Measuring facilities

As a workgroup for electroacoustics, we have to perform various kinds of acoustical tests and measurements. Examples are our work on piezoelectret transducers and micromachined sensors and actuators.

For this purpose, we are equipped with various instruments from Brüel & Kjaer, Rhode & Schwarz, and other manufacturers.

Our labs include an anechoic chamber and a reverberation chamber.

The anechoic chamber is a cube with approximately 5 meter sidelength. The absorption coefficient is about 99 % for soundwaves above 70 Hz. To reduce sound propagation through walls, the whole chamber stands on 28 spring beams each carrying 12 tons. The walls and the door are covered with wedge shaped absorber. The anechoic chamber is used for calibrating loudspeakers and microphones, psychoacoustical tests, noise measurements, etc.

The reverberation chamber has a similar size. Prohibiting standing waves, the angle between two walls differs from 90 degrees, and the walls are not parallel to each other. The reflecton coefficient is about 99 %, and the reverberation time is 20 sec. The aim is to get a diffuse sound field which means constant sound pressure level in the far field. Similar to the anechoic chamber, the reverberant chamber stands on 14 spring beams each carrying 12 tons. It is used for measuring absorption coefficients of materials, radiated power of loudspeaker, etc.

For our work in materials research, we have equipment for permanently charging dielectrics and instrumentation for measuring charge density, charge stability, trapping parameters, charge dynamics, charge distribution and piezoelectric properties of these materials.

The charging equipment consists of several corona setups and of a modified scanning electron microscope (SEM). The corona setups can be used with and without control grids; needle voltages up to 60 kV are available. The SEM may be operated with accelerating voltages up to 40 kV; the electron beam is available in a focussed, scanning mode or in a diffused, stationary mode.

The instrumentation for determining physical parameters of the charged dielectrics include electrostatic voltmeters for measuring surface potentials, heating chambers for sample annealing, thermally-stimulated discharge (TSD) equipment for measuring TSD currents with different heating rates, static and laser vibrometer setups for determining the quasistatic and the dynamic d33-coefficients of piezoelectric samples in the frequency range from 1 mHz to 1 MHz, and a pulsed laser to determine charge distributions in polymers with µm resolution.