Quality Evaluation of Engine Start and Idle Sounds

One of the interesting questions in vehicle acoustics is how do we evaluate the instationary sounds. What are the relative contributions of the different sequences to the overall quality judgment, if the sounds have two characteristic time sequences? Normal operation of a vehicle covers stationary and instationary sounds depending on driving condition. “Engine start” (instationary) and “Idle” (quasi-stationary) are two examples of these driving conditions which are coupled with each other functionally. Therefore we hear the sounds of these conditions sequentially. Once such a hearing experience is over, we can form an overall evaluation of it. This study deals with the rules behind the evaluation process. We examine the hypothesis stating that the experience over time is not a simple combination of its discrete components. The influences of the extreme part (feature-based), beginning part and the final part of the experience to the overall evaluation are investigated. In order to approach this aim in a systematic way, psychoacoustical experiments were conducted. In the first experiment, the binaurally recorded engine start and the idle sounds of 12 cars from different brands were presented to the nonexpert subjects. They were asked to describe what they like and dislike about engine start and idle sounds. Some of the verbal descriptors which were used by the subjects were selected for the further experiments. In these experiments, the engine start and idle sounds were evaluated separately and variously combined by the subjects using a quasi continues scale. The results of the experiments give some important hints for the experience summation of multiple events. INTRODUCTION Quality evaluation of instationary sounds is an important topic for product designers, due to the fact that most of the products have instationary operational conditions and generate instationary vibration and sounds. Recently, different studies have focused on the loudness perception and evaluation of instationary sounds [1,..,9]. Most of them have discussed the suitable pychoacoustical measurement methods for the instationary signal evaluation. Kuwano and Namba have found that instantaneous and global judgments can be well correlated with acoustic level in dBA and Leq, respectively [6]. In another study, the instantaneous and global judgments were well correlated with a specific value of loudness, N4 (ISO 532B [7]). While the results of some investigations show that the global loudness judgment of a timevariant signal is the simple average of the continuous judgments, other studies claim that it is not a simple average. If we take into account the hypothesis of Kundera about our memory, according to which “our memory does not take film, it takes photographs”, it is possible to say that extreme events dominates our experiences, but also our evaluations [10]. Possibly not only extreme parts but also the final part (end) of the signal stimulates our evaluations, because of the short-term auditory memory effect called the "recency effect" [5]. This study focuses on the quality evaluation of complex time-variant vehicle sounds which have two characteristic time sequences, such as engine start and idle. Besides of the loudness, other quality features of the vehicle sounds are in interest of this investigation. Engine start sound is an important factor influencing the quality impression received from a vehicle. It has a complex nature in time and frequency domain and can emphasize the feeling of a sporty, solid, or a rather cheap, flimsy vehicle. As an example, interior noise spectrograms of two different vehicles that have been recorded during an engine starting condition are shown in Figure 1. Spectrograms were obtained with 32768 FFT points (a: better frequency resolution) and 1024 FFT points (b: better time resolution). Combination and interaction of a number of sources, such as starter motor, intake system and combustion engine cause a number of instationary engine start sounds. A rapid increase of the engine order sound pressure level and frequency gives powerful and sporty feeling (see Figure 1a, time interval: 0.6 – 1.25 sec.). Across a range of engines, there is a large variation of the durations of the different sound source events. Long duration of the starter motor noise and late start of the combustion engine gives impression that the car is powerless and in Figure 1. Interior noise spectrograms during engine start. 100 200 500 1k 2k 5k 20k