Design Con 2015

Make Great-Sounding $40 Bucket Speakers in an Afternoon

-April 24, 2014

Although being told you “carry your tunes in a bucket” is usually considered an insult, this sweet-sounding pair of outdoor speakers gives new meaning to the expression. OK, they’re a bit odd looking, but they’re rugged, waterproof, and surprisingly efficient. In fact, all they need to fill a small backyard with party music is an inexpensive 10W/channel amp which runs on either 120V AC or 12V DC and can be tucked into one of the buckets when not in use. Best of all, you can build them yourself in a long-ish afternoon for about $40.

As with many of the best DIY projects, this one began with a problem. Both My buddy Mark and I enjoy having a lively soundtrack for our outdoor adventures but none of the bookshelf speakers we had lying around sounded good when they were cranked up to the levels needed to produce even a moderate sound level in an open picnic area. Besides, we both knew that any normal speaker we took on our camping trips would be one cloudburst away from being a pile of soggy junk.

Being the cheapskates we are, we came up with the idea of converting a pair of orange utility buckets sold by a leading big-box home improvement supply store into rugged, waterproof speakers. Although we weren’t the first audio hackers to build a set of “bucket speakers,” the other ones we listened to sounded mediocre at best. But Mark’s a real audio genius who specializes in low-budget, high-performance designs and swore he could do better than that.

One of the biggest secrets to achieving great sound on a budget was to select speakers whose frequency responses complimented each other, and whose acoustic characteristics closely matched those of the buckets we’d house them in. First, the mid-range driver’s acoustic displacement had to match the bucket’s internal volume for efficient sound production.

We hit the clearance/surplus section of our favorite audio components catalog www.parts-express.com and identified a 5-inch “full-range” woofer, which had the correct displacement, a relatively clean frequency response profile (approx 40Hz-3500Hz) and didn’t cost much more than $10. From there, we located a tweeter element whose 3dB point on its low-frequency response curve was located fairly close to the 3db point of the mid-range speaker’s high-frequency roll-off curve.  By carefully overlapping the speaker’s response profiles, we were able to produce a relatively flat composite output and get away with a very simple (1 resistor, 1 capacitor) crossover network.


My wife loves our $40 bucket speakers and says that they sound just as great at a picnic as they do in the living room - even if they don't match the carpet.

We mounted the speakers on a disk of ¾-inch plywood with a circumference of 11¼ inches, about 3/8-inch smaller than the top of the bucket so it drops in and stops about 1 to 2 inches below the lip.  

From Bucket to Acoustic Enclosure
Besides providing a place to mount the woofer and tweeter, the plywood disk is also part of the speaker housing’s acoustic design.

For example, the 4 ½-inch hole for the woofer and the 2 ½-inch tweeter cutout should be separated by approximately 1 inch so the wave fronts they produce appear to be coming from roughly the same point but don’t produce any excessive acoustic interactions.  The woofer mounts to the disk using the screw holes conveniently located around its perimeter. The piezo tweeter must be modified a tad by sawing off its passive radiator horn before press-fitting and gluing the naked driver element into the 2 ½-inch hole so that its top rests ¼ inch below the disk’s front surface. This creates a small cavity that helps align the tweeter’s acoustic center with the woofer.

In addition, we used ¾-inch plywood because it is just the right thickness to create an efficient bass reflex port by simply cutting a hole through the wood. But, instead of cutting out a regular circular hole, we’ll create two smaller ports by taking a 4½-inch tangential slice off each side of the disk at a 2½ degree angle, creating a slightly narrower end which allows the disk to be tipped on-edge and slid into the bucket for safe transport or storage.  In fact, both speaker mounts can be slid into the same bucket and the now-empty bucket can swallow its companion to save lots of space in the back of your SUV or storage closet.

It took a little experimenting, but we managed to eliminate the tinny audio quality that the bucket’s thin un-damped shell tends to produce. We killed off a good portion of the unwanted resonances by heating the bottom of the plastic bucket and re-shaping the flat surface into a concave dome (½ to ¾ inch deep) which is much more rigid and resists “oil canning” in response to the speakers’ outputs.

You can soften the plastic just enough by pouring a gallon or so of boiling water inside the bucket and waiting about a minute before pouring it out, inverting the bucket, and pushing the bottom inward with a large weight (i.e. a knee attached to a heavy adult, insulated with a towel). This process may need to be repeated several times.  For those of you with less patience (and a higher tolerance for risk) using an industrial heat gun or a blow torch will speed up the process nicely.
 

A frequency response plot of the finished bucket speakers. We achieved the speaker’s relatively flat low frequency characteristics by deforming the bottom of the bucket into a concave shape which got rid of rid of its tendency to resonate at 100Hz.

Subsequent experiments with various damping materials putting an 18” length of 10” x 1” open-cell polyurethane foam inside the bucket flattens the enclosure’s response without killing its efficiency. It also comes in handy as packing material to keep the two plywood speaker faces from banging together inside the bucket during transport. 

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