2.1 Audio System

November–December 2015

This audio system was built for the individual speaker project of my freshman fall advising seminar, 6.A01; we were tasked with building a speaker system, with the only constraint being a $250 budget.

Initially, I wanted to build a system with many speakers for different parts of the room that could be switched on and off as needed, so that the active speakers could be differently configured depending on the position of the listener. However, I didn’t want to sacrifice sound quality to do this, and I found that parts for that many speakers would exceed my budget. Instead, I decided to build the best-sounding system I could make within the budget.

In researching components for this project, I discovered the large DIY audio community that exists on various pages and in various forums on the internet. These online resources taught me much of what I needed to know for this project, from material choice to box construction to painting technique.

Desktop speakers and subwoofer.

This is a 2.1 system, with two desktop speakers and one subwoofer. The desktop drivers are Dayton Audio PS95-8 drivers, chosen for the reasonable price and fairly flat frequency response from 100 Hz through 20000 Hz. It has a paper cone and a copper-anodized aluminum phase plug. The subwoofer driver is a GRS 8SW-4; it is 8” in diameter and has a sealed F3 of 48 Hz. I chose the subwoofer driver to maximize bass extension while keeping in mind power needs and enclosure size recommendations - a bigger driver would have required a more powerful, expensive amplifier and a bigger, costlier enclosure.

There are two amplifiers in the system. Audio input goes to the Lepai LP-2020A+, the output of which is connected to the Dayton Audio SA70 (mounted in the back of the subwoofer enclosure). The SA70 powers the subwoofer and outputs signals for the left and right desktop speakers. The SA70 has controls for subwoofer gain and crossover, which are useful in tuning the balance of the system between the subwoofer and the desktop speakers.

The Dayton Audio SA70 amplifier in the back of the subwoofer. The wires at the left are coming from the Lepai amplifier, and the wires at right are going to the desktop units.

The enclosures are made of MDF (chosen for reasonable cost and acoustic properties), wood glued together and spray painted satin black. The subwoofer enclosure is made of 3/4” thick MDF, while the desktop speakers are made of both 3/4” and 1/2” MDF. Dimensions were chosen to get as close to the Parts Express recommended enclosure volumes as possible while using the least material possible; because of the seminar’s purchasing system, I was limited to using 12” x 24” pieces of MDF from Inventables. The subwoofer enclosure has a wooden brace made of scrap wood across the interior. Acousta-stuf fiber filling was added to the enclosures, although I’m not sure whether this causes an actual audible improvement in sound quality. The drivers are attached with T-nuts and screws.

I ended up choosing to build sealed, rather than ported, enclosures because 1) I wasn’t confident enough in my ability to design ports that would do more good than harm to the sound quality, 2) the large subwoofer meant that bass extension would be sufficient for what I wanted without ports, and 3) I decided to value sound accuracy over bass strength, as this would be primarily used for music.

Enclosure edges are beveled to mitigate any Olson diffraction, mostly on the desktop speakers. The subwoofer sits on cabinet feet, and the desktop speakers have small rubber feet.

Enclosure pieces were cut using a circular saw. I used a laser cutter to mark where holes were needed in each piece, then cut the holes out with a jigsaw. These holes were very rough, but the speaker drivers cover up the imperfection in the final product. A chopsaw was used to bevel the front faces.

The pieces were then glued together to form the enclosures.

The boxes were made smooth and (mostly) seamless by sanding and filling with wood putty.

Spray primer and black satin paint were used to finish the enclosures. I forgot to seal the MDF before priming, so I had to do this step after priming, which worked but was not ideal. Sealing the MDF using a 50/50 mixture of wood glue and water seals pores in the MDF and prevents it from absorbing paint. The end result is clearly imperfect, but better than what I was expecting.

The end result is a system that blows my old $40 Creative Audio 2.1 system out of the water. Bass is strong but not overwhelming or excessive when adjusted correctly, and the desktop speakers provide a very high level of detail throughout their range. When placed about three feet apart and slightly toed in, the soundstage is very wide, and I’m discovering localization effects in songs that I’ve never heard before. Listening to these is also very comfortable and fatigue-free. The system is capable of filling a medium-sized room (e.g. a typical classroom) with sound, but also works well for medium to quiet personal listening at my desk.

This is not at all a perfect system (so many paint imperfections!), but for a first attempt at building speakers, I’m more than happy with how this turned out.