Flint Acoustics: "The Arthur" Outdoor Subwoofer

[Flint]

To go along with the fun “Little Joker” coaxial speakers I built for patio use (described in this thread), I built a 12” sealed subwoofer to fill out the bottom end in the outdoor system. The idea was to keep the price down and make the woofer as resistant to weather conditions as possible for a decent price. Since I am using a great Dayton Audio mini 2.1 Class-T amp with 50 W per stereo channels and 100W into the woofer, I made this subwoofer passive without an amp or crossover.

I waited until I received the woofer and measuring it’s parameters before I designed the cabinet to ensure I get the best results. With inexpensive drivers, such as this one, the spec sheet parameters are not always the same as the delivered driver. This driver was fairly close to the factory specs, but I did have to adjust the volume of the enclosure by about 7% from the recommended design from the seller.

So, with a solid design in mind, I built and finished the enclosure, which I chose to make with equal sized parts to make cutting the panels easier. That led to an enclosure which is a perfect 15.5 inch cube. Nice!

I used an old input terminal cup I had in a drawer, as well as the cutouts from speaker holes in other projects to make the feet to lift the down-firing woofer off the ground. The predicted response said I should have a -3dB point of about 53Hz and a -10dB point (the effective bass limit) of about 35Hz. In reality it was just slightly deeper in the range, but not enough to make a huge difference.

Photo 1: Subwoofer installed and tuned up

I’ll post a bunch of photos in the following posts along with measured performance and several comments.

 

[Flint]

Here's some woodworking photos:


Photo 2: Panels prior to gluing up


Photo 3: Gluing up the initial pieces. I didn't use a brace for the top because I put a double wall on it and assumed it wouldn't need more bracing.


Photo 4: Final glue-up


Photo 5: Finished and ready for staining

 

[Flint]

Since this is going to be an outdoor speaker (protected under the roof of a patio), I wanted to use Spar Urethane to protect the MDF. That limited the options for coloring or painting the MDF. If I wanted to use enamel or acrylic to color the wood, I'd have to allow it to dry for nearly a week before applying the Spar Urethane. But with dye or stain I can apply Urethane in just a couple of day, so that's the route I took. I immediately realized that creating a faux Walnut grain finish on the MDF surfaces was very simple and it looked darn near real.


Photo 6: Front facing panel after faux Walnut with "Antique Walnut" colored stain


Photo 7: Additional surfaces after staining


Photo 8: Faux walnut including top


Photo 9: After painting with Spar Urethane


Photo 10: Finished speaker with gold painted feet (rubber bumpers on bottom)

 

[Flint]

Once I finished the final assembly, I tested the performance and got a very decent pseudo-anechoic response for this type of subwoofer:


Chart 1: Close Mic (Pseudo-Anechoic) response


Once installed on the patio using the great little 2.1 digital amp and getting it tuned so it measured okay and sounded good to my ear, I measured the response of the speakers with and without the subwoofer.


Chart 2: Frequency response from listening position with (black) and without (red) the subwoofer.

The 1.5 octaves of range is really nice out on the patio and makes for much more pleasant listening. I am very pleased.

This is an extremely affordable little subwoofer which sounds plenty fine and makes my music much more fun while sitting on my porch or hanging out in the back yard. I am pleased.

 

[Flint]

In an offline conversation I was asked about how I made measurements, before or after burn-in. Since this is a lower end mass produced subwoofer driver, I chose to burn it in for 48 hours using a slow sinewave sweep that also varied in amplitude with a 125 watt amp pushing it. Since it was just sitting on the floor in a remote guest room in the winter wing of my house, I could hardly hear it while burning in. I didn't bother to measure it before and after the burn-in period, which might have been interesting. But I was just concerned with getting a good & affordable subwoofer out of the project. I believe I have succeeded.

This is definitely NOT an audiophile sub or even a very good home theater sub. It is a better than every sub I've heard which came with a soundbar, and it is better than most of the budget subs I've seen at Best Buy or Fry's, but I wouldn't recommend it for a primary sub in a serious music nut's listening room.

 

[Randy]

Very cool and thank you again for documenting these projects.

 

[Flint]

You'll notice in the installed frequency response that in blending with the main speaker the sub causes a dip in the 150Hz region (the fixed crossover point on the amp I am using). The measurement I made was with the subwoofer in reverse polarity which was the better of the two options I have. The in phase cabling resulting in a large wide dip from about 125Hz to over 200Hz which was not only ugly to look at in the measurements, but sounded anemic as well.

The fact I cannot get a perfect match is why I have always advocated for a continuously variable phase control on sub amps with built in crossovers. I would bet that if I had the ability fine tune the phase at the crossover point I could almost eliminate any nulls when mating with my main speakers.

 

[Flint]

Yesterday I sat on the porch and did some work on my 2-in-1 portable PC while listening to music on this new speaker system. At first I felt the subwoofer was a tad too loud, so I futzed around with the level control on the amp until I settled on a balance of the sub being clearly present while remaining reasonably mellow.

The experience was great.

The speakers are a tad bright, but that works at the lower listening levels I tend to use on the porch (don't want to piss off my neighbors). Basically, I have created a set of speaker with a built in inverse loudness curve to I can get the details in the treble and impact of the bass without having to turn the levels up to 90dB SPL where I typically tune my high fidelity speakers for balance. really. As such, at 70dB the treble and bass is appropriately louder to give a timber which is perceptually balanced.

By the way, this is one reason I audition speakers and do my critical listening at higher SPLs.

 

[Flint]

Had a couple of neighbors over yesterday and the weather was outstanding so we sat on the porch. They saw the speakers and asked for demo and, of course, I was happy to oblige. I asked Alexa to play my Light Jazz playlist and turned them up a bit. It sounded pretty good to me, but my neighbors who last summer spent $2,000 on a bunch of outdoor speakers from their contractor (they couldn't remember the brand) were blown away - and disappointed in their speakers. I am not surprised - there are few commercial outdoor speakers which sound great at low to moderate SPLs. Mine are tuned to sound their best at fairly low levels - turned up they are bright and boomy.