The Oily Rag - British Saloon Car Club

“No Noise Is Good Noise” Part One
The vehicle NVH (Noise Vibration & Harshness) engineer has four physical properties available to him to reduce/control noise
& vibration within a vehicle.
- Vibration or panel damping
- Noise barriers
- Noise absorption
- Active cancellation
Over my four or so decades in the industry available materials & manufacturing technologies have changed significantly. To a
degree, driven by various factors, they have gone full circle. In the sixties available processes and materials resulted in only
fairly lightweight products being available to function as either noise barriers or absorbers. At this time available vibration
damping materials were applied by spraying. Whilst noise absorbing materials have remained lightweight noise barrier
systems have transitioned from lightweight, through increasingly heavy, moulded, components to today’s dual density,
significantly lighter, moulded products.
The amount of text required to effectively cover this subject is felt to be too much for a single newsletter edition. Therefore
the plan is to serialise into three or four editions.
1. Vibration or panel damping materials.
These materials are applied directly to the body sheet metal prior to painting; they are heat cured and bond intimately to the
steel. Often, in older vehicles, those applied in sheet form, bridged the swages in the sheet steel resulting in holes in the
material. The intention was that the material melted to conform to the actual shape of the steel.
Such materials are said to be ‘visco-elastic’ they resist the vibrational behaviour of the vibrating sheet steel, converting what
is essentially vertical vibration into horizontal movement within the material. Such shear energy is converted into heat via
friction as mineral elements rub together.
Damping materials are most effective against truly resonant behaviour that is where the panel is freely resonant & not being
forced into vibration by surrounding stiff structure. It is all a question of the available correction force.
As stated earlier original damping materials were sprayed onto the vehicle body, generally these materials were
manufactured from waste paint products their effect being more about added mass than high performance vibration
damping. There are two ways to control a vibrating system, introduce damping or add mass; in those early days it was more
likely to be the added mass effect.
At Triumph the introduction of the front wheel drive Triumph 1300 lead to a lot of work to obtain a material with increased
performance to treat the front wheel housings.
Spraying of materials in those days was hit or miss being manually applied.
Obviously the ultimate test of the material was in its effect upon the vehicles interior noise & vibration levels (vibration can &
does cause noise) However less costly testing was required to test material performance as new iterations were developed.
In the UK a British Standard procedure was adopted, BSAU125-1965, which utilised a test bar nominally 24” x 2” x1/4”thick.
This was of a specific steel type it having been determined that when supported on knife edges at its nodal points (zero
vibration) it resonated at 100Hz when struck a sharp blow. Electronic equipment was used to determine the rate at which the
vibration resulting from the blow decayed. Comparison is always made with an untreated bar. For convenience, as most
electronic measuring equipment utilised Decibels as a unit of quantity, the results obtained were expressed in dB’s/second.
The higher the reading the better the material.
Interestingly the NA industry also utilised a thick steel plate to determine damping materials performance, the so called
Geiger Plate Method. The plate used was thinner than the UK one, probably 1/8” and much larger in area, probably 18”
square. This plate was supported on; I think three sharp points, again at nodal points.
Both the UK & US methods tested at around 100Hz, most vehicle damping problems occurring at low frequencies
The world industry standard today is the Oberst Methodology developed in Germany. Unlike the UK & US methods Oberst
utilises strips of body gauge steel in a fixture. The system relies upon electronic excitation to vibrate the test steel. It is
therefore capable of determining performance at a range of frequencies. The test stand can also be installed in a small
climatic chamber which allows for the automatic testing over a range of temperatures. As with most materials damping
materials change their state with temperature, it is important to determine the full range of performance
Materials & application techniques have both evolved significantly. Much lighter weight, high damping efficiency materials
have been developed which can be applied as flat die cut sheets or very accurately sprayed with robots thereby meeting the
demands of high performance with lightweight.
Detailed dynamic analysis of body structures enables us to accurately determine both where & how much material is required
to alleviate/cure problems thereby increasing occupant comfort.
Adding a stiff layer to the upper surface of the material serves to “constrain” that surface. This focuses all of the energy into
the core of the material. Such Constrained Layer Systems may have a simple aluminum layer or a stiff cardboard as the
constraining layer. Being less flexible they are more difficult to apply in end use as ideally they require a flat surface, which
few vehicles have.
We, as end users of the vehicle, can make little use of such visco-elastic materials because of the requirement of intimate
contact with the sheet metal. However we can make use of another effect, “inter-face frictional effect”. That is a material laid
upon a vibrating panel will resist the movement of the panel to a degree, dependent upon the material & its weight. Thus
fibre batts laid onto your cars panels will have a positive effect on vehicle noise. Gluing negates the effect
As we will see later in the discussion fibre products have additional beneficial effects.
Tony Hughes, aka NVH ONE