01054137

Component

With the increasing awareness of consumers to product noise combined with stringent noise regulations this paper describes how acoustic comfort has become a primary interest in the design and manufacture of mechanical systems. A popular way to reduce the undesirable or harmful effects of sound is to block the sound transmission paths with passive acoustic material treatments. These treatments are generally multilayered and consist of some type of dense barrier material in combination with porous, open celled absorptive materials. The barrier layers can be made of materials from rubber to hard plastic. Typically the absorptive materials are made of either polyurethane foam or some type of compacted fibrous material. It is important for manufacturers and engineers to understand how effectively an acoustical treatment can block incident sound when designing a mechanical system. Transmission loss is a key quantification of the effectiveness of acoustical treatments for engineering applications. The sound transmission coefficient of an acoustical treatment is a function of frequency defined as the ratio of sound energy transmitted through the treatment to the amount of sound energy incident on the source side of the material. Sound transmission loss (STL) is then simply defined as the transmission coefficient expressed in decibels. Most engineers and scientists who deal with these types of noise control treatments perform experiments to compare the STL effectiveness of different treatment options. The standard test methods for STL involve using two adjacent rooms with an adjoining transmission path. The treatment under test is placed between the two rooms in the adjoining transmission path, sound is generated in one room, and measurements are taken in both the source and receiver room to characterize STL. These methods are well defined, time tested, and reliable. Unfortunately, implementing these testing methods reliably requires large and expensive test chambers. In many situations where STL tests are necessary but infrequent, this cost and space burden is unacceptable. A STL testing procedure that is less costly and requires less space would be of great interest in this situation. A method exists that can fit these low cost and space specifications. It utilizes a modified impedance tube with four microphones. This method measures what is known as the transmission loss matrix of the acoustical treatment material, from which STL can be extracted. This paper, conducted at Michigan Tech University (MTU), attempts to implement and validate this transfer matrix method by testing four materials with traditional two-room methods and the modified four microphone impedance tube. Brief theory of the two-room techniques and extended theory of the modified impedance tube technique is presented in this paper. The strengths and weaknesses of the three two-room methods and the transmission loss matrix method are also described.

01054353

English

pp 209-216

2004

Noise-Con 04. The 2004 National Conference on Noise Control Engineering

CD-ROM

Figures (8) ; References (8)

Acoustic measuring instruments; Acoustics; Microphones; Noise barriers; Noise control; Noise sources; Polyurethane foams; Products; Rubber; Sound level meters; Sound transmission

Energy; Environment; Highways; I15: Environment

TRIS, TRB

Jul 16 2007 2:30PM