in case of ascending vs. descending sequences has a diagnostic importance. In the final step, the average of the previously calculated means will result in the absolute threshold. Method of constant stimuli: Stimuli of varying intensities are presented in random order to a subject. Intensities involve stimuli which are surely subthreshold and stimuli which are surely supra-threshold. For the creation of the series, the approximate threshold judged by a simpler method (i.e.: by the method of limits). The random sequences are presented to the subject several times. The strength of the stimulus, perceived in more than half of the presentations, will be taken as the threshold. Adaptive method: Stimulation starts with a surely supra-threshold stimulus; then further stimuli are given with an intensity decreased in previously-defined steps. The series is stopped when the stimulus strength become subthreshold (this is called the turn phenomena). Then the step is halved, and the stimulation is repeated, but now with increasing intensities, until the subject perceives the sound again. This process is repeated several times, until the step size reaches the preset minimal value. With this method, the threshold value can be delineated very accurately. The initial size of the step can be selected depending on the expected accuracy. In measuring sensory threshold, noise must be accounted for. Signal noise is defined as the presence of extra, unwanted energy in the observational system which obscures the information of interest. As the measurements come closer to the absolute threshold, the variability of the noise increases, causing the threshold to be obscured. Different types of internal and external noise include excess stimuli, nervous system over- or under-stimulation, and conditions that falsely stimulate nerves in the absence of external stimuli. A universal absolute threshold is difficult to define a standard because of the variability of the measurements. While sensation occurs at the physical nerves, there can be reasons why it is not consistent. Age or nerve damage can affect sensation. Similarly, psychological factors can affect perception of physical sensation. Mental state, memory, mental illness, fatigue, and other factors can alter perception. Aviation use When related to motion in any of the possible six degrees of freedom (6-DoF), the fact that sensory thresholds exist is why it is essential that aircraft have blind-flying instruments. Sustained flight in cloud is not possible by `seat-of-the-pants' cues alone, since errors build up due to aircraft movements below the pilot's sensory threshold,