14.05.2007
By the free-field reciprocity calibration of quarter-inch measuring microphones, the basis for the realization of the sound pressure unit has been created for frequencies up to 160 kHz.
The frequency range above 40 kHz is becoming increasingly important in acoustic metrology as the use of ultrasound increases permanently in many fields of medicine and technology and, beyond this, in daily life. Examples of this are therapy methods in medicine, non-destructive materials testing, cleaning procedures, the welding of metal and plastic materials, length measurements, parametric loudspeakers with high directional effect, and ultrasound toothbrushes. For all these fields of application, there is the - at least potential - risk of an impairment through high-level airborne ultrasound which can be harmful for the hearing or can have other negative biological effects on a person. In order to determine reliably the airborne ultrasound emissions and immissions, but also to specify limiting values and dose-damage relationships, it must be possible to measure airborne ultrasound levels with calibrated instruments which are traceable to a national standard.
As yet, no such measuring service exists in Germany. For this reason, by means of the free-field reciprocity calibration of quarter-inch condenser microphones, the basis has been created for realizing the sound pressure unit in the frequency range between 20 and 160 kHz.
During the reciprocity calibration, use is made of the fact that condenser microphones are reversible reciprocal quadrupoles and thus do not only - as in 'normal' operation - generate a certain output voltage when a certain sound pressure is applied, but that they also generate a volume velocity when an alternating current is fed in. Three microphones are arranged in pairs in such a way that each of the three microphones works once as a transmitter and once as a receiver. The quotients of the open-circuit voltage generated by the receiving microphone and the current through the transmitting microphone are measured and serve, together with the description of the acoustic coupling in the free field, for the calculation of the free-field transfer functions of the individual microphones. In the case of the free-field reciprocity calibration, the microphones are mounted opposite to each other in a sufficiently anechoic environment in such a way that they face each other directly.
Extensive pre-investigations were carried out to test whether the available measuring rooms are suitable for this calibration. It turned out that reflections from the room limitations have no significant influence on the measurements.
Through a thorough optimization of the measurement set-up as well as an extraordinarily narrow-banded correlation analysis and a microphone preamplifier developed specially for this calibration, a stably working measurement set-up could be put into operation which, in future, is to be used for free-field calibrations in the airborne ultrasound range.
Contact person:
Thomas Fedtke, FB 1.6, AG 1.61, Thomas.Fedtke@ptb.de