End-fire subwoofer array is becoming more & more common. Also it’s super easy to configure with Sub like AUDIOFOCUS S series which have dedicated Plug & Play factory presets for endfire (and cardioid btw) configuration*.
The goal of an end-fire configuration is to re-enforce sound in the front while cancelling on the back (and also some how on side).
End-fire configuration consist on at least two row of subwoofer facing audience in the same direction :
To have a coherent sound in the front, they must be time aligned so the second row is delayed by the exact distance between the sources, not to be mixed up with subwoofer gap.
The result is that the sources are in phase at the front 0° and sliding out of the phase when going to the back. This is what makes the end-fire configuration directive.
Note that this effect do not take extra « wasted » subwoofer like cardio but it takes more space. This is also for this reason that I want to stay practical and write « only » about two row of subwoofer end-fire. Yet, the principle can be applied virtually to as much subwoofer as you want.
Let’s take a look at what happens at 60Hz :
- One subwoofer, almost omni :
- Two subwoofers WITHOUT delay :
- Two subwoofers with delay applied :
So how to choose the distance between sources? My approach is to choose the frequency you want to cancel.
This can be determined by the frequency response of the subwoofer. You don’t need to cancel 200Hz, nor 15Hz, and also by the acoustic response of the venue, so you could target resonating frequency.
The goal is than to have both sub in phase at the targeted frequency in the front (the delay applied match the distance) and out of phase in the back. To be out of phase at the targeted frequency, the delay must be of an half wave length.
The delay between sources in the cancellation zone is two time the space between sources: the front subwoofer is « late » in the cancellation zone because of the delay applied to him and because of the space.
So: Delay = space between source = 1/4 cancelled wave length
In this example, with the MTsub218a MKII, I decide to focus on 60Hz. I also want to have a rounded delay to milliseconds so not only processor but any mixing desk could be used to apply the proper delay (as some mixing desk have 0.5ms step), and a rounded gap measurement between subs so once can remember easily this magical number for his setup.
Considering speed of sound at 340m/s, for 60Hz the space between sources is 1416mm and 4,1ms. The MTsub218a MKII is 800mm deep. So the GAP between subwoofer would be 616mm.
Let’s round to 60cm GAP and 4ms delay and measure the result :
Note that for the measurement, front and back microphones are equally spaced from the back subwoofer.
In green, we can see the measurement of one subwoofer, front and back. Those make us realise how much omni a sub can be.
In red, we can see the front and back measurement of the total end fire system. The front is 6dB higher than one sub alone. This is a good thing for two sub to be 6 dB louder than one! In the back, we have cancellation varying between 6 to almost 10 dB less than one subwoofer alone.
This is a shoot inside a quiet reflective area so this explain the irregularities.
Food for tough, on the picture on the next picture you can compare curve measured with mic on the ground like in the first one and the mic in audience and stage height position:
As before, the red are front and back ground level measurement and the blue are front and back with mic at 170cm front and 260cm (stage would be 90cm) aside (as if the sub were in a Left Right configuration, which is not ideal but a reality often met).
End-fire can also be combined with other technics like cardio stack or virtual arc sub, like below :
NOTE ABOUT TIME DOMAIN:
Keep in mind that cancellation is late of a half wave length of the target frequency so end fire is more effective on long sound than it is for short transient impulse.
Conclusion for the MTsub218a MKII
You can use this setup with all the preset of the MTsub218a MKII except the cardio alone of course.!
*End fire on S series involves more than just classic end fire technics
Destrée Thomas, Sound Application Engineer.