Room Low Frequency Response Estimation using Microphone Averaging

Little has been published about the repercussions of different source locations and measuring positions for the Low-Frequency Effects (LFE) loudspeakers in cinemas and dubbing theatres (cinema sound-track mixing rooms). The aim of this study is to determine the effects of the number and positioning of the loudspeakers on the uniformity of the response over the listening area, and to assess the effect of the measuring of those responses by the choices made regarding the positioning of the microphones within typically used arrays. Some organizations issuing recommendations for the installation layout specification, such as Dolby, [1] have long suggested that the installation of low frequency effects loudspeakers below the screen should be made asymmetrically. The reason given for this is to avoid the symmetrical driving of the low-frequency modal responses of the rooms. If two loudspeakers or groups of loudspeakers are used, one may typically be centred about 20% of the distance fromone side-wall, with the other centred about 33% of the distance from the opposite side wall. However, other organisations, and many designers, have recommended the mounting of the LFE loudspeakers in tight-packed clusters. Furthermore, it has long been suggested that a single microphone position is inadequate for measuring the response of an LFE channel because the long wavelengths involved make spacial variation in the measurements inevitable. Typically, 4, 5, 8 and 10-microphone arrays have been used, but as rooms differ so much in size, shape and acoustic properties, no universal instructions exist about precisely how to place the microphones in such arrays. This paper examines how the choice of low-frequency source positions and the microphone positions in the common, 5-microphone array can affect the measured average responses over the delineated listening areas. It also discusses how the positioning of the loudspeakers can affects the evenness of coverage. The results of the spacially averaged responses will be compared with the responses at individual microphone positions, in order to assess how representative the averages are of the responses at specific locations.