Cinderella Speaker Project for Middle School Technical Outreach

by

Paul S. Russo

LSU

chruss@lsu.edu

225-588-8104

 

 

INTRODUCTION

You might think a speaker named Cinderella would be a dainty lightweight, but not this one! The Cinderella Middle School Speaker (CMSS) is built around a cinderblock! This and other features make the speaker easy to construct in typical middle school environments, which often do not include a shop. The result is a rock (literally!) solid speaker. The CMSS is very heavy (15 - 30 pounds) and passes the audiophile's knuckle rap test with aplomb. A stack of these will not only hold down your floor but provide a line source capable of very high output level and astonishing efficiency.

GENERAL FUNCTIONAL PRINCIPLES

It is possible to build many versions! Look at all the cinderblocks you can choose! In all cases, the speaker drivers (woofer, midrange, tweeter) go on wood faces that are glued to the cinderblock using Gorilla urethane glue. The weight of the block itself holds the pieces tight and makes a very good, airtight seal. Drivers are affixed to the wooden boards, and usually NOT flush-mount (extra woodworking is required for that). A grill cloth board (typically 3/16" or 1/4") restores the flush surface to minimize diffraction. Systems can be one-way (single, full-range driver), two-way (woofer + tweeter) or three-way (woofer + midrange + tweeter). In two-way or three-way systems, it is recommended that each driver be connected directly to the back of the speaker, so that students can tweak the crossover components, relative phase. They can even use multiple amplifiers and an electronic crossover. Drivers should be fused to prevent disaster!

SKILLS STUDENTS CAN LEARN BY DOING

General design: shape, internal volume, selection of drivers, computing prices.

Measurement of internal volume.

Drilling holes for speaker connections.

Twisting screws into the wood to hold drivers.

Painting or staining.

Performance measurements.

APPEARANCE OPTIONS

Plain cinderblocks can be painted, covered with contact paper, set into beautiful wooden boxes that are stained and finished or painted (flat, shiny, or decorative).

EXAMPLE

Here is the simplest Version: a single 8" x 8" x 16" cinderblock with two chambers sandwiched between two pieces of high-density fiberboard. The wooden face was given an automotive-level paint finish in a powder blue. The drivers are high-quality Tang Band bamboo 3.5" full-range. The sides were covered with thick paper (e.g., posterboard). One chamber is left unused for future expansion--e.g. bass driver. In this case, greater internal volume is probably desirable. That can be achieved by penetrating the separator between the two chambers of the cinderblock using a masonry drill. Alternately, a small box can be added, made of wood (more tooling required).

CLICK ON IMAGES FOR LARGER PICTURES

Parts List Below Mark hole for speaker binding plate. Drill corners.

Saw the edges with jigsaw. Lay in speaker binding plate; mark holes; drill. Urethane glue (usually Gorilla; here Brand X)

 Newspaper prevents glue from getting on floor.  Glue expands; use chisel to remove excess. Front piece...just sitting here for now.

Mark speaker cut-out. Drill using hole saw.  Slanted internal baffle fights standing waves.

 Sound absorber.  Spray insides with sound absorber.  Chisel away any rough edges. Goggles here!

 Glue on front pieces.  List of stuff needed to wire speakers together.  Painted blue & placed in my office!
FUTURE IMPROVEMENTS FUTURE IMPROVEMENTS FUTURE IMPROVEMENTS
FUTURE IMPROVEMENTS FUTURE IMPROVEMENTS FUTURE IMPROVEMENTS

 

PERFORMANCE

No very serious measurements have been made on the Cinderella yet, but here's an impedance trace showing resonance about 140Hz. So, the lower chamber should be expanded and a woofer added.

SOUND

Well, not bad considering there is no real bass. This Tang-Band driver is highly regarded, and for good reason. For awhile, I used these in my office at about ear level with an old pair of KLH Model Sixes to supply the bass below about 180 Hz. A high-frequency driver as large as this one is very directional, but the high frequencies are there (not the very, very high frequencies kids can hear, but anything most over-40 people can hear). It is a very simple sound...no crossover. 

PARTS LIST

Quantity      
 2a

Tang Band W3-1364SA 3" Bamboo Cone Driver

Parts Express

http://www.parts-express.com/pe/showdetl.cfm?partnumber=264-844

 $40  $80
 2b

Square Speaker Wire Terminal Cup 3-1/8" Gold Spring-Loaded

Parts Express http://www.parts-express.com/pe/showdetl.cfm?partnumber=260-280

$6  $12
 1

Cascade VB-1S PRO Quiet Kote Acoustic Damping Spray 18 oz. Net Wt.

http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-250

$17  $17
 2c

Cinder blocks

Home Depot, Lowes, Etc.

$3  $6
 4d

Wood or Medium- or High-density fiberboard for faces

Home Depot, Lowes, Etc.

Varies  $10
 ~8 oz  Gorilla Glue or equivalent polyurethane glue $12  $12
 25 feet 14 Ga. Speaker wire (Lowes, Home Depot, etc. $7  $7
 N/A   $5  $12
  Total    $156e

Notes:

a. You could buy four and put one in each chamber of the cinder block, or buy a dedicated woofer to expand. The Tang-Band is a nice driver, but an expensive one. You can find many other similar drivers at lower cost! Sources include PartsExpress, Madisound and MCM. Just Google these. The same basic plan can be used with a 4" or even 5" full-range driver. High-frequencies will not be heard unless the speakers are aimed right at the listener then, but bass will be better. A nice discussion of drivers can be found here: http://www.zaphaudio.com/  (You can see find nice designs here, but they may require more woodworking skills than available in a middle school).

b. There are easier ways to hook the speakers up that do not require cutting out the hole. Consider these easily installed posts instead. http://www.parts-express.com/pe/showdetl.cfm?partnumber=091-1245  Only a drill is required for these.

c. We are not using the lower chamber in the present design, so a "half block" might work as well: 8" x 8" x 8" cube. I chose the full cinderblock in order to get the speakers at ear level in my office...and I figured I would add a woofer to it one day.

d. You need 4 pieces. Take a tape measure when you buy the cinder blocks and measure the size you need. Many stores will cut the wood pieces for you on a panel saw or table saw, which will result in nice rectangular pieces. For the speakers pictured here, the wood pieces are 15.75" x 7.75". They are 3/4" thick.

e. The project can be completed for much less using drivers, scrap wire, alternative ways to connect speakers, etc.

 

SAFETY

This is a very safe construction project, but:

SOME TIPS

When screwing down the drivers and speaker binding mount, students should be careful not to let the screwdriver slip. If you can get them, socket head screws with hex drivers are better.

Ideally, after you cut the opening for the drivers with the hole saw you can run a router bit around the inside edge to relieve the underside a bit. Provides better air flow, so I am told. I did not do this.

Although we did not place the driver in a recessed hole--no smooth front baffle as in most modern speaker designs--I will eventually put another thin board on, covered in speaker grill cloth. The thickness of that piece matches the protrusion of the driver, resulting in a smooth front. The second piece helps to absorb some resonances from the front baffle too. This is a 1960s-1970s solution. In those days, no one wanted to look at the drivers all the time. 

REUSABLE ITEMS

Hole-cutter bit, 1/4" or 3/4" drill bit, electric drill, sabre saw, soldering gun (the trigger kind is better than a pen soldering tool for this work). For students, hand tools might be better in some cases.

OTHER NOTES

Some people would build a small speaker like this using a ported design (a hole somewhere in the box). Perhaps they would knock out part of the wall between upper and lower chambers to get more volume too. I don't like the sound of ported speakers, so even if a larger woofer is added I will find more internal volume without using a port. Maybe I will drill out the lower chamber and add an extra box. There are many other options, too...for example the back plate could be made with some convoluted "exhaust" system--a transmission line. Its purpose is to dampen out most sound waves launched inside the cabinet but pass out certain low frequencies to extend the bass response. The design of transmission lines is a difficult thing.