Threshold of pain

Figure 7-75. Heat radiation intensity vs. exposure time for bare skin at the threshold of pain. By permission, Kent, Hydrocarbon Processing, V. 43, No. 8 (1964), p. 121 [60].

For convenience, the usual measurement of sound is expressed in decibels (dB), and ratings go from threshold of hearing to threshold of pain (135 dB). Figure 34.11 illustrates the common noise criteria, which can be expressed in sound-pressure levels (SPL). The human ear can detect 1 dB but 6 dB represents a doubling of the SPL, although it would need a 10 dB increase to make it sound twice as loud. 

Because ACTH synthesis originates from the POMC precursor peptide, its production by the pituitary is closely tied to the secretion of endogenous opiate peptides, such as P-endorphin. The physiological effects of endogenous opiates include (1) sedation, (2) an increased threshold of pain, and (3) autonomic regulation of respiration, blood pressure, and heart rate. These peptides are also involved in modifying endocrine responses to stress and water balance... 

Figure 8.6 also shows the threshold of pain superimposed. Fortunately for us, the threshold of pain is relatively flat. Generally a SPL of over 120 dB at any frequency will cause pain. An important observation that can be derived from the two curves is that if a sound is audible at very low or very high frequencies, persons subject to this sound are very close to experiencing pain. 

Thermal radiation 2.9 kW/m (threshold of pain reached after 30 s) - number of affected people large... 

A peak sound pressure of above 200 pascals or about 120dB is considered unacceptable and 130dB is the threshold of pain for humans. If a person has to shout to be understood at 2 metres, the background noise is about 85 dB. If the distance is oniy 1 metre, the noise level is about 90dB. Continuous noise at work causes deafness, makes people Irritable, affects concentration, causes fatigue and accident proneness, and may mask sounds which need to be heard in order to work efficientiy and safely. 

Lees (1986) summarizes the data from which this relationship was derived. The probit method has found less use for thermal injury than it has for toxic effects. Mathematical models of thermal injuries can be based on a model detailed description of the skin and its heat transfer properties. Experiments have shown that the threshold of pain occurs when the skin temperature at a depth of 0.1 mm... 

Now that the relationships between the various levels have been established, we can relate them to everyday life. The threshold of hearing for a healthy human ear is considered to be 0 dB at 1,000 Hz. The threshold of pain is considered to be 140 dB at nearly all frequencies. 

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