AN ANALYSIS OF THE DYNAMIC CHARACTERISTICS OF A SALOON CAR STRUCTURE APPLIED TO THE GENERATION OF LOW FREQUENCY NOISE

This report describes the first stages of an investigation into the mechanisms of generation of noise in the passenger compartment of a medium sized saloon car over the frequency range 30-100 c/s, when excited at a single point by an electromagnetic vibrator, the vibrator serving as a source of structure borne noise analogous to the engine or road surface. In the first instance, the investigation was carried out on a bare body, i.e., one without doors, windows, etc., the structural response of which was measured in terms of the driving point mobility, vibrational mode shapes and associated damping. The noise generated by the vibrations of the structure was measured in terms of the sound pressure distribution in the passenger compartment. The noise generated in the passenger compartment is mainly the result of vibrations of the floor and roof sections of the vehicle at a number of closely spaced resonant frequencies, a direct correlation was observed between the parallel resonances of the driving point mobility curve and the sound pressure measured in the passenger compartment. Bending modes were found to be more efficient generators of noise than were torsional modes, and where out-of-phase conditions existed between the floor and the roof motions there was found to be up to 6 dB increase in sound pressure over the in-phase condition. When doors, windows and bonnet cover were fitted to the bare body they produced a marked increase in the damping of the structure which reduced the dynamic range of the driving point mobility by up to 30 dB. In fact, the mobility over the greater part of the frequency range was then found to be constant within plus or minus 5 dB. This approximately constant input mobility combined with the spring-like character of the closed volume to control the acoustic response at frequencies below 60 c/s, which was markedly greater than with the bare body. Acoustic resonance of the closed volume controlled the sound pressure level at frequencies above 60 c/s. A more complete investigation of the assembled structure, which is still in progress, should identify the fundamental properties of a vehicle which control the noise produced in the passenger compartment, when the structure is excited by such sources as the road and the engine.

  • Corporate Authors:

    Motor Industry Research Association

    Lindley Near Nuneaton
    Warwickshire,   England 
  • Authors:
    • Dunn, J W
  • Publication Date: 1967

Media Info

  • Features: Appendices; Figures; References;
  • Pagination: 25 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00261288
  • Record Type: Publication
  • Report/Paper Numbers: 196718 Intrm Rpt
  • Files: TRIS
  • Created Date: Oct 22 1974 12:00AM