268 THE EAR threshold masking of the opposite ear must always be used because of the ease with which bone conduction sound is transmitted through the bones of the skull. In speech audiometry the ability to understand speech is measured. A recording of phonetically balanced word tests is used. These are played to the patient at increasing intensities and the proportion of words correctly repeated by the patient is expressed as a percentage; the results are charted (.Fig. 148). When hearing is normal all the words will be understood if they are 100 80 - I 60 £ 40 I 20 10 90 100 20 30 40 50 60 70 80 Speech Intensity-Decibels above normal threshold Fig. 148. Speech audiogram. 1, Normal hearing; 2, Conduction deafness; 3, Perceptive deafness. played loudly enough. In conductive deafness all the words will be under- stood but they will require to be played louder than for the normal subject. In sensorineural deafness 100 per cent understanding of words will not be achieved and increasing loudness above the optimum may produce a reduc- tion in understanding. If increasing the loudness can produce an understand- ing of more than 40 per cent of the words, it may be predicted that 90 per cent of sentence intelligibility can be obtained. This gives a useful method of assessing the degree of disability produced by the deafness and is also helpful in predicting the usefulness of a hearing aid or the benefit which might be obtained by an operation. Tests for Recruitment Recruitment is the inability to hear quiet sounds, yet loud sounds can be heard as loud or even louder than normal. Recruitment produces diminished understanding of speech and intolerance of loud sounds, particularly when a hearing aid is worn. Recruitment is present when the cause of the deafness is in the organ of Corti. Demonstration of recruitment may be used to localize the site of the lesion in sensorineural deafness and, in particular, to differentiate deafness due to a cochlear lesion from that due to an auditory nerve lesion. Recruitment in unilateral deafness is best demonstrated by the Fowler's Alternating Loudness balance test in which a tone is played alternately into the normal ear and into the deaf ear, the intensity in the deaf ear being adjusted until the sounds in both ears are heard equally loudly (Fig. 149).