METHOD FOR INDICATING HOLES IN A TARGET HAVING AN ACOUSTIC CHAMBER

Abstract: In a method for indicating holes in a wall (1) upon firing at a target having an acoustic chamber (4) which is defined forwardly by said wall (1) and in which microphones (5) are arranged to receive the soundwaves generated by a fired projectile (6) entering said chamber, a quantity corresponding to the sound velocity within the acoustic chamber (4) is calculated on the basis of differences in the transit time of the soundwaves to the microphones (5). The calculated quantity is then compared with a reference value. A hole indication signal is provided when the deviation of the calculated quantity from the reference value exceeds a predetermined value.

Description: METHOD FOR INDICATING HOLES IN A TARGET HAVING AN ACOUSTIC CHAMBER The present invention relates to a method for indicating holes in a wall during firing at a target having an acoustic chamber which is defined forwardly by said wall and in which microphones are arranged to receive the soundwaves generated by a fired projectile entering said chamber. A known target having an acoustic chamber will be described 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 shows the lower part of this target in an en¬ larged sectional view. 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 impermeable 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 soundwaves generated by a fired projectile entering said chamber 4. The propagation of these sound¬ waves 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. 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 assumed to travel at supersonic velocity, and the sound¬ waves emanating from the point of the projectile therefore propagate towards the target in the form of a cone having the 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 projec¬ tile 6 is circular in the chamber 4, the projectile being the centre. These internal soundwaves reach the four micro¬ phones 5 at different times depending upon the hit posi¬ tion of the projectile, i.e. the distance to the respec- tive microphone, and the sound velocity in the chamber 4. On the basis of these transit time differences and known mathematical relationships, the hit position of the pro¬ jectile 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 material of the sheet 1 is such that it closes up again after the projec¬ tile has passed therethrough. However, this characteris- tic deteriorates after many piercings, and finally the sheet may be worn to such an extent that lasting holes may occur in certain especially exposed parts of the sheet. 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. The deterioration is accelerated upon use of the novel type of projectile or bullet, so-called hollow-point ammunition, which at its front end has a hollow instead of a point and which during its passage through the sheet 1 punches a small and lasting hole in the sheet. Such a defect in the form of a hole is assumed to exist in the sheet 1 at the point 8 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 8, 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 8 before the internal soundwaves reach the same point. Since the point 8 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 po¬ sition determination will thus be impossible. In view hereof, it is desired to provide an indi¬ cation showing that holes or similar sound-transmitting defects occur in the sheet defining the chamber in the forward direction, and that a correct hit position de¬ termination thus has been made impossible. This object is achieved by means of a method for indicating holes in a wall upon firing at a target having an acoustic chamber which is defined forwardly by said wall and in which microphones are arranged to receive the soundwaves generated by a fired projectile entering said chamber. The method according to the present invention is characterised in that a quantity corresponding to the sound velocity within the acoustic chamber is calculated on the basis of differences in the transit time of the soundwaves to the microphones, that the calculated quantity is compared with a refer¬ ence value, and that a hole indication signal is pro¬ vided when the deviation of the calculated quantity from the reference value exceeds a predetermined value. The reference value preferably is established on the basis of a predetermined number of immediately pre¬ ceding calculations of the said quantity. The hole indication signal preferably is connected to an indicator located in a position at a distance from the target, such as the position of an instructor or the marksman's position. The microphones utilised preferably are four micro¬ phones adapted to provide a simultaneous hit position determination. In the coordinate system illustrated in Fig. 1, an assumed hit position has the coordinates (x; y), while the four microphones 5 have the coordinates (x. ; y 1 ) , ( x 2 ; y 2 ? ) f ^ - 2 '' y 3-* and * x 4 ; v 4-* • τhe following relationship is obtained wherein i = l, 2, 3, 4; v is the sound velocity within the chamber 4; t is the transit time of the soundwaves from the hit position point (x; y) to the microphone closest thereto; and t. + t is the transit time of the soundwaves from the hit position point (x; y) to the respective microphone. This equation system thus contains four equations with four unknowns, viz. x, y, t and v. The hit position (x; y) and the sound velocity v within the chamber 4 may thus be determined. The sound velocity in air is substantially dependent upon the temperature only and is practically independent of the air pressure, the air density and air humidity. In one day the temperature changes but slowly, and this means that also the sound velocity within the chamber, as calculated in the above-mentioned manner for each shot, changes but slowly. A rapid change of the calculated sound velocity thus indicates that something is wrong, and that in all probability there is a hole in the wall or sheet 1. This rapid or abrupt change of the calculated sound velocity is due to the fact that, as has been men- tioned above, the "external" soundwaves via the hole may reach one or more of the microphones 5 before the internal soundwaves, whereby the transit times tl. + to on which the hit position and sound velocity determinations are based, will be inaccurate. A rapid or abrupt change of the calculated sound velocity may thus be utilised for indicating that there is a hole in the wall or sheet 1. The calculated sound velocity is then compared for this purpose with a reference value which is based on, for example, the next preceding sound velocity determinations and which consti¬ tutes the mean value of a specific number of such deter¬ minations. If the deviation of the calculated sound velo¬ city from the reference value is greater than a predeter¬ mined value, an error signal or hole indication signal is provided. The said predetermined value must be so great that a considerable jump is required to provide a signal, and that a minor change of the calculated sound velocity, caused by a change in temperature, shall not provide an error signal or hole indication signal. The error signal or hole indication signal prefer¬ ably is connected to an indicator at the instructor's position or the marksman's position.

Claims: CLAIMS 1. A method for indicating holes in a wall (1) upon firing at a target having an acoustic chamber (4) which is defined forwardly by said wall (1) and in which microphones (5) are arranged to receive the soundwaves generated by a fired projectile (6) entering said chamber, c h a r a c t e r i s e d in that a quantity corresponding to the sound velocity within the acoustic chamber (4) is calculated on the basis of differences in the transit time of the soundwaves to the microphones (5), that the calculated quantity is compared with a reference value, and that a hole indication signal is provided when the deviation of the calculated quantity from the reference value exceeds a predetermined value. 2. A method as claimed in claim 1, c h a r a c - t e r i s e d in that said reference value is estab¬ lished on the basis of a predetermined number of immediately preceding calculations of said quantity. 3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the hole indication signal is connected to an indicator located in a position at a distance from the target. 4. A method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the microphones (5) are four microphones for simultaneous hit position determination.

Inventors: AXNER YNGVE, AXNER, YNGVE
Applicants: SAAB TRAINING SYSTEMS AB, SAAB TRAINING SYSTEMS AKTIEBOLAG
Application year: 1985
Main classification: F41J5
IPC codes: F41J5