Theory surrounding Squeaks & Rattles

Why does noise occur in automotive interior trim?

Some materials may be not be compatible when they come in to contact with each other. Other materials maybe smooth against other surfaces but will be rough when rubbed against another surface of the same type, e.g. ABS or Glass.

Unwanted noise generated by two surfaces can be due to the accumulation of multiple factors such as:

• Generation of stick slip
• Bending of surface peaks
• Compression to spring over peaks
• Squeezing out of air to cause a partial vacuum
• Stretching of polymer strands which cause potential energy rises at 1000’s of points. At a certain point the energy is released and causes a noise.

When two surfaces rub, the true area of contact is usually only a tiny fraction of the whole surface, where peaks and troughs of materials meet.

One of the keys to understanding the concept of noise generation is understanding the surface grain frequency of the materials you are using. Grain frequency is the pattern of the microscopic surface structure formed during the manufacture of the material. The relationship between two grain patterns, such as size (depth and height) of grain is very important in their potential to generate noise when dry sliding.

Grain frequency (opposite). The microscopic physical shape effects the compatibility between two connecting surfaces. Like a jig-saw puzzle, some surfaces have a grain frequency that results in the interlocking of ridges under pressure (green colour), the outcome of which is a possible noise.

Some surfaces have low surface affinity under pressure (blue colour) due to their low grain frequency - not as much chance of noise resulting from this scenario.

One way to gauge how materials interact is finding out the coefficient of friction (COF) between two surfaces. Coefficient of friction (aka the frictional coefficient) is a physics value that describes the ratio force of friction between two materials and factoring in the amount of force pressing them together.

The coefficient of friction obviously depends on the materials used; an example of this would an ice skater. Metal glides smoothly and easily over ice. The COF between ice and metal would be very low. There are other factors involved with COF and the explanation given is very simplistic. A good site for more theory on COF is available by clicking here.

Compression to spring over peaks

• Squeezing out of air to cause a partial vacuum
• Stretching of polymer strands which cause potential energy rises at 1000’s of points. At a certain point the energy is released and causes a noise

So, unless problem surfaces are separated or a specialist physical barrier is introduced between the two materials (e.g. felts or foams), there three main factors that must be overcome when trying to solve a noise issue between to two trim materials:

1. Surface grain frequency
2. Shear strength of the microscopic grain
3. Yield pressure of the microscopic grain

Squeaks & Rattles/NVH In the Automotive Industry

The passenger car manufacturers have enjoyed major improvements in their interior noise levels in recent years. However with the reduction in engine noise, many relatively little sounds that are generated both inside and outside the passenger area, that were previously hidden by such factors as road, power train and wind have become more audible to the customer.

Interior noises can annoy vehicle owners and such noises can give the impression of a lack of quality and result in customer complaints and bad press.

While in warranty, after market complaints can become a significant and expensive source of irritation for OEM’s.

NVH/S&R reduction is becoming an ever increasing concern for automotive manufacturers. Engineers spend considerable resources trying to eliminate or reduce noise sources. Designers have many issues to consider when creating the various components that make up a vehicle interior and material incompatibility sometimes is not considered until after a noise problem is identified. Also, what should work on the ‘drawing board’ (maybe CAD software!) or in the lab may not actually be true in the real world.

A squeak and rattle can be discovered during initial testing or after product launch. If discovered early, designs can be altered, parts can be re-engineered. After product launch squeaks can be found due to the various complexities vehicle design, construction and the parts supply chain:

• Tolerances between connecting surfaces may vary or be tighter than intended
• Inconsistencies in the assembly process
• Variations in raw materials used to formulate compounds
• Variation in surface structure of leather and synthetic surfaces

Premium automotive brands and models usually include leather in their interior designs. Leather seating, dash, panelling etc is all a sign of luxury and quality; however leather is the source of many squeaks and ticking noises while a vehicle is in motion. Of course, most are familiar with the sound of leather when positioning yourself in a leather seat.

A quality aniline leather will appear quite smooth but as with most surfaces, under a microscope the situation is very different. (see picture opposite). So, leather is a big source of noise issues within the interior in areas that a customer would not expect to hear noise from. These noises could be very irritating for customers, for example the plastic seating base/surround that houses electrical seating controls; this plastic can interact with the leather seat when the seat is occupied and a ticking or squeaking sound can result.

It is expected that the extra number of electric vehicles on the road will only increase the amount of interior trim noise problems. Engine noise will have previously helped to minimise the audibility of trim noise problems by customers, but the increasing popularity of electric vehicles will make the work of S&R/NVH engineers all the more important.