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"The holy grail of loudspeakers is a sound source that provides a sound field whose three dimensional radiation pattern is constant over a wide frequency range. This type of source provides an acoustic output whose spectral content does not vary with direction. Particularly challenging is a speaker that couples these characteristics with high directivity. Traditionally, these speakers are called constant-directivity or constant bandwidth devices. Various methods have been used in the sound industry to approximate this behavior including horns, omnidirectional sources, and arrays, higher-order sources, etc" Syn-Aud-Con Newsletter, 2010. If you have been reading our series of articles on speaker directivity you'll know that none of the approaches we have looked at so far really achieve the 'holy grail'. The CBT approach we examine in this article gets the closest because its beam pattern and directivity is essentially independent of frequency. The CBT approach originated in research into underwater transducers, which was then applied to speakers by Don Keele in a series of AES articles. To my knowledge there are only two commercially available products utilizing the approach - the Audio Artistry speaker kit available via Parts Express and the JBL CBTs such as the 70-J.
Parts Express / Audio Artistry CBT[/caption]
A line source speaker tends to take the form of a vertically stacked array of small drivers. CBTs can look a lot like line sources in their physical appearance, but from a directivity perspective they are quite different, as the following graphic, from Floyd Toole's Sound Reproduction book (excerpt here) shows.
CBT vs. point and line source directivity[/caption] The diagram shows a point source (a), line sources (b, c) and various types of CBT (c) through (f). The term 'shaded' refers to a design that tapers off the SPL output of drivers towards one end of the line. You can see that designs (e) and (f) are pretty much the same but with one implementation using delay and the other distance to achieve the same effect (delay and distance being interchangeable). Note that JBL have implemented delay in their designs using passive all pass networks (see this tech note for further details). The amazing directivity control and vertical dispersion of CBTs can be clearly seen in these diagrams. There is a complete lack of lobing in the vertical plane and flat directivity through the whole audio bandwidth! Don Keele has published an extensive collection of measurements in a very interesting comparison of the B&W Matrix 801 and his CBT implementation.
Nyal Mellor, Founder, Acoustic Frontiers