Figure shows noise results for a prototype van measured on a rolling road. The vehicle had a four-cylinder-in-line engine. The engine speed was varied in 3rd gear from just above stall to about 4500 rpm. The sound pressure at a representative interior position was measured in dB using the standard reference, namely 20 ?Pa. Figure Q2 shows the overall noise level and also the component associated with the second harmonic of the engine speed.
a) The noise peak at 3600 rpm is due to an acoustic resonance with a standing wave between the floor and roof of the van. What is the frequency of this resonance and what is the vertical dimension of the van interior?
b) Sketch a graph of the microphone signal at 3600 rpm. Your graph should include time and sound pressure axes with scales to show the ranges for the signal you have sketched.
c) Comment on the overall results in Figure Q2 indicating if they are acceptable or if they indicate a specific NVH problem.
d) It would have been better to have measured the sound pressure in dBA. Explain why.