Testfacility for correct free-field reproduction of low frequency sound

Project Details


Correct reproduction of low frequency sound in psycho-acoustic experiments often involves great practical problems. An anechoic room must have very large inner dimensions and accordingly large damping wedges on the surfaces in order to obtain free-field conditions. Likewise, proportionally large amounts of electrical power for efficient loudspeakers are needed in order to raise the sound pressure sufficiently above the hearing threshold. In this project, an alternative facility is constructed, a smaller room with many loudspeakers distributed uniformly over the entire surface of two opposite walls. With this arrangement, it is possible to let one wall create a plane forward traveling wave, which is actively absorbed by the loudspeakers on the opposite wall. This is possible by using digital signal processing that adjusts the signal for each loudspeaker individually. The verification of the signal processing shows good free-field conditions in a substantial part of the centre of the room. The ventilation system for the room has been modified so that the noise is significantly below the hearing threshold, while maintaining an airflow that is large enough for having two persons in the room for longer durations. In order to achieve this, it has been necessary to find and seal leakages in the ventilation ducts while performing extensive measurements on the entire ventilation system. Since the test facility will be used for psycho-acoustic experiments at very low sound levels, a modification of the power amplifiers has been performed in order to fulfil the extremely strict noise limits for equipment in this type of experiments. The facility has been used for the project: "Explanations of annoyance from low frequency noise at very low levels ". The test facility has been improved by implementing a new sound-field control combining both free-field and pressure-field conditions have been designed and more measurements have been made for the implementation of the digital signal processing. Furthermore, the final verification measurements have been made and the documentation of the test facility is in progress. Supported by STVF and NEDO (New Energy and industrial technology Development Organization, Japan ), see the project "International Cooperation on Hearing Thresholds and Equal Loudness Contours").
Effective start/end date19/05/201031/12/2017


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