I was told in about 1973/4 that there had been a trial at Dallas for the use of an extremely steep approach angle in order to bring the noise footprint of a B707 to within the airfield boundary ... I'm fairly sure the angle was into double figures!!
In the same seminar, run by Southampton University, we were told of the experiment in which a B707 was used to measure pure airframe noise in fight by turning off all 4 engines (I don't know about the APU). It was only attempted once ... the pilot refused to do it again.
