10 The perception of intensity
10.1 Absolute thresholds
The human ear has incredible absolute sensitivity and dynamic range. The most intense sound we can hear without immediate damage to the ear is at least 140 dB above the faintest sound we can just detect. This corresponds to an intensity ratio of 100 000 000 000 000:1. In this section, we examine how the loudness of a sound can be measured and how the perception of loudness is affected by the intensity and duration of the signal.
You know from Section 9.2 that the absolute threshold is the smallest value of some stimulus that a listener can detect. In order to investigate our perceptual capabilities, it is useful to generate an absolute threshold curve, which relates the frequency of a signal to the intensity at which it can be detected by the ear. Figure 34 is a plot of the thresholds of hearing for a range of frequencies.
Activity
From the graph, describe the relationship between signal frequency and the threshold of audibility of a signal.
Answer
Auditory thresholds are lowest for tones around 3000 Hz and increase for tones of higher and lower frequency.
This means that people are therefore most sensitive to tones of frequencies around 3000 Hz, with sensitivities decreasing for tones that are either higher or lower in frequency. There will be very high and very low frequencies to which, no matter how intense the sinusoidal wave, the auditory system is insensitive. These frequency limits define the bounds of the auditory system's sensitivity to frequency.
In order to generate an audibility curve like that shown in Figure 34, you would determine the level required for a listener to detect the presence of a sinusoidal wave at each of many frequencies. One method of doing this involves delivering the sound using loudspeakers and measuring the sound pressure at the entrance to the auditory meatus at threshold. A threshold measured in this way is known as a minimum audible field (MAF). In contrast when sounds are delivered through earphones the threshold measured is called the minimum audible pressure (MAP). MAP thresholds are plotted as a function of frequency in Figure 34.
The threshold sound levels displayed in Figure 34 produce extremely small physical displacements at both the tympanic and basilar membranes. In humans, the sound level of frequencies to which we are most sensitive cause movements of the basilar membrane of about 0.2 nm – about the diameter of two hydrogen atoms.
Figure 34 shows auditory thresholds for young people. As we grow older, we become less sensitive to stimuli of all frequencies, but the maximum hearing losses occur for high frequency tones.
Activity
Can you think of a reason why this may be the case?
Answer
Remember that low frequencies stimulate the basal end of the basilar membrane, even though the peak in displacement is towards the apex. This part of the membrane in fact is stimulated by all frequencies to some extent and the hair cells in this region have a greater potential for being ‘worn out’.