Speech Intelligibility 7-9. People must be able to hear in the range of 300 to 3,000 hertz to understand speech communication. Speech outside these ranges may result in incoherence or misinterpretation. Vibration affects the body most in low frequencies, usually confined to frequency ranges below 100 hertz to displace body parts. These effects vary greatly with the direction, body support, and restraint.
Intensity is a measure that correlates sound pressure to loudness.
For every 20-decibel increase in loudness, sound pressure increases by a factor of 10. The same sound pressure moving through the air that stimulates the ear to hear may also cause hearing loss under certain conditions. Table 7-1 shows the effects of various sound intensities on listeners. Duration is the length of time that an individual is exposed to noise or vibrations. It is a variable factor that may be measured in seconds, minutes, hours, or days or any other selected unit of time.