Enhanced acoustic material testing in a tube

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Demands from the Industry
 
A growing number of customers demand quieter vehicle cabins, and cabin acoustic comfort has become a major concern for the automotive industry. Optimization of the acoustic insulation performance has, therefore, been given a more prevalent role in the design process. In the aircraft industry, acoustic comfort for customers and the use of new, lighter structures for reducing the aircraft’s fuel consumption, are both focus areas and require new innovative acoustic insulation solutions. In general, more stringent requirements regarding sound barrier performance of multilayered noise control treatments - often with tight weight constrains - demand the use of sophisticated prediction models in the design phase to tune the transmission loss in desired frequency ranges. An accurate experimental determination of the acoustical and mechanical properties of the constituent materials is then necessary to validate and calibrate such models.

Due to these industry challenges, there has been a growing interest in measuring acoustic insulation properties of noise control materials. In addition to standard measurement methods that require expensive two-room facilities, typically two adjacent reverberation rooms, there has been a demand for new, quick, and cost-effective techniques that can provide accurate and reproducible data on the acoustic insulation performance.

Transmission Loss Solution
 
Some years ago, Brüel & Kjær expanded its standard two-microphone impedance tube kit(used to measure normal incidence sound absorption properties only) to a four-microphone, plane-wave tube kit, which can also measure normal incidence sound transmission loss.

Although the sound power transmitted through the sample generally depends on both its properties and the tube termination conditions, a method was implemented in PULSE (two-load method) providing normal incidence transmission loss as if the sample were backed by a perfectly anechoic termination, independent of the actual tube termination conditions used during measurement. That is, the solution does not require a perfectly anechoic termination, which would be difficult and very expensive to realize.

The measurement equipment and the test setup are more practical and less expensive than two-room facilities. Only small samples are required, and these can be quickly mounted in the measurement tube. Moreover, the plane-wave sound field generated in the tube guarantees highly repeatable test conditions. However, it is worth noting that there is not necessarily a close relation between the normal incidence transmission loss obtained with the tube and the most commonly assessed random incidence transmission that is typically measured using a sample tested in a two-room facility.

Enhancements in PULSE 11

As part of our commitment and continuous attempt to create innovative solutions that respond to market demands, Brüel & Kjær has enhanced the PULSE Transmission Loss Software (part of Acoustic Material Testing in a Tube Type 7758).

A transfer matrix representation (widely used in scientific literature) is adopted, implementing the two-load method. Moreover, when the sample under test is symmetric front-to-back, a new procedure (one-load method) that halves the measurement time is implemented. Transfer matrices of homogeneous acoustical elements are particularly useful when predicting the transfer matrix of a complete, multi-layer, noise control treatment. As part of our commitment and continuous attempt to create innovative solutions that respond to market demands, Brüel & Kjær has enhanced the PULSE Transmission Loss Software (part of Acoustic Material Testing in a Tube Type 7758).

Besides the normal incidence transmission loss, the new software determines other quantities such as the fraction of incident energy dissipated within the sample, the characteristic impedance and the complex wave number of the sample in the case of porous materials that may be modelled as an effective Fluid (like glass fibres and fibrous materials). The last two quantities (or their derived quantities complex density and complex sound speed) are most often required when validating and calibrating Finite Element or Boundary Element models of arbitrarily shaped materials.

The software includes extensive new features that substantially improve the user interface for optimal operator convenience and dramatically increase the productivity of testing. Data can be stored in a dedicated database, increasing the efficiency of browsing, comparing, and sharing data across the company. It also makes it easy to create statistical reports on batch measurements, such as mean and standard deviation calculations.

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