Aircraft Noise Measurements Can Be Fun Steven Cooper - The ... -...

Aircraft Noise Measurements Can Be Fun

Steven Cooper - The Acoustic Group Typically in acoustic circles when one mentions aircraft noise assessments there is a consideration of undertaking an evaluation of noise intrusion into a residential building in accordance with Australian Standard 2021. Measurements for such the assessment of a development with respect to that Standard are not particularly exciting or fun. Over 30 years of conducting acoustic assessments there have been other forms of aircraft noise measurements undertaken which are somewhat more interesting and at times required significant deliberations or analysis to address complex acoustic issues. This talk to the NSW Division of the Australian Acoustical Society is intended to highlight some of the more interesting and thought provoking materials obtained from such measurements/investigations.

Helicopter Noise 1970’s Back in the late 1970s, acoustics in Australia was primarily governed by Australian Standard AS1055 and utilised instrumentation that some members of the audience may never have seen, such as measurements conducted with an analogue meter. Such analysis required observations of a needle whether it be A-weighted or octave bands and relying on what the observer heard. Digital analyser instrumentation was only available for government departments or those with vast funding where measurements may have also been recorded in the field on reel to reel tape recorders for subsequent analysis. I was involved in compliance testing for the helicopter at Channel 9 and noticed there was some variation in the noise level for different approaches and takeoffs. That led me to ask the pilot a few questions about the method of landing a helicopter, although in terms of acoustic criteria at the residential receivers, there was no particular issue. Some years later that same helicopter pilot had a charter business involved in picking up cheques at various regional centres and then delivering to the bank’s processing centre (well before the internet). This resulted in him using various helicopter landing sites that gave rise to complaints and required investigations. Testing revealed there could be different techniques in flying the helicopter that had significant variations in terms of noise. We worked out flight procedures to reduce noise. Move to 1982 and the State Pollution Control Commission (SPCC) introduced a set of helicopter noise guidelines that placed limits on the operation of helicopters when assessed at residential premises. The limits were presented in terms of the maximum level and a Leq level. To determine the Leq , the method of assessment utilised a graphic level recorder and marking off the A-weighted levels in one second increments to determine what was described then as SENEL (sound exposure noise event level) in Chapter 212 of the SPCC’s Environmental Noise Control Manual, which now, in modern days, has simply become a Sound Exposure Level (SEL). The major noise issue was associated with the landing phase. However, the noise criteria imposed by the SPCC caused severe restrictions in terms of both existing and proposed helicopter landing sites. In order to resolve the noise issues one had to understand how helicopters operate, I was taken up in a helicopter and given a basic flying lesson to better understand how it worked. Then the pilot and I went to a bush site to conduct measurements by flying the helicopter at a location similar to other helipads.

Aircraft Noise Measurements Can be Fun of 22

AAS NSW Division July 2011

What became apparent was that when pilots were taught to land helicopters they do not work on gradients or slopes as one thinks in engineering terms - they use a reference point (or gate) out from the landing site and basically fly through the gate to the helipad whilst adjusting the rate of descent, the forward speed and power settings of the helicopter. In many cases the techniques taught in those days was to follow a glide slope approach used at airports typical for private or commercial fixed wing operations. Using a direct line from a reference point to the helipad requires the forward speed of the helicopter be gradually reduced so that it is zero at the helipad, which means changes in the rate of descent and power through the entire landing profile. For helicopters, the main rotor twists ± 90 o through each revolution and generates both positive and negative pressure pulses off the end of the rotor. When one blade passes through the tip vortex shed by another blade at certain speeds and rate of descent this interaction can generate a distinctive ,thwat thwat’ noise. This noise in Europe is identified as blade vortex interactions, whilst in the US is identified as blade slap (see Figure 1). Blade slap is impulsive and results in a substantial increase in noise levels across the entire spectrum. Therefore, the concept, from an acoustic perspective, is to determine approach profiles that avoid the blade slap region. In those days the common commercial turbine engine helicopter was the Bell JetRanger II. The acoustic mechanisms for optimising the flight path were either to reduce the speed and the height quickly, and then transition at a lower level to the helipad, or to remain high above the typical glide slope whilst bleeding off speed and use a steeper approach with little power (see Figure 2). In developing the above techniques the JetRanger pilot essentially got to the point of flying by the seat of the pants. With experience the pilot could actually determine a critical area of concern before generating the noise impact. Because there are helicopters with the different types of rotor heads and different number of rotor blades, the generic profile determined for the JetRanger may not apply to other helicopter types. In investigating different helicopter types we have had to fly in the helicopters to determine the optimised flight profile - a little bit more interesting than standing there and conducting measurements. If you get to do any of this testing then the opportunity of a flight in a helicopter is somewhat of a plus factor and, if possible, get the front seat because it is much better than in the rear. After having carried out a significant number of helipad assessments I conducted workshops for the Helicopter Association of Australia (in Sydney, Brisbane and Melbourne) where the concept of those controls and EPA criteria were discussed. That then led to practical flying demonstrations where I had instruments setup and the pilots on the ground would be communicating with the pilots flying the helicopter. This assisted in obtaining an appreciation of the different techniques used to obtain different noise reductions/increases. Today it may appear that some of that work and noise restrictions for the Helicopter Industry were somewhat redundant because it has been established the SPCC did not have the power to control helicopters when in the air. The SPCC only had the power to control helicopters when on the ground. Subsequent acoustic assessments with respect to helipads have concentrated on the noise from the helicopter when on the ground and, accordingly, those investigations really weren’t that much fun anymore. Airship Noise Sydney has experienced a number of different airships. The early ones (identified as the “Bond” airship) were reasonably noisy whereas the second generation airships (the “Whitmans Chocolate”) are much quieter. Re-introducing the second generation airships to Sydney (for the 2000 Olympics) was the subject of review by the



Administrative Appeals Tribunal. There was a small amount of noise data to use for assessment purposes so we had to go and measure one airship operating in Melbourne. AirServices Australia went to the Gold Coast to measure the noise associated with a Grand Prix. There were no real acoustic issues with the proposed operations and the Airship operations were approved. After the work I carried out in analysing the Airship the UK owners (being aware of my involvement in Australian aircraft noise Standards) asked if I could assist with writing a Standard for the noise from Airships. By this time I had private pilot’s licence and understood a bit more about how aircraft flew but that didn’t help with an Airship. Therefore, I had no alternative but to arrange for a flight to understand the ballast and flight terminology for Airships. That experience was quite interesting as my flight had to fit in with their advertising schedule. I had no alternative but to go in the air for 2 hours flying around the northern beaches. When we flew over the northern beaches area I rang the staff at B & K and they came outside their Terry Hills Head Office to wave at us flying overhead. By the way, they couldn’t hear our airship noise either. After that flyimg experience, I took the ICAO certification test method for helicopters and re-wrote it for Airship terminology and added an environmental section. The document was submitted to the ICAO ANNEX 16 Committee by the UK CAA only 3 hours after I sent it out and later that day was passed by member nations as a working document. Helicopters in 2000 and the INM Some 10 years ago, I was requested to undertake work for the Department of Defence involving helicopters undertaking flight training exercises on a military base that resulted in a significant number of circuits. This was a precursor to including helicopters in the ANEF for that base. The INM model (version 6) in use at the time had a few helicopters in the database, but the data was in terms of the A-weighted results. No EPNL material was in INM for helicopters (at present the situation is still the same). . The A-weighted data was from the “Rainbow Series” conducted in the US in 1984 at Dulles International Airport that tested eight different helicopters for different flight profiles techniques, gradients, etc, with very comprehensive documentation that to an acoustician is certainly interesting to digest. But that documentation did not include the military helicopters of concern. We undertook testing of the military helicopters and had results both on-base and off-base with a preliminary view to providing input material to the INM database. That exercise started some very interesting investigations. The investigations are still on-going in a number of areas and have been the subject of a few acoustic issues to be discussed tonight. To conduct the measurements and then analyse the material to determine the Effective Perceived Noise Level (EPNL) is in itself a complicated task. What became interesting was that when that material was imported into the INM the persons running the program advised that our measurements were wrong because they couldn't agree with the INM output. Since the persons running INM for this project had 150% confidence in the accuracy of INM, we had to be wrong. Those persons were not trained in acoustics but certainly had a vast knowledge of military aircraft operations and the INM modelling. By the way, none of those INM people were a pilot or air traffic controller. This became somewhat of an interesting dilemma in that we had measurements of the helicopter flying at different levels and undertaking various operations which should agree with the INM predictions, this is, if the program was working correctly. Examination of the predicted levels from INM found anomalies that simply defy acoustic logic and created interesting discussions with the (US) INM programmers. According to our testing and the laws of physics for the helicopter producing a constant noise when we double the height (between the ground and the helicopter) then there would be a reduction in the noise levels, and the area of a



Leq noise level contour should reduce. However, INM actually increased the noise level and expanded the noise contour. So it was up to us to find where the problem was. If you look in the INM 6.0 Users Guide, Section 8.3.2,you will find the following equation in relation to lateral attenuation which, depending upon the angle between the receiver and the aircraft, allocates excess attenuation. As we change the height of the helicopter in relation to receiver positions on the ground for low level circuit work the angle of the receiver to the helicopter dramatically changes; hence the attenuation. When the airplane is on the ground:

G = 15.09(1 – exp(‐0.00274 D) ) 0 <= D <= 914 m

G = 13.86 D > 914 m

Where G is ground‐to‐ground attenuation (dB), and D is the horizontal lateral distance to the airplane (meters).

When the airplane is airborne:

L = (G/13.86) (3.96 – 0.066 β + 9.9 exp (‐0.13 β) 0 <= β <= 60

L = 0.0 60 < β <= 90

Where L is the total lateral attenuation (dB), and β is the elevation angle to the airplane (degrees).

Figure 3 shows the relative angles (β) for a slant distance of 10,000 ft for different circuit height, which from the above equations identifies allocated attenuations for each NPD location shown in Table 1.

Relative Angle

(degrees) 2.3 4.6 6.9

Lateral Attenuation

(dB) 11.1 9.1 7.5

Table 1. Lateral attenuation (dB) for 10,000 ft Slant Distance Calculated from the airborne equation Based on my post graduate research work I have found that it is appropriate to review the source material as sometimes the formulae may be based upon a series of measurements that may not relate to the use of concern or may be from a theoretical basis and “sound” principles. For the young acousticians I can thoroughly recommend the book Collected Papers by Wallace Clement Sabine to comprehend how, with the use of a stop watch, his observations and shear brain power determined the reverberation formulae. Reading the book, I was certainly humbled as to my capabilities. That book gave me a greater respect to research in acoustics and is the reason why I still question a lot of “findings and conclusions” proffered in many reports that I review. The research material upon which the lateral attenuation was established was done quite a few years ago (1965). It involved measurements of engines mounted in aircraft and engines mounted on test fames. For large distances, where the source is only a few metres above the ground, you would expect excess attenuation from ground absorption. However, when the aircraft is above the ground, say a few hundred feet, it is difficult to accept that you would be



getting the same degree of attenuation. The actual measurements suggest otherwise. In my opinion, when the aircraft are 500 feet above the ground or 1000 feet above the ground it is difficult to accept that a position, say 3000 ft to the side of the flight track, there would be an excess ground attenuation from the aircraft to the receiver location. To test the hypothesis we conducted a series of measurements. We were looking at a helicopter operating at different heights relative to a receiver position to compare the noise level that were recorded versus what was predicted by INM. That testing consistently found INM to under predict the helicopter noise for low heights. To make INM work I reverse engineered the lateral attenuation calculations and added back into the Noise Power Distance (NPD) curves to overcome the anomaly. Simply correcting the lateral attenuation allowance resulted in good agreement with the measurement results. I was then asked to look at conducting measurements to determine the NPD curves upon which an ANEF could be derived from the helicopter operations at a base. I could not find any material that identified how NPD curves had been measured or derived. I was aware of certification requirements that set rigorous regimes for aircraft operations in testing and therefore considered that we could utilise a series of microphones under a flight path to take the measurements and derive the NPD curves. I decided to try this method. Figure 4 Troop Carrying Helicopter – NPD Max Level presents the noise level from the aircraft of concern under various operating power settings for standardised distances used in aircraft noise calculations. To produce correctly the NPD Max Level curves it was necessary to determine the noise level at various distances. These distances are not vertical distances but are slant distances perpendicular from the nearest point of the flight track to the receiver location (as shown in Figure 5). This required determination of the position of the aircraft in three dimensions (vertical and horizontal displacements) and time, which must be synchronised with the sound level measurements. Obtaining the exact position of the aircraft is not an easy exercise and because we were dealing with multiple locations in different heights of the aircraft. The certification procedures (as has been used for certification in Australia) consisted of lying on the ground and having a camera to photograph the aircraft; at the same time a piece of rope suspended between two poles was used to scale the size of the aircraft. In my opinion, this is not exactly the most appropriate to the task at hand. We didn't have the resources to do the modern/sophisticated certification testing which involves laser instrumentation, radar tracking and Doppler shift tracking. Therefore, we had to find another way of obtaining the necessary aircraft position. We determined that, if using GPS tracking in the aircraft synchronized with another GPS on the ground, both synchronized, then you could find a position of the aircraft. We then found a more complex method Differential GPS (DGPS) was available at a cost. We had to arrange for the fixing of a DGPS antenna to the tested aircraft. After the testing we went through a somewhat involved process of extracting the data and converting it to reference locations. With the position information and the time coding of our signals we could conduct an analysis to determine the position of the aircraft relative to the microphone. Figure 5 AB Take off of twin engine military jet shows tracking information for a twin-engine military jet which operates at a much faster speed than a helicopter. At this point in the discussion may be used to identify where the aircraft may be at the time that creates the maximum level versus the “distance” (under certification procedures) that relates to that maximum level. When dealing with fast moving aircraft the speed of the aircraft versus the speed of sound dramatically alters the allocated position to be used in an NPD derivation. This highlights the complexity of the task and that it is simply not a matter of standing on an airfield and measuring the noise of the aircraft flying overhead. Furthermore, Figure 5 shows that at the position the aircraft was when it gave rise to the maximum level, may be the location used for calculation of atmospheric corrections the reference distance for that position is the slant distance. In dealing with published data in relation to helicopters it is necessary to understand not only the operational



procedures of helicopters but also the significant difference between normal operations and certification operations. Certification data will provide a higher noise level than that that will occur in normal operations. Figure 6 Helicopter Normal versus Certification Flight Testing provides a simplified diagram to show the difference in relation to normal helicopter operations versus certification operations. It is noteworthy to say that certification testing only utilises three microphones for determining the noise levels whereas NPD testing involves a greater number of microphones. Therefore, the NPD testing method is more precise and reflects the noise effects of the true operation of a given helicopter. I was given two aerodromes to a conduct the noise testing of military helicopters and at each of those sites I was given three helicopters to conduct the testing and derive the NPD curves. The logistics of taking control of a military airfield such as running cables up-and-down runways and across runways to a central recording system needed a team effort and certainly was an interesting (and sometimes challenging) exercise. For example, in the middle of measurements we were told to stop because the base required the main runway to be operational for an aircraft carrying senior military personnel… As the microphones were located directly under the flight path, the lateral attenuation issue does not occur in any of the calculations. On processing the data and utilising the correction factors that are set out in ICAO Annex16 (for certification procedures) we found two distinct lines of fit for the NPD. We obtained a linear regression for distances out to about 1,000 feet; but for larger distances the gradient of the curve changed dramatically. Furthermore, the curves did not appear to look like the dB(A) INM curves that have been nominated for the same helicopter. In result, we had three lines of fit with respect to the regression curves: two from our testing and the other from the INM prediction.. We had an inkling that the discrepancy was more related to high frequency attenuation. For testing of the other aircraft we would include additional testing to investigate this aspect. These issues, together with the lateral attenuation matter, were discussed with AirServices Australia as a possible mechanism for explaining the discrepancy in relation to an INM validation exercise conducted for the main flight path to Sydney airport. In the process of undertaking the tests for the purpose of obtaining NPD curves we had to consistently check the measurement results against what would be obtained from the program. In carrying out that exercise we uncovered a number of interesting outcomes that had not been identified anywhere else in published literature or guidelines for the use of the INM. When deriving NPD curves for helicopter operations where the speed is increased we find that the tail rotor generates more noise and for some aircraft results in an entirely different spectrum. That in turn changes the slope of the regression curve for the larger distances. If the speed of the aircraft is related to the power setting of the helicopter then for the increased speed/power the right-hand side of the NPD curve can cross over other curves for lower power settings because of the different rate of attenuation as a result of the different spectrum (as shown in Figure 7 NPD with Crossover). When generating an INM with NPD curves crossing over, the INM output collapses and produces unusable results as per Figure 8 INM output with Crossover. Slight modification to the NPD curves in Figure 7 eliminates the crossover and results in curves shown in Figure 9 NPD of Figure 7 without Crossovers and leads to the output shown in Figure 10 INM output for Figure 9 NPD. I attended the American aircraft standards noise committee (SAE-21) to discuss the issue of lateral attenuation and found that we had pioneered testing that simply had not been undertaken previously for helicopters. It was interesting to find that the NPD curves for civilian aircraft in INM had basically been taken from certification data utilising the ICAO reference points and then applying theoretical extrapolations to the standardised distance points and then variations in power settings. Military aircraft use NPD data from the NoiseMap Omega 10 output file. Military Jet Noise



With the proposed introduction of the Joint Strike Fighter aircraft (JSF) there were matters raised as to the accuracy of INM with respect to the military base operations. As the JSF was initially to replace the F111 aircraft we were tasked with conducting compliance testing of F111 operations and to see whether its operations agreed with the INM predictions. Firstly, we conducted testing at a busy operational base but found extraneous noise was an issue. Therefore, we were given a sporadically used base in the desert in South Australia where we had ambient levels less than 30 dB(A), an uninterrupted open terrain and no interfering noise from operations of other aircraft. As part of the testing we had microphones spread up to 12 nautical miles from the end of the runway and multiple microphones on diagonals off the threshold. I proposed a number of tests to evaluate the normal operations and a few atypical operations to continue our investigations into the INM I needed to test the difference in the noise from the aircraft at different heights for a constant power. We conducted one set of tests with the aircraft at full power but stationary on the ground where we determine the noise level over 180° (at appropriate safe distances) to determine an effective sound power level of 174 dBA for the aircraft on full afterburner (see Figure 11: Twin Engine Military Jet Polar Plot) The staff conducting occupational noise measurements at the wingtip and front of the aircraft (to ascertain noise levels for the ground crew) thought the measurements were pretty cool and were thrilled to put the testing down as “work experience”. I included in the test program to have a simulated take off at full afterburner but continuing the afterburner up to 18,000 feet. At all attended monitoring locations the personnel could clearly hear when the aircraft turned on its afterburner. As a side issue, Figure 12 Departure AB Profile demonstrates that about 500 m above ground level there is a noticeable change in the angle of climb. In this case it is relating to the undercarriage being fully closed in the body of the aircraft such that the aircraft was cleaner and therefore had a greater rate of ascent. Normally what happens is the afterburner is turned off before the airfield boundary and the aircraft continues to climb out on maximum military power, not afterburner power. When the aircraft was relatively close to the ground and we sought to derive the NPD curve for the afterburner operation we obtained similar results for determination of the effective sound power of the aircraft obtained in the stationary full power test. However, when applying the atmospheric attenuation nominated by ICAO to the measured spectrum (i.e. in reverse) the sound power level increased to such an extent that is shown in Figure 13 Twin Engine Military Jet – Departure on Full AB. The effective sound power level of the aircraft was greater than 500 dB(A) which is simply impossible to occur. In seeking to resolve the above anomaly we went back to the reference material that provided the atmospheric attenuation in terms of dB/100 metres which is referred to in various textbooks and noise models. I would expect that the measurement apparatus to determine such an attenuation coefficient would be relatively large. I was somewhat surprised that the testing, as described in the reference text books (see Figure 14 Test System to derive Atmospheric Attenuation), was conducted in a stainless steel sphere having a diameter of only 1.68 m. Are we to use results of this laboratory test in our airfield exercises where the distances between a source and a receiver is measured in hundreds and even thousands metres? Now we had an answer as to why there was an anomaly with the atmospheric attenuation calculations. To obtain measurement data to consider the possibility of what might be the correct answer to the lateral attenuation enigma, we re-examined the noise results for various aircraft testing and included in future programs level flights of various heights to evaluate noise propagation under different temperature and relative humidity. By no means were we funded to undertake such a research exercise. However, it is an important side issue in terms of the analysis that considered the possibility of what may be appropriate propagation coefficients.



Figure 15 Excess Attenuation – Military Twin Engine Jet and Figure 16 Excess Attenuation - Helicopter provide experimental test results for the F111 aircraft and a new helicopter yet to come into service in Australia. Examination of the experimental data and the formula from the work by Schultz (being the material from Figure 14) would suggest that the equation should be one of a power series along the lines of the dotted curves in the two figures and that significant errors occur after slant distances of thousand metres for the high frequencies. For the mid band and low frequency the error in the atmospheric attenuation is not as severe. Joint Strike Ffighter Invariably, people want to ask information about the Joint Strike Fighter (JSF). I have had the privilege in 2008 to physically touch the F35 AA1 aircraft (prototype JSF) and attend the extensive noise testing at the Edwards AFB in California, USA, initiated by the Australian client to obtain accurate data for the purpose of noise modelling. The technical material in relation to the noise testing is classified and is not permitted to be placed in the public domain; nor am I permitted to discuss various components of the testing or the results. However, the Australian Department of Defence has previously authorised the release of some of the material for my ICA papers which I am permitted to discuss. Back in the early 90s NASA proposed a testing for helicopters described as the Rotor Noise Model which involved flying the helicopter between two cranes from which were suspended microphones so as to determine a three-dimensional plot of the noise levels for the operating helicopter. Such testing is expensive and only a few helicopters had been tested under the procedure to evaluate that process. In the ensuring period the three-dimensional testing has been the subject of further refinement. In particular, its application to fixed wing aircraft has been considered with testing conducted on a number of other military aircraft. In addition to the derivation of NPD flyover results (for Australia) the Edwards AFB testing also included the provision of two 300ft (US measurement system) 90 metres cranes, with multiple quarter inch and half inch microphone arrays hung from each crane, to provide the 3D noise measurement data. When dealing with fixed wing aircraft the persons involved in such evaluation referred to the system as ANM (aircraft noise model). The following information summarises the complex arrangement that occurred for the test flights:

• Edwards AFB noise test is the most extensive test carried out to date on fighter aircraft – 60 microphones on two 300 ft cranes collect 3 dimensional measurements

• Winds typically less than 3 knots – nearly ideal for ground engine run-up and flyover noise data collections • Detailed weather measurements at 8 locations for flyover tests • Approximately 200 GB of raw data • 660 separate flight profiles • Each flight profile contains uniquely defined aircraft performance parameters and specific flight track

Apart from the three acousticians from the US Air Force Research Laboratory, two American acoustical consulting engineers, and two Ph D students researching shockwave phenomena, there was one acoustical engineer representing the Netherlands Defence Force and me. The remainder of the persons are military and defence personnel being the physical (forced) labour needed for the testing, and observers (see Figure 17 Edwards Air Force Base Test Flights). Experiencing the JSF flights and being part of the testing team was a unique and definitely a fun “aircraft noise” experience.



Aircraft Noise Monitoring As part of the ongoing research and validation of aircraft noise at military bases an advanced version of the civilian aircraft noise monitoring systems has been installed at various aerodromes in Australia. Due to the unique operations associated with military aircraft there has been a significant development in terms of monitoring and tracking military aircraft operations which have required acoustic certification/validation of the automatic testing regimes. This has required attendance at various military bases in Australia and conduct measurements leading to an evaluation of INM predicted levels. Certainly not as exciting as a JSF testing (at dawn out on a salt bed lake and, just by-the-way, watching the splendid sun rise) but there are certainly some interesting and challenging aspects as to the capability of the monitoring systems which are still the subject of further investigation for the acousticians. The attached Figures 18 - 21 provide a number of tracking outputs for aircraft operations into a base (Figure 18) and for a weapons range (Figures 19-21).

REFERENCES

Harris C, Absorption of Sound in Air in the Audio‐Frequency Range, JASA 35 (1) pp11 – 17, Jan 1963

Parkin P & Scholes W, The Horizontal Propagation of Sound from a Jet Engine Close to the Ground, at

Hatfield, J Sound Vib (1965) 2 (4), 353‐374

SAE 1751, Protection Method for Lateral Attenuation of Airplane Noise During Takeoff and Landing,

March 1981

SAE 1845, Procedure for the Calculation of Airplane Noise in the Vicinity of Airports, issued March 1986

SAE AIR 1989, Helicopter External Noise Estimation, May 1989

Integrated Noise Model, US Department of Transportation, Federal Aviation Administration

ISO 9613‐1:1993 Acoustics – Attenuation of sound during propagation outdoors – Part 1: Calculation of

the absorption of sound by the atmosphere, International Standards Organisation, 1993

North, A. & Kenna L, Sydney Airport, Joint Study of Aircraft Noise, February‐March, 1996, AirServices

Australia Report 1265

Fleming, Olmstead, D’Aprile, Gerbi, Guldng, Bryan & Mirsky, Integrated Noise Model (INM) Version 6.0

User’s Guide, September 1999 US Department of Transportation, Federal Aviation Administration

ICAO International Standards and Recommended Practices, Environmental Protection Annex 16 to the

Convention & International Civil Aviation, Vol 1, Aircraft Noise Third Edition 1997, Amendment 7

(2001)

Steven Cooper Acoustics, HMAS Albatross 2013 ANEF, Derivation of NPD Curves, SCA Report

33.4185.R111, 2003



Cooper, S, The INM program is a much better program than HNM for helicopter modelling, but ….

SAE‐A21 Helicopter Noise Working Group, Las Vegas 2004

Cooper, S, Noise Certification is the Helicopter Industry selling itself short?, HeliExpo 2004, Las Vegas,

March 2004

The Acoustic Group, Derivation of NPD Curves, F‐111, TAG Report 34.4421.R15, July 2006

Cooper S, Derivation & Use of NPD Curves for the INM”, Helicopter Noise Workshop, American

Helicopter Society Conference, June, 2005

Cooper, S, Problems with the INM: Part 1 – Lateral Attenuation, Acoustics 2006, New Zealand

Cooper, S, & Maung, J, Problems with the INM: Part 2 – Atmospheric Attenuation, Acoustics 2006, New

Zealand

Cooper, S, Problems with the INM: Part 3 – Derivation of NPD Curves, Acoustics 2006, New Zealand

Cooper, S, Problems with the INM: Part 4 – INM Inaccuracies, Acoustics 2006, New Zealand

The Acoustic Group, Tiger ARH NPD Curves, TAG Report 37.4510.R15, 2007

Cooper, S, INM Getting it to work Acoustically, 20th ICA 2010, Sydney

Cooper, S, Alternative Aircraft Metrics – Useful or like moving the deck chairs on the Titanic?, 20th ICA

2010, Sydney

Cooper, S, Military Aircraft Noise in the Community, 20th ICA 2010, Sydney

Lochard Webtrack – example for Sydney Airport http://www331.webtrak‐lochard.com/webtrak/syd3

Wasmer Consulting, NoiseMap, Version 7.353, Florida



FIGURE 1: Main Rotor Blade Slap Boundary – Typical Light Helicopters

FIGURE 2: Noisy Flight Operations – Typical Medium Helicopters



FIGURE 3: Relative Angles for a Slant Distance of 10,000ft

FIGURE 4: Troop Carrying Helicopter – NPD Max Level



FIGURE 5: AB Take off of Twin Engine Military Jet



FIGURE 6: Helicopter Normal versus Certification Flight Testing



FIGURE 7: NPD with Crossover

FIGURE 8: INM Output with Crossovers



FIGURE 9: NPD of Figure 7 without Crossover

FIGURE 10: INM Output for Figure 9 NPD



FIGURE 11: Twin Engine Military Jet Polar Plot

FIGURE 12: Departure AB Profile



FIGURE 13: Twin Engine Military Jet – Departure on Full AB

FIGURE 14: Test System to derive Atmospheric Attenuation



FIGURE 15: Excess Attenuation – Military Twin Engine Jet

FIGURE 16: Excess Attenuation - Helicopter



FIGURE 17: Edwards Air Force Base Test Flights



FIGURE 18: Military Jet Approach, Circuits and Landing

FIGURE 19: Military Jet Operations at Weapons Range



FIGURE 20: Expanded View of Figure 20

FIGURE 21: View of Figure 20 from Opposite Side

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