#Standing wave tube calibration
Calibration techniques employed and perfected at Weles Acoustics ensure a precise and reliable measurement of particle velocity, sound pressure and sound intensity.
All particle velocity sensors and microphones are calibrated in a controlled laboratory environment. Each probe manufactured at Weles Acoustics is delivered with an individual traceable calibration certificate. State of the art equipment is used to complete the calibration process and ensure a high quality product.
Calibration is carried out in a stainless steel standing wave tube. Each manufactured PU intensity probe is positioned in the middle of the tube, while a reference microphone (pref) is fixed at the rigid end of the tube. The particle velocity sensor (u) and the microphone (p) are calibrated separately. The ratio between particle velocity (u) and reference sound pressure (pref) is determined. Direct comparison of measured u/pref ratio with the known theoretical value of the same ratio, allows obtaining the amplitude calibration curve of the velocity (u) transducer for any given frequency. The same technique is employed to establish the amplitude calibration curve of the sound pressure (p) transducer. Phase mismatch between p and u sensors is determined, by measuring the transfer function between u and p - the reference sound pressure is not used in this case. Details of the employed calibration technique along with the description of the considered factors is laid out in [1 to 6].
The standing wave tube calibration technique has well known frequency limitations. Depending on the tube diameter and relative tube length certain frequency ranges can be calibrated. Full acoustic bandwidth calibration (20 Hz - 20 kHz) is achieved at Weles Acoustics by using different tube sizes and their right thermoviscous acoustic models. Further validation is carried out with free-field measurements.
 H.Tijdeman, "On the propagation of sound waves in cylindrical tubes", J. of Sound and Vibration, 39(1), 1-33 (1975)
 F.Jacobsen, V. Jaud, "A note on the calibration of pressure-velocity sound intensity probes", May 2006, JASA, (2006)
 F. J. Fahy, "Sound Intensity, 2nd ed." E & FN Spon, London, (1995).
 K. Beissner, “On the plane-wave approximation of acoustic intensity,” J.Acoust. Soc. Am. 71, 1406–1411 (1982)
 F. Jacobsen, “Active and reactive, coherent and incoherent sound fields,” J. Sound Vib. 130, 493–507 (1989)
 J.W. van Honschoten, H-E.de Bree, F.J.M. van Eerden, G.J.M Krijnen, "The influence of viscothermal effects on calibration measurements in a tube", 109th AES Convention, Los Angeles U.S.A (2000)