19th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 DESIGN OF PRODUCT SOUND BY BLENDING OF SOUND SOURCES

Product sound usually consists of a mix of several sources. Models for prediction of sound quality based on combinations of psychoacoustic metrics like loudness, sharpness and roughness have limitations when compound sounds are analysed. When a compound sound is divided into separable sources through auditory streaming such models fail. The character of each source is given by physical properties. The character is usually hard to change without complete redesign of the product. The quality of the final product sound is determined by the mix of sound sources. Efficient product sound design requires tools for recording or simulation of sources, realistic modelling of transfer functions from source to listener and tools for mixing the sources. With such tools a sound source can be auralised as part of sound environments. Recordings of sound sources in a car were used to get source information. The sound from each source was filtered through idealised transfer functions, mixed and reproduced in a 5.1 surround sound studio. The resulting sound was evaluated through listening tests. This approach was shown to be a powerful tool for evaluation of the effect of auditory streaming on subjective judgments of compound product sound. INTRODUCTION Psychoacoustic metrics like loudness, sharpness and roughness [1] and combinations of such metrics into more sophisticated models like annoyance [2, 3], sportieness, powerfulness and luxary [4] are commonly used for prediction of product sound quality. Such models have been shown useful in many applications, but fail if the analysed sounds deviate too much from the bank of sounds used for development of the models. We have also encountered problems when sounds composed of several blended sources have been analysed. Even basic psychoacoustic indices like loudness and sharpness fail in some cases. In one experiment sound of electrical power windows in passenger cars were studied. The sounds were mainly composed of two sources, the electrical motor and the scratching of the window sliding over the seal. These two sources were fairly well separated in the frequency domain, and could hence be separated by spectral filtering. Analysis of the sources separately revealed subjects’ tendency to judge only the motor sound, neglecting the window-seal-scratching. When loudness and sharpness were measured only for the motor sound agreement with subjective judgements was achieved. The human ability to separate sound sources have been thoroughly investigated by Bregman [5]. The theories for auditory stream segregation supports the assumption that psychoacoustic models based on the total sound will fail in applications where listeners focus only on part of the sound. Carosone [6] has described Ferrari’s approach to handle blended sound sources. This approach served as starting point for the design of an experiment with the aim to study whether typical interior sound of a passenger car is heard as a unity or if attention is drawn to some of the sound sources. Four main sound sources in a passenger car were sampled by near field recording. To get an approximation of each source contribution to the interior sound, the signals were filtered using idealised transfer functions. Through mixing of the filtered sounds four different sounds with the same overall loudness level but different levels of the separate sources were created. These four sounds were evaluated in a listening test at two different loudness levels. The subjects were asked to judge the overall loudness of the sound, and they were given no information about the way the sounds were created. By this approach