Ford Falcon. Manual — part 496

501-25G-3

Body Repairs - Noise, Vibration and Harshness

501-25G-3

DESCRIPTION AND OPERATION (Continued)

Noise transmission in a vehicle

Low frequency oscillations can usually be

Item

Description

seen (large amplitudes).

1

Sound through air

High frequency vibrations can be heard, but

2

Sound vibrations

will also be felt as vibrations in the floor,

- Example

steering wheel or seat.

The source of the noise is the engine

Resonance

Engine noises are:

Every body has a natural frequency of vibration.

radiated directly through the air

If a system able to vibrate (a vehicle) is excited at

transmitted as vibrations from the engine to the

its natural frequency of vibration, it will start to

bodywork and radiated into the passenger

resonate.

compartment as air borne sound, for instance by

Uneven engine running appears for instance

the bulkhead.

when the resonant frequency is reached (critical

The speed of transmission (speed of sound) in liquids

engine speed).

or solids depends on the material, but is generally

Once the engine speed increases above this

faster than that in air (about 5 times faster).

speed, the engine will very soon run smoothly

Combination of transmission through air

again.

and bodies

Damping

NOTE: Sound transmission through both air and

If a damper is installed parallel to a spring, the

bodies is of prime importance in vehicle technology.

damping characteristic acts to reduce the

movement of a body.

Vibration Technology

This is also the way in which a shock absorber on

Frequencies below 20 Hz (low frequencies) and

a vehicle works.

those above 20,000 Hz (20 kHz; high frequencies)

Damping affects the resonance of an object or

are not heard by the human ear.

system.

The engine is made to oscillate vertically in its

The mass being moved is damped by a correctly

flexible mountings by unevenness in the road.

operating shock absorber, so that shaking of the

Engine mountings which are not tuned will

bodywork stops immediately.

transmit oscillations to the bodywork and hence to

The shock absorber in a vehicle is an oscillation

the occupants.

damper, which allows the bodywork to oscillate in

- The following basic rules apply:

a damped manner.

In vehicle technology, not only audible

frequencies but also inaudible (low)

frequencies must be considered.

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501-25G-4

Body Repairs - Noise, Vibration and Harshness

501-25G-4

DESCRIPTION AND OPERATION (Continued)

Oscillation

Isolation

In oscillation technology, the term isolation means

decoupling (separation) of components and systems.

Example:

An engine is mounted in sprung elements, so

that as little oscillation as possible is passed

to the vehicle.
In automotive technology, the isolation

technique used is nearly always rubber

mounting. The elasticity of the rubber acts like

a spring.
The effectiveness of this type of decoupling

depends on the construction of the

component.

Item

Description

1

Undamped oscillation

2

Damped oscillation

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501-25G-5

Body Repairs - Noise, Vibration and Harshness

501-25G-5

DESCRIPTION AND OPERATION (Continued)

Directional loading of a rubber mounting in order to achieve optimum isolation

Item

Description

1

Engine

2

Engine mounting

Absorption

Sound waves are reflected from hard surfaces
If they meet soft surfaces, they are absorbed.

Here both the material and the material thickness

play important roles.

All sound deadening components such as

Item

Description

door trim, carpets, headlining, parcel shelves

1

Reflection of sound waves

and the seats are removed from a vehicle.

2

Absorption of sound waves

Example:

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501-25G-6

Body Repairs - Noise, Vibration and Harshness

501-25G-6

DESCRIPTION AND OPERATION (Continued)

The result of this is that the sound is directly

Noise and oscillation behaviour in a

reflected (the sound level rises), and a

vehicle

concert hall effect is produced.

The occupants of a vehicle experience different levels

of sound or noise depending on where they are

sitting.

Sound/noise wave (standing wave) in a vehicle

necessary for these type of oscillating

Item

Description

components to be connected to the body

1

Driver sits in a "wave calm"

using stiff connection points.

2

Rear passenger sits in a "wave swell"

The exhaust system especially is an

oscillating component and requires optimal

Noise and oscillation behaviour of intake

isolation through its connection to the floor

and exhaust systems

and also good routing and positioning of the

fixing points in order to reduce the solid body

The vehicle exhaust system as an air borne sound

sound transmission.

and solid body sound source

Intake system

Intake system

Ways in which sound is transmitted

The top illustration shows the ways in which

sound can be transmitted from intake and

Item

Description

exhaust systems.

1

Intake manifold

Apart from air borne sound, solid body sound

is predominant in both systems. In order to

2

Connection hose

keep this proportion as small as possible, it is

3

Fixing component (rubber mounted)

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