Automotive Vibration and Sound


When driving on a highway, if the steering wheel or the body vibrates or an unpleasant booming noise is heard from some unknown place, the driver would feel anxious not only about the discomfort but also about possible malfunctions, even if the symptom is not damaging the vehicle’s function. This body vibration and noise is referred to as the “vibration and noise,” the contents of which you will learn about here.
When troubleshooting vibration and noise, randomly starting without any theoretical basis often results in wasting a lot of time.
It is important to have a thorough knowledge of vibration and noise, and understand the mechanisms that generate them, and to perform the appropriate inspection and repair. The noise explained here refers to the sound that is generated by body vibration. Abnormal sound in the interior are not included.

What is Vibration and Sound?
Click on the bulb mark or the underlined sentence.
We experience vibrations with our hands and feet or body, and sounds with our ears. How vibrations and noises are sensed varies depending on the person who experiences them. Even if vibration and noise is generated simultaneously, some people sense both, others sense only vibration or noise, and the rest do not sense anything. This is because vibration and noise are related to one’s sensitivity.
1.Vibration and sound is essentially the same
A sound is the vibration (pressure fluctuation) of the air, and vibrations and sounds are both expressed as “waves.” They are represented by frequency, the number of waves per second. By the variation in their frequency, waves are sensed sometimes as a vibration, and in other cases, as a noise.
How vibration and sound is sensed depending on the frequency
20Hz or less → Vibration only
20 to 200Hz → Vibration and sound
200Hz to 20kHz → Sound only
20Hz to 20kHz → Audible range
Over 20kHz → Ultrasonic waves
(High frequency sound that is not audible to the human ear)
A rough estimate of the frequency can be done depending on whether a vibration or a sound is felt.

How We Sense Sounds
We can only sense a sound when it is within a certain frequency range and strength as a “sound.” Therefore, a sound is not always heard when something vibrates. A vibration is not sensed as a sound even if it has a lot of energy, unless its frequency exceeds a certain level.
Audible range
Kinds of Vibration
1.Rigid body vibration
When an object is held by a spring and is functioning only as a weight, its vibration is called a rigid body vibration.
The rigid body vibration can be thought of as a simple relation between a spring and a weight.
The resonance point in the rigid body vibration
Generally, an object has a natural frequency of its own. The vibration becomes large at a specific resonance point. So, in the rigid body vibration, when a vibrating force is added, the vibration begins at that frequency. Then resonance occurs when the frequency reaches natural frequency of that object, which in turn greatly increases the vibration.
For the vehicle,
Body = weight
Suspension spring = spring
Engine = weight
Engine mount rubber = spring
2.Elastic vibration
The vibration of a continuous body which occurs when plucking a string with a finger is called the elastic vibration.
When the frequency is lower, regarding the problem like “riding comfort”, the vehicle body can be thought of as a weight which does not bend or deform. But when the vehicle body vibrates at a frequency which causes the problem like “body shake”, the vehicle body is not only a weight (rigid body), but should be thought as an elastic vibrating element which is accompanied with deformation such as bending or twisting of the body itself.
The resonance point in the elastic vibration
In elastic vibration, there is more than one resonance point. When a string is plucked, vibrations occur in various frequencies as shown in the figure. In these frequencies, as for a to n which are remarkable, their relationship is the following: making a as the first order, b is the second order (twice), c is the third order (three times), n is the “n” th order (“n” times). The a to n can be thought of as a resonance point of the string. In the elastic vibration, as seen above, the vibration becomes large at each resonance point when the vibrating force is added.

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