What Is a Vibration Level?
Vibration levels are the acceleration of vibrations calculated from physical quantities plus a correction for human perception of vibrations.
The unit is a dB. Vibration can be measured in terms of displacement, velocity, and acceleration, but it is recognized that acceleration is more appropriate as the basis for vibration felt by the human body. Vibration levels are established to regulate vibration pollution.
Vibration has vertical and horizontal directions, and the way people perceive vibration differs according to the direction. Vertical vibration is felt more strongly than horizontal vibration.
Applications of Vibration Levels
1. Specific Factories, Specific Construction Work
As a strain of rapid postwar economic growth, various types of pollution have occurred, and regulations have been enacted to deal with them. The vibration subject to regulation is specified factories, etc., specified construction work, and road traffic vibration. Vibration levels are determined based on the human body’s sensory perception during the daytime and nighttime.
Specified factories, etc., are factories and workplaces in designated areas set by local governments. Facilities that generate significant vibration are designated as specified facilities. Examples of specified facilities include metal processing machinery, large compressors, earth or mineral crushers, concrete block machines, injection molding machines for synthetic resin, and wood processing machinery. Vibration limits are established for each region.
Specified construction work is construction work that generates significant vibration; for example, piling machine work, work that destroys buildings, etc., and breaker work are subject to regulation. Vibration levels are set at the boundary of the site, depending on the area, time and duration of the work, and day of the work.
2. Road Traffic Vibration
Road traffic vibration is not directly regulated, but is required. If the living environment around a road is deemed to be significantly damaged, the mayor of the municipality will request the prefectural public safety commissioner to make improvements.
Principle of Vibration Levels
1. Calculation of Vibration Levels
Vibration Levels are calculated as the ordinary logarithm of the ratio of the effective vibration acceleration (a), corrected for vibration sensation, to the reference vibration acceleration (a0), times 20. The equation is shown below. The unit is dB.
VL = 20log10^(a/a0)
VL: Vibration Levels (dB), a: RMS value of vibration acceleration with vibration sensory correction (m/s^2), a0: Reference vibration acceleration (m/s^2)
For a0, the Japanese vibration code uses 10^-5m/s^2. Vibration levels are calculated using the ordinary logarithm because, according to Weber-Fechner’s law, the sensation of vibration is proportional to the logarithm of the intensity of the stimulus, as is the case with hearing.
The sensory correction used for vibration levels is more sensitive in the frequency bands of 4 to 8 Hz in the vertical direction and 1 to 2 Hz in the horizontal direction than in the other bands.
2. Guideline for Vibration Levels
Vibration levels are published by the Japan Meteorological Agency as seismic intensity classes in dB according to human senses as a guide; a level of 110 dB or higher is a level at which people are tossed about by the shaking and cannot act on their own volition, while a level of 55 dB or lower is a level at which people do not feel the shaking.
Other Information on Vibration Levels
1. Measurement of Vibration Levels
If the indicated value is irregular and fluctuates significantly, more than 100 measurements are taken at 5-second intervals, and the value is determined using the LV10 method. In the past, such a method was read by a human, but currently vibration levels meters can calculate it automatically; LV10 is a statistical value, meaning that it accounts for 10% of the time that it is measured.
2. Measurement of Ground Vibration
In principle, the measurement location for ground vibration is at the boundary of the site or property. However, if vibration is measured at the ground level, it may pick up abnormal vibration levels depending on the measurement location. The cause is due to the nature of the ground. It is necessary to measure vibration at multiple points to understand the propagation of vibration.