Logo of the Physikalisch-Technische Bundesanstalt

Secondary calibration for microphones and sound level meters in the infrasound frequency range


A key component of calibration facilities for traceability tasks is a secondary calibration method, which provides a way to transfer the calibration results obtained in a well-equipped standard laboratory to practical devices in the field. At PTB a sound tube apparatus has been developed, which enables this transfer for measurement devices for airborne infrasound.

The sound tube consists of an acrylic tube which is sealed off by a lid at the top. A loudspeaker at the bottom provides a sinusoidal excitation signal, a device under test and a reference transducer can be mounted via the lid. Both are subjected to the sound field created by the loudspeaker. The particularly designed excitation configuration ensures a homogeneous sound field at the locations of the microphones and sound level meters. Mounting brackets in the lid allow positioning the venting holes of the reference microphones either inside or outside of the sound field, depending on the primary calibration method used and the addressed application. The calibration system can be used for a wide variety of currently available measurement microphones and sound level meters with very different functioning principles and properties.


Bild 1: New calibration facility

The sound tube covers a frequency range from 0.5 Hz up to 100 Hz. For higher frequencies the sound field becomes more and more inhomogeneous and at frequencies below 0.5 Hz leakage occurs, which results in a limited excitation sound level. The uncertainty from the primary calibration of the reference microphone is currently the largest contribution to the uncertainty of the measurements in the sound tube. A repeatability of 3/100 dB is achievable for suitable measurement microphones, while the primary calibration method currently contributes a systematic uncertainty of 0.23 dB at 2 Hz.

The presented work is part of the EMPIR project infraAUV. Methods which will be developed in the infraAUV project will provide a significant improvement of this contribution of the primary calibration. In the future, this setup can be used for low frequency calibrations and test measurements as, for example, part of type approvals for sound level meters. Another application is the low frequency calibration of microphones of any kind for infrasound measurements outside the laboratory.



[1] Website of the EMPIR project IntraAUV: Opens external link in new windowLink



Marvin Rust, FB 1.6, E-Mail: Opens local program for sending emailmarvin.rust(at)ptb.de