A PU probe can be used to determine the reflection coefficient in situ:

• PU surface impedance measurements on curved liner materials in the presence of a grazing flow,
  Emiel Tijs, Hans-Elias de Bree, Piergiorgio Ferrante, Antonio Scofano, CEAS, Bilbao, 2008
• Acoustic absorption measurements of moving structures and under influence of flow,
  Emiel Tijs and Hans-Elias de Bree, ISMA, 2008
• A handheld device to measure the acoustic absorption in situ (poster presentation),
  HE de Bree et al, SNVH, 2008
• A study of influences of the in situ surface impedance measurement technique (2 pages DAGA version),
  A study of influences of the in situ surface impedance measurement technique (extended version),
  Martin Nosko, Emiel Tijs and Hans-Elias de Bree, DAGA, 2008
• Non destructive and in situ acoustic testing of inhomogeneous materials,
  Tijs et al, ERF33, Kazan, Russia, 2007
• In-situ measurements of the complex acoustic impedance of porous materials,
  JD Alvarez and F Jacobsen, INTER-NOISE 2007
• An ultra miniature measurement tool to measure the reflection coefficient of acoustic damping materials in situ,
  de Bree et al., SAE, 2007
• Two complementary Microflown based methods to determine the reflection coefficient in situ,
  de Bree et al., ISMA 2006
• Measuring the free field acoustic impedance and absorption coefficient of sound absorbing materials with a combined particle velocity-pressure sensor,
  R.Lanoye et al., JASA May 2006
• A practical device to determine the reflection coefficient of acoustic materials,
  R. Lanoye et al., ISMA 2004 Leuven, Belgium
• Broad band method to determine the normal and oblique reflection coefficient of acoustic materials,
  Hans-Elias de Bree, Reinhilde Lanoye,  Simon de Cock, Jos van Heck, SAE 2005
• Experience with different free field techniques to evaluate the surface impedance,
  R. Lanoye et al.,  Proc. 18th ICA. Kyoto, 2004,1983-1984.
• In-situ Impedanzmessung mit einem kombinierten Schnelle- und Drucksensor,
  Roland Kruse, Volker Mellert, Daga 2006, Germany
• In situ impedance and absorption coefficient measurements compared to poro-elastic simulation in free, diffuse or semi-statistical fields using microflown p-u probes,
  A. Duval et. al., DAGA 2006
• In situ measurements of acoustic impedance for car applications: influence of forming effect,
  Céline Sandier et al, Euronoise 2006
• Measuring Method of Sound Reflection and Absorption Characteristics Based on the Particle Velocity and its Applications to Measurements on Such as Drainage Pavement of Road Surface,
  Iwase et. al, Internoise 2003
• Impedance measurement of materials by use of ambient noise for computational acoustics,
  Otsuru et al., Internoise 2006
• Evaluation of Electro-acoustic Techniques for In-Situ Measurement of Acoustic Absorption Coefficient of Grass and Artificial Turf Surfaces,
  Niranjan R. Londhe et al, SAE 2007

PU probes can be used to determine the reflection coefficient in a tube:

• Measurement of absorption with a p-u sound intensity probe in an impedance tube,
  Yang Liu and Finn Jacobsen, ASA, 2005
• A novel technique for measuring the reflection coefficient of sound absorbing materials,
  H-E. de Bree et al,  ISMA25, International Conference on Noise and Vibration, September 13-15, 2000 - Leuven, Belgium
• Comparison of two methods for measurement of horn input impedance,
  H. Schurer et al., Published in the Proceedings of the 100th AES convention, Copenhagen, 1996
• On the use of a P-U sound intensity probe for the qualification of complex surface properties,
  Bonfiglio et al, ICSV 13, 2006

The reflection coefficient in a tube can be determined with 2 Microflowns:

• Experiments with a new acoustic particle velocity sensor in an impedance tube,
  F.J.M. van der Eerden et al., Sensors and Actuators A69, (1998) 126-133

Energy based method:

• Measurement of sound power and absorption in reverberation chambers using energy density,
  David B. Nutter et al., JASA, 2007

A medical application:

• A method for measurement of the vocal tract impedance at the Mouth,
  Malte Kob et. al, Medical engeneering & Physics, 2002 

Some general papers:

• Een nieuwe methode voor het ter plekke meten van akoestische eigenschappen van materialen,
  Tom Basten et al, Geluidnieuws, mei 2008
• Experimental energetic analyses of an actively controlled one-dimensional acoustic waveguide,
  Donadon et. al, Applied Acoustics (2006)
• Analysis of a perforated panel for the correction of low frequency resonances in medium size rooms,
  Andrea Panteghini et al., Applied Acoustics (2006)
• Acoustic impedance measurement, Part I: a review,
  J.-P. Dalmont, Published in the Journal of Sound and Vibration, 2001 (also at www.idealibrary.com doi:10.1006/jsvi.2000.3428)
• Calibration of the p-u Microflown probe and some considerations on the physical nature of sound impedance,
  D.Stanzial and D. Bonsi, EURONOISE, Naples, Italy, 19-21 May (2003)
• Comparative analysis of several acoustic impedance measurements,
  J. González Suárez et al., 2002
• The concept of 3-D acoustic resistance and its evaluation at the open end of an organ pipe,
  Domenico Stanzial and Davide Bonsi, SMAC, Stockholm 2003
• In situ acoustic impedance measurement using abient noise - Investigation and discussion using p-u sensor,
  By Toru Otsuru, Reiji Tomiku, Yasuo Takahashi and Kenji Nabeta (Oita University)
• Input impedance of brass instruments with velocity measurements
  Daniel O. Ludwigsen, Kettering University