Relevance of component level validation from Vehicle NVH point of view

Often, with an objecitve of performance cascading from vehicle level NVH, its components or subsystems are tested in an idealistic way or in a stand alone condition. A question comes to mind how much is relevance of such evaluations.

An example is taken here which is related to sound absoroption measurements of automotive seats in an ARAI reverberation chamber as per ISO 354 / ASTM C 423: first the test was done without human beings occupying seats and then the test was repeated with 4 persons.

There was found a good difference acorss all octave frequency bands between 2 operating conditions w.r.t. Reveraberaion Time (RT60) and hence the Sound Absorption (SA) per seat. With the human occupancy, RT60 came down and SA increased .

In order to support this, when a diesel engine-powered car was placed in an anechoic chamber with its engine at idling, and its In-cab noise was measured with nobody sitting inside and 4 persons occupying the seats, a difference in Sound Pressure Levels (SPL dBA) was found to be as high as 8 dB at the Front Seat especially for 1/3rd octive frequency centred around 100 Hz which is close to acoustic cavity mode of the car-cabin.

Obviously, the engine-induced In-cab noise came down due to the good absorption of the sound due to clothes & skin of the car passengers over a wide frequency band of 100 to 6300 Hz.

A point is that in an above case, physical validation of the seats alone condition has a little relevance from realistic vehicle-level NVH point of view.

Of course, considering variability of customer driving pattern ( with single driver versus fully occupied car driving, different sizes of passengers, etc), one may go in for the empty-seats testing; especially the seat-suppliers & CAE engineers may prefer it w.r.t. engineering sign-off of their components.

But the NVH Managers need to take ahead mainly the system level results or with realistic boundary conditions which would be useful for assessing & refining their vehicles for actual road driving conditions.