Klippel, Werner Measurement of Turbulent Air Noise Page 1 of 28 Measurement of Turbulent Air Noise Distortion in Loudspeaker Systems Wolfgang Klippel, wklippel@klippel.de Robert Werner, r.werner@klippel.de Abstract Air leaks in the dust cap and cabinets of loudspeakers generate turbulent noise which highly impairs the perceived sound quality as rub and buzz and other loudspeaker defects do. However, traditional measurement techniques often fail in the detection of air leaks because the noise has a large spectral bandwidth but a low power density and similar spectral properties as ambient noise generated in a production environment. The paper models the generation process of turbulent air noise and develops a novel measurement technique based on asynchronous demodulation and envelope averaging. The technique accumulates the total energy of the leak noise radiated during the measurement interval and increases the sensitivity by more than 20 dB for measurement times larger than 1s. The paper also presents the results of the practical evaluation and discusses the application to end-of-line testing. 1. INTRODUCTION The generation of sound in loudspeaker systems requires the compression of air and generates high air pressure within the enclosure. If the enclosure is not perfectly sealed air at high velocity is expelled at those leaks causing turbulences which generate characteristic air noise. The signal contains spectral components all over the audio band and beyond. In passive loudspeaker systems those leaks usually occur at the dust cap, at the surround or somewhere in the cabinet. Modern active loudspeaker system using a sealed enclosure and active filtering to extend the bandwidth to lower frequencies are much more prone to air noise than passive system because the pressure inside the box is significantly increased and electronic components such as switches and connectors require careful sealing. In vented-box systems high air velocity in the port generates a similar noise signal.