Just noticeable differences of loudness and sharpness for earth moving machines

This paper describes some results of a research concerning the improvement of the noise climate at the operator station of construction machines during real working conditions. Binaural noise signals were previously recorded and then used in subjective listening tests aimed at identifying the set of acoustic and psychoacoustic parameters which affect the auditory perception of these signals with respect to the annoyance sensation. Results showed that loudness and sharpness are the parameters best correlated to the annoyance. In order to verify the efficacy of some noise control solutions in improving the operator comfort conditions, the next necessary step is to build a specific metric able to show the minimum differences in these parameters which are subjectively perceived. This paper describes the results of specific listening tests carried out in order to evaluate the differential thresholds of loudness and sharpness by the Method of Limits. The subjects were asked to detect the just noticeable differences for both loudness and sharpness sensations, the step size of the stimulus being 0.3 sone and 0.02 acum, respectively. The test was repeated at three different signal presentation levels. Results show that the just noticeable difference in loudness becomes greater as the overall sound pressure level of the signal increases. On the contrary, the just noticeable difference in sharpness has very small variations with the overall level. Focusing on the highest presentation level, 75% of subjects perceives a different sensation when sounds have a loudness difference of at least 0.8 sone and a sharpness difference of 0.04 acum.