GMW 14468 Sound Transmission Loss and Sound Power-Based Insertion Loss Buck Evaluation Procedure

The “GMW 14468 Sound Transmission Loss and Sound Power-Based Insertion Loss Buck Evaluation Procedure” standard, developed by General Motors Worldwide (GMW), is an engineering standard relating to the acoustic evaluation of vehicle components and assemblies using a Buck evaluation procedure. Its purpose was to provide a controlled method for evaluating the acoustic performance of components by measuring sound transmission loss (STL) and sound power-based insertion loss (PBIL). However, this standard has now been discontinued and replaced by the “GMW 14176 Sound Transmission Loss (STL) and Sound Power-Based Insertion Loss (PBIL) Buck Evaluation Procedure”.

Vehicle noise is affected not only by noise from the engine, powertrain, tires, and other sources, but also by how effectively vehicle structures and acoustic materials prevent this noise from reaching the passenger compartment. This standard addresses this issue by providing an engineering procedure to evaluate the acoustic performance of components in a controlled test rig configuration. The basic measurements are as follows:

• Sound transmission loss (STL)
• Sound power-based insertion loss (PBIL)

These measurements help engineers determine whether a component or material provides adequate acoustic insulation or noise reduction.

Sound transmission loss (STL) describes how effectively a partition or component reduces sound transmission from one side to the other. It is usually expressed in decibels. A higher STL value indicates better sound insulation. For example, an automotive dashboard, floor assembly, door system, or acoustic barrier can be evaluated to determine the amount of airborne noise prevented from passing through the component. Sound transmission loss (STL) is particularly useful for engineers during the development phase, allowing them to compare alternative materials, structures, thicknesses, and acoustic treatments.

The second important measurement related to the process is insertion loss based on sound power (PBIL). Insertion loss evaluates the reduction in acoustic output resulting from the application of a component or acoustic treatment. Rather than simply asking how much sound passes through a material, the measurement takes into account the change in radiated sound power produced by adding the component to the test configuration. A higher PBIL value generally indicates that the treatment being tested provides a greater acoustic benefit. This is particularly valuable for evaluating components such as:

• Acoustic barriers
• Instrument panel insulators
• floor coverings
• Engine bay acoustic packages
• Tunnel insulation
• Body cavity linings
• Gaskets and caps
• Flooring and interior acoustic components

A key feature of the process is the use of a test prototype. A test prototype is a controlled physical representation of a vehicle or a part of a vehicle structure. Instead of testing the entire production vehicle, engineers can assemble the component or assembly into a repeatable laboratory structure. This offers several advantages:

• Reproducibility: Different samples can be tested under comparable conditions.
• Development speed: Engineers don't need a complete tool for every acoustic experiment.
• Component comparison: Competing materials or designs can be evaluated objectively.
• Reduced testing costs: Laboratory prototype testing can reduce the need for full vehicle testing in the early development phase.
• Controlled conditions: Undesirable variations associated with a complete vehicle can be minimized.

The principles addressed in the GMW 14468 standard are applicable to many vehicle noise, vibration, and harshness (NVH) improvement activities:

• Instrument panel and fire wall systems: The fire wall separates the engine compartment from the passenger compartment. Acoustic treatments applied to this area are important to reduce the following:
  • Engine noise
  • Powertrain noise
  • Suction noise
  • Exhaust noise
  • Airborne mechanical noise

STL and insertion loss measurements can help determine whether a firewall application provides adequate acoustic insulation.

• Floor systems: Vehicle floors can transmit road, tire, drivetrain, and exhaust noise. Therefore, acoustic carpets, floor insulators, barriers, and damping applications can be evaluated using acoustic performance measurements.
• Door systems: Doors are relatively large pathways for acoustic transmission. Their construction, seals, floor panels, barriers, and absorbent materials can all affect passenger compartment noise.
• Engine bay applications: Acoustic shielding and insulation around the engine bay can be evaluated to determine how effectively they reduce noise transmission into the cabin.
• Body acoustic applications: Other body structures may also be examined in the vehicle NVH development process, especially when engineers need to identify and reduce airborne noise paths.

During testing, the component is assembled in a defined configuration, and acoustic measurements are taken under controlled conditions. The aim of STL testing is to determine the acoustic attenuation provided by the test specimen. In PBIL testing, engineers compare the acoustic performance of the configuration with and without the component or process. The data obtained can then be used to compare designs.

The GMW 14468 standard is important because automotive acoustic enhancement requires more than simply measuring the overall noise level inside a finished vehicle. Engineers need to know where noise is entering the vehicle and how effectively individual components are blocking it. Therefore, component-level acoustic testing provides valuable engineering information before full vehicle validation. For example, if two instrument panel insulators have similar material properties but one produces significantly higher sound loss, the higher-performance design might be preferred for vehicle enhancement.

STL and PBIL are related but should not be treated as identical measures:

• Main objective:
  • STL: Evaluates the transmission through a component.
  • PBIL: Evaluates the acoustic reduction produced by an implanted device.
• Typical output:
  • STL: dB
  • PBIL: dB
• Focus:
  • STL: Sound insulation / transmission.
  • PBIL: Reduced sound intensity.
• Application:
  • STL: Acoustic performance of material/component.
  • PBIL: System/component acoustic benefit.
• Engineering applications:
  • STL: Comparing transmission characteristics.
  • PBIL: Comparing the effectiveness of acoustic treatments.

This distinction is important because a material that exhibits good conduction loss properties in the laboratory will not necessarily deliver the same improvement in complex vehicle assembly.

The GMW 14468 standard approach offers several advantages for automotive development:

• Controlled evaluation: A standardized test configuration makes comparisons between components more meaningful.
• Objective acoustic data: Instead of relying solely on subjective listening, engineers can use quantitative acoustic measurements.
• Early design optimization: Components can be optimized before costly vehicle-level testing.
• Material comparison: Different acoustic materials and structures can be compared using a common engineering methodology.
• NVH troubleshooting: This approach can help engineers identify weak acoustic pathways within a vehicle structure.

Today, General Motors engineering operations are required to adhere to the GMW 14176 standard, rather than the previous GMW 14468 standard which applied automatically.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services in accordance with the GMW 14468 standard.