Noise monitoring and extraction of new information from older instruments and archives

Several thousand permanent monitoring terminals have been deployed around the world for
monitoring aircraft noise. A large proportion of these were designed more than twenty years ago,
primarily to record A-weighted levels, but later enhanced to include levels pertaining to C- and Zweighting,
third octaves, effective perceived noise, stationary loudness, and more recently, nonstationary
or dynamic loudness. In addition, acoustic recording of events with levels above a
prescribed threshold and neural network classification were added. Noise events are frequently caused
by more than one type of sound source, so that it becomes important to quantify the aircraft
contribution to the noise impact. In addition to the enhanced facilities of noise monitoring terminals,
the sonograms of the acoustically recorded noise events prove to be especially useful. In many cases a
quick glance at the sonogram will reveal the presence of several other noise sources, such as barking
dogs, loud birds, trains and other ground-based sources. Moreover, the sonogram permits the speed of
trains and aircraft, with surprisingly good agreement with aircraft speed data obtained from radar, to be
estimated. Distinctions between different types of jet, turbo-prop and helicopter aircraft show up
readily on the sonograms. In this paper, examples of sonogram evaluations will be presented, including
cases where non-aircraft noise events have been wrongly ascribed to certain aircraft movements, or the
wrong type of aircraft has been ascribed to the noise event. The methodology can similarly be applied
to other situations of environmental noise to reveal more details of the noisescape.