Greetings to All: I am building a surround system for the playback of live recordings and video. Naturally, as with all of us, there are economic as well as technical constraints. The purpose of this post is suggest (and receive feedback) for a system that uses multiple subwoofers in order to obtain the directional components of very low frequency sounds. One could justifiably argue that we don’t have a keen sense of direction for the low lows (and why subwoofers can be placed in inconspicuous places), but that is not the issue at hand. In fact, I’ve always been a proponent of center-channel subs since my introduction to hi-fi decades ago. Today my low end needs direction to accurately recreate industrial sounds.
The system is comprised of 16 channels. The advantage of 16 (or less) channels is that I don’t have to use a dedicated Word Clock to sync my MOTU FireWire interfaces. I’m fond of these interfaces, and two of them can sync-up via the FireWire link without complication. I intend to use two hexagonal arrays of small-sized loudspeakers (a bit larger than the 3-inch coned Genelecs, but not much more so). One array will be near floor level, whilst the second array is proximal to the ceiling. According to the literature (Malham, Rumsey, and others come to mind, but I’m shooting from the hip), diametrically opposed pairs may be preferred when the listener is centered in the array. Further, it is purported that six speakers provide immunity against drawing signals toward a single speaker. Eight speakers is probably overkill and doesn’t leave me the four channels needed for a square array of subs. Any thoughts as to whether the two hexagonal arrays providing horizontal and height information should be offset or vertically aligned? Regarding the need for subs: With ‘normal’ music content, twelve speakers working in concert would provide more than adequate low-frequency energy. But I’m going to be using live recordings where a particular low-frequency sound could be coming from an extreme R, L, front or back direction. In this scenario, I’d rather have the subs handle the load but I still need to preserve ‘direction’ as stated above. Because four speakers can provide adequate surround sound, my intent is to frequency-divide the B-formatted signal and send the highs and lows to their respective feeds via ‘conventional’ Ambisonic decoding. To be clearer, I will digitally filter the B-format signal so that each of its four components (W, X, Y and Z) are divided into a high and low-frequency signal component. The low-frequency components will be decoded and sent to the square (and likely horizontal) array of four subs. The highs will be decoded based on the position of the 12 ‘full-range’ speakers. I use full-range loosely here because the added bass channels aren’t for enhancement, but to alleviate the 12 speakers from their low-end duty. I haven’t determined the best crossover frequency, and this may be determined in part by a combination of the speakers used and the stimuli to be presented. I wish to use the lowest possible frequency, but not to the point of driving the small speakers to distortion. I’m guessing a digital (crossover) filter that is both maximally flat and phase coherent is best, though slight dips caused by frequency response anomalies are easy to EQ out. I use EQ judiciously because it is generally just a marginal cure for a loudspeaker's deficiencies. Upping the response at some frequency extreme merely adds to distortion that is ‘measured’ (in SPL) as a boost at the deficient frequency (or third-octave band or whatever). Only a spectrum analyzer or critical listening reveals where the real boost is occurring. Although my proposed strategy focuses on subwoofers and low frequencies, it may find purpose at higher frequencies. For example, I read about Oticon’s carefully placed 39-speaker array in the Hearing Journal (circa 2010). I recall that the speakers were positioned within 1 cm of their ideal position. But what about the ‘acoustical centers’ of loudspeakers? How would this be determined? Justification of such extreme placement would require knowledge of phase characteristics and an exacting acoustic center. However, by applying frequency division to the B-formatted signal, each speaker within a bi- or tri-amplified enclosure could receive its own, unique decoded signal based on its absolute position within an array. Of course, this means a s- load of channels and amps, but perfectionists and the technically inclined may find this appealing. Anyway, I’m curious as to what others may have attempted with regards to bi-amping the B-format signal (or whether it’s a remotely good idea), use of multiple subwoofers, and whether the hexagonal arrays providing height information should be offset or vertically aligned. As always, I appreciate the help and insights of the experts and experienced Ambisonics aficionados. Best, Eric C. -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20130220/217102ab/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
