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Pyramid Powered Isobaric Subwoofer

Part I

April, 2006

Kevin Lichterman

 

Introduction

A wonder of an ancient world, a shape with mystical powers, and thanks to Stargate Franchise a parking lot for aliens, pyramids have always had a special allure for mankind. From the mystical to the mundane, many qualities have been attributed them. For me, they look kind of cool a perfect shape for a speaker that I'd love to add into my home theater.

When my two commercial subs died, I had a perfect opportunity to justify a better performing replacement. Since I had been taking a woodworking class for a number of years and needed a new project, I had the perfect opportunity to work on two of my hobbies at one time.

This article will walk through how I moved from the idea of replacing my subs to the measured performance of my personally designed and built Pyramid Power Sub.

Design Goals

I've always struggled with my desire for the new and cool with my cheapness. I guess I can describe myself as a "Champaign on a beer budget" type of guy. Plus I've always loved the feeling of accomplishment I get from doing it myself (just like my 3 year old). Although I knew I could pay for the performance of one of the well known manufacturers such as Velodyne or SVS that produce the subs that could be described as sub-sonic beasts, I couldn't justify the cost. So, I decided to take a stab at similar performance myself.

For the most part, my main goal was to replicate the high SPL, flat response, deep extension, and tight bass that the high dollar subs offer plus mine had to look both cool and distinctive. Not just another black box. I wanted a sub that would typically not be built by a mainline manufacturer due to its size, complexity, and cost.

Secondary goals of course included a low price I know shocking! Since my previous two subs both died due to failures in the internal amp, I also wanted to use an external amp that offered me the versatility to use with the sub or to supplement by main home theater rig. Finally, I wanted to pick a project that would broaden my own woodworking skills by offering new design challenges.

The Pyramid Power Sub project fit the bill.

Audience

When writing this article, I had to make a choice with respect to the level of detail to include. Either I could target the Norm Abrahms of the world, or people like me someone willing to tackle a wood working project without their own TV show. I of course chose the latter, so the detail I include may be cumbersome at times for some. Norm will just have to forgive me if he reads this!

Starting Out - the Design Process

I could say that when I sat down to design my sub, I had an elegant plan based on sound theory and years of experience which, of course, was supported by extensive research and development. I'd be lying of course. What I did know was what I wanted to achieve based on my design goals. Designing a well performing speaker is a complex process involving many decisions with respect to box size, shape construction, etc. Without a background in this design process, I had a few options to make things work. One option was spending my time to educate myself on the intricacies of speaker design. The other was to cheat. I wanted to get started as soon as possible, so I decided to cheat (I guess add impatience to my list of attributes).

Luckily, a friend of mine did have many of the attributes that I alluded to earlier: he had the experience of many projects under his belt (heck he even commissions his own drivers!). He suggested that I sit down with him and his copy of BassBox Pro, and with a few tips from him, brute force my way through the process.

This was a surprisingly educational process. I won't go into the details on how BassBox Pro works, but in summary it allowed me to simulate the real world performance of a sub by manipulating the various parameters that go into the box itself: type, size, shape, drivers, ports, electrical connections, etc. Using this tool, I ran through a few (ok hundreds really) of simulations where I played with these contributing parameters until I zeroed in on my design goal.

So what did I end up with? First, I chose to use Adire Audio's Shiva 12" subwoofer drivers (yup I said drivers two were used in this beast). Based on reviews at Secrets and elsewhere on the Internet, these had the reputation of being a driver both of high quality and of reasonable price. The design of the enclosure revolved around the acoustic and electrical properties of these selected drivers.

Speaking of the enclosure, I ended up with a pyramid as I desired a big heavy one at that! The enclosure is 40" tall overall with a base (width x depth) of 24" x 16" and the top of 16" x 10.5" for a total volume of 4.84 cubic feet. It has two 2" x 11" vents on the top panel for a bass reflex design (more noise with less power versus other options) - each port flared to reduce port turbulence noise (as air pumps in an out). It may strike you as an unusual design choice to place the ports on top, but I had a very good reason. At nearly a foot long each, that's the only place they fit! And, if I do say so myself, they look pretty cool on top as well the smokestacks of my subsonic factory.

As scary as it seems, this great big box could have been bigger nearly 10 cubic feet and 8 feet tall! The BassBox Pro simulations demonstrated that by using a compound Isobaric Chamber, I could reduce the size of the box in half while achieving my aforementioned design goals. In short, an Isobaric Chamber can be seen as a small sealed speaker inside of a larger box. In my case, the two 12" Shiva drivers I used were mounted one behind the other in an Isobaric configuration. Each driver was oriented in the same direction and worked in phase.

Click Here to Go to Part II.

Copyright 2006 Secrets of Home Theater & High Fidelity

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