Sound Power Estimation of a Mounted Car Engine

Abstract

The NVH optimization process of a power train often consists in a target setting for the acoustic power radiation of the engine in free field working conditions (in an anechoic or semi-anechoic room). This method requires the engine to be dismounted from the car and to be measured in an anechoic or semi-anechoic room which is costly and time consuming. Moreover the free-field characterization is not a good predictor of the acoustic behavior of the power train when it is mounted in the engine bay of a car (very reactive field). This paper presents a number of existing methods to determine the acoustic power radiation pattern of the engine mounted in a car using an intensity probe which is based on a pressure sensor and a particle velocity sensor. For the lower frequencies the velocity probe is used, for the higher frequencies both pressure and velocity is used to measure intensity. A new method for the mid-low frequency range is presented. A measurement in a non-anechoic engine test room is done to determine the relation between the particle velocity close to the surface of the engine to the intensity at a certain distance. This relation is used to estimate the intensity at lower frequencies from the surface velocity. INTRODUCTION With a Microflown sensor it is possible to measure the acoustic particle velocity directly at a well-defined location. A P-U probe consists of a pressure microphone and a microflown, which makes it possible to measure velocity, pressure, but also sound intensity and sound energy. The particle velocity sensor is made by two hot micro-wires. When a flow pass through the wires one of them is cooled down more than the other. That causes a variation in the electric resistance that is measurable and proportional to the velocity of the flow (fig.1).