Time and costs can be saved by performing numerous acoustic measurements with PC-based measurement systems. This is true for experiments in research and development as well as quality control of acoustic properties of materials and components in the production process. Using the PC as a hardware platform, the signal processing tasks are realized in the form of "virtual" instruments. Powerful development kits (e.g. LabWindows®/CVI) support their design and implementation. Know-how and special requests of the user concerning data evaluation and presentation can easily be taken into account. This concept profits from the rapid development of the PC market with continuously falling prices. It simplifies electronic data communication via standard networks.
Example: Measurement of Acoustic Impedance and Absorption Coefficient at Normal Sound Incidence (KUNDT´s Tubes)
The determination of impedance and absorption coefficient of material samples or construction components employing the Transfer-function method (ISO 10534-2) is a standard measurement task in acoustics. A possible method of implementation is to pickup the sound pressure with respect to a reference signal sequentially at two positions using a movable microphone (Fig. 2, inside the tube) or microphone array. With these two transfer functions, the acoustic impedance and the absorption coefficient can be determined applying the known formulae. It would be a waste to use an expensive all-round analyser for this simple signal processing task. A virtual instrument together with a PC data acquisition board are sufficient to carry out precise and cheap measurements.
Further aspects when choosing a suitable system for the measurement of absorption coefficients are the preferred geometric form of the samples (round or square surface) and the maximum sample depth as well as the material type (solid plates, elastic layers, loose granules etc.). These points can be taken into consideration without limiting bandwidth or precision of the measurements by choosing either a measurement tube or a duct, and an adequate sample holder.
So software and hardware for the measurement of absorption coefficients allow for a whole variety of different demands. These opportunities may ease not only laboratory work at the Fraunhofer-Institute of Building Physics but meanwhile also quality control at several manufacturers of sound absorbing materials and construction components.