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Technical Information
Fundermentals of Vibration, Shock, and Audible Noise Solutions
What is Vibration?
"A phenomenon where the magnitude of a certain quantity increases or decreases from a reference value over time." (As defined by the acoustic industry dictionary published by the Acoustical Society of Japan)
A change in which the magnitude of a quantity related to a certain coordinate system alternates between states greater and smaller than its average or reference value. (As defined by JIS B 0153 "Glossary of terms used in mechanical vibration and shock")
Vibrations we experience in daily life range from simple constant oscillations, like the pendulum of a clock, to complex irregular vibrations felt when riding a train, which may include periodic vibrations caused by rail joints.
The sources of these vibrations range widely: from those that generate shock, such as breakers used in road construction, to those caused by unbalanced loads during the spin cycle of washing machines, and even to vibrations stemming from the inertia forces of reciprocating mass in reciprocating compressors.
Frequency f (Hz)
The period (T) is the time taken for one complete cycle from peak to peak, measured in seconds (S). The reciprocal of the period (T), defined as f = 1/T is called the frequency, measured in hertz (Hz). The amplitude (A) is the value from the baseline to the maximum value.
Natural frequency
Natural frequency refers to the frequency at which any system (object) vibrates most easily. When the system is subjected to an external frequency equal to its natural frequency, the vibrations are amplified, resulting in significant oscillations. This phenomenon is known as resonance, and the associated frequency is referred to as the resonance frequency.
In the spring and mass example, natural frequency is determined by two factors: mass size and spring stiffness.
When mass decreases or stiffness increases, natural frequency increases. Conversely, when mass increases or stiffness decreases, natural frequency decreases.
Decibels [dB]
Generally, when expressing the ratio of physical quantities, the unit [dB] (decibel) is used.
Particularly, when dealing with values that are thousands or millions of times the reference value, using the logarithmic unit [dB] rather than using the unit directly makes calculations easier and has been proven to be more in line with human perception.
[dB]Formula of [dB] unit
The relationship between [dB] and values (multiple) is summarized in the table below.
| Decibels [dB] | Value (multiple) A1/A0 |
|---|---|
| -60 | 0.001 |
| -40 | 0.01 |
| -20 | 0.1 |
| -10 | 0.3 |
| -6 | 0.5 |
| -3 | 0.7 |
| 0 | 1 |
| 3 | 1.4 |
| 6 | 2 |
| 10 | 3.2 |
| 20 | 10 |
| 40 | 100 |
| 60 | 1000 |
The sound pressure level is based on the minimum audible sound pressure's effective value of 2×10 −5 Pa.
How to Read Vibration Transmission Graph
One key characteristic of vibration-damping materials is their ability to transmit vibrations. In graphical representations, the horizontal axis typically denotes frequency [Hz], while the vertical axis indicates response magnification [dB]. The frequency corresponding to the graph's peak value is known as the resonance frequency (f₀), and the response magnification at this peak is referred to as the resonance magnification.
As frequency increases, the response magnification decreases. When the frequency reaches √2 times the resonance frequency (f₀), the response magnification becomes 0 dB (equivalent to 1x).
The region above the zero-crossing point is called the vibration isolation region, where vibrations are effectively damped. In contrast, the region below the zero-crossing point is known as the resonance region, where vibrations are amplified.
These characteristics can change based on factors such as load and temperature, so they should be carefully considered in practical applications.
Overview of Vibration, Shock and Audible Noise Reduction Techniques
| Buffer |
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Methods to Reduce Impact Force Using an Arbitrary Stroke: 1) Attach buffering material to the target object 2) Increase the thickness of the buffering material 3) Adjust the buffering material to the appropriate hardness and support area |
|---|---|---|
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Vibration Damping (Vibration Isolation) |
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Methods to damp and insulate vibrations transmitted from the vibration source to the target object: 1) Insert vibration damping/isolation material between the vibration source and the target object 2) Use lower ""spring constant"" damping/isolation material that has a natural frequency lower than the vibration source |
| Vibration Control |
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Methods to convert vibration energy into thermal energy and dampen vibration: 1) Attach damping material to the target object or vibration source 2) Use higher ""loss factor"" material that can convert vibration energy into thermal energy efficiently |
| Sound Insulation |
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Methods to block sound propagating through the air and reduce sound power transmission: 1) Cover the noise source with sound insulation material 2) Increase the density of the sound insulation material 3) Increase the thickness of the sound insulation material |
| Sound Absorption |
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Methods to absorb sound propagating through the air: 1) Cover the noise source with sound-absorbing material 2) Use sound-absorbing material that has a high absorption rate for the problematic frequency 3) Increase the thickness of the sound-absorbing material |