Ultrasound is used in wide fields of industry and medicine as, for example, in medical diag-nosis and therapy, the analysis of chemical and physical processes, cleaning, welding of plastic materials, for distance measurements, monitoring systems and remote controls. In the case of many of these applications, ultrasound is - intentionally or unintentionally - radiated into the air with sometimes considerable energies. To protect persons against this exposition, it must be possible to measure ultrasound levels in air with calibrated devices whose trace-ability to the national standard can be proved. Realization of the sound pressure unit in this frequency range with primary procedures is at present in the development stage. This is why a method for easy and cost-effective dissemination of the calibration is required which can be realized by determining the relative frequency characteristic with the aid of electrostatic ac-tuators. The absolute sensitivity at low frequencies which is also required, is measured with sound calibrators. Within the scope of a study it was investigated which measurement uncer-tainty is to be expected for such a calibration in the case of quarter-inch condensor micro-phones.

With the aid of an electrostatic actuator of type B&K UA0033, a quarter-inch free-field micro-phone of type B&K 4939 was calibrated as described above in the frequency range between 100 Hz and 200 kHz. Up to 20 kHz, the maximum standard deviation under repeatability conditions for such a set-up amounts to 0,05 dB and between 20 kHz to 200 kHz to 0,15 dB. Taking other disturbing influences as, for example, cross-talk, into account, it can be assessed that in the frequency range up to 160 kHz, the expanded measurement uncertainty (with coverage factor k = 2) remains smaller than 0,5 dB. Figure 1 shows this measurement uncertainty over the whole frequency range.