After building quite a few subwoofers and 2-way speakers over the years, I was looking for something different, and line arrays fit the bill nicely. They use a large number of small-diameter drivers –sometimes as many as hundreds– and have some intriguing characteristics such as little loss of volume as you get further away, low distortion, and very slim enclosures.
Designing one isn’t simple, though, since drivers interfere with each other, causing anomalies in both frequency response and sound dispersion. To minimize destructive interference and use it to our advantage, it’s vital to minimize driver-to-driver spacing.
When making line array speakers, some companies use a large number of 1" tweeters coupled with several small mid-bass drivers. Unfortunately, the distance between two 1" tweeters can’t be made much smaller than 40 mm, and they don’t really go much below 2 kHz without a waveguide anyway. On the other hand, while 2" full-range drivers result in a slightly wider driver-to-driver spacing of around 60 mm, this is a small price to pay given their ability to reproduce frequencies down to 150 Hz, which is particularly useful for a home theater surround speaker use case I have in mind.
I’ve settled on using eight 2" Lavoce FSN020.71F drivers, featuring neodymium magnets and 3/4" voice coils. I could have used more, but in a typical sized room, they should be more than enough.
The drivers will be arranged in four banks, with all but the center bank’s high-frequency responses shaped by 2nd-order low-pass filters. This aims to reduce high-frequency interference and achieve very narrow vertical directivity, providing near-constant volume regardless of listening position (within reason, of course). This is especially useful if the speakers are angled towards the furthest listeners. That way, you wouldn’t be blowing the ears of people close to the speakers, something that’s only possible with line arrays. More on this later.
A line array using 2" drivers will never produce enough bass by itself, so it’s only natural to couple it with a subwoofer. And since my latest subwoofer build had generous rounded edges, I thought adding curves to the slim line array enclosure would not only make it look less like a column, but also help it match the subwoofer’s appearance better.
The cabinet walls are made from nine layers of 18 mm MDF.
Curves certainly look nice, but they aren’t easy to make. One method for creating a curved enclosure is to form it from layers cut from a sheet of wood. I’m not very fond of this technique, as a lot of material goes to waste, but in this case, the enclosure will be very slim, resulting in little waste. Overall, I’ve cut nine layers from 18 mm MDF on a CNC router. Some layers have inner lips to mount the speaker baffle and the rear panel, plus one in the middle for central support. I also applied 1/2" roundovers to the speaker cutouts.
Full-range drivers rear-mounted to the baffle.
Eight drivers mean a lot of unsightly screws, which led me to mount the drivers from the back. However, that requires either a removable rear panel or baffle (or both) to access the drivers in the future. Since space is tight, I decided to make both removable by using threaded inserts at the back of the baffle and long screws, inserted from the rear panel, to attach the front and rear together.
Baffle front view.
I painted the baffle and rear panel black and the side walls, which form the shell, white for what I think is a nice contrasting look.
Baffle and rear panel, inside view.
There are four banks of drivers, each with two 8-ohm drivers connected in series. The banks are then connected in parallel, resulting in a nominal load of 4 ohms. However, two filters are necessary to flatten the frequency response for all drivers: a low-shelf cut and a notch. These bring the nominal impedance up to 6 ohms.
From top to bottom, drivers 1 and 8, 2 and 7, 3 and 6, and finally 4 and 5 form each bank. High frequencies are increasingly shaded for each bank as they move further from the center, which helps reduce destructive interference and control vertical directivity.
There are two very short ports made from 45 mm inner diameter PVC pipe to extend the low-frequency response.
Rear of the cabinet on a scrap wood stand.
The speaker was originally designed for wall mounting, but I made a quick stand from scrap wood, incorporating the wall mount, so that it could stand on its own.
Line array speaker placed on a subwoofer.
So, how’s the sound? The speaker needs a subwoofer to fill in the missing low end, but once I’d dialed in the subwoofer integration, I was greeted with clean, smooth sound, particularly in the mid-range, and the speaker can go loud for sure. At 95 dB/1m, there was no noticeable distortion thanks to the eight drivers working in tandem. As for the controlled vertical directivity, you can tell the upper frequencies decrease in volume when your ears are not on the speaker’s center axis. The effect is subtle but quite noticeable, especially when you go increasingly off-axis.
Measured outside at ~2 meters from the ground.
It’s difficult to make quasi-anechoic measurements of the line array due to the number of drivers involved. So, the speaker was measured outside, roughly 2 meters from the ground, to minimize reflections.
On-axis frequency response measured outside at 1 meter (gated at 10 ms).
The frequency response is mostly within ±3 dB between 150 Hz and 17 kHz. The critical 1 to 5 kHz region, where the ear is most sensitive, is quite flat with only a ±1.5 dB variation. There are some response anomalies starting at 10 kHz since the 2" driver isn’t really suited to reproducing these frequencies well, and there are also some destructive effects from nearby drivers. Fortunately, there’s very little content at these frequencies.
Horizontal directivity is wide up until 5 kHz, but narrows somewhat after that.
Horizontal directivity is wide at ±60 degrees nominal up until 5 kHz, but it starts to narrow somewhat after that.
Vertical directivity is very narrow as intended and controlled well for the most part.
Vertical directivity is near ruler flat at ±15 degrees nominal down to 1 kHz and gradually gets wider below that. This extremely well-controlled and narrow directivity –if I may say so myself– allows precise steering of sound, which is especially well suited for applications such as surround sound speakers in a home theater, where some listeners are closer to a speaker than others.
The nominal impedance is 6 ohms.
The impedance and phase plot shows no major issues other than a few minor resonances. The minimum impedance stays above 5 ohms across the band. The bump centered at around 2.3 kHz is due to the notch filter that flattens the response.
Finally, a complete build video is available below:
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