Bookshelf Speakers

Introduction

In high school, I took a subject where we chose a problem in a category (i.e. a commercial, personal, domestic problem, etc), designed and then manufactured a product to solve that problem. We created a folio to document the design and build process. This webpage is an excerpt from sections of such a folio (images were scaled awkwardly to fit the layout of the folio).

At the end of high school, our final category was a domestic problem. My mother complained about the quality of her speakers at her desk and I wanted to do a ‘big’ project for the end of the year; so, I decided to design and make speakers to fix the domestic problem.

Problem

The client needed a set of powerful yet small speakers to fit behind their computer monitor. They needed to have significantly more bass response than regular PC / desktop speakers. Portability was a requirement as they liked to move them for listening in another room. The client did not want headphones or earphones as they dislike wearing them. I was given a budget of no more than $300AUD.

Computer on a desk
The desk environment in which the speakers needed to fit

Client Problems

  • Many Hi-Fi speakers are too big to fit behind the monitor and be portable
  • Speakers with sub-woofer units are annoying to move around
  • Many Hi-Fi speakers are too expensive for their budget
  • The client does not like wearing headphones
  • The characteristics of mainstream computer speakers are not to their liking
  • There is not enough bass presence in the client’s current speakers

For the issues at hand meant that bookshelf speakers were the most appropriate solution, as the client didn’t want a portable speaker either.

Design Criteria

  • Units must not cost more than $300 to produce
  • The expense must be proportional to the sound quality
  • The speakers must be aesthetic in some way i.e. not just a blank wooden box
  • The sound must satisfy the client
  • The speakers must have longevity
  • The speakers must be safe
  • The speakers must be portable
  • The unit should not have obstructive wires or be annoying to move
  • Only essential features should be present to minimize cost

Design

Research showed that natural wood did not have good audio qualities as natural wood has non-uniform density. Most said that MDF was the best sounding of man-made wood so that was selected as the primary material for the project.

It was difficult to source speaker drivers as they were very expensive, or the supplier was so far away that shipping was too expensive. I decided to use Response speaker drivers from Jaycar as it was the only financially viable option considering the budget constraints.

I chose a 1” self-sealed tweeter unit and 6.5” woofer for each unit. The tweeter went down to 2kHz so I bought a crossover at that frequency. The wiring was done with stock audio cable I had a spool of at home.

Diagram of speaker enclosures
Comparison of sealed and ported speaker enclosures

I chose to make the enclosure sealed rather than ported as sealed speakers are more forgiving to mistakes in construction than ported enclosures. Ported enclosures also need to be bigger than sealed ones, and as they need to be portable and compact, I went with a sealed style.

A sketch of a speaker unit
Initial hand-sketch of final speaker design

I then formalized the design in CAD. It was a requirement to make drawings that could be fully worked from.

A 3D model of speaker unit
CAD model of a single speaker unit
CAD drawing of wooden box
CAD sketch of box dimensions
CAD drawing of a wooden panel
CAD sketch of front panel dimensions

Manufacturing

All manufacturing was done in the school workshop apart from cutting the MDF cut for me at my local hardware store. I simply had to cut the holes for the drivers, make a hole for the wire terminals at the back and then assemble the box together with glue and screws. I did all the wiring into the terminals, the crossover and the drivers at home.

Cutting a wooden panel
Using a jigsaw to cut the holes for the drivers
A sander sanding wood
Sanding the driver holes
A speaker front panel
The drivers test seated into the front panel
Screwing into wood
Screwing the drivers into the front panel
A speaker front panel
The painted front panel
Interior of a wooden box
Gluing the attached side panels to make a seal
Holes in a wooden panels
Drilling for the top panels
Interior of a wooden box
Top and side panels attached and sealed
The interior of a speaker unit
Mounting the crossover in the unit
A square hole in wood
The hole for the screw terminals
A screw being drilled into wood
Attaching the wire terminals
An assembled wooden box
Attaching the back panel
Paint being applied to wood
Painting the speakers

Evaluation

A computer desk with speakers
The speakers set up in their designated environment

The speakers fit in their design scenario well. They could be moved over to the side table easily. The client was able to make comments and adjustment during the manufacturing process. For example, the units were going to be painted completely black, but the client wanted the wood to show as a highlight. The client stipulated nearly everything with aesthetics. However, due to time constraints the enclosures were not planed and sanded square.

The sound of the speakers was quite unique as putting large drivers in such a small enclosure created a unique response characteristic. The fully sealed enclosure also contributed to the sound quality. The speakers needed to be played loud as the lack of ports reduced the volume of the bass. Once the levels were correct, the speakers had an excellent sound.

Two speaker units on carpet
The two completed speaker units

The client was disappointed with the weight of the speakers, they were indeed portable but the large drivers and the choice of MDF made the units hefty. The enclosure should also have been routed to remove all the sharp edges. The client also expressed fears that the lack of a cover on the speakers left them vulnerable to damage. A mesh should’ve been placed over the front of the unit or the woofer drivers.

The way this whole project turned out was a fluke. Most of the time I was going on information from audio forums and hearsay. I had rough calculations that the enclosure size should work but nothing concrete. With so many unorthodox decisions in the design I am lucky the sound was ‘unique’ and not downright terrible. I recommend other to try sealed speakers as their sound is pleasant, especially with pre-90s music.