During my interview with Dave he explained a subwoofer array that he developed during a Blink182 tour. Here’s what it looks like.
Here’s where Dave explains it in the interview, starting at 45 minutes.
This came out of the sub testing that I did, primarily on the Blink 182 tour and then finished or got farther on the SoundGarden tour. I tried multiple arrays and out of it I came up with this fanned array.
The way to make it is, find your zero point, your rear sub location, the center of the grille. I would just have someone step on a tape measure there or put a road case on it. Then walk out 6.25ft for a 45Hz maximum rear cancellation. Then I draw an arc with the string.
Then you would place along that arc the front radiating points. One, two, three, however many you want. The 0ms point is your rear sub. Set your front ones at +6.25ft, which is about 5.8ms.
I see, so if you soloed up any of those front subs, they should be arriving at the same time as the rear subs.
Yeah, so the concept is the sound radiates from [the rear sub] and travels 6.25ft to sum with the subs in the front.
The closer these are together the beamier it gets. As you spread them out on the arc, the wider it covers.
[You have to put a sub stack in the back equal to the ones in the front. Each stack has three.] It’s making sound that will actually cancel out everywhere else and sum in the front.
Dave’s design is a combination of end-fire and physical arc. The end-fire takes care of the cancellation in the rear and the arc controls the coverage width.
End-fire array: an array of multiple subwoofers, placed in a line, one behind the other, with a specific spacing and delay strategy in a timed sequence that creates forward addition and rearward subtraction.Bob McCarthy, Sound Systems: Design and Optimization
The array achieves summation in the front by delaying the front elements to the rear, causing everyone to arrive at the same time. It achieves cancellation in the rear through delay as well (output processing + distance offset), except in the rear everyone is arriving late causing a chaos of phase relationships.
Physical arced arrays exhibit a coverage angle equal to the segment of the arc segment of virtual circle whose origin lies behind the array.Merlijn van Veen
A physical arced array turns a line source into a point source. Where all elements were arriving at equal level and time on-axis, they are now steered outward with a virtual origin from behind the array. As arc angle increases so does coverage width as the various spatial crossovers move away from on-axis.
How did Dave come up with the 6.25ft spacing?
spacing = ¼λ
spacing = speed of sound / frequency * 0.25
6.2778 = 1130 / 45 * 0.25
Dave’s design breaks some rules, which is fun. Normally we avoid a two-element end-fire array because it is limited to a single cancellation in the rear and then comes back with a nasty peak an octave up.
It is for this reason that if you design this array in Merlijn van Veen’s Subwoofer Array Designer, it will recommend high and low-pass filters at 29 and 60Hz, respectively.
With preferred filters.
I haven’t tried this in the field, yet, but we can look at some models in MAPP XT.
I’ll start with the widest spacing to get the widest coverage.
Here’s a prediction at 44.2Hz showing an opening angle of 176º.
In the measurement viewer we have a nice F2B ratio of 50dB at 50Hz.
But why 50Hz? I expected to see it at 45Hz.
It turns out that I spread the subs out so far that the distance offset changed from 6.28ft to 3.77ft from the rear. This changed the combined phase of the front subs to 150º apart at 45Hz and 180º apart at 50Hz, compared to the rear sub.
I reset the placement and delay based on the outside subs instead of the center one and was able to shift the null down to 47Hz. Hurray!
Now let’s try the narrow coverage version.
Here’s the prediction at 44Hz. Looks like it’s 10º more narrow than the previous design.
Nice F2B rejection of 31dB at 44Hz.
Dave breaks some rules here, which is fun.
Never end-fire with just two elements. It’s a one-note-wonder on the back side. Use the gradient in-line instead (same physical, different settings).Bob McCarthy
This make me wonder what an in-line gradient array would look like with the same design.
Gradient array: A cardioid configuration commonly used for subwoofer arrays with front and rear elements. The rear element is delayed and polarity reversed to effectively cancel behind the speakers.Bob McCarthy
The gradient array has frequency dependent coupling in the front in exchange for broadband cancellation in the rear.
You can use the exact same speaker placement, with different output processing.
It looks very similar to the end-fire at 44Hz, but is quite different across its operating range.
Where we had a limited range of cancellation with the end-fire array, we now have broad-band cancellation with the gradient. You may be worried about the dip at 100Hz, but notice that that the expected response is already 20dB down, moving it into isolation if you are running a linear system.
What’s it for?
As we discussed on the podcast, Dave was looking for a way reduce LF interaction in the center of the audience and total LF energy behind the arrays.
Compare a traditional setup of left and right sub stacks…
To two of Dave’s end-fire arcs splayed 90º apart.
If you would like to play with these MAPP designs, you can download them here. Please let me know if you discover any improvements. 🙂
Have you tried something like this in the field. What were your results?