Noise, vibration and harshness

Introduction

This chapter provides a brief overview of how noise, vibration and harshness (NVH) parameters manifest themselves in a vehicle and what corrective measures are possible.

What is meant by the term NVH in the automotive industry

• N = Noise - Sound, can be heard
• V = Vibration - Oscillation, can be felt
• H = Hardness, its impact can be heard and felt

Types of noise in NVH technology

• Noises in cars are classified according to their tonality:
  • Low pitch - roar, hum
  • Medium tone - buzzing, crackling
  • High pitched - howling, whistling, squeaking
• Loud howling and whistling are painful to the ears.

How different noise penetrates into a car:

• Low-pitched noise is mainly produced by the engine.
• Low-tone noise is also caused by the road surface, especially if the road surface is uneven. This is a form of humming that can be felt by people in the car as vibration.
• However, high-pitched noise that is perceived as a howling or whistling sound is often airflow noise (wind noise) or comes from components such as the alternator, power steering pump, or drive belt.
• There is also a rattling noise that can occur when driving on uneven roads. "Jerking" noises are produced, for example, by shock absorbers, chassis components or loose components inside the car.

Propagation of sound waves in air

Pos.	Spare Part No	Name
1	-	Sound source, such as an engine
2	-	Sound wave
3	-	Amplitude

Sound is the propagation of longitudinal waves in the form of changes in air pressure.

Sound transmission through the body

Sound is transmitted by liquids or solids, such as the body of a car.

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

Combination of airborne and solid state transmission

NOTE: Sound transmission through air and solid bodies is of primary importance in automotive engineering.

Noise transmission in a car

Pos.	Spare Part No	Name
1	-	The passage of sound through the air
2	-	Sound vibrations

Example:

• The source of noise is the engine. Engine noises:
• pass right through the air
• are transmitted as vibrations from the engine to the body and are "radiated" into the car"s interior through the air, for example, by a partition.

Vibrations

• Frequencies below 20 Hz (low frequencies) and above 20 kHz (high frequencies) are not perceived by the human ear.
• The engine is installed in such a way that, due to unevenness of the road, it oscillates vertically on its elastic supports.
• Misaligned engine mounts will transmit vibrations to the body and therefore to the people in the car.
• The following basic rules apply:
  • In automotive engineering, not only sound frequencies must be taken into account, but also inaudible (low) frequencies.
  • Low frequency vibrations can usually be noticed (large amplitudes).
  • High frequency vibrations can be heard, but they will also be perceived as vibrations in the floor, steering wheel or seat.

Resonance

• Each body has its own vibration frequency.
• If a system capable of vibrating is excited at its own vibrational frequency, it will begin to resonate.
• Uneven engine operation occurs, for example, when the resonant frequency (critical rotation frequency of the engine crankshaft) is reached.
• Once the engine speed has been increased above this value, the engine will very soon begin to run smoothly again.

Damping

• If the damper is installed parallel to the spring, the damping characteristic is aimed at reducing body movement.
• The shock absorber on the car also works.
• Damping affects the resonance of an object or system.
• The moving mass is damped by a properly functioning shock absorber, thus immediately stopping the vibration of the body.
• A shock absorber in a car is a damper (vibration damper) that allows the body to vibrate with gradual damping of vibrations.

Oscillation

Pos.	Spare Part No	Name
1	-	Undamped oscillation (undamped)
2	-	Damped oscillation

Insulation

In vibration technology, the term "isolation" means separation of elements and systems. Example:

• The engine is mounted on spring elements so that the vehicle is transmitted the smallest possible vibrations.
• In the automotive industry, a rubber cushion (support) is almost always used as insulation. The elasticity of the rubber acts like a spring.
• The efficiency of this type of separation depends on the design of the element.

Directional load of the rubber cushion to ensure optimum insulation

Pos.	Spare Part No	Name
1	-	Engine
2	-	Engine support

Absorption

• Sound waves reflect off hard surfaces
• If they meet soft surfaces, they are absorbed. Here, both the material and the thickness of the material play an important role.

Pos.	Spare Part No	Name
1	-	Reflection of sound waves
2	-	Absorption of sound waves

Example:

• All soundproofing elements such as door trim, carpets, headliner, storage shelves and seats are removed from the vehicle.
• The result of this is that the sound is reflected directly (the sound level increases), and a concert hall effect is created.

The nature of noise and vibrations in a car

People in a car perceive sounds or noises of varying levels depending on where they are located.

Sound/noise wave (standing wave) in a car

Pos.	Spare Part No	Name
1	-	The driver is in the "blind spot"
2	-	The rear passenger is in the "wave shaft" zone

The nature of noise and vibrations in the intake and exhaust systems

The automobile intake system as a source of airborne sound and solid-borne sound



Automobile exhaust system as a source of airborne sound and solid-borne sound




Paths by which sound is transmitted

• The top drawing shows the paths that sound can take from the intake and exhaust systems.
• In addition to airborne sound, solid-body sound predominates in both systems. To keep this ratio as small as possible, this type of oscillating element requires that it be connected to the body using rigid connections.
• The exhaust system is a particularly vibrating element and requires optimal insulation by connecting it to the floor, as well as good routing and positioning of the mounting points to reduce the transmission of sound by the solid body.

Intake system

Pos.	Spare Part No	Name
1	-	Intake manifold
2	-	Connecting hose
3	-	Fixing element (fixing using rubber)
4	-	Air filter
5	-	Resonator

• The volume and length of the intake systems are exactly the same. No changes can be made during maintenance or repair. When connecting components (hoses and pipelines), pay attention to the airtightness of all connections.
• The surfaces of all components subject to strong pressure changes in the intake system are particularly subject to strong vibrations.
• Temperature effects, such as heating of the air filter housing, can cause changes in surface hardness.
• To prevent the transmission of sound caused by vibrations of the intake system surface to the vehicle body, the following points should be taken into account during maintenance:
  • The entire intake system must be isolated from the body and secured in rubber bushings without causing any stress.
  • The air ducts of the intake system must not come into contact with body elements.
  • Inserted foam insulation at the point of contact with the body or other elements.

Exhaust system design

Pos.	Spare Part No	Name
1	-	Catalytic converter
2	-	Dividing element
3	-	Front muffler
4	-	Rear muffler

• Exhaust systems must be installed without any deformation.

NOTE: The rubber mounts that the exhaust system is suspended in also transmit vibrations and under certain conditions can also cause noise problems. For this reason, it is important that the exhaust system is installed without deformation.

• The rubber cushions should not be "stretched".
• The exhaust manifold and catalytic converter (if mounted directly to the engine) must be rigidly connected to the engine (no connectors in brackets).
• Simple check of the problem:
  • Remove all supports that support the exhaust system.
  • Hang the exhaust system on cords in two places, maximum.
  • Check the noise level inside the vehicle during a road test.
  • If this solves the problem, the noise is coming from the exhaust mounts.
  • Install the exhaust system mounts into place, one at a time, and road test the vehicle after installing each one.
  • Continuing step by step in this manner, you can find the support that is causing the noise in the car's interior.

Complete body

Purpose

• The car body is subject to various requirements, especially when the car is moving. The predominant vibrations are those caused mainly by other elements of the car, such as the engine and supports. Therefore, the body is one of the most important elements of the car in terms of NVH parameters.
• The described elements of the car in accordance with their functions were individual independent sources of noise. But the body connects and places all the elements of the car.
• The body is subject to vibration from airborne sound and sound carried by solid bodies.
• The assembled body must absorb all vibrations and impulses from all elements and, if possible, not transmit them into the car interior.
• The largest NVH parameters on the body are characterized by steel roof, side and floor panels. They are strongly stimulated by the body structure and act like a loudspeaker transmitting noise.

Additional requirements for the body

• Good collision behavior
• Large interior space
• Corrosion resistance
• Small mass
• Good aerodynamic shape

A compromise must be made between these points. One example is between high body rigidity for good NVH characteristics and good collision energy absorption through the use of deformable zones.

Example:

• Waterproof does not mean soundproof, so the following points should be taken into account:
  • Correct installation of windows.
  • Correct installation of seals.

• Additional points
  • Holes in the engine compartment
  • Door and window seals
  • Heater and ventilation holes
  • Sunroof Drain Hoses

• The body side members and cross members are often channels for sound transmission. Possible corrective measures:
  • Installing baffles at critical locations such as the joints between the spar and the A-pillar to provide good sound insulation for airborne sound.
  • Installing foam plugs in problem areas.
  • Filling hollow sections with foam.

NOTE: Foam elements and foam padding are used in automobile manufacturing to seal airborne sound transmission points. If wind, tire, engine or road noise is a problem, the foam element placement should be checked.