# How are Vibration and Noise Transmitted

Basically, vibration and sound are transmitted in the same way. Let us take a look at the process of how a vibration is generated and transmitted to the human body as a vibration or a sound. Transmission process of vibrations and sounds
(1)Vibration is generated
(2)Vibration grows
(3)Vibration is transmitted
(4)Vibration or sound appears
The case of an electric guitar
(1)Strings vibrate
(2)Vibration is amplified by an amplifier
(3)Vibration is transmitted via a wire
(4)Sound is heard via a speaker
An example of the case of a vehicle

Vibrating force
Even if there is an object which can vibrate, vibration does not occur without some kind of external force. The vibrating force is the force which makes vibration occur forcibly.
As the vibrating force in the vehicle, there are some forces such as combustion pressure in the engine, unbalanced tires or propeller shaft, run-out generated in each portion.
Typical vibration force in vehicle
(1)Torque fluctuation in engine
(2)Tire imbalance
(3)Tire run-out
(4)Tire uniformity
(5)Imbalance in propeller shaft
(6)Second order vibration of propeller shaft

(1)Torque fluctuation in engine
In each cylinder of a four-cycle engine, one combustion for every two crankshaft rotation is repeated generating the torque fluctuation.
In-line four-cylinder engine:
Four combustions per two crankshaft revolutions occur, causing two torque fluctuations per one crankshaft revolution.
Two vibrations per one revolution (second order vibration) are generated.
In-line six-cylinder engine:
Six combustions per two crankshaft revolutions occur, causing three torque fluctuations per one crankshaft revolution. Three vibrations per one revolution (third order vibration) are generated.
(2)Tire imbalance
Wheel balance can be divided into “static balance” and “dynamic balance”. If there is any insufficient balance, the tires will vibrate.
Static balance
The balance that exists when a tire is not rotating. The radial weight balance as that from the center of the wheel.
Dynamic balance
The balance that exists when a tire is rotating. The balance of the centrifugal force which causes lateral vibration.
(3)Tire run-out
Because tires are made by bonding rubber together layer by layer, there are thick portions and thin portions. So the circumference of tires is not always perfect circle. Also, even if the circumference is completely circular, a tire will become eccentric and then fluctuate if the center of axle and tire rotation does not coincide.
When a tire with radial run-out is rotated, that makes the vehicle fluctuate up and down.
Lateral run-out
If lateral run-out exists in a tire or a disc wheel, it will cause uneven wearing of the tire, and also lead to unstable driving.
(4)Tire uniformity
Tire uniformity refers to uniformity of weight, dimensions and rigidity. However uniformity of weight is the same as wheel balance and uniformity of dimensions is the same as run-out.
Due to this, the word uniformity here is used to mean “uniformity of rigidity”.
The force that is generated when rotating a tire with uneven rigidity:
Force fluctuation in the direction of radius of tire (vertical direction)
LFV (Lateral Force Variation)
Force fluctuation in the lateral direction of tire
TFV(Tractive Force Variation)
Force fluctuation in the direction of tire rotation.
In the uniformity, the most important is RFV. A tire with a high RFV imposes a vertical vibration on the axle, which can lead to excessive vibration during high-speed driving.
(5)Imbalance in propeller shaft
If there is any imbalance in the propeller shaft that causes vibration and noise. (“Imbalance” means the same thing as in the case of tire.)
Main causes of imbalance
Run-out of propeller shaft
If propeller shaft fluctuates, one vibration per one revolution is generated.
Run-out of differential companion flange and propeller shaft center flange
If there is run-out (radial/lateral run-out) in each flange, that offsets the rotational center of the propeller shaft. This is a big factor in lost balance.
Run-out is the same thing as the offset of the rotational center, which causes the lack of rotational balance.
(6)Second order vibration of propeller shaft
The joint of the propeller shaft has some angle due to the structure of a vehicle.
Torque fluctuation
In the case of Hooke’s joints, joint angle creates torque fluctuation in the output shaft. This fluctuation is generated twice per one propeller shaft revolution, which appears as a second order vibration.
Secondary couple
At a joint portion, a force proportional to the joint angle and input torque is generated. This force is called secondary couple, because this force fluctuates twice per one revolution of the propeller shaft. Secondary couple, different from the torque fluctuation, exists in each joint angle.