Part 1 of this feature discusses noise sources and analysis methods.
A case study was performed using the National Instruments CompactDAQ USB data acquisition system and SenSound's Helmholtz Equation Least Squares (HELS)-based NAH software. The application involves a noise diagnostic test on a vehicle door assembly to understand system-level interaction of an electric motor and the door module when the motor powers the window up and down, to complement continued efforts to refine the motor to manufacture quieter assemblies.
The electric motor was the source of energy that produced the noise. Accordingly, the effort had always been focused on improving the design of the electric motor to make it quieter. Yet after spending a lot of resources to design and manufacture a quieter motor, the noise problem still persisted.
The door was tested with (below, left) and without (below, right) trim. Various motors were tested, including a known "good" motor and known "bad" motors with different issues.
A 63-microphone array, covering the entire door surface, measured the acoustic pressures. The microphones were connected to NI 9233 dynamic signal acquisition modules with four acquisition channels each in three cDAQ-9172 chassis. The chassis were synchronized via a shared clock and trigger using NI 9401 bidirectional digital I/O modules. The NI CompactDAQ systems were then connected to a laptop over Hi-Speed USB with the data input to SenSound's HELS formulations to reconstruct the surface acoustic maps.
For best results and to ensure the accuracy in reconstruction of the surface acoustic quantities, a conformal array was used. The microphones in the array were positioned as close to the surface as possible to capture the evanescent waves. These evanescent waves are very important because they provide all the details of an acoustic field generated by a noise source but they decay exponentially with distance from the surface.