Eight subjects were required to localise a sound source (white noise through a speaker) which varied in position on both sides of the head over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degree to 180 degrees) at 10 degrees intervals. The perceived position of the source was indicated by pointing a special gun. Depression of the trigger activated a photographic system which recorded two views of the subject, the sound source, and the gun. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. In a second experiment, four of the same subjects performed the same task but in this case visually located the sources. This experiment provided an estimate of inherent motor error in the pointing task. No differences in localisation acuity between sides were found, but there were significant differences between front and back regions. Azimuth and elevation error were well matched and low in the front. However, azimuth error increased in the regions behind the head, particularly for azimuth positions 120 degrees to 160 degrees. Larger increases were found for positions in the upper elevations of this region. Elevation error also increased in the upper elevations behind the head. A comparison of the auditory and visual data indicates that this pattern of error is not due to motor factors. The results are discussed in relation to the structural characteristics of the pinnae and modifications that they impose on incoming sound energy.

The acuity of azimuth and elevation discrimination was measured under conditions in which the cues to localisation provided by the pinnae were removed. Four subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degree to 180 degrees), at 10 degrees intervals, on the left side of the head. Pinna cues were removed by the insertion of individually cast moulds in both pinnae. Each mould had an access hole to the auditory canal. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The variability of azimuth and elevation error was also computed. The performance of the subjects was compared to their performance under normal hearing conditions. Insertion of the pinnae moulds was found to increase substantially elevation error and the number of front/back reversals. The importance of the cues provided by the pinnae in these discriminations was thus confirmed. However, the increase in elevation error did not result in a corresponding increase in azimuth error. These findings provide support for the proposition that azimuth and elevation discrimination are coded independently.

No abstract

The ability of subjects to detect and discriminate spectral peaks and notches in noise stimuli was determined for center frequencies fc of 1 and 8 kHz. The signals were delivered using an insert earphone designed to produce a flat frequency response at the eardrum for frequencies up to 14 kHz. In experiment I, subjects were required to distinguish a broadband reference noise with a flat spectrum from a noise with either a peak or a notch at fc. The threshold peak height or notch depth was determined as a function of bandwidth of the peak or notch (0.125, 0.25, or 0.5 times fc). Thresholds increased with decreasing bandwidth, particularly for the notches. In experiment II, subjects were required to detect an increase in the height of a spectral peak or a decrease in the depth of a notch as a function of bandwidth. Performance was worse for notches than for peaks, particularly at narrow bandwidths. For both experiments I and II, randomizing (roving) the overall level of the stimuli had little effect at 1 kHz, but tended to impair performance at 8 kHz, particularly for notches. Experiments III-VI measured thresholds for detecting changes in center frequency of sinusoids, bands of noise, and spectral peaks or notches in a broadband background. Thresholds were lowest for the sinusoids and highest for the peaks and notches. The width of the bands, peaks, or notches had only a small effect on thresholds. For the notches at 8 kHz, thresholds for detecting glides in center frequency were lower than thresholds for detecting a difference in center frequency between two steady sounds. Randomizing the overall level of the stimuli made frequency discrimination of the sinusoids worse, but had little or no effect for the noise stimuli. In all six experiments, performance was generally worse at 8 kHz than at 1 kHz. The results are discussed in terms of their implications for the detectability of spectral cues introduced by the pinnae.

The only difference between nanotechnology and many other fields of science or engineering is that of size. Control in manufacturing at the nanometre scale still requires accurate and traceable measurements whether one is attempting to machine optical quality glass or write one's company name in single atoms. A number of instruments have been developed at the National Physical Laboratory that address the measurement requirements of the nanotechnology community and provide traceability to the definition of the metre. The instruments discussed in this paper are an atomic force microscope and a surface texture measuring instrument with traceable metrology in all their operational axes, a combined optical and x-ray interferometer system that can be used to calibrate displacement transducers to subnanometre accuracy and a co-ordinate measuring machine with a working volume of (50 mm)3 and 50 nm volumetric accuracy.