Assessment of the change in similarity judgements of auralized engine sounds caused by changes in frequency resolution of transfer functions Arne Nykänen a,⇑ , Roger Johnsson a , Anna Sirkka b , Örjan Johansson c a Division of Sound and Vibration, Luleå University of Technology, 971 87 Luleå, Sweden b Scania, RTRN Acoustics, Truck Development, 151 87 Södertälje, Sweden c ÅF-Ingemansson, Box 1551, 401 51 Göteborg, Sweden article info Article history: Received 26 September 2008 Received in revised form 25 August 2010 Accepted 16 October 2010 Available online 16 November 2010 Keywords: Binaural Transfer function Auralization Sound quality Psychoacoustics Vehicle acoustics abstract Auralization facilitates aural examination of contributions from different sound sources, individually and as parts of a context. Auralizations can be created by filtering sounds of the perceptually most salient sources through binaural transfer functions (BTFs) from source positions to a listening position. When psychoacoustic analysis is based on auralizations, the auralizations need to give the same impression as real sounds. The objective of this study was to determine the frequency resolution required for aural- izations to be perceptually equivalent to recordings made with an artificial head. Auralizations of the con- tribution of engine sounds to interior sounds of a truck were examined. In listening tests auralizations based on simplified BTFs were compared to artificial head recordings. The BTFs were simplified by low- ering the frequency resolution and by smoothing in the frequency domain. Auralizations made through BTFs with a resolution of 4 Hz or higher or smoothed with maximum 1/96 octave moving average filters were perceived as similar to artificial head recordings. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Product sound is commonly evaluated as a unity, either through listening tests or psychoacoustic analyses [1]. In many cases this is not sufficient since human hearing is able to separate sounds from different sources. These separation mechanisms are described as auditory stream segregation [2]. When there are multiple sources autonomous stream segregation effects and cognitive effects may influence the results. Someone may consciously or unconsciously listen to only some sources. This may effect even basic perceptions like loudness [3] and stresses the importance of presenting correct binaural signals to subjects in listening tests. Auralization through binaural transfer path analysis and synthesis is a useful tool for analysis of how contributions from different sources affect the per- ception of sound [4]. Few studies addressing the precision required in such auralizations have been published. Some studies concern- ing audibility of irregularities in electro-acoustical transfer func- tions show a human sensitivity to defects in transfer functions used for sound reproduction [5–8]. However, common electro- acoustical transfer functions are smooth when compared to trans- fer functions typically found in mechanical structures of, for exam- ple, vehicles. The precision of measurements of transfer functions has been discussed in several papers [4,9–12]. The results are com- monly presented as graphs. However, it is difficult to draw conclu- sions on the audibility of differences in transfer functions from such graphs. Since the results of transfer function measurements usually have to be interpreted as effects on sound, the understand- ing of the audibility of alteration of transfer functions is essential. Within the automotive industry artificial head recordings and reproductions have become a standard method for sound quality evaluations [13]. Artificial head recordings result in true to life reproductions but require existing products or useable prototypes for making the recordings. In the industrial design process sketch- ing and visualisations play a central role for conceptualisation, refinement and concept selection [14]. Therefore, auralization which is analogous to visualisation is assumed to be essential for structured product sound design [15]. Depending on need, aural- izations require different levels of detail. Nykänen introduced the concept of sound sketching as a strategy for the evaluation of sounds through hearing early on in the design process [15]. In early conceptualisation phases, simple sketches called thumbnail sketches are commonly used [14]. Such thumbnail sketches serve as basis for expressing ideas and evaluating possibilities. When auralization is used as a tool for making such thumbnail sound sketches, audible errors and artefacts may be acceptable as long as the main character of the sound is realistic. What is considered important for preservation of a sound’s main character may be 0003-682X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2010.10.007 ⇑ Corresponding author. Tel.: +46 920 492139; fax: +46 920 491030. E-mail addresses: arne.nykanen@ltu.se (A. Nykänen), roger.johnsson@ltu.se (R. Johnsson), anna.sirkka@scania.com (A. Sirkka), orjan.johansson@afconsult.com (Ö. Johansson). Applied Acoustics 72 (2011) 115–123 Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust