Wheel RPM Research Articles

Device for reducing tire/road surface noise

A device for reducing certain sound ranges of tire/road surface noise of a motor vehicle includes sound absorbers in the form of resonators positioned at the inner wall of a wheel well of the vehicle in the area of sound projection of the tire/road surface noise. The device has an electronic control device for receiving a signal preferably derived from an ABS system in the form of the wheel rpm. The control device adjusts the resonators according to the number of tread blocks of the tire, the sound emission characteristics, and the wheel rpm. In another embodiment the electronic control device is supplied with signals received from a sound receiver that records the course over time of the sound pressure. The control device adjusts the resonators such that the measured sound pressure signal is minimized. In another alternative the device has a data processing unit including an electronic control device. The control device is supplied with a signal from sound receiving devices that record the course over time of the sound pressure. The data processing unit together with an adjusting device adjusts the resonators such that the measured sound pressure signal for selected frequency ranges is minimized.

Some calibrations and applications of the aerodynamic fan reference sound source

Introduced by H. C. Hardy [Noise Control (May 1959)], the mechanical reference sound source (RSS) provides a broadband sound field by turbulent air shear at the perimeter of a rotating open centrifugal fan wheel. For a given wheel rpm, geometry, and air density, the emitted sound level is constant within less than 1 dB when averaged over a few seconds. The sound power is relatively uniform in octave or third‐octave bands from 125 to 8000 Hz. Usable levels of sound power are emitted above and below these frequencies, with field measurements in the 63‐Hz octave band feasible in large rooms. Small motor‐driven units have endured for decades. Hemi‐anechoic, reverberation room and intensity methods JR. Peppin et al., NOISE‐CON 88, and W. D. Gallagher (private communication)] for RSS calibration will be described. Low‐frequency discrepancies between free‐field and reverberation room calibration methods [P. Brnel, B & K Review (1978, No. 3)] have diminished as the reverberation room method is improved (better sound diffusion, space averaging, and decay analysis) [P. K. Baade, 1989 ASHRAE Trans. (January 1989)]. The portability and reliability of this device suit it especially to worldwide use in hostile environments such as factory assembly rooms and construction sites. Some recent experiences in application of the Acculab RSS to small and large device sound power measurement and room sound absorption will be described.

Inlet turbulence—rotor noise

The present contribution is concerned with aerodynamic noise from isolated, subsonic tip rotors. With large axial spacing between blade rows and elimination of inlet guide vanes, at least the forward radiated noise from modern single-stage acre-engine fans is dominated by isolated rotor noise. In an early study, Sharland described three agencies by which such a rotor would produce noise, namely, vortex shedding, turbulent boundary layer noise, and noise due to inflow distortion or inlet turbulence. The present study develops means of computing inlet and discharge PWL spectra due to impingement of inlet distortion or inlet turbulence on an isolated rotor. Both dipole and quadrupole mechanisms are analyzed as only the latter afford a good explanation of why fan noise is pressure ratio dependent at constant wheel rpm. The input parameters needed are a description of the inlet distortion far upstream of the rotor and of the inlet turbulence and steady-state design parameters of the isolated rotor. Preliminary ideas of the author have been expanded in several directions by the author's colleagues at GE, especially in the direction of experimental verification of the analysis. This study will emphasize such comparisons.