Piston on a sphere calibrator

pistononaspherelow pistononaspherehigh Click to enlarge

The procedure for calibrating a particle velocity Microflown sensor is different compared to a standard microphone calibration. A method called "Piston on a Sphere calibration" has been developed to calibrate the Microflown sensor. There are two measurement steps in the procedure to calibrate the full bandwidth of 10Hz - 20kHz, a low frequency method covering a bandwidth of 10Hz till 400Hz and high frequency method covering a bandwidth of 300Hz till 20kHz. The 100Hz overlap  the two methods have is used to fit the two models together. Basically said the calibration is performed by calculating a FFT and Transfer Function. The high frequency method for the calibration requires a special loudspeaker that has known acoustic impedance. The Microflown sensor and a reference pressure microphone (with a known, calibrated sensitivity) will be positioned at defined distance in front of the defined speaker. The microphone and the Microflown sensor are positioned at nearly the same position as the reference microphone. With the pressure measured with the reference microphone and the known normalized impedance at the measurement location, the Microflown sensor can be characterized and calibrated. At lower frequencies, the described high frequency calibration procedure has some drawbacks. The Microflown sensor and reference are not in the far field but in relative near field at lower frequencies. Next to this the loudspeaker is not powerful enough at lower frequency range. In an ordinary room, at lower frequencies the background noise has higher pressure levels than the noise that is generated by the source. That why the low frequency method is required as second measurement step. Now the reference microphone is put into a hole in the sphere and tightened with rubber rings. The reference microphone measures the interior pressure variations in the sphere and the relation between the interior pressure and the particle velocity at a distance in front of the sphere will be used for calibration. The advantage is that the pressure inside the sphere is sufficiently high down to lower frequencies and there is a simple relation between the interior pressure and the particle velocity at the probe position. After the fitting of the models the calibration report can be generated.