermediate (FIGS. 5 a , and  5   b ) having a lower density than the density of any of the first two layers as seen in the thickness direction thereof. The first layers may for example be made of carbon fiber baked into a matrix with a density of about 1500 kg/m3. The intermediate layer may be of cellular plastic or honey comb structure with a density of about 300 kg/m3. 
     The above designs is also suited for Public Address systems (PA). The invention also include end driven flextensional devices. 
     REFERENCE NUMERALS IN 
     FIG. 1, a simplified sectional view of a flextensional sound generator: 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the reference numerals denote the following elements: 
       1 . force from driving unit 
       2 . membranes 
       3 . endbeams 
       4 , 5  flextensional ends 
       6 , 7  membrane movement