Abstract :
Deafness and loud noise have the same effect on the reception of speech through telephone receivers: they increase the minimum distinguishable speech signal. When either condition becomes severe, satisfactory intelligibility is attained only if the speech is reproduced very loudly, and at times with intensities approaching those which give rise to a sensation of pain. This common feature is a reason for associating in the paper two otherwise distinct experimental researches. In the first part the range of hearing loss for which hearing aids may be useful is indicated, and their design then considered in a general way. A theoretical study indicated that, when speech signals have to be very loud in order to be distinguished, the greatest intelligibility can usually be expected if the amplification varies with frequency in a given way. Experiments which confirmed this conclusion by tests with a large number of deaf people are described, and the most useful amplification/frequency characteristic for a hearing aid is given. In the second part data are given concerning the noise existing in some armoured fighting vehicles, and the extent to which it may be excluded by ear-pads. The overall amplification/frequency characteristic of an intercommunication, or radiotelephone, system is not considered in such detail as is that for a hearing aid, but subjective tests are described which showed that, for the best results, the response should have no sudden changes with frequency.
