Active Design of Automotive Engine Sound

For many years, active noise control (ANC) is a concept strongly discussed for the reduction of unwanted sounds. In addition to minimizing noise, this technique can be extended to achieve a target sound which is different from silence. This concept is called Active Sound Design (ASD). It does not only reproduce a target sound, but monitors the sound and uses a closed control loop . This results in a high quality reproduction and allows the target sound level to be lower or higher than the original sound level without system. One major application is the automotive engine noise. ASD allows to merely freely define the engine sound independent of the physical engine used in the car. The paper presents the concept of ASD and reports on applications of this technique for automotive interior and exterior sound. INTRODUCTION Active Noise Control (ANC) is quite an old concept in acoustics [1] and became the topic of an increasing number of textbooks during the last years [e.g., 2,3]. The base idea is to add some sound sources at the area of interest and let them radiate some sound coherently with the primary noise in order to sum up with a lower residual noise. The task of an ANC system therefore is to reproduce the primary sound field in counter phase at the whole area of interest, for all times of interest and always in time. This has to be done very precisely, as can be seen in figure 1. This figure shows the level of a residual tone which results of summing two sine tones of the same frequency, but with different amplitude and phase. To achieve this necessary accuracy, closed control loops are required. Typically an ANC system uses monitoring microphones to measure the residual noise and some digital adaptive filters to calculate the speaker output signal, often based on some advanced information as e.g. the rpm information of rotating machinery. Although reducing some noise is often very beneficial, it is not always the best thing to do in acoustics. This is definitely true for the sound of products, which is often used for personal judgments on a product or as a part of the user interface of a product. As a result, sound design, i.e. the intended shaping and design of the sounds, is gaining an increasing importance for many products. Active Sound Design (ASD) is the technique to use active means (e.g., speakers) to intentionally modify the sound of a product. A key application area for ANC [e.g., 4], sound design and ASD [5,6] is the automotive industry. ACTIVE SOUND DESIGN (ASD) CONCEPT Conceptionally, Active Sound Design (ASD) is best understood by having a look on Active Noise Control. Figure 2 shows a sketch of a typical ANC system for the interior engine noise of a car. To minimize the interior noise, the signal processing implicitly assumes a target sound of zero (silence) and minimizes the residual error microphone signal. SCHIRMACHER ACTIVE DESIGN OF AUTOMOTIVE ENGINE SOUND AES 112 CONVENTION, MUNICH, GERMANY, 2002 MAY 10–13 2 0 45 90 135 180 225 270 315 360 -10 -7 -4 -1 2 5 8 11 14