Ford Workshop Service and Repair Manuals

• The sound is spread by longitudinal waves in the form of pressure changes in the air.

Sound transmission through a body

Sound is passed through liquids or solids e.g. through the car bodywork.

The speed of transmission (speed of sound) in liquids or solids depends on the material, but is generally faster than that in air (about 5 times faster).

Combination of transmission through air and bodies

NOTE:Sound transmission through both air and bodies is of prime importance in vehicle technology.

Noise transmission in a vehicle

• Example
  -

  The source of the noise is the engine Engine noises are:
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  radiated directly through the air
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  transmitted as vibrations from the engine to the bodywork and radiated into the passenger compartment as air borne sound, for instance by the bulkhead,.

Vibration technology

• Frequencies below 20 Hz (low frequencies) and those above 20,000 Hz (20 kHz; high frequencies) are not heard by the human ear.
• The engine is made to oscillate vertically in its flexible mountings by unevenness in the road.
• Engine mountings which are not tuned will transmit oscillations to the bodywork and hence to the occupants.
• The following basic rules apply:
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  In vehicle technology, not only audible frequencies but also inaudible (low) frequencies must be considered.
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  Low frequency oscillations can usually be seen (large amplitudes).
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  High frequency vibrations can be heard, but will also be felt as vibrations in the floor, steering wheel or seat.

Resonance

• Every body has a natural frequency of vibration.
• If a system able to vibrate (a vehicle) is excited at its natural frequency of vibration, it will start to resonate.
• Uneven engine running appears for instance when the resonant frequency is reached (critical engine speed).
• Once the engine speed increases above this speed, the engine will very soon run smoothly again.

Damping

• If a damper is installed parallel to a spring, the damping characteristic acts to reduce the movement of a body.
• This is also the way in which a shock absorber on a vehicle works.
• Damping affects the resonance of an object or system.
• The mass being moved is damped by a correctly operating shock absorber, so that shaking of the bodywork stops immediately.
• The shock absorber in a vehicle is an oscillation damper, which allows the bodywork to oscillate in a damped manner.

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.
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  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.

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

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.

• Example:
  -

  All sound deadening components such as door trim, carpets, headlining, parcel shelves and the seats are removed from a vehicle.
  -

  The result of this is that the sound is directly reflected (the sound level rises), and a concert hall effect is produced.

Noise and oscillation behaviour in a vehicle

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

Noise and oscillation behaviour of intake and exhaust systems

The vehicle intake system as an air borne sound and solid body sound source

The vehicle exhaust system as an air borne sound and solid body sound source

• Ways in which sound is transmitted
  -

  The top illustration shows the ways in which sound can be transmitted from intake and exhaust systems.
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  Apart from air borne sound, solid body sound is predominant in both systems. In order to keep this proportion as small as possible, it is necessary for these type of oscillating components to be connected to the body using stiff connection points.
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  The exhaust system especially is an oscillating component and requires optimal isolation through its connection to the floor and also good routing and positioning of the fixing points in order to reduce the solid body sound transmission.

Intake system

Intake system

• The volume and length of intake systems are matched exactly. No changes may be carried out during service or repair work. When attaching components (hoses and tubes), pay attention that all connections are air-tight.
• The surfaces of all components affected by the strong pressure variations in the intake system are especially subject to strong oscillations.
• The effects of temperature, such as warming of the air filter housing can cause changes in the rigidity of surfaces.
• In order to prevent solid body sound caused by surface oscillations of the intake system from being transmitted to the vehicle body, the following points must be observed during service:
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  The whole intake system must be isolated from the bodywork and mounted in rubber bushes without tension.
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  The air channels of the intake system must not contact the bodywork components.
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  Insert foam insulation at points of contact with bodywork or other components.

Exhaust system

Exhaust system construction

• Exhaust systems must be installed without strain.
• 

  NOTE:The rubber mountings in which an exhaust system is suspended also transmit oscillations and under some circumstances can also cause a noise problem. For this reason it is important that the exhaust system is installed free of strain.
  The rubber mountings must not be over-stretched.
• The exhaust manifold and catalytic converter (if it is mounted directly at the engine) must have a solid connection to the engine (no splits in brackets).
• Simple problem testing:
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  Remove all mountings from which the exhaust system is suspended.
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  Support the exhaust system with rope at two positions at the most.
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  Check the noise level in the passenger compartment during a road test.
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  If this solves the problem, the exhaust mountings are the source of the noise.
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  Put back the exhaust supports one at a time, and road test the vehicle after fitting each one.
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  By proceeding in steps in this way, the mounting that is causing the noise in the passenger compartment can be found.

Bodywork

Function

• Different types of demand are made on the bodywork of a vehicle, particularly when it is being driven. Predominant here are vibrations caused mainly by other vehicle components, such as the engine and mountings. The bodywork is therefore one of the most important vehicle components where NVF is concerned.
• The vehicle components described so far were, according to their function, individual self-contained noise sources. The bodywork however, connects and houses all the vehicle components.
• The bodywork is subject to vibration from both air borne sound and solid body sound.
• The bodywork must absorb all oscillations and impulses from all components and if possible transmit none of these to the vehicle interior.
• The largest proportion of NVH behaviour of a bodyshell is in the steel panels of the roof, sides and floor. They are strongly stimulated by the bodyshell structure and work like a loudspeaker transmitting noise.

Further demands on the bodywork

• Good crash behaviour
• Large interior space
• Corrosion resistance
• Low weight
• Good aerodynamic shape

A compromise must be made on some of these points. One example is between high body rigidity for good NVH performance and good energy absorption in an accident by the use of crumple zones.

Example:

• Watertight does not mean sound tight, therefore the following points must be taken into account:
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  Correct installation of the windows.
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  Correct installation of the seals.
• Further points
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  Openings into the engine compartment
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  Door and window sealing
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  Heater and ventilation openings
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  Sliding roof drainage hoses
• The side- and crossmembers in the body structure are often linking channels allowing sound transmission. Possible corrective measures are:
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  Installation of dividing walls at critical points such as the joint between the sidemember and the A-pillar in order to ensure good air borne sound insulation.
  -

  

  NOTE:Foam pieces and foam filling are used in production to seal air borne sound transmission routes. If wind, tyre, engine or road noise is a problem, the position of the pieces of foam should be checked.

  Installation of foam plugs in affected areas.
  -

  Filling hollow sections with foam.