A convenient and theoretically satisfactory method for measuring the sound attenuation of flying helmets is described in which a miniature microphone is employed to measure the sound intensity inside the helmet capsule. A theory of sound transmission through helmet capsules is proposed the validity of which is established by a direct comparison of theoretical with experimental results.It is thus possible to design helmets which are considerably better than standard R. A. F. types. Comparison tests on one such helmet are described.