You're getting into something that will effect us all. In Michigan there is a joint project funded by the Michigan Legislature and the Michigan Lake and Stream Association which is working with Professor Radcliffe at MSU to develope a "Noise Gun". This gun will measure noise at any distance up to 300 feet, measure distance, and measure background noise. It will then calculate the noise being produced by the boat and give a reading to an officer who will then be able to issue a ticket based on the calculated reading. The gun has been developed anbd is currently under US Patent submission. If approved, Michigan is planning to change their law to mimic Wisconsins. The reasoning is it is too dangerous to set up the 25 foot course and have a boat drive through at wide open throttle, this gun will measure and calculate what the boat would have done in the test. Or the 75DB from shore. Fun reading. This is the short version.
FIELD OF THE INVENTION
 The present invention relates to noise measurement equipment and, more particularly, to equipment configured to repeatably measure noise from a moving object at variable distance.
BACKGROUND & SUMMARY OF THE INVENTION
 Excessive noise levels on lakes are the source of many community problems. Given the increasing numbers of boats on lakes, shoreline residents who wish to maintain a peaceful environment have a vested interest in controlling noise pollution.
 Current standards for measuring noise levels of boats are written in various State Acts. The current standards often cited are testing procedures SAE J2005 (SAE 1991b) and SAE J1970 (SAE 1991a), which tend to discourage the state law officials who try to apply and enforce them. The SAE J2005 standard requires that the target boat be tethered to either another boat or a dock. The engine motor is set to idle, and the measurement is taken three feet away. This requires extraordinary cooperation not only from the vehicle operator, but also from all other boats in the immediate environment. The SAE J1970 standard is a shoreline-based measurement of the boat noise. The measurement is taken from the shore, as long as the boat is not within 30 seconds of leaving or returning to shore. SAE J1970 can only be used when the offending boat is alone on the water and near to shore, thus making citation of the offending vehicle operator difficult.
 The J2005 standard is intended as a stationary test for motorboats. This test is designed to determine whether a boat's muffling system is adequate to reduce the sound power of the boat. The basic procedure is as follows:  The boat must be docked or tied to another boat.  The boat must be in a neutral gear, or at its lowest idle speed possible.  The microphone must be placed 1.2 to 1.5 m (4 to 5 ft) above the water, and no closer than 1 m (3.3 ft) from the boat itself.  The background noise level must be at least 10 dB lower than the level of the boat. The J2005 test was designed mainly because stationary tests are easier to conduct than tests performed while boats are in motion. There are many problems with stationary tests, however. The process of identifying a noisy boat, chasing it down, lashing it to a police boat, and then administering the test is long and cumbersome, besides being impossible to conduct in rough water. Furthermore, newer boats have a "captain's choice" exhaust, which allows boat operators to switch between underwater exhaust and unmuffled air exhaust. Obviously, when a test is administered, a captain would switch to the quieter underwater exhaust system. The J2005 test simply does not address the problem of unreasonably noisy boats on the water.
 The J1970 standard is meant to test the sound level of boats as perceived on the shoreline by riparian owners, the originators of most of the complaints. The basic procedure in this test is:  The measurement must be taken either on shore or on a dock not more than 6 m (20 ft) from shore.  The microphone must be placed 1.2 to 1.5 m (4 to 5 ft) above the water.  There is no distance requirement to the target boat. The boat must not be measured 30 seconds after it launches or its last 30 seconds coming into the dock.  The background level must be at least 10 dB lower than the level of the boat. Theoretically, the use of a shore measurement should be enough to satisfy shoreline residents regarding boat noise. However, there are problems with this test as well. First of all, it is not easy to perform unless there is only one boat on the water, which never happens in the busy summer season. Secondly, the noisy boat can be over a mile away, but this test is impossible to conduct at such a distance. These tests, if enforcement were easy to carry out, would alleviate some of the noise problem. However, the many intricacies of measurement procedures, coupled with the variables that affect noise level which are not taken into account, produce loopholes so that citations can be challenged successfully in the court. Therefore the J1970 and J2005 standards are inadequate for enforcing a noise measurement standard for boats in-use on the water.
 The SAE J34 standard (SAE 2001) is referred to in the laws of fourteen states. It seeks to provide a comprehensive test to determine the maximum sound level of the boat in use. A summary of this test is as follows:  A test site must be created as shown in FIG. 1.  The boat must pass within of 3 m (10 ft) on the outside of the buoys.  The boat must be at +/-100 rpm of its full throttle rpm range.  The microphone must be placed 1.2 to 1.5 m (4 to 5 ft) above the water, and no less than 0.6 m (2 ft) above the dock surface.  The background level must be at least 10 dB lower than the level of the boat.  The wind speed must be below 19 km/h (13 mph).  The peak reading as the boat completes the course shall be recorded.  Two readings will be made for each side of the boat.
 The J34 standard successfully measures the peak in-use noise level of the boat as it traverses the course. However, the complexity of setting up a course, and the range of variables which must be recorded, necessitate that there must be an officer in the boat as well as on the dock. It requires skillful piloting, extremely calm conditions, and patient and qualified officers to administer the test. This standard is meant as a way for boat manufacturers to certify their boats are compliant with noise standards, rather than as an on-lake test for noisy boaters. The J34 test provides a measured value that is related to the sound power of the boat. However, the difficulty in administering this test limits its usefulness for enforcement of noise statues.
 The solution to the noise standards enforcement problem lies in the creation of a noise measurement standard, which allows the accurate measurement of the in-use noise level of a boat or vehicle. The goal of the standard is to compute a value representative of the acoustic power of the noise source without requiring operator cooperation. Enforcing this standard would require a device that would compute a value representative of the noise level of a boat unaffected by distance, background noise level of other boats, and weather. In order to compute this representative value of acoustic power, a model of sound propagation is needed. With this model, and a distance measurement, the point sound pressure (dB) measurement can be related to the sound power of the boat. A measure of the background noise level is also needed so that the influence of other noise sources can be removed from the measurement. The purpose of this work is to create such a "sound measuring device." This device, coupled with redrafted statues, would finally allow law enforcement officers to enforce a reasonable noise level standard not only for boats, but for ATV's, snowmobiles, and other vehicles.
 A noise measurement device that is more advanced than a standard noise meter is required by the new standard. The meter will need to output a predicted minimum possible sound level of the boat at a standard distance away, after compensating for various possible measurement errors including, but not limited to, background noise and noise propagation characteristics. This will allow for measurements to be compared to other boat measurements no matter how far the meter operator is from the boat. Sound propagation and measurement techniques need to be reviewed in order to make this prediction.
 The noise measurement device will need to do these predictions in an invisible manner from the operator. The corrected noise level at a standard distance away will eliminate the problems of reliability of the measurement. The integration of this procedure into a single electronic instrument will eliminate the difficulty in use that plagues the other standards. With this device, the new noise measurement standard can be used easily for law enforcement.
 The noise measurement device utilizes how sound propagates over water for noise level calculations. As such, it is an object of the invention to measure the sound level at the position of the observer, and convert it into what the equivalent sound level would be at a standard distance from the source. Two models of how sound propagates are used to develop the noise measurement device, a point source and an infinite plate source. The affect of background noise on the measurement of sound level is determined.
 Spherical sound propagation is one way to idealize the acoustic propagation field produced by a boat. The point produces a level of acoustic power, which then propagates uniformly away from the point over a sphere. The acoustic power is assumed constant at any distance from the point. The energy is spread over the sphere of radius r from the sound origin. This derivation is based on standard sound propagation theory.
 The acoustic intensity (I) is an energy flux (W/m.sup.2). The acoustic power (P.sub.source) is the integral of that flux over some sphere (radius r, surface area A) surrounding the source, P.sub.source=.intg.IdA=I(4.pi.r.sup.2) (1) Acoustic intensity is related to the square of the acoustic pressure (p), where .rho. is the air density, and c is the speed the wave (sound), I = p 2 .rho. .times. .times. c ( 2 ) Relating the acoustic power to the acoustic pressure with (1) and (2), p 2 .rho. .times. .times. c = P source 4 .times. .pi. .times. .times. r 2 ( 3 ) The relationship between the ratios of pressures to the ratio of distances is determined using: p 2 p 1 = .rho. .times. .times. c .function. ( P source ) / 4 .times. .pi. .times. .times. r 2 2 .rho. .times. .times. c .function. ( P source ) / 4 .times. .pi. .times. .times. r 1 2 = r 1 r 2 ( 4 ) Sound pressure level is a function of acoustic pressure. It is specified in decibels, defined as SPL = 20 .times. log 10 .function. ( p p ref ) ( 5 ) where p.sub.ref is a reference pressure (2.times.10.sup.-5 Pa), The change in sound pressure level for two points is .DELTA. .times. .times. SPL = SPL .function. ( r 2 ) - SPL .function. ( r 1 ) = 20 .times. log 10 .function. ( p 2 p ref ) - 20 .times. log 10 .function. ( p 1 p ref ) = 20 .times. log 10 .function. ( p 2 p 1 ) = 20 .times. log 10 .function. ( r 1 r 2 ) ( 6 ) For a doubling of distance, r.sub.1=1 and r.sub.2=2, the .DELTA.SPL is -6 dB.
 An infinite plane source is the other limiting case of a boat sound propagation field. This approximation can be used very close to the boat. In this case, an infinite wall radiates sound at the same intensity at every point on the wall. Since the sound wave travels linearly away from the wall and does not expand, the sound intensity from an infinitesimal patch radiates over a rectangle. The acoustic power remains constant since the area is constant, and therefore the sound level at any point away from the wall is constant.
 The acoustic intensity (I) is an energy flux (W/m.sup.2). The acoustic power (P.sub.source) is the integral of that flux P.sub.source=.intg.IdA=I(A) (7) Acoustic intensity is related to the square of the acoustic pressure. I = p 2 .rho. .times. .times. c ( 8 ) Relating the acoustic power to the acoustic pressure with (7) and (8), p 2 .rho. .times. .times. c = P source A ( 9 ) The relationship between the ratio of pressures to the ratio of areas is determined by: p 2 p 1 = .rho. .times. .times. c .function. ( P source ) / A 2 .rho. .times. .times. c .function. ( P source ) / A 1 = A 1 A 2 ( 10 ) Since the surface is infinite, the acoustic energy radiates outward into the same area at every radius from the surface. So A.sub.1=A.sub.2, and p 2 p 1 = A A = 1 ( 11 ) The change in sound pressure level for two points is .DELTA. .times. .times. SPL = SPL .function. ( r 2 ) - SPL .function. ( r 1 ) = 20 .times. log 10 .function. ( p 2 p ref ) - 20 .times. log 10 .function. ( p 1 p ref ) = 20 .times. log 10 .function. ( p 2 p 1 ) = 20 .times. log 10 .function. ( 1 ) = 0 ( 12 ) The change in sound pressure level, .DELTA.SPL, is always 0 dB.
 The National Marine Manufacturers Association (NMMA 1987) set out to measure the sound propagation field from motorboats. A boat is neither a point source nor an infinite wall source; its decay is somewhere between these two cases. These tests measured the propagation decay of a group of actual boats. Sound level meters were placed on poles at 50, 75, 100, and 200 feet away from a straight buoy course that the boat traversed. This allowed simultaneous readings of the boat noise at different distances. This test was conducted for a wide range of boats (with horsepowers from 10 to 370) in a single set of conditions. FIG. 5 displays the data gathered in this experiment.
 The Marine Manufacturers Association study determined experimentally that on average boats had a 5 dB drop per doubling of distance. Further testing is needed to determine the average sound level decay for most watercraft, since this set of testing was not as rigorous and complete as is needed to stand up to court challenges. The data shows that most vehicles exhibited a decay value of between 4 and 6 dB per doubling of distance.
 FIG. 6 displays the differences between various decay values. If a boat noise level is 85 dB at 200 feet, which model used would affect your prediction of the sound level at 50 feet. If, for example, the model assumed that sound decayed at 4 dB per doubling of distance, the model would predict that the boat would be 93 dB at 50 feet. If the model assumed a 6 dB drop, the model would predict 97 dB at 50 feet. It is important to determine what this decay value is in order for the device to make an accurate prediction.
 Background noise is key to making a precise noise measurement. A noise source can only be measured when it is louder than the surrounding noise level. Even when the source is above the background, the reading taken from a source is a combination of the source noise and the background noise. The SAE noise standards only allow measurements when the measured source is 10 dB higher than the background. Because of this, the sound measuring device must correct for the background sound level.
 Since the noise reading is a linear combination of the sound intensities from the background and the source, we can subtract the background contribution. The total mean squared measured sound y.sub.m is the sum of the source y.sub.s and background noise y.sub.b, for a broadband random noise, y.sub.m=y.sub.s+y.sub.b (13) This total measured sound Y.sub.m is expressed on a decibel (dB) scale as Y.sub.m=20log.sub.10(y.sub.m)=20log(y.sub.s+y.sub.b) (14) where the measured background level Y.sub.b Y.sub.b(dB)=20log.sub.10(y.sub.b) (15) and the desired sound source level Y.sub.s Y.sub.s(dB)=20log.sub.10(y.su- b.s) (16) Solving for the source and background levels in (13) yields y.sub.b=10.sup.(Y.sup.b.sup./20) (17) y.sub.s=10.sup.(Y.sup.s.sup./20) (18) These results can now be substituted into (16) and (13) to solve for the source pressure y.sub.s and source level in decibels Y s .function. ( dB ) = 20 .times. log 10 .function. [ y s ] = 20 .times. log 10 .function. [ y m - y b ] = 20 .times. log 10 .function. [ 10 ( Y m / 20 ) - 10 ( Y b / 20 ) ] ( 19 ) Rearranging (19) to collect terms and compute compensation in dB, Y s .function. ( dB ) = 20 .times. log 10 .function. [ ( 10 ( Y m / 20 ) ) .times. ( 1 - 10 ( Y b / 20 ) 10 ( Y m / 20 ) ) ] = 20 .times. log 10 .function. ( 10 ( Y m / 20 ) ) + 20 .times. log 10 .function. ( 1 - 10 ( ( Y b - Y m ) / 20 ) ) ( 20 ) This compensation equation (20) can now be written as Y.sub.s(dB)=Y.sub.m+C (21) where the compensation C=20log.sub.10(1-10.sup.[(Y.sup.b.sup.-Y.sup.m.sup.)/20]). Because the argument of the log function is always less than 1, the compensation C will always be negative.