Vibro-Acoustic Reciprocity Analysis Using Dodecahedral Sound Source and its Application for Material Assessment
Authors:   Mahindra & Mahindra Ltd., Anerao Nitesh , Tulesh Kuma Tulesh Kuma ,   Mahindra & Mahindra Ltd.
Published: April 2022

NVH (Noise, Vibration and Harshness) is an important aspect of vehicle design since it is one of the biggest influencing factors for customer while making decision to buy a vehicle. Hence, automotive engineers strive to design superior cabin compartment to maintain pleasant levels of noise, vibration and the overall experience within the vehicle. Transfer function analysis is primarily used to quantify the relative relationship between source and receiver region. The focus of this paper is to measure vibro-acoustic transfer function using reciprocity approach instead of direct impact test measurement to evaluate the frequency response of heavy layer barrier materials. Normally, it is difficult to mount and quantify the correct response of such heavy damped materials in usual random incidence sound transmission test since its performance is very sensitive to leakage (less than 1% required). The purpose of demonstrating the vibro-acoustic reciprocity in a vehicle is to show that the principle works good even for non-linear system like vehicle, in most cases where mutual coupling effects are negligible and hence at most area the frequency response for tested material holds true even at vehicle level. The reciprocal measurement involves usage of dodecahedral sound source to acoustically excite the cabin from desired seating location and accelerometers being placed at all the response locations of interest on vehicle body. In case of direct measurement, vehicle body is excited using impact hammer and acoustic pressure is measured at desired seating location. It is observed that transfer function calculated with direct and reciprocal measurements showed a good match with overall deviation of less than 2 dB on an average. The reciprocal measurement based transfer function can be used as a logical input for prediction of receiver level sound pressure data/sound quality or prediction of source strength depending on the subject of requirement for dynamic tests. The validation helps in optimizing the traditional approach for measuring structural response using vibro-acoustic transfer function in frequency range of 80 - 5000 Hz. The traditional method involving impact hammer test for predicting response at various points in vehicles is a time taking process and inaccessible measurement locations are prone to error. The linearity of the reciprocity measurement is checked for various locations in vehicles and found to be working satisfactorily in frequency range of 80 - 5000 Hz where frequencies till 1000 Hz are very well correlated and deviations are observed above 1000 Hz comparatively for all measurement locations. The main idea of this paper is the application of above reciprocity principle for assessment of acoustic materials used for noise control. The reciprocal approach of predicting transfer function is used to capture frequency response of heavy layer barrier material which uniquely characterizes its behaviour. To verify that material comparison holds true, comparison of two test samples with known performance are also made and it is seen to correctly quantify the frequency behaviour of one sample with respect to other.