1 PREDICTION OF THE SPEECH TRANSMISSION QUALITY IN THE PRESENCE OF BACKGROUND NOISE USING THE RAY TRACING TECHNIQUE Abderrazak Mejdi ESI North America, San Diego, California, USA email: amj@esi-group.com Bryce Gardner ESI North America, San Diego, California, USA email: bga@esi-group.com Chadwyck Musser ESI North America, San Diego, California, USA email: cmu@esi-group.com Noise levels inside commercial aircraft and automobile impact both the quality of the speech communication between the crew members and the communication between the crew and passengers. Noise measurements are expensive and have significant variability. Therefore, geometrical techniques can be used as an alternative to predict the steady state frequency response or the impulse response. This paper presents a ray-based technique to predict the acoustic response and speech transmissibility indices inside aircraft and automobile. The ef- fect of the background acoustics levels on speech communication is analyzed. Moreover, the effect of the acoustic treatments on the reflected field and its interference with the direct field perceived by the listener is analyzed. Keywords: Ray Tracing, acoustic wave, background noise, Speech Intelligibility 1. Introduction Speech intelligibility constitutes an essential characteristic of the quality in vehicles such as aircraft, cars, trains, etc., The sources of noise causing high-level interior background noise which may affect the quality of the speech vary from one type of vehicle to another. The sources most responsible for noise are the aerodynamic flow, fans, internal turbomachinery, propeller, rotors, exhausts, engine combustion etc., [1]. These sources represent either airborne or structure borne excitations that are transmitted through the outer section of a vehicle and generate interior background noise. This noise provokes serious discomfort and fatigue of crew and passengers, and potentially endangers the safety of flight or driving [2]. Measurement of the transmitted noise level is often expensive or difficult to perform during early stages of design before test hardware is available; therefore, numerical simulations are used as an alter- native. Often, discretization methods such as finite element and boundary element (FE/BEM) are used