EP0233886A1

EP0233886A1 - Target

Info

Publication number: EP0233886A1
Application number: EP19860900296
Authority: EP
Inventors: Bo Svensson
Original assignee: Saab Training Systems AB
Current assignee: Saab Training Systems AB
Priority date: 1984-11-27
Filing date: 1985-11-25
Publication date: 1987-09-02
Also published as: WO1986003284A1; SE458719B; SE8405960L; SE8405960D0

Abstract

Une cible possède une chambre acoustique (4) à l'intérieur de laquelle sont disposés des microphones (5) pour recevoir les ondes sonores émises par le tir d'un projectile entrant dans ladite chambre. Les microphones (5) sont reliés à des moyens conçus pour établir la position d'impact du projectile sur la base des écarts de temps de transit des ondes sonores jusqu'aux microphones (5). La chambre acoustique (4) est délimitée vers l'avant par une paroi (12) retardatrice du son, d'une épaisseur telle que les ondes sonores émises par le projectile et passant à travers la paroi vont atteindre chaque point éventuel de la face intérieure de ladite paroi, et ainsi chaque microphone (5), plus tardivement que les ondes sonores émises par le projectile au moment où il entre dans la chambre (4), et cela également lorsque ledit point est situé dans un trou précédemment percé dans la paroi (12).A target has an acoustic chamber (4) inside which microphones (5) are arranged to receive the sound waves emitted by the firing of a projectile entering said chamber. The microphones (5) are connected to means designed to establish the impact position of the projectile on the basis of the transit time differences between the sound waves and the microphones (5). The acoustic chamber (4) is delimited towards the front by a sound-retarding wall (12), of a thickness such that the sound waves emitted by the projectile and passing through the wall will reach each possible point of the interior face of said wall, and thus each microphone (5), later than the sound waves emitted by the projectile when it enters the chamber (4), and this also when said point is located in a hole previously drilled in the wall (12).

Description

TARGET

The present invention relates to a target of the type having an acoustic chamber within which microphones are arranged to receive the soundwaves generated by a fired projectile entering said chamber, the microphones being connected to means adapted to establish the hit position of the projectile on the basis of differences in the transit time of the soundwaves to said micro¬ phones.

A known target of this type and the chief disadvan- tage thereof are described in more detail below, reference being had to Figs. 1 and 2 of the accompanying drawing. Fig. 1 is a front view of a known target, and Fig. 2 is an enlarged' sectional view of the lower part of this target. The known target has a front sheet 1 of rubber or plastics material so designed that the sheet 1 will not be torn open or become frayed when pierced by a projectile, but will close up again after each piercing. The target also has a rear sheet 2 of the same material as the front sheet 1. The sheets 1 and 2 which are im- permeable to sound, are stretched over a circumferential frame 3 defining, together with said sheets 1 and 2, an acoustic chamber 4. In the lower part of the chamber 4, four microphones 5 are arranged to receive the sound¬ waves generated by a fired projectile entering said chamber 4. The propagation of these soundwaves in the chamber 4 is circular, the projectile being the centre. The microphones 5 are connected to means (not shown) adapted to establish the hit position of the projectile on the basis of differences in the transit time of the soundwaves to the four microphones.

In a known alternative embodiment, thin plastics films are provided on either side of the target to protect the sheets 1 and 2 against sunlight.

Fig. 2 shows a projectile 6 fired at the target and its trajectory 7. The projectile 6 which is shown at the moment when its point enters the chamber 4, is here as¬ sumed to travel at supersonic velocity, and the soundwaves emanating from the point of the projectile therefore pro- pagate towards the target in the form of a cone whose front has been designated 8 and which has a cone angle 2α. These external soundwaves do not enter the chamber 4 since they are stopped by the sheet 1 which is impermeable to sound. As has been mentioned above, the propagation of the soundwaves generated by the projectile 6 is circular in the chamber 4, the projectile being the centre. These internal soundwaves reach the four microphones 5 at different times depending upon the hit position of the projectile, i.e. the distance to the respective icro- phone, and the sound velocity in_. the chamber 4. On the basis of these transit time differences and known mathe¬ matical relationships, the hit position of the projectile is established by the above-mentioned means (not shown) to which the microphones 5 are connected. The established hit position is recorded, and the marksman is provided with an indication thereof on a screen or strip.

As has been mentioned above, the sheet 1 is of such a character that it closes up again each time it has been pierced by a projectile. This applies to pointed projec- tiles. Recently, a novel type of projectile or bullet has been introduced, especially in sport shooting, and this projectile has no point. This novel projectile or bullet, which is called hollow-point ammunition, is provided in its front end with a concave hollow and punches a lasting hole in the sheet 1 during its passage therethrough. As a result of such holes, the sheet 1 can no longer be regarded as impermeable to sound and therefore may give rise to inaccurate hit position recordings.

Such a defect in the form of a hole is assumed to exist in the sheet 1 at the point 9 of Fig. 2. The hit position of the projectile 6 is located at a distance r from the hole, i.e. the path of the internal soundwaves from the hit position to the hole is r. When the external soundwaves have reached the point 9, they have travelled the distance a from the moment at which the projectile penetrated the sheet 1. It will be appreciated that a < r, which means that the external soundwaves enter the chamber 4 through the hole at the point 9 before the internal soundwaves reach the same point. Since the point 9 as shown in the drawing lies between the hit position and the level of the microphones 5, the "external" soundwaves will now reach at least one of the microphones before the internal soundwaves. In this manner, the transit time differences on which the determination of the hit posi¬ tion is based, will be inaccurate, and a correct hit position determination will thus be impossible. It therefore is the object of this invention to provide a target in which this shortcoming has been eliminated and which thus makes it possible to effect a correct determination of the hit position also at repeated shooting with hollow-point ammunition. This object is achieved in accordance with the present invention by means of a target which is of the type mentioned by way of introduction and which is char¬ acterised in that the acoustic chamber is defined for- wardly by a sound-delaying wall of such thickness that the soundwaves generated by the projectile and passing through the wall will reach each optional point on the inner side of the wall, and thus each microphone, later than the soundwaves generated by the projectile as it enters the chamber, and this also when the said point is located in a hole previously punched into the wall.

In a preferred embodiment, the wall is built up of small closed cells, preferably of plastics material. In a variant, the wall has two surface layers defining between themselves a space which is filled with free cells. In another variant, the wall is a panel consisting of cells bonded to one another. The invention will now be described in more detail, reference being had to Figs. 3-6 of the accompanying drawings.

Fig. 1 is a front view of the above-mentioned known target.

Fig. 2 is an enlarged sectional view of the lower part of this target.

Fig. 3 shows a part section of a first embodiment of the target according to the present invention. Fig. 4 shows a part section of a second embodiment of the target according to the present invention.

Figs. 5 and 6 are enlarged views of a portion V and VI, respectively, of the part sections in Figs. 3 and 4. The target shown in Fig. 3 has a front sheet 1 and a rear sheet 2 which may be of the same type as the front sheet 1 and the rear sheet 2 in the target shown in Figs. 1 and 2. Thus, the front sheet 1 also here is of such a character that it cannot be torn open or become frayed as a result of being pierced by pointed projectiles, but will close up again after it has been hit by a pro¬ jectile. Furthermore, it is impermeable to sound. On the outside of each of the sheets 1 and 2, there is provided in this embodiment a further sheet la and 2a, respectively, the front sheet la preferably being of the same type as the sheets mentioned before, whereas the rear sheet 2a may be a plastics film protecting the sheet 2 against sunlight. Between the sheets 1 and 2 and a circumferential frame 3 an acoustic chamber 4 is defined, the lower part of which accommodates four microphones 5 in the same manner as in the target according to Figs. 1 and 2.

The space between the sheets 1 and la is filled with small light-weight plastics cells 10 having a diameter which is at least equal to the diameter of the remaining bullet holes in the sheets and amounts to 2-3 mm. These plastics cells which are tight so as not to absorb moisture and which preferably are filled with air, are not bonded to one another but may, as far as space is available, be entirely freely movable relative to one another. This filling of plastics cells forms a wall in which soundwaves penetrating thereinto will obtain, because of the cell structure, a far longer transit path. It will be appreciated, upon comparison with the prior art target shown in Fig. 2, that "external" sound¬ waves reaching the wall with the front 8 also in geomet¬ rical respect will travel a distance to the sheet 1 which is longer than the transit path a in Fig. 2. The total transit time of these "external" soundwaves will thus be so long that the internal soundwaves which propagate circularly from the projectile entering the chamber 4, will reach any optional point on the inner side of the sheet 1 before the "external" soundwaves, also when this point is located in a hole 11 made in the sheet 1 (see Fig. 5) or even in a hole which, in the path 13 of an earlier projectile, extends through the entire wall formed by the sheet la, the cells 10 and the sheet 1. However, the risk that a projectile passing through the wall will produce such a lasting hole therein, is ex¬ tremely small since each such hole will be closed up by overlying plastics cells sliding down into the hole. This means that the pierced wall portion will be imme¬ diately restored to its original state, as is shown in Fig. 5. On the other hand, lasting holes 11 may be punched into the sheet 1 if hollow-point ammunition is used. Nevertheless, such holes.do not affect the deter¬ mination of the hit position since the internal sound¬ waves, because of the sound-delaying wall of plastics cells 10, will reach the microphones 5 before the

"external" soundwaves. Thus, a correct hit position determination is obtained in spite of the said holes, regardless of whether these extend merely through the sheet 1 or all the way through the wall la, 10, 1. The target shown in Fig. 4 distinguishes from the target of Fig. 3 merely in that the wall formed by the plastics film la, the cells 10 and the sheet 1 has been replaced by a panel 12 consisting of cells bonded to one another and preferably prepared from the polyethylene cellular plastics material which is being marketed by Dynamit Nobel AG, Troisdorf, Federal Republic of Germany, under the trade name TROCELLEN. The panel 12 has similar sound-delaying characteristics as the wall 10 of plastics cells, but distinguishes therefrom in that the cells bonded to one another are unable completely to fill up a hole produced by, for example, hollow-point ammunition. However, the cell material has been shown to form channels having extremely "fluffy" or uneven walls, as will appear from Fig. 6, and this means that these channels per se provide a sound-damping and sound-delaying effect. Like the wall of plastics cells of the target according to Fig. 3, the panel 12 has a thickness such that the total path of the "external" soundwaves, which here naturally includes the distance a, as in the known case according to Fig. 2, will be considerably longer as compared therewith. As a result of the delay, the inter- nal soundwaves which propagate circularly from the projectile 6 and cover the distance to the hole 11 (Fig. 6), will reach the hole and any other optional point on the inner side of the panel 12 before the "external" soundwaves, also when this point is in a location where one or more projectiles on their way 13 through the panel 12 have completely pierced this panel. It should be noted, however, that the panel thickness must be selected in dependence upon the hole diameter that may be expected, and this diameter naturally is dependent upon the ammunition employed. The greater the expected hole diameter, the greater should be the thickness chosen for the panel 12.

Claims

1. A target of the type having an acoustic chamber (4) within which microphones (5) are arranged to receive the soundwaves generated by a fired projectile entering said chamber, the microphones being connected to means adapted to establish the hit position of the projectile on the basis of differences in the transit time of the soundwaves to said microphones, c h a r a c t e r ¬ i s e d in that the acoustic chamber (4) is defined forwardly by a sound-delaying wall (la, 10, 1; 12) having a thickness such that the soundwaves generated by the projectile and passing through the wall will reach each optional point on the inner side of the wall, and thus each microphone (5), later than the soundwaves generated by the projectile as it enters the chamber, and this also when the said point is located in a hole (11) previously punched into the wall (la, 10, 1; 12).

2. A target as claimed in claim 1, c h a r a c ¬ t e r i s e d in that said wall (la, 10, 1; 12) is built up of small closed cells, preferably of plastics material.

3. A target as claimed in claim 2, c h a r a c ¬ t e r i s e d in that said wall has two surface layers (1, la) defining between themselves a space which is filled with free cells (10).

4. A target as claimed in claim 2, c h a r a c ¬ t e r i s e d in that said wall is a panel (12) con¬ sisting of cells bonded to one another.