All Is Quiet?

Listen! Can you hear it? Is it a buzz, a squeak or a rattle (BSR)? With the never-ending quest for quiet vehicles, these words are all too common today in the automotive industry.

To improve product quality and sustain or improve customer satisfaction, automobile manufacturers are mandating that new test requirements be implemented in existing durability tests for automotive interior components. These new specifications define quantifiable and acceptable BSRs for components, subassemblies and higher-level assemblies within the automobile passenger area, including the ashtrays, glove box doors, instrument clusters, sound systems, steering columns, door panels and seat assemblies.

Automotive manufacturers and their suppliers are attempting to quantify the cacophony of sounds within the automobile so designers can make the automobile interior quiet. Quantifying these sounds is a difficult process because every person perceives sound differently. What is acceptable to one person may not be acceptable to someone else.

What is consistent, however, is that automobile interiors that exhibit BSRs are perceived to be inferior or lower quality. In fact, BSRs can sometimes indicate actual design or assembly errors.

The Quiet Car

Most acoustic measurements are made between 200 Hz and 10,000 Hz. Any responses above the ambient noise level are evaluated for corrective action. Those exceeding the ambient noise floor by more than 6 dBa are usually unacceptable and require design or assembly changes.

The detection of BSRs is a lengthy, involved process and continues from the design phase through manufacturing and sales efforts in the form of customer feedback. To detect and eliminate BSRs before the product reaches the customer, manufacturers are studying and implementing a variety of tests, using special test equipment, early in the design phase to help precipitate and identify BSRs that would otherwise arise within the service life of the automobile.

Qualification, in-line production testing and lot field testing are common methods for verifying the synergistic relationship between the various components and assemblies within the automobile. Testing designs in the laboratory provides many benefits, including control over environmental conditions such as temperature, humidity, vibration or radiation; test repeatability; accelerated product maturity; and lower test cost.

BSR Testing

Three basic environments are used for BSR testing: vibration, temperature and humidity. In various combinations and sequences, these environments recreate real-world conditions that can precipitate BSRs and accelerate vehicle aging.

Although the most effective laboratory setup would include the capability to control all of these environments, vibration testing is essential in all setups for stimulating BSRs. The most common types of vibration are swept sine and random.

Swept sine excitation is a time-varying frequency of a single sine tone at various acceleration amplitudes. It is used primarily during the acoustic measurement section of the BSR test.

The classical input provides a simple excitation for studying the dynamic behavior of the test specimen. A typical frequency bandwidth is from 2 Hz to 2,000 Hz for very small items and 2 Hz to 200 Hz for larger specimens (Figure 1).

Random excitation, a broadband frequency excitation, is used primarily for vehicle aging. Random testing excites multiple structural resonances at the same time. The typical frequency bandwidth is 5 Hz to 2,000 Hz for very small items and 5 Hz to 500 Hz for larger specimens (Figure 2).

Without vibration stimulation of the specimen, BSRs will not be generated. Consequently, one of the most essential pieces of test equipment is the quiet vibration test system.

The quiet vibration test system, an oxymoron in the vibration industry, must reproduce a variety of vibration environments and, under certain conditions, remain quiet to allow detection and measurement of BSRs within the specimen. The system also must support small to very large packages, depending upon the specific test configuration, and not structurally couple the vibratory energy into the surrounding test area.

Most of the quiet vibration test systems on the market provide closed-loop servo control of the vibration environment. This means that the dynamic response of the fixture and hardware can be measured in real time and the drive input to the shaker can be adjusted so the response of the control spectrum meets a specific vibration requirement. In this fashion, testing in the laboratory can be controlled and correlated to the actual vibrations encountered by the automobile when subjected to various road conditions.

For really large structures, special control setups may be required. Even with sophisticated control systems, the importance of designing the interface fixturing with sufficient stiffness and acceptable dynamic response (as related to the test level and highest frequency) and selecting the proper vibration test system cannot be overstated. The dynamic behavior of the total test-equipment package is paramount to the success of a BSR program.

In a typical application, an instrument panel (IP) with all associated subassemblies–such as the dashboard, instrument cluster, radio, ashtray, HVAC unit and glove compartment–would be mounted to the special head expander, or to a horizontal slip table assembly for horizontal testing. Then the IP would undergo a rigorous set of controlled, sequential vibration stresses, all based on specific components and their location within the panel.

The sequence usually represents a significant accumulation of service stress to simulate the out-years of the designed service life. At certain intervals within the test sequences, the test level and frequency spectrum of the vibration environment are adjusted so the specimen can be studied and all sources of BSRs identified and systematically eliminated.

Figures 1 and 2 show a generalized range of operating requirements for the vibration test equipment. During the acoustic measurement sequence, the shaker system and fixturing must not produce typical ambient noise levels within the vibration bandwidth in excess of 75 dBa to 80 dBa.

In general, the vibration test equipment must provide the proper stimulus (acceleration and frequency) to excite the fundamental structural resonances of the test specimen without interfering with its acoustic behavior. The dynamic behavior and interaction between the various components of the test specimen are the sources of BSRs.

The sources of BSRs can exist at all levels of assembly. To detect, classify and eliminate them, testing is conducted from the smallest subassemblies to the full automobile.

A common method for testing components within the passenger compartment uses limited full-scale mock-ups. Depending on the configuration requirements, testing may be conducted with a 1/4 buck or 1/2 buck model. These models represent the forward quarter section and half section of the passenger compartment, respectively.

Full-scale models provide a closer correlation in dynamic input between the actual automobile and lab simulation. A number of suppliers test their components within limited full-sized mock-ups of the passenger compartments.

The detection and elimination of BSRs are vital to the success of the quieting process. By using a quiet vibration test system, actual vibratory conditions can be used to study and design quieter components for the total automobile.

About the Author

John Raymond IV is an Application Engineer at Unholtz-Dickie. Before joining the company in 1991, he was affiliated with Hughes Aircraft Co. and Martin Marietta Orlando Aerospace. Mr. Raymond received a B.S. degree in mechanical engineering from the University of Arizona in 1972. Unholtz-Dickie Corp., 6 Brookside Dr., Wallingford, CT 06492, (203) 265-3929.

Copyright 1995 Nelson Publishing Inc.

July 1995