Loudspeaker Directivity: An Ongoing Experimental Survey
This work is sponsored by the Sony Corporation of America
Loudspeaker directivity is the extent to which loudspeakers focus the sound in a particular direction (typically towards the listener) instead of broadcasting it in all directions around the room. It is the single most important loudspeaker property for 3D audio with crosstalk cancellation (XTC), since room reflections directly degrade the level of XTC. There is a very high correlation between loudspeakers with high directivity and high XTC levels. Consequently, the 3D3A lab is conducting detailed measurements of the directivity of various loudspeakers measured in the lab's anechoic chamber. Such a study on loudspeaker directivity is also important for our ongoing Sony-sponsored research on head-externalization of 3D sound through headphones, since some of the relevant techniques rely on emulating the radiation characteristics of real loudspeakers.
The table below consists of the loudspeakers measured to date. The loudspeakers have been ranked and sorted, by default, according to their suitability for use in an XTC system. The rank, in general, gives an indication of the loudspeaker's directivity in 3D space. The loudspeaker likely to be most effective in an XTC system is ranked the highest (Rank 1).
Guide to understanding the table
Clicking on the appropriate column headers in the table will enable sorting by rank, loudspeaker name, or average frontal horizontal directivity index. Note that the thumbnail contour plots show frontal horizontal directivity only and provide at-a-glance information on the loudspeaker's directivity in the frontal horizontal plane.
The image to the right depicts the frontal horizontal plane and the three images below are a set of frontal horizontal contour plots arranged from left to right in order of decreasing directivity. These contour plots depict the departure from the on-axis (straight ahead) frequency response of the loudspeaker as a function of azimuthal angle. Regions of red indicate very similar output levels to the on-axis response, whereas regions of violet indicate a significant decrease in output level. The spacing of the intermediate contours indicates the rate of change of the loudspeaker's output with angle. A plot with a narrow region of red surrounded mostly by violet typically denotes a loudspeaker with high directivity, whereas wider regions of red (or, alternatively, plots consisting of mostly non-violet regions) denote less directive loudspeakers.
Click on a loudspeaker name or on the corresponding frontal horizontal directivity contour plot thumbnail to view more detailed information on the directivity of that loudspeaker.
Click here to download a .zip file containing all directivity plots. (Last updated November 12th, 2014)
All directivity measurements are carried out in the anechoic chamber of the 3D3A Lab using a B&K Microphone Type 4189. The loudspeaker is placed on a rotation stage in the chamber and rotated in increments of 5°. At each orientation, the loudspeaker's impulse response is measured with, unless otherwise noted, an exponential sine sweep at a sampling rate of 96 kHz. To align the microphone, the loudspeaker is placed at 0° (facing straight ahead) and the microphone is positioned such that the barrel of the microphone points straight at the center of the loudspeaker cabinet (or some other logical alignment point). Due to the physical limitations of the anechoic chamber, a time-window is applied to each measured impulse response to remove any reflected sound. The measured data are processed and plotted using custom software developed in-house at the 3D3A Lab.
The measurements shown above have been made by Lukasz Mosakowski, Tim Matchen, Joe Tylka, Rahulram Sridhar, Tony Jin, and Gianfranco Colombi under the direction of Prof. Edgar Choueiri.
If you know of any loudspeakers that may have particularly high directivity, please inform the 3D3A Lab. We will try to acquire these loudspeakers and measure their directivity.