Earthquake 21 — 21-Inch Sealed Subwoofer Build Plans
DIY SUBWOOFER PLANS

Earthquake 21-inch Sealed Subwoofer Build Plans

21-inch sealed bass in a 24-inch cube — designed around room gain for bass that you can feel.

The Earthquake 21 started with a question: could I put a massive 21-inch driver into a relatively compact sealed enclosure and still get the kind of deep bass I wanted for home theater?

Built around the SB Audience ROSSO-21SW800, the answer in my room was better than I expected. The combination of the large driver, sealed cabinet, room gain and DSP produced incredibly deep, clean bass that I didn't just hear — I could feel it throughout the room.

$20.00
▶ Watch the Build
Instant PDF download Personal use only
WATCH THE BUILD

Can a 21-Inch Subwoofer Work in a 24-Inch Cube?


This build started as a theory. Follow the complete build, see how the enclosure and internal bracing come together, and then watch what happened when I measured the finished subs in my theater.

▶ Watch on YouTube
WHY BUILD THE Earthquake 21?

Big Driver. Surprisingly Small Box.

Normally, getting serious output from a huge driver makes you think of an equally huge enclosure.

The idea behind the Earthquake 21 was different.

I wanted to see whether a 21-inch driver could work in a sealed enclosure only about 24 inches in each direction, then use the natural room gain of my theater and DSP to extend the response where I wanted it.

The result changed what I expected from a sealed home theater subwoofer.

21 Inches in a 24-Inch CubeThe enclosure is approximately 24" × 24" × 24" externally with roughly 6 cubic feet of internal volume — surprisingly compact for a design built around a 21-inch driver.
Designed Around Room Gainnstead of trying to get all of the low-frequency extension from the enclosure alone, the Earthquake 21 was designed around what actually happens once the subwoofer is placed inside a room.In my theater, room gain allowed me to achieve response far deeper than the raw sealed-box response would suggest.
Sealed and Seriously BracedMoving this much air creates a lot of pressure inside the cabinet. A substantial internal bracing matrix helps control cabinet flex and unwanted resonance, while the sealed alignment keeps the design simple and delivers the clean bass I was after.
WHAT'S INCLUDED

The Measurements, Layouts & Instructions to Build It

Panel Cut List

Exact dimensions for cutting the main enclosure panels

Internal Bracing Layout

Shows how the individual braces are positioned and assembled inside the cabinet

Internal Bracing Cut List

Measurements for cutting the heavy-duty bracing matrix

Basic Assembly Guide

Straightforward guidance for gluing and assembling the enclosure

Cabinet Dimensions & Layout

Overall dimensions and the information needed to lay out the sealed enclosure

PARTS LIST

What you'll need

Prices and availability can change. Use the links below to check current pricing and stock before starting your build.
Affiliate disclosure: Some links below are affiliate links. TOID may earn a commission if you purchase through them, at no additional cost to you.
ComponentQtySource
SB Audience ROSSO-21SW800 21" Subwoofer 1 per speaker Check Price & Stock →
MDF Approx~ 2 Sheets Source Locally →
Speakon Connector 1 per speaker Check Price & Stock →
T-Nuts + Machine Screws 1 Set Details in Plans
Internal Dampening Material As Needed Your Choice
DIMENSIONS & COST

Finished size & estimated cost

Height24
Width24
Depth24
Estimated parts costApprox. $500

Prices fluctuate — check current links before ordering.

READY TO BUILD?

Ready to Build the Earthquake 21?

Get the complete plans and start your build today.

Instant PDF download • Personal use only. Commercial production or resale is prohibited.

$20.00