Distributed Bass Array configuration
"All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband." Yes, that’s because they are really delay controls, and a given delay is equivalent to different fractions of a wavelength (phase) at different frequencies. But as far as I can see, delay is what’s needed to align speakers that are different distances from the listener (and might have different latencies). At least, that's my understanding of the situation. |
The following quote was copied from a Stereo Times interview with Duke Lejeune that took place many years ago, There was no date listed on the article that I could find. So, I'm not certain of the exact date but I believe it was published around the year 2000 give or take a few years. I thought the entire interview might be of interest to members and this is a link to the complete interview: http://www.stereotimes.com/comm081710.shtml The interview begins with a fairly lengthy portion about his regular speaker designs. The copied portion below is a portion near the end of the interview, which I thought was more directly relevant to this thread since it is the portion at which "LB", Stereo Times interviewer Larry Borden, first asks Duke, "DL", about his "SWARM" 4-sub distributed bass array system: "LB: Let’s turn now to bass. What is “SWARM,” and how does it work? DL: The concept behind the Swarm arises from a brief conversation with Earl Geddes, as I was driving him to the airport after CES in 2006. I had been trying for years to come up with a subwoofer system that would match up well with Magneplanars and Quads, trying various enclosure types in a quest for very good “pitch definition” in the bass region, along with good impact (good planars excel at the former but not the latter,). Anyway, Earl mentioned that scattering multiple subs asymmetrically around the room resulted in a net smoothing of the in-room bass, as each sub would interact with the room differently so that the sum would be smoother than any one alone. The lightbulb went off in my head, and I asked him for permission to use the idea. He said yes. By the time we got to the airport, I was already designing the Swarm in my head. Let me digress for a minute into acoustics and psychoacoustics. The ear/brain system tends to smooth out peaks and dips that are fairly close to one another, but if they are more than 1/3 octave or so apart then the peaks and dips are usually audible. Now we get room-induced peaks and dips all up and down the spectrum, but only in the bass region are they typically far enough apart (due to the wavelengths involved) that the ear cannot smooth them out. So in the bass region the problem is not too many peaks and dips – the problem is that they are too few and far between! Another factor is that it takes the ear a fair amount of time to hear bass frequencies. The ear cannot even detect the presence of bass energy from less than one full cycle, and it takes several cycles to detect the pitch. So considering the wavelengths and room dimensions, by the time we can hear bass tones the room’s effect is in full swing. Perceptually, in our home listening rooms there is no such thing as “direct sound” in the bass region; by the time we even begin to hear it, it’s all reverberant sound. The Swarm consists of four fairly small subs and a single kilowatt shelf-mounted external amplifier. The subs are “voiced” to have a gentle rolloff over most of the bass region that is the approximate inverse of anticipated room gain (the vented version comes closer to this ideal than the low-Q sealed version does). The amplifier has a steep 24 dB per octave lowpass filter so that the subs can be scattered without betraying their locations by leaking lower midrange energy, along with a single band of parametric EQ in case the scattering alone doesn’t do the trick. To the best of my knowledge, none of my customers are using the parametric EQ because the in-room bass is sufficiently smoothed as it is. Not only does the Swarm result in a smoothing of the in-room peaks and dips, but the peaks and dips that remain are more numerous and closer together, so that the ear’s smoothing mechanism can be effective. Now at first glance it might seem that multiple bass sources with multiple arrival times results in loss of impact and/or mud, but that is not the case in practice. As explained above, we cannot hear the bass wavefront before the room has its say. The ear responds primarily to frequency response (rather than to time-domain behavior) in the bass region, so when we smooth the frequency response we are solving the biggest problem. Because the low fundamentals and first few overtones are present in proper proportion, the pitch definition is very good. The argument for a single large equalized sub can of course be made, and there are some exceptional examples on the market, but equalization is a local rather than a global solution. In other words, the room-induced peak-and-dip pattern varies so much from one location to another within a room that fixing the frequency response at one location via equalization will almost inevitably make it worse elsewhere. In contrast, the multi-sub approach smoothes the bass throughout the room (decreases the spatial variance), ironically making equalization (if needed) even more effective." I thought his statement, "So considering the wavelengths and room dimensions, by the time we can hear bass tones the room’s effect is in full swing. Perceptually, in our home listening rooms there is no such thing as “direct sound” in the bass region; by the time we even begin to hear it, it’s all reverberant sound.", was especially relevant to this thread discussion. He's basically pointing out that the very long and omnidirectional bass sound waves behave very differently in domestic sized rooms than the much shorter and highly directional midrange and treble soundwaves. Which means that sound wave time coherence, the timing of sound waves actually reaching a listener's ears, is very important, necessary and doable for the shorter midrange and treble frequency sound waves for good stereo imaging. But sound wave time coherence is not very important, doable or even necessary for the much longer bass frequency sound waves since there is no such thing as "direct sound" in the bass region, it's all reverberant, or reflected, sound. The fact that humans are poor at locating the source of sounds below 80 Hz and progressively better at locating the source of sounds above about 80 Hz, the fact that we require the detection of a long and full cycle bass sound wave to even perceive bass sound present in the room and several full cycle bass sound waves in succession to perceive pitch, also distinguishes the differences in bass versus mid/treble sound tone perceptions in our rooms. Another interesting fact about bass sounds is that, perceptually, we're much better and more naturally able to perceive them outside or in a very large concert hall because the very long, full cycle bass sound waves, due to the much larger space, have the capability to arrive at our ears as "direct sound" rather than arrive as reverberant sound that has reflected off one or more room boundaries on its path. Just some food for thought, Tim |
