Another example of the importance of determining the noise transmission path is
demonstrated in the solution of an intense tonal noise in the cockpit of a fighter airplane which
was thought to be due to a pump, as the frequency of the tone corresponded to a multiple of the
pump rotational speed. Much fruitless effort was expended to determine the sound transmission
path until it was shown that the source was the broadband aerodynamic noise at the air
conditioning outlet into the cockpit and the reason for the tonal quality was because the cockpit
responded modally. The frequency of strong cockpit resonance coincided with the multiple of
the rotational speed of the pump but was unrelated. In this case the obvious lack of any
reasonable transmission path led to an alternative hypothesis and a solution.