Nick Henriksen recently published a paper in the new Journal of Linguistic Geography (edited by Bill Labov and Dennis Preston). With his characteristic combination of careful sociolinguistic fieldwork and theoretically motivated phonetic analysis, Nick documents and analyzes variation in the realization of /r/ in different varieties of Peninsular Spanish. It is due to research like this that Nick is regarded as one of the leading voices on the sociophonetics of Peninsular Spanish. The full bibliographic information of Nick's paper, together with an abstract, is given below.

Henriksen, Nicholas. (2014)  Sociophonetic analysis of phonemic trill variation in two sub-varieties of Peninsular Spanish. Journal of Linguistic Geography, 2, 4-24.

In this paper we provide a preliminary characterization of the phonemic trill (i.e., /r/) produced by twenty-four speakers of northern and central Peninsular Spanish. The acoustic analysis revealed a considerable number of non-canonical variants containing one or zero apical  occlusions. The quantitative results showed robust effects of the following three factors on trill articulation: Speaker dialect, gender, and preceding vowel. Regarding social factors, central Peninsular speakers and male speakers showed the greatest propensity to produce fewer occlusions per phonemic trill. Regarding linguistic factors, non-canonical variants were especially common in contexts of preceding /u/; we interpret this result on articulatory grounds given the antagonistic gestures required for the trill and the high back vowel. All in all, these findings offer empirical support that geographically-oriented studies within a sociophonetic framework offer critical information on the diachrony of trill consonants.