Submarine Propeller Cavitation Noise Simulator

Abstract

Simulation of submarine propeller cavitation as it varies with speed and th of submergence is effected by feeding a frequency proportional to blade rate to a counter which is periodically read out and reset at a rate proportional to the square root of pressure. The read out is used to control noise attenuator means including a one of N decoder and N attenuators scaled to provide relative noise according to a curve characteristic of the submarine to be simulated. The noise output is modulated in pulse width and repetition rate by a function generator also controlled by the counter read-out.

Description

BACKGROUND OF THE INVENTION

This invention relates to the art of sonar simulation for training purposes or the like, and more particularly to improved apparatus for simulating propeller cavitation noise.

Propeller cavitation noise is usually simulated by modulating the output of a random noise source at the blade rate which is proportional to the number of propeller blades and the shaft RPM of the ship or vessel being simulated. Provision is normally made for accentuating the pulse representing one of the blades inasmuch as it has generally been observed that the actual sound from a ship's propeller is usually characterized by such accentuation. The noise spectrum selected is, of course, chosen to resemble that of a known ship or class of ships. U.S. Pat. No. 3,341,697 of Kaufman et al. and U.S. Pat. No. 3,165,734 of Grodzinsky et al. are representative of the art of propeller noise simulation of the character mentioned.

In the case of simulating the propeller noise of submarines an additional factor, not heretofore known to be considered, is important, namely the effects of depth on the inception and degree of cavitation, the principal source of propeller noise. It has been shown by R. Urick in his book "Principles of Underwater Sound for Engineers" published by McGraw-Hill, 1967, that the shaft speed at which propeller cavitation begins is approximately proportional to the square root of the static water pressure. He has also shown that the relative noise output can be plotted as a function of the ratio of the speed in knots to the square root of the static water pressure in atmospheres.

SUMMARY OF THE INVENTION

With the foregoing in mind, it is a principal object of this invention to provide improved apparatus for simulating propeller cavitation noise and which is particularly well suited to simulating the propeller noise of a submerged submarine.

As another object this invention aims to provide such improved apparatus which serves to provide a signal simulative of propeller cavitation noise as a function of vessel or shaft speed and the depth at which a simulated submarine vessel is operating.

Still another object is to accomplish the foregoing in a manner which is compatible for use with either analog or digital simulation systems and can take advantage of the recent availability of low cost solid state shift registers, counters, multivibrators, and the like.

Other objects and advantages of apparatus embodying the invention will become apparent from the following description of the presently preferred embodiment when read in conjunction with the accompanying sheets of drawings forming a part hereof.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration, in block form, of a propeller cavitation noise simulator apparatus embodying the invention;

FIGS. 2a, 2b, and 2c are graphical illustrations of various levels of output corresponding to different degrees of cavitation;

FIG. 3 is a graphic illustration of relative noise level as a function of vessel speed and depth of operation; and

FIG. 4 is a diagrammatic illustration, in block form, of a function generator forming part of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the form of the invention illustrated in FIG. 1 and described hereinafter, there is provided a cavitation noise simulator apparatus which is generally indicated at 10 and serves to provide an analog output signal which is a function of vessel speed and depth of operation. In this regard, when representing cavitation noise at the onset thereof the output of the apparatus 10 is graphically shown in FIG. 2a as a series of noise pulses 12a having an overall amplitude excursion 14a and each having a duration which is short relative to the periodicity of occurrence 16 thereof. The periodicity is, of course, the reciprocal of the blade rate. FIG. 2b shows an example of noise pulses 12b when cavitation is well established. This example, wherein the period 16b is the same as 16a while the amplitude 14b and the duration of each pulse is notably greater, is indicative of operation at a shallower depth than in the prior example. In the example of FIG. 2c the cavitation has progressed to a general condition wherein the amplitude 14c and the duration of the pulses 12c is still greater and noise 18 occurs between the pulses 12c resulting from cavitation from the vessel hull appurtenances other than the propeller. This last example is indicative of still shallower operation.

Of course, if depth were held constant, and speed were increased from that of FIG. 2a, the cavitation noise would increase from that of FIG. 2a to that of FIG. 2b, but the period 16b would be decreased. The analogy can be pursued by further increases of speed at the same depth to reach general cavitation as shown in FIG. 2c, but compressed into still shorter periods. Referring to FIG. 3, the relative noise generated by a submarine can be plotted as a function of the ratio of speed in knots (related of course to blade rate) to the square root of the static water pressure P in atmospheres as indicated by curve C. The particular shape of curve C will vary from submarine to submarine, and the curve shown is a hypothetical one for purposes of example rather than a curve representative of any actual submarine. The apparatus 10, in addition to effecting the desired modulation of noise, serves to provide simulation wherein the relative noise is a desired function of the ratio of the speed or blade rate to the square root of static pressure.

Reverting to FIG. 1, the apparatus 10 receives as its principal inputs an analog voltage via line 20 representative of static water pressure P at the depth of simulated operation, an analog voltage via line 22 representative of blade rate, and band limited noise via line 24 from a suitable noise source. The apparatus 10 comprises means 30 for providing as an output on line 32 a voltage proportional to the square root of the pressure represented by the voltage input on line 20.

A voltage to frequency converter 34 is connected to receive the output on line 32 and provides a frequency output f.sub.1 on line 36 where f.sub.1 = k.sub.1 .sqroot.P. The voltage to frequency converter 34 may be of any conventional construction but conveniently comprises a voltage controlled oscillator and a divider to provide an output frequency f.sub.1 which is compatible with the remainder of the apparatus. The output on line 36 is applied to trigger a one-shot 40, the output 42 of which is fed via lines 44, 45, a delay element 46, and line 48 to the reset connection of a digital counter 50 having, by way of example only, a four bit output represented by lines 50a, 50b, 50c, and 50d.

The input to the counter 50 is derived by a voltage to frequency converter 52 which provides a corresponding frequency f.sub.2 on line 54 to the counter 50, where f.sub.2 = k.sub.2 (shaft RPM).

The output lines 50a-50d are connected to a parallel AND gate means 60 having parallel output lines 60a, 60b, 60c and 60d connected as the input to a hold circuit 62. The AND gate means 60 is connected to receive an enabling input from the one shot 40 via lines 44, 66, a delay element 68 and line 70. The AND gate means 60 may comprise a plurality of individual AND gates such that when an enabling pulse exists at line 70 logical ones existing at any of lines 50a-50d will be shifted to the hold circuit 62. The hold circuit 62 is periodically reset by the output of the one-shot 40 via line 44.

The output of the one-shot 40 is in the form of a short pulse 42 of duration T.sub.1 and the delays D.sub.1 and D.sub.2 imposed respectively by delay means 68 and 46 are conveniently such that D.sub.2 >D.sub.1 +T.sub.1. The leading edge of pulse 42 is used to reset the hold circuit 62, following which the leading edge of the delayed output of means 68 is used to shift the count from counter 50 into the hold circuit, and following which the leading edge of the delayed output of means 46 is used to reset the counter 50. This sequence of events occurs each time the one-shot 40 is triggered by the output of the converter 34.

Inasmuch as the input to counter 50 is supplied by the voltage to frequency converter 52 which generates the output frequency f.sub.2 proportional to shaft RPM and blade rate, and since the counter 50 is read out and reset at a rate f.sub.1 which is proportional to .sqroot.P, the count shifted out of counter 50 to hold circuit 62 is equal to

where n equals the number of propeller blades. Keeping in mind that speed may be measured by blade rate and that depth may be measured by pressure, it may be stated that the net effect of the two inputs (f.sub.1 and f.sub.2) to the counter 50 is to make the count registered at read out directly proportional to speed, and inversely proportional to the square root of pressure at a given depth. For example, if speed doubles but pressure increases by a factor of 4, the count registered remains the same.

The readouts from the counter 50 are used to control electronic attenuator means about to be described which adjusts the level of a modulated noise signal to provide relative noise output which varies with speed and depth according to a desired curve such as C of FIG. 3.

The circuit 62 serves to hold the count shifted thereto while counter 50 is working up a new count. The count being held in 62 is, in this example in four bit, binary form and is applied via lines 62a, 62b, 62c, and 62d to a one-of-ten decoder 76. The decoder 76, as its name implies, has 10 output lines 76a, 76b - 76j only one of which exhibits a change of voltage for any given digital combination applied 62a-62d thereof. Such decoders are well known to those skilled in the art to which the invention pertains, one example being that marketed by Fairchild Semiconductor Division of Fairchild Camera and Instrument Corporation, Mountain View, California as their MS1 9301.

The output lines 76a - 76j are connected as shown to the control electrodes of a plurality of field effect transistors 78a - 78j which are connected in series with a corresponding plurality of attenuator resistors 80a - 80j. The resistors 80a -80j are connected to a common line 82 from which cavitation noise output is taken, and which is separated from nominal ground by a load resistor 84. One or more of the output lines, e.g. lines 76i, 76j, may be connected to the control electrodes of other field effect transistors such as 86, 88 which are connected in series with resistors 90, 92 for a purpose which will presently be made apparent.

In order to effect the desired modulation of the noise signals in accordance with speed or blade rate, the count shifted by the AND gate means 60 out of the counter 50 is also applied as shown by lines 94a, 94b, 94c, and 94d to a function generator 96. The function generator 96, one satisfactory form of which is later described with reference to FIG. 4, provides a voltage output on line 98 to a voltage variable pulse width one-shot 100, which voltage is a function of the input count to the function generator. The purpose of this, as will presently be seen, is to control the width of the noise pulses 12a, 12b, 12c such that the pulses are narrow when the threshold for cavitation inception has just been exceeded and such that the pulses increase in width as cavitation progresses toward the general condition. Conversely, of course, the pulses go from wide to narrow as cavitation decreases.

To this end, the blade rate analog input on line 22 is also applied via line 22a to a voltage to frequency converter 104, the output of which is fed via line 106 as the triggering input to the one-shot 100. The one-shot 100, may conveniently be of the type described in U.S. Pat. No. application Ser. No. 50,259 of Arthur B. Moulton, assigned to the assignee hereof, and is adapted to have its unstable or triggered period controlled in duration by the voltage input from the function generator 96.

The output of the one-shot 100, represented by line 108 is applied as an enabling signal to a gate 110 which controls a portion of the band limited noise input via line 24. Thus, the noise input signal is connected via line 24, gate 110 and lines 114, 114a, 114b - 114j to the plurality of field effect transistors 78a, 78b - 78j. The noise input is further connected directly as shown by line 24a to the additional field effect transistors 86 and 88.

Referring now to FIG. 4, the function generator 96 may conveniently comprise a hold circuit 120 which periodically holds the binary, four bit input from the counter 50 for use by a one of ten decoder 122 as shown by lines 120a, 120b, 120c, 120d. The decoder 122 has its output lines 122a - 122j connected to the control electrodes of field effect transistors 124a - 124j, respectively. These transistors serve as electronic switches to control application of a voltage from a source 128 to respective resistors 130a, 130b - 130j. These resistors are connected through a resistor 132 to nominal ground and the output line 98. Selection of the resistors 130a - 130j, only one of which will be connected by its associated transistor to the voltage source at any given time, will result in a desired voltage output on line 98 for a given input to the hold circuit 120. Accordingly, resistors may be selected so that as the blade rate increases, the output pulses of the one-shot 100 will be widened, thereby enabling the gate 110 to pass noise for wider pulses on output line 82.

The resistors 80a - 80j are selected to attenuate the noise in accordance with a curve such as curve C. For any given analog inputs of pressure and blade rate, only one of the transistors 114a - 114j will be rendered conductive and the associated resistor, together with resistor 84 will provide an appropriate relative noise level output, modulated of course at the blade rate by the gate 110 and having pulse widths as determined by the function generator 96 and one-shot 100.

Present information indicates that as the speed of a submarine increases past the threshold of incipient propeller cavitation noise, the propeller beat phenomena becomes less noticeable although the level of cavitation noise increases. This is shown in FIG. 2c by the presence of noise between the beat pulses. The purpose, therefore, of additional transistors such as 86 and 88 is to include in the output unmodulated noise at the higher speed to square root of pressure ratios in addition to the propeller beat modulated noise.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. Apparatus for generating signals simulative of propeller cavitation noise from a source of first analog signals representative of water pressure P, a source of second analog signals representative of blade rate, and a source of band limited noise signals, said apparatus comprising:

first means for converting said first analog signals to a first frequency which is proportional to the square root of P;

second means for converting said second analog signals to a second frequency which is proportional to blade rate;

third means for periodically providing an output which is directly proportional to said blade rate and inversely proportional to the square root of P in response to said first and second frequencies;

fourth means for modulating said noise signals to provide pulses thereof at said blade rate and having a pulse width which is a function of said output of said third means; and

fifth means for attenuating said pulses in amplitude by an amount which is a predetermined function of said output of said third means.

2. Apparatus as defined in claim 1, and wherein:

said first means comprises first voltage to frequency converter means;

said second means comprises second voltage to frequency converter means;

said third means comprises counter means for providing a count in binary form of input pulses at said first frequency, first gate means for reading out said count, hold circuit means for holding said count after read out until a new count is developed, first and second delay means for effecting read out of each new count after resetting of said hold circuit and before resetting of said counter means; and

said fifth means comprising one-of-N decoder means for rendering a condition change at a predetermined one of N output connections of said decoder means corresponding to each count read out of said counter means, a plurality of attenuator elements, a plurality of electronic switching means each being responsive to a change of condition at one of said connections to connect a corresponding one of said attenuator elements in series with said fourth means.

3. Apparatus as defined in claim 2, and wherein:

said fourth means comprises third voltage to frequency converter means for converting said second analog signals to a series of pulses at said blade rate, function generator means for providing a voltage which is a predetermined function of the output of said third means, voltage variable pulse width one-shot means for rendering a train of pulses at said blade rate and having widths determined by the output of said function generator means, and second gate means for passing said noise signals to said electronic switching means.

4. Apparatus as defined in claim 3, and wherein:

said attenuator means comprises at least one additional attenuator element and at least one additional electronic switching means responsive to condition change at one of said decoder output connections to connect said additional attenuator element directly in series with said source of noise signals.