Scaling Studies of Perceived Source Width Juha Merimaa Institut für Kommunikationsakustik Ruhr-Universität Bochum

Outline Introduction Background on listening tests Description of the conducted pilot test Analysis methods & preliminary results Discussion & summary

Introduction A room or a hall broadens the perceived width of auditory objects Traditionally auditory source width (ASW) has been investigated as a descriptor for concert halls How does the broadening depend on source signals?

In other words... In a scene based paradigm – source broadening is due to the part of room effect that is grouped with source signals – the rest of room effect is resolved into a separate percept What are the spatial features related to auditory “deconvolution” of reverberation

Listening test basics Quantifying auditory perception Levels of measurement Interval Ordinal Nominal Ratio Short Long

Possible test methods for assessing ASW Direct scaling – Rating – Rank ordering – Assigning stimuli in successive categories Constant reference – All stimuli are judged relative to a single reference stimulus

Possible test methods (contd.) Method of adjustment – Listeners adjust a variable reference to correspond to each stimulus Adaptive procedures – Reference is adaptively adjusted based on listeners judgements Pairwise comparisons – Each stimulus is judged relative to all others

Why pairwise comparisons? Source broadening is expected to be a sum of several interaural signal features All except pairwise comparison methods force the results onto a linear scale – Weighting of dimensions implicit in the data Can be accessed with factor analysis – Weights may vary between individuals, which will result in noisy unidimensional data

Pilot listening test Gathering both preference and distance data between pairs

Stimuli – Speech (sp) – Cello, f 0 = 196 Hz (ce) – Snare drum (sn) – Two harmonic complexes, f 0 = 196 Hz, -12 dB/oct No modulation (h1) Freq. mod. 1%, 6 Hz (h2) – Pink noise 100 Hz – 10 kHz (ns) Anechoic samples convolved with binaural room responses

Binaural room responses Diffuse field and system compensated responses – Medium size diffuse concert hall (p) RT = 2.2 s, 1-IACC E3 = 0.78 – Large multipurpose hall (a) RT = 2.4 s, 1-IACC E3 = 0.02 – Small listening room (s) RT = 0.5 s, 1-IACC E3 = 0.32

Altogether 18 stimuli resulting in 153 permutations

Analysis of preference data A single run comparing all the the pairs results in a preference matrix that can be used to rank order the stimuli In an ideal case each run will yield the same perfectly ordered set of data ABCDAB1C00D111ABCDAB1C00D111

Real world comparative judgements Each stimulus has a dispersion on a psychological scale Each judgment of distance and order depend on current points of perception

Checking for consistency Circular triads Mean for random answers with 18 stimuli: 204 Average in collected data approx. 40 All data matrices consistent with significance p < 0.01 A B C

Unidimensional scaling Simplest scaling method: count the number of times a single stimulus is prefered over all others

Wincount statistics, CR = 60

Comparison with stimulus IACC

More sophisticated scaling Thurstone's law provides a method for mapping pair comparison data on an interval scale – Assumes normally distributed unidimensional data – Includes tests for checking the fit Results – Significance of deviation from data p < 0.01

Multidimensional scaling Uses distances between stimuli to construct a spatial representation of data in n dimensions Metric (interval) and nonmetric (ordinal) procedures Few assumptions on data Works well with a relatively small number of test subjects

p_ns p_sp p_h1 p_h2 p_ce p_sn s_sp s_ns a_ns a_sp a_sn a_h2 a_ce s_sn s_ce s_h2 s_h1 a_h1 3-D scaling of all stimuli

Comparison of the large halls

Multipurpose hall vs. Listening room

Concert hall vs. Listening room p_h1 p_h2 p_ns p_sp p_ce p_sn s_ns s_sp s_sn s_ce s_h2 s_h1

Discussion & conclusions The perception of auditory source width is clearly multidimensional – Results between the most similar spaces suggest separate source and room dimensions with some interaction – Euclidian metric of MDS might not reflect human perception between extreme cases The pilot data is insufficient to draw more firm conclusions

Future work A larger listening test with a reduced set of stimuli Interpreting the dimensions in terms of binaural cues – Breaking the experiment into several unidimensional studies – Use gained results in choosing stimuli Similar investigations into envelopment