9.2 Absolute thresholds
The absolute threshold or absolute limen is the smallest value of a stimulus that an observer can detect. The concept of an absolute threshold assumes there is a precise point on the intensity or energy dimension that, when reached, becomes just perceptible to the observer and he or she responds ‘yes – I can detect the stimulus’. It follows that when the stimulus is one unit weaker it will not be detected. If this were the case then some form of hypothetical curve, like the one shown in Figure 32 would be the result. However this rarely happens, as illustrated in the following example where an auditory threshold is derived using a traditional psychophysical method, the method of limits.
Figure 32 A hypothetical curve linking stimulus intensity to absolute threshold. The vertical axis plots the number of trials in which the subject responds ‘yes’, he/she can detect the stimulus. The threshold value is 14.0 dB SPL, i.e. below 14.0 dB SPL the stimulus is not detected