Research Focus Area: Design Dynamic Systems

Subject: DEVELOPMENT OF AN IN-SITU MEASUREMENT TECHNIQUE FOR THE
MEASUREMENT OF ACOUSTIC ABSORPTION COEFFICIENT OF GRASS
AND ARTIFICIAL TURF SURFACES

Keywords: NVH, Acoustics, Vibration, Digital Signal Processing Techniques

Description: A proper vibro-acoustic design of garden and lawn equipments require the measurement of sound power radiated from the machine in an operating environment. The sound power radiated by these equipments is currently measured by operating them on a simulated grass like artificial Astroturf surface inside semi-anechoic and reverberant rooms. The accuracy of such experimental simulation is not known, as it requires precise knowledge of the sound absorption of the acoustically treated rooms. This absorption is proposed to be different from the actual grass surface in the outdoor operating environment. In this research the applicability of a novel in-situ technique outlined in the ISO 13472-1, 2002 to grass has been investigated and is further developed for measurement of sound absorption coefficient of outdoor grass lawns. The in-situ method is based on acoustic impulse response measurement of the material surface. A maximum length sequence (MLS) signal excites a sound power source and a single channel microphone response from a grass field is recorded. A digital signal post-processing algorithm based on Fast Hadamard Transform and Fast Fourier Transform provides sound the absorption coefficient. Two digital signal processing and four data acquisition and measurement chains modules with an efficient graphic user interface programmed in Matlab and commercial software were developed for data acquisition and post processing. An inexpensive speaker-microphone system redesigned and developed at MTU and a commercial speaker-microphone system supplied by John Deere Corporation were implemented. A set of four microphones of different makes and four material surfaces including grass was tested. Several important key points for accurate outdoor data acquisition using MLS signal were reported. Notable changes in sound absorption coefficient results were observed while post processing the windowed and reflected wave impulse responses. A monitoring technique was developed to provide accurate results. The results for all materials from various combinations of the developed hardware and software were compared. The sound absorption coefficient of Dow Quash surface was consistently found to match from the In-Situ method and from the Impedance Tube ASTM E 1050 98 method documented earlier by the John Deere Technology Center. Significant saving in testing time and cost was observed with the In-Situ method as compared to the traditional methods.