V i s i t o r   r e s p o n s e s 

R e :   E r n a g e   -   D e c e m b e r   1 1 ,   1 9 8 9   
 Received : 10/27/07  Estimates of size and distance - 1   Roger PAQUAY   

According to your web site, when the craft turned and moved in the direction of the witnesses, it was at a distance of 1 km (0.62 mile or 3,280 feet), a distance that can be measured on the scaled map....  read more 

 Received : 04/20/08  Helicopter noise - 1   Martin SHOUGH   

In this case, an Army officer, Lt. Col. AMOND [now retired], and his wife, stopped their car on what Wim [VAN UTRECHT] describes as a "lonely road" and wound down the window with "ears pricked" to try and detect any sound from an object of evidently large angular size showing details of lit "panels" or windows as well as various other separate lights which circled apparently nearby for up to 8 minutes.....  read more 

 Received : 05/13/08  Helicopter noise - 2   Martin SHOUGH   

It is correct that an inversion will refract sound towards the surface, and the recommendation to avoid flying in an inversion makes sense therefore. But the inference that flying above the top of an inversion will have the inverse effect of abnormally refracting sound upward is unwarranted I think.....  read more 

 Received : 05/17/08  Helicopter noise - 3   Martin SHOUGH   

Re the Ernage case in particular and silent helicopters in general, I looked a bit further into the sound propagation question and helicopter noise sources. Perhaps this will help us decide the merits of the theory. Anyway I've collected some information for future reference - and just because it's interesting.

First, I wonder if the sound refraction diagram found by Wim [see the diagram in "Our reply" to Martin's message received 04/20/08], is too simplistic and possibly misleading for our purposes? Sound does not travel in "rays" like this of course (I realise we all know that). The rays are a device used to represent the wave normals of expanding wavefronts, and the shadow zone underneath the ray is also an idealisation. In practice I think the situation for an isotropic emitter is really more like this (sorry for rough sketch)

expanding wavefronts The ray diagram gives the impression of a clearly defined dead zone where no upward curving rays can go because they are intercepted by the ground, and some descriptions I've read also give this impression. But as far as I understand it the wave phenomena of diffraction, reflection, interference and scattering due to turbulence mean that it's much more complicated and that sound levels in the "shadow" are only ever partially attenuated from the free-field spherical (^1/2) loss.

Research in Austria by a Belgian university team (I didn't keep URLs for this source but we can google it), namely "Comparison of measurements and predictions of sound propagation in a valley-slope configuration in an inhomogeneous atmosphere", T. Van Renterghem and D. Botteldooren, Ghent University, P. Lercher, Medical University of Innsbruck, Austria found that:

"In the case of an upward refracting atmosphere, turbulent scattering into the acoustic shadow zone that is formed becomes important. Neglecting this effect often results in unrealistically large attenuations. (...) Based on experiments, it was found that the sound pressure level relative to free field propagation stays more or less constant in the acoustical shadow zone formed by an upward refracting atmosphere. This constant value depends on the geometry of the problem and on the strength of the turbulence. A value of -20 dB is common in acoustical literature. (...) [but] (...) The temperature profiles observed (...) contain upward and downward refracting parts [whose] influence ranges from -3 dBA to +10 dBA [+10 for the downward refracting parts]".

This was in a mountain valley where drainage winds and topography complicated the normal picture. As I understand it, the shadowing effect of upward refraction at distance from the source in temperature lapse conditions was mitigated (to only -3 dBA, which it's important to remember is a small difference in subjective loudness on the dBA scale, although it's 1/2 power on the normal dB scale) by the fact that the rising ground of the valley followed the refraction curve. Presumably the downward amplification was enhanced a few dBA by the same factor, but I don't find this to be very clear. Anyway it is clear that the refraction effects are generally only a few dBA, up to about 10 dBA.

That seems to fit with a similar study near Phoenix, Arizona. Arizona Transportation Research Center project SPR 555, "Evaluate the Atmospheric Effects Associated with Highway Noise Propagation" identifies 4 typical acoustic conditions that are well understood:

"(A) Uniform geometric spreading without any excess attenuation or amplification from refraction effects.
(B) Upward refraction creating an acoustic shadow zone with lower than normal sound levels. Sound levels with this condition will often be about 5 dB lower than for condition (A).
(C) Downward refraction creating an enhanced sound fields and higher than normal sound levels. Sound levels with this condition can be 5 to 10 dB higher than for condition (A).
(D) Sound focusing in localized regions. Depending on the amount of focusing, sound levels for this condition could be 15 to 20 dB higher than for condition (A) and more than 20 dB higher than for condition (B)

Note that the sound amplifications occurring in C and D (downward refraction by inversion and local focusing by more complex atmospheres) tend to be larger than the attenuation due to the shadow zone in B. This echoes the results found in the Austrian study (lapse attenuation -3 dBA; inversion amplification +10 dBA).

The Phoenix study found that in the local conditions at distances beyond a few hundred metres, where the effect of up- or down-refraction due to temperature gradients becomes significant, its contribution was typically limited at +/-10 dB.

So it would seem reasonable to infer that if there had been a fairly uniform temperature lapse at Ernage in fairly open level country it might be expected to cause an acoustic shadow where noise from a helicopter at ~1km is attenuated from (say) 70-80 dB down to somewhere in the range 60-70 dB, which is equivalent to about a doubling or halving of subjective loudness, though the log difference in power is much greater), perhaps more if conditions were just right. But attenuation from helicopter-like levels to anywhere near 0 dBA (the faintest normally detectable sound) just does not occur.

Anyway this is academic, because such a lapse condition, typical of a daytime atmosphere, is probably not the relevant one for Ernage.

Radiative cooling of the earth under a clear sky after sunset typically accompanies a change from lapse to inversion in the surface layer. This would be the normal tendency of the atmosphere. If we had no radiosonde data at all, an inverted gradient, or at least an isothermal gradient on the way to an inversion, would be our best guess. The measurements we do have support this.

As Wim says, it's unfortunate that the profile from De Bilt, the nearest ascent in terms of time, is not a useful profile above 188mAGL. Nevertheless within this surface layer the profile is that of an inversion (about 1.5°C/100 m) on top of an isothermal layer (i.e. averagely on the super-refractive side of neutral) and it looks very possible that this trend continues at least some distance into the unsampled 1230 m of overlying air. And there is evidence of general low level inversion conditions at each of the other stations Idar-Oberstein, Uccle & Essen. All of this leads one to think that a surface inversion would be likely at Ernage also.

So we should assume that some earthward refraction and amplification of sound is probable, not shadowing, and the crude figure of 70-80 dBA - based on the spherical-loss assumption - ought if anything to be a minimum, which would be enhanced further by favourable low level wind direction (light, but generally SSE according to Wim's synoptic chart - even SW/W at De Bilt within 15min of the sighting time) and small wind velocity gradients which were also generally favourable for downward refraction.

The only other factor (AFAIK) that we haven't yet considered is the attenuation by atmospheric absorption, which is proportional to temperature and inversely proportional to humidity. That is, warmer and drier air attenuates sound more effectively than cooler and moister air. This is why sound travels better at night when the air is cooler, and travels best of all at night in a damp atmosphere. So what about temp and humidity at Ernage?

The balloon station which is the closest to Ernage in terms of elevation above sea level is Essen only a few metres below Ernage (160 m) at 153 m. Surface temp here at midnight was +1.6°C, and the mean temp through the first three levels (330 m ascent) was +1.3°C. See temp figures for 4 stations tabulated below:

timesurf temp (°C)mean of 3 levels (°C)depth of 3 levels (m)
De Bilt418:00+3.4+3.8188
De Bilt400:00+0.6+0.3103
Idar-Ob.  37700:00-9.3-6.1145

Humidity figures (%RH) for the first 3 levels at the same 4 stations are:

LevelDe Bilt (18:00)De Bilt (00:00)UccleEssenIdar-Ob

These figures show that the air over the whole region was cold (close to freezing) and very humid (overall mean relative humidity 90%). Attenuation would be low. Sound propagation in these conditions should have been very good.

One other factor is sound frequency. Apparently much of the annoyance factor of chopper noise is the higher frequencies, which are most affected by atmospheric absorption, usually dropping off significantly beyond about 4-500 m. Higher frequencies occur mainly in the tail rotor, transmission and engine spectra. The attenuation of these high frequencies in particular will be minimal in the cold, humid air of Ernage.

Mid to low frequencies, coming mainly from the main rotor, go from the very low fundamental blade pass frequency (rate of blade rotation), which is in the order of 10 Hz, right across the spectrum of sensitivity of the human ear which has a peak at a few KHz. The ear is least sensitive to the lowest frequency parts, but this will be compensated somewhat because low frequency (long wave) sound is most susceptible to being amplified by down refraction. (In any case the published noise measurements are done in terms of A-weighted decibels, or dBA, which is a scale adjusted to compensate for the frequency spectrum of human sound response.)

Helicopter noise is highly directional. According to various sources, engine noise generally propagates vertically, much of it upwards. But the main components of the rotor noise - "thickness noise" (bulk air displacement by the rotor), "blade-vortex interaction" (BVI, when the advancing blade tip runs into the vortex shed from the previous plade tip) and "high-speed impulse" noise (HSI, which is the shock wave caused by the blade tip exceeding the speed of sound) - are projected into an ellipse extending predominantly forward and down (to a lesser extent rearward), and the vertical angle of peak noise occurs within a depression angle of about 15° from the helicopter nose.

All of which adds up to the following conclusions :

- a helicopter at lowish altitude approaching the Ernage witnesses (and to a lesser extent a helicopter receeding, during the circling and recession SW) has the sort of aspect, bearing and elevation most favourable for sound projection towards the witnesses;
- the atmospheric conditions of temperature, humidity and wind also appear to be most favourable for sound propagation;
- the witnesses arranged their own circumstances (engine off, window open, ears pricked) so as to be most favourable for sound detection.

Re the suggestion that the witnesses' own talking - or even shouting - could have drowned out the helicopter: The normal level of human conversation at 3 ft is about 65 dBA. It's even possible that they would have to be shouting to hear one another over the helicopter at an expected noise level in excess of 70 dBA. According to this "Helicopter Noise Analysis"

"U.S. Environmental Protection Agency guidelines for normal levels of voice effort and hearing ability at listener-speaker distances of about three feet (...) [show that] sentence intelligibility drops dramatically when the steady A-weighted sound level exceeds 65 dB. The following guidelines are conservative predictors of interference with conversation: Speech interference may occur outdoors when the sound level is 60 to 65 dB or higher (...) Speech interference may occur indoors, with windows open, when the outdoor sound level is 70 to 75 dB or higher. ...

HMMH obtained LAmax contours from the INM output for each aircraft type to examine the effects of the helicopter noise on normal speech intelligibility. Using the speech interference guidelines previously discussed, the 75-dB LAmax contours represent the area where the helicopters begin to interfere with normal communication levels indoors with windows closed for older construction not meeting current Title 25 requirements [i.e. presumably inside houses without double-glazing]. (...) levels normally exceed the 75-dB threshold along the route of flight when in level flight and when arriving and departing the helipad. Outside the 75-dB contour, the helicopter flight may potentially interfere with normal outdoor conversations. The arrival (from the east) tends to dominate the shape of the contour as the arriving helicopter has the 'blade-slap' effect and a shallower descent angle than the ascent angle on departure

BTW, "LAmax" represents individual peak noise events weighted for human audio frequency response, unlike some other time-averaged types of dBA measure that are used for various purposes. The whole dB/dBA thing can be very confusing and it isn't always clear what environmental and protocol variables have been factored in to certain measurements quoted, or what variables have been allowed for in comparisons of sound level. So I wouldn't be confident that my crude spherical-loss calculation of >70 dBA for a Super Puma approaching at 1 km is very accurate. An acoustics expert should really be consulted with all of this information - if anyone knows of one prepared to spend the time. But I think it's indicative and the trends of the environmental factors are probably reliable.

Hopefully some of this will be useful.

Strathconon, Scotland


 Received : 05/26/08  Estimates of size and distance - 2   Martin SHOUGH   

I'd like to point out that Mr. PAQUAY treats Lt. Col. AMOND's "2x moon diameter" light as relating to the angular size of a light source, when clearly this should be treated as an impression of the circle of glare (overloaded retina) caused by this "huge" and "brilliant" light shining into the witness's face......  read more 

 Received : 06/05/08  A response from the prime witness   André AMOND, Col. (Ret.)   

I would like to correct the approach of Mr. PAQUAY who claims that the observation of the "spotlight" was made while the craft was 1,000 m from my observation point. This is COMPLETELY WRONG.......  read more 

 Received : 06/17/08  Estimates of size and distance - 3   Roger PAQUAY   

In his response, dated June 5, 2008, Col. AMOND claims that my distance estimation of 1,000 m for the phase during which the craft turned towards him is completely false and that the real distance at that moment was 100 to 200 m. He also disputes my calculations and qualifies them as unscientific.......  read more 

 Received : 06/22/08  Not a helicopter - 1   Wilfried DE BROUWER, Maj. Gen. (Ret.)   

The Belgian airspace is surveyed by four powerful radars, two military and two civilian, which are all interlinked, i.e. any duty controller can select the image of any of these radars at any one time. All radar registrations are recorded and these recordings are kept during a well determined period......  read more 

 Received : 06/23/08  Witness reliability - 1   Jean-Michel ABRASSART   

A little comment about the strawman argument at the end of Wilfried DE BROUWER's email......  read more 

 Received : 06/23/08  Witness reliability - 2   Grégory GUTIEREZ   

In my opinion, the important information in DE BROUWER's e-mail is not in his last paragraph about the reliability of Col. AMOND. Of course human testimony is not always reliable, but Jean-Michel ABRASSART tends to say that it's NEVER reliable. I think he pushes his argument too far here, turning it into some kind of indisputable doctrine.....  read more 

 Received : 06/23/08  Plasma stealth chopper - 1   Jean-Pierre PHARABOD   

Now I am beginning to wonder : how could it be that the four Belgian radars did not detect these objects in the sky which were neither aircraft nor helicopters...  read more 

 Received : 06/23/08  Not a helicopter - 2   Martin SHOUGH   

Gen. DE BROUWER's first-hand testimony confirms not only that no helicopter should have been flying (as previously reported by Col. AMOND) but that, in point of fact, no illicit helicopter or other aircraft was detected by radars covering the area....  read more 

 Received : 06/26/08  Plasma stealth chopper - 2 / Map confusion   Prof. Auguste MEESSEN   

I will try to contribute to this debate by sharing with you that I have the following documents concerning the Ernage case....  read more 

 Received : 07/02/08  Recapitulating   Martin SHOUGH   

Re your reply to Prof. MEESSEN, you're right, the statement about flight plans in the Defence Minister's reply to the Parliamentary question is different from what we understood Gen. DE BROUWER to say....  read more 

 Received : 07/03/08  Estimates of size and distance - 4   Martin SHOUGH   

Thanks for Mr. PAQUAY's follow-up. I was not (as Mr. PAQUAY believes) suggesting that Col. AMOND was "dazzled" to explain a large image. As he says, an "overloaded retina" is not a very good explanation....  read more 

 Received : 07/06/08  Not a helicopter - 3   Wilfried DE BROUWER, Maj. Gen. (Ret.)   

Allow me to clarify a few points. 1. Your statement: "A helicopter is perfectly capable of executing such a manoeuvre". The report of André AMOND suggests that the object was making a tight turn with a considerable angle of bank (45 degrees?) at very slow speed (20-30 km/h?)....  read more 

 Received : 07/14/08  Helicopters noise - 4 / Banking angles - 1   Joe McGONAGLE   

I don't claim to have the detailed aeronautical experience of General DE BROUWER, but during my own military service, I did have some exposure to helicopters as a passenger and as an observer from the ground.....  read more 

 Received : 07/14/08  Helicopters noise - 5 / Banking angles - 2   Martin SHOUGH   

Thanks for your input Joe. Yes, clearly it can happen that when the conditions for sound propagation and other witness circumstances are unfavourable people can fail to hear helicopters......  read more 

   See our contact page to find out how you too can contribute.