Loudspeaker Design

This information is partly based on my own experience and research into designing loudspeakers. It covers the basics and hopefully provides some useful hints. It may not be entirely accurate. It is recommended that you look at books on the subject if you intend to build your own speakers.

There are two main parts to designing loudspeakers:

Designing the speaker enclosure based on required performance and driver specifications

Designing the filter network to compensate for the enclosure and driver performance to give ideally flat frequency response

Materials & Dimensions

Good quality thick wood is recommended for building the enclosure. The majority of the enclosure of a large speaker should be made of at least 18mm thick good quality chipboard.
The front panel (baffle) should be made of 18mm thick MDF.
The inside of the enclosure should be covered in accoustic wadding, to prevent reflections of sound which can result in 'booming'. As accoustic wadding can be expensive, I have found that the inside of a thick duvet is a good alternative!

The dimensions of the enclosure should not be exact multiples of each other, to help prevent reflections. Don't use ratios of 3:2:1, use 2.3:1.6:1 for example.

Many large DIY stores will cut wood to a good accuracy at a small cost.

You can download my loudspeaker Design Spreadsheet lsdesign.xls. You simply put in the specifications of the bass driver you want to use at the top, and hey presto, the dimensions of the pieces of wood you need will appear at the bottom!

Construction

All joints should be sealed so the enclosure is air tight. If the enclosure is not air tight, at high power levels, hissing may be heard as air is sucked/pushed through the gaps.

Lots of screws should be used to ensure that the case is strong.

It is important that the speakers are identical.

Filter Design

The calculations for this are also fairly complicated, so I devised another spreadsheet for this: x-over.xls. This spreadsheet gives you two options:

First Order Filter - Slow roll off, typically 3dB/octave, cheap and used in most loudspeakers except the most expensive. Often, the inductor can be omitted, as the bass driver will roll off quite rapidly anyway, and the capacitor should be chosen for the tweeter to take over where the bass driver 'left off'.

Second Order Filter - Faster roll off, typically 6dB/octave, slightly more expensive as it uses more components but better quality.

I have used a first order filter without the inductor for my loudspeakers and it seems to work ok.

The compensation required for the second order filter is to prevent a peak in amplitude in the overlap between the bass driver and tweeter 'swapping over'.

The connections to the tweeter must be reversed for the second order filter because it would othewise be 180 degrees out of phase with the bass driver.

My Example

The photo below shows what one of my speakers finally looked like. Wood and hardware cost less than £10. The ports were made of 75mm drain pipe. The tweeters (good quality 70w RMS ones, from Maplin Electronics, now discontinued) cost £20 each. The bass drivers (13" x 6") are very old second hand ones, and not very good, but the enclosure brings out the best in them ;-)

One major mistake I made was that the tweeter is too far away from the bass driver - making it less suitable for 'near-field' domestic use. It is fine at high volumes in large rooms.

Each speaker weighs 20kg - most of that is wood. The capacity of each speaker is about 77 litres. I didn't cover them with speaker cloth as it is too expensive. At some point I may paint or varnish them. They definitely aren't designed for outdoor use in their present state!

I used a first order filter with just a 10uF capacitor in series with the tweeter for approximately a 3kHz crossover frequency.

I used thick screened cable for the connections to the amplifier - this wasn't nearly as expensive as 'audiophile' cables, which are very often not screened. The screened cable certainly improved the sound quality over the original cheap loudspeaker cable.

My Example - Upgraded

The photo below shows one of my new speakers.

This new design corrects some of the problems mentioned above. As you can see, the bass driver is now much closer to the tweeter, improving near-field performance. The front baffle is now made of MDF, and is much more solid. The new bass driver is an Eminence Beta 12 rated 150W RMS. (see www.eminence.com for more details). The speaker network was slightly modified to take into account the different frequency response of this driver and the improved sensitivity. I have not had the opportunity to test them with more than about 0.5 watts input power - they are very loud even at this power level!