Comparison between Road Test, Loudspeaker and Headphones Evaluations of the Sound Quality of Automobiles MALCOLM J. CROCKER 1 , CÉDRIC BÉCHET 1 , RAN ZHOU 1 and HE LINGSONG 2 1 Sound and Vibration Laboratories Mechanical Engineering Department Auburn University, AL 36849, USA www.iiav.org 2 School of Mechanical Engineering, Huazhong University of Science and Technology Wuhan, Hubei, 430074, China Abstract: This paper describes sound quality evaluations of five different automobiles. The primary purpose of the study was to compare sound quality evaluations made of interior vehicle noise during road tests with recordings of the noise of the vehicles played back to a jury of subjects using different playback methods. The experiment consisted of recording the sound fields inside the five cars with dual channel microphones situated in a dummy head. The study involved using a jury comprised of a large number of student subjects who first evaluated the sound quality of the five vehicles during the road tests, as well as later evaluating recordings of the same five vehicles played back through loudspeakers and headphones. It was found that there is a high correlation between the three different methods of evaluation used for the same vehicle. This study demonstrates that it is possible to evaluate the sound quality of a car in the laboratory with some confidence using the playback methods described in this paper. It also shows that laboratory evaluations, which are less expensive to conduct than road test evaluations, are good enough for manufacturers to determine the sound quality of a vehicle and to decide on the important sound quality factors needing improvement. Key words:- Sound quality, product sound quality, noise, vehicle noise , automobile noise 1 Introduction Sound quality is a term that was coined in the late 1980s by acoustical engineers who wanted a subjective description of the noise output of manufactured products. In the past 20 years, the term has more commonly become known as product sound quality and has become an important property of manufactured products, particularly of automobiles. Manufacturers try to improve the sensation of ride comfort for occupants of vehicles. Previously, improving the sound quality of a vehicle simply consisted of decreasing the overall sound pressure level. Now manufacturers tend to focus as well on additional properties of the manufactured product’s sound such as loudness, frequency content, tonal components, impulsiveness, and level fluctuations. Noise control of manufactured products through design procedures and the use of acoustical materials to control noise are well known. However, it is more difficult, time consuming and expensive to evaluate the interior sound quality of a vehicle and to quantify the evaluations. It is hard to determine the sound quality of a manufactured product because it involves a psychological judgment. The dominant factor in the judgment about the sound quality of a product is its loudness. It is well known that people do not judge pure tone sounds of equal sound pressure level, but of different frequencies, to be equally loud. Most people find that pure tone sounds at about 4000 Hz seem to be the loudest. But the bandwidth of a sound also affects its loudness. People have the feeling that a sound is louder as its bandwidth increases even if the sound pressure level is unchanged. Masking effects are also important in our evaluation of sounds. Even young people with perfect hearing will find that some important noise properties of a vehicle will be masked by other background noises. One aspect of masking phenomena is that a loud sound at one frequency can mask a quieter sound at the same frequency or another close in frequency so that it is not heard. Another masking effect occurs when loud broadband sounds mask a pure tone or a narrow band sound. The human ear is also still unable to 9th WSEAS Int. Conf. on ACOUSTICS & MUSIC: THEORY & APPLICATIONS (AMTA '08), Bucharest, Romania, June 24-26, 2008 ISBN: 978-960-6766-74-9 106 ISSN 1790-5095