To understand why human beings are more sensitive to some frequencies than to others, it is useful to consider the human body as having sub-systems, where each sub-system has its own resonance frequency band and the interactions between sub-systems are influenced by the body's position, for example, standing or sitting.
Fig. 7 Simplified human body sub-systems
Each part of the human body has its own resonance frequency, therefore, it reacts differently to different frequencies.
One of the most important parts of the system with respect to vibration and shock seems to be the abdominal part with the resonance occurring in the 4-8 Hz range. The other main resonant effect is found in the range 20-30 Hz related to the head-neck part. Also, a vibration in the region 20-90 Hz correlates with the eyeball resonance.
Above 100 Hz, the model from Fig 7 is not very useful, and other more complex analyses have to be used. Some of the analyses show that the skull itself has a fundamental mode of vibration in the region of 300-400 Hz with resonances for higher modes around 600-900 Hz, (Bruel & Kjaer, 1988).
From the above, you can see that it is the resonant amplification of response and the dissipation of vibration energy within the human body that results in various physiological effects on humans. It can manifest itself as increased pulse rate or respiratory rate or more seriously as ailments of spinal muscle, ano-rectal or gastro-intestinal systems.
The human responses to vibration depend also on which part of the body is affected. There are two major types of human exposure to vibration:
vibration transmitted to the whole-body through a supporting surface,for example, the feet of a standing person or the buttock of a seated person; and
vibration applied to a part of the body ie. segmental vibration. When vibration is applied to the hand, it is termed "hand-arm" vibration.
Whole-body and segmental are the two major types of human exposure to vibration.
Whole-body vibration and segmental vibration will be addressed in separate lectures later in the course, as they are measured using different standards, require different engineering controls and have their own unique effects on the human body.