Biomedical subjects
Publications and source records attributed to M S Shipton.
A number of new British Standards have been published which affect users of audiometers and audiometric calibration equipment. These are identical to their international counterparts, giving us agreed values for thresholds of hearing by both air and bone conduction. This article draws attention to the important changes that have taken place and looks forward to likely future changes.
The diaphragm of a condenser microphone is exceedingly fragile and, in normal use, is covered by a protective grid. However, when the microphone is incorporated into the IEC 303 reference coupler for the calibration of audiometers or audiometric earphones, the Standard demands that the diaphragm be left exposed. Nevertheless, in order to avoid inadvertent mechanical damage, some users leave a protective grid on the microphone. The effects of two particular grids have been investigated and the magnitude of the resulting errors is indicated.
Air-(ac) and bone-conduction (bc) thresholds were determined for 96 subjects in the age range 16-30 years. The bc tests were carried out using a B-71 vibrator applied to left and right mastoid under three conditions at the contralateral ear: unoccluded, masked at 25 dB and at 40 dB sensation level; ac audiometry and masking noise used a TDH-39 earphone. By a systematic process of otological classification of the subjects, the bc threshold for 'normal hearing' could be estimated with an uncertainty of less than 1 dB and interpolated for masking noise levels from zero to 40 dB sensation level. Results are expressed as alternating force levels in an artificial mastoid of current commercial design. Variance of the air-bone difference for individual ears is found to be less than the sum of ac and bc threshold variances calculated separately, implying strong air-bone correlation. Comparison is made with other recent studies using B-71 and KH-70-type vibrators, and average results are presented in the context of a proposed international standard for the normal threshold of hearing by bc.
The international standard of audiometric zero for normal hearing by air conduction was originally given only for laboratory reference earphones. Subsequently, it has been extended to cover earphones in general audiometric use, and in other ways. A new determination is described in which these derived standards are tested by comparison with the measured thresholds of hearing of 108 subjects in the age range 18-30 years. Particular attention is paid to the otological classification of the subjects and its relation to the description of otological normality in the ISO standards. Suggestions for adjustment of the standards at certain frequencies are made.
The effect of air-borne sound radiated from transducers used in bone conduction audiometry has been known for many years. This paper seeks to quantify this effect for the vibrator types (B70A, B71, and B72. Using three examples of each vibrator sound pressure levels were measured at the entrance to the external ear canal on four human heads. Values obtained have been related to hearing theshold levels for otologically normal young persons using two different methods: firstly using international standard values of open-ear free-field theshold with corrections for monaural listening and for diffraction, and secondly by direct measurement of the subjects' monaural open-ear thresholds. None of the vibrators tested meets international standard requirements for air-borne sound radiation at all frequencies, but in practics, provided a high quality ear plug is fitted to the test ear at 3 and 4 kHz large errors are unlikely to be recorded for any of the vibrator types tested.