A report commissioned by the Bloody Sunday Tribunal states that it is possible for a person to mistake the noise of a helicopter blade for the firing of a sub-machinegun.
The report, carried out by the Institute of Sound And Vibration Research in England, said the Sioux helicopter, which was the type flown over the Bogside area of Derry on Bloody Sunday, produced a regular series of sharp beats or thumps commonly known as "Blade Slap".
"It most frequently occurs during transient manoeuvres and is often associated with steep turns, shallow descents and with the `flare' approaching a hover. Under certain conditions with some helicopters, continuous blade slap can occur in level flight", the report states. "Many times, blade slap noise has been mistaken for machinegun fire and vice versa," it adds. "The peak sound pressures from the blade slap received by a listener on the ground from a Sioux helicopter at a height of 1,000 or 2,000 feet were between 0.6 Pascals and 1.4 Pascals.
"A sub-machinegun would need to be a few hundred metres away for the peak pressures to be this low, or else shielded by buildings.
"The sound of individual blade slap impulses is subjectively lower in pitch than nearby gun fire and the character of the blade slap often changes perceptibly from slap to slap, whereas the repeated shots in a burst of submachinegun fire are more uniform. "Our opinion is, that under conditions where a helicopter or sub-machinegun is clearly audible above any background noise, their sounds would be sufficiently distinctive that the likelihood of confusion would be small.
"However, taking into account that the repetition rate of blade slap is similar to the rate of firing of some sub-machineguns, it might be possible under some conditions for an observer on the ground to mistake a brief period of helicopter blade slap for the firing of a sub-machinegun at a moderate distance", the report adds.
Meanwhile, the chairman of the Bloody Sunday Tribunal, Lord Saville of Newdigate, said yesterday he and his two colleagues, Sir Edward Somers and Mr William Hoyt, were becoming "increasingly restive" at the delay by the security services in providing the tribunal with threat assessments in respect of soldiers involved in Bloody Sunday giving their evidence in Derry. Mr Michael Mansfield QC told Lord Saville that he and his legal colleagues who represent the families of the Bloody Sunday victims as well as those wounded, believed it was essential that the legal representatives of the soldiers concerned, who would be arguing for a change of venue, should make known what their arguments would be.
Lord Saville said the tribunal was becoming "increasingly restive" about the delay. "One of the reasons is apparently that the tribunal indicated that it wanted the assessments to be in a form in which one could be compared with the other, that is the RUC one with the English security authority's one, because otherwise if they were using different language or conventions, it might be very difficult, if not impossible, to make any real sense out of them.
"We put that point up to the security organisations many weeks ago. We have since received a number of promises of estimated times of delivery. The latest is that these assessments will be delivered tomorrow. Bearing in mind the track record I cannot possibly suggest that is a warranted date, but you may be assured the tribunal has made it quite clear that it does require these assessments as a matter of the highest priority in order not to delay the proceedings of the tribunal", said Lord Saville.