During the war period, effort has been concentrated on the design of military aircraft, in the performance of which operational efficiency has been an overriding consideration. Speeds and powers have increased, resulting in a greater production of noise; but since noise reduction usually involves some loss of performance, perhaps from the additional weight of a soundproofing treatment, or the slight loss of engine efficiency which may accompany the use of an exhaust collector system, noise has had to be largely ignored. In the military machine, moreover, the incidence of noise on the ears of the crew is mitigated to an appreciable extent by the use of flying helmets incorporating well fitting earphones–a measure which it is hardly practicable to apply to civil machines.