Sound transmission paths through a statistical energy analysis model of mechanically linked aircraft double-walls

Sound transmission loss (TL) through mechanically linked aircraft double-walls is studied with a statistical energy analysis method. An overview of the method is given with details on acoustic and structural transfer path analysis. The studied structure is composed of a thick composite sandwich panel representative of a skin panel, lined with an acoustic insulation layer (glass wool), and structurally connected via vibration isolators to a thin composite sandwich lining panel representative of a trim panel. Two types of vibration isolators are considered: a soft and rigid mechanical link. Various experimental methods were used to assess the accuracy of this model. This study shows the robustness of the simple four-pole modeling of isolators, which depends mainly on the importance of correctly determining the experimental dynamic stiffness of typical aircraft vibration isolators. The prediction of the TL while acceptable was, however, found less satisfactory for the soft configuration. This is traced to the uncertainties on the used coupling loss factor. Finally, a transfer path analysis is performed to identify the contribution of each transmission path in the entire frequency range of interest. Results show that non-resonant airborne transmission dominates in low frequencies, the airborne radiation is significant in the critical frequency region of the panels, while the structure-borne radiation increases the noise transmitted in the mid- and high-frequency ranges.

Influence of mobility completeness and source behavior on the robust-ness of Transfer Path Analysis and Source Characterization methods: A numerical study.

Structure borne noise is considered a major contribution to the noise generated inside aircrafts. In order to analyze it, engineering methods have been developed such as Transfer Path Analysis (TPA) and Source Characterisation (SC). These methods are based on active and passive properties of the source and the receiving structure being coupled or decoupled. The theoretical formulation requires mobility according to all Degrees Of Freedom (DOFs) and rotational DOFs represent a challenge for experimental application. To fulfill the mobility matrix, indirect method have been developed and specific sensors have been proposed, resulting in a more complex experimental set-up and an increase in measurement uncertainties. The necessity of assessing the full matrix completeness is thus still questionable. The robustness of these methods with respect to the matrix completeness and the source behavior is investigated numerically in this work. A numerical model has been developed to simulate vibrating sources with simple or complex vibratory behavior and to assess the mobility matrices for any completenesses. Velocity on the receiving structure is used as a target indicator. The influence of source behavior and completeness are discussed and the results show that the required mobility completeness depends on the source behavior.

On the resonance sound generated in a vehicle cabin moving at low speed due to breaking force

To improve the comfortability in a vehicle cabin, unwanted noise which is recognized as an allophone generated from automobile wheels was experimentally studied to investigate its generation mechanism and to develop its reduction countermeasures. In this experiment, simultaneous measurements of sound pressure and vibrational acceleration of the wheel surface were performed. Then, frequency analysis, vibrational modal analysis and operational transfer path analysis were performed by using measured data. The results show that this kind of noise started in a low frequency first and then became higher. Furthermore, the high-frequency noise was mainly generated by vibrational acceleration at its center and near the rim when the wheel spoke gets close to the brake caliper. The high-frequency noise is around 250Hz, 750Hz, 1000Hz and 1250Hz, and the wheel spoke easily gets vibration and resonance mainly from around 750Hz and 1000Hz. Vibration at 750Hz occurs on the side of the wheel spoke in the rotation direction, while vibration at 1000Hz occurs at the midpoint of the wheel spoke. The closer to the brake caliper, louder noise was generated at the wheel spoke.

Application of Blocked Force Methodology in NVH development of Electrical Machines

To determine excitation forces of electrical machines from measurements, Transfer Path Analysis is commonly used. Transfer Path Analysis yields input forces indirectly utilizing measured responses and transfer functions. When conducting transfer function measurement, it is recommended that the source of excitation is mechanically isolated from the receiver structure. However, in practice this is difficult to achieve without affecting the transfer path itself. The concept of the Blocked Force method introduces blocked forces which are independent of the receiver structure, thus allowing measurement of transfer functions without isolating the source. In this research, a stator / rotor assembly is considered as the source. This assembly is bolted to a test-housing, considered as the receiver. Blocked forces are determined at the mounting locations between stator and test-canister. The correctness of the calculated blocked forces is verified by comparing the predicted and measured responses at selected target points which were not used for determining the blocked forces.