The design of a multiple driver loudspeaker, overview

Design of a multiple driver loudspeaker requires quite a few tools. Without these tools the design / trial and error process is lengthy and often doesn't show the required results.

Absolutely mandatory for designing are multiple driver loudspeaker is a tool to measure the sound pressure level depending on frequency, and - if possible - best in such a way that room resonance doesn't come into account.

The design process using the Alcone Dirac XT as an example

Selection of drivers

The individual drivers need to compliment one another. You should bear in mind that usually the volume of the bass driver cannot be reduced by using a resistance divider. The bass driver should not me louder than tweeter or midrange driver.

Each driver requires its own compartment, if the rear is not encapsulated (two of the same drivers may be placed in the same compartment).

One of the first steps should be the calculation of required compartment size for each driver.

The Alcone Dirac XT uses an encapsulated XT300 tweeter and two Alcone AC 5.25, one in a closed compartment and one in bass reflex cabinet.

Using the following calculators:
- simple calculator for a closed cabinet
- simple calculator for a bass reflex cabinet
- LspCAD Lite(also for band pass) the required volume for a 7 litre closed cabinet and a 22 litre bass reflex cabinet may be calculated, where also the DC resistance of the 1mH coil with 0.28 Ohm was taken into consideration. Since there are two drivers the coil acts like two parallel 2 mH coils with 0.56 Ohm, where one affects just one driver.

For all calculators you will need the Thiele and Small parameters which we list for all our drivers. If we design a loudspeaker for you the Thiele and Small parameters are measured and confirmed by us to be sure.

Cabinet construction

We need to design one 7 litre closed compartment for the first AC 5.25 and 22 litre bass reflex compartment for the second AC 5.5. That's what we are going to do a bit later.

Measuring of the fitted drivers

The drivers' properties are very much dependent on the cabinet. Therefore, frequency and phase response should only be measured when the drivers are fitted.

Crossover design

The simplest and quickest way is to transfer the frequency and phase response results to a crossover simulation software. The software then simulates the crossover with total frequency response and phase response. This enables you to choose a crossover that best suits your loudspeaker.

Suitable software:
- LspCAD
- LspCAD Professional
- Netcalc (on Waldo CD) a DOS based programme that runs by using a clock speed reducer.
- for mathematicians: Microsoft Excel + plug-in for complex numbers and Solver

The easiest to use is surely LspCad, Netcalc has got the best price / performance ratio.

Crossover design and checking of measuring results

Under the condition that the tools have been used correctly the crossover design should provide a loudspeaker with excellent frequency and phase response, the same as simulated using the software.

Finally the total frequency response and the phase response should be measured again for confirmation; afterwards it's time for a listening test.

There is no guarantee that a loudspeaker with perfect frequency response also sounds perfectly. Nevertheless, it's still a high quality loudspeaker, that would not have been possible without the use of measuring tools.

Finishing touches

There is still time to experiment:
- What are the implications when crossover components and damping materials are exchanged
- what are the implications of a slightly adjusted bass reflex tube (may be temporarily extended by using paper)
- what happens when the cabinet is de-coupled from the floor by using spikes (try using drawing-pins)
- what are implications of using different speaker cables

If the speaker's sound shows too much presence, try to reduce the midrange frequencies.

...we hope you enjoy your new loudspeakers....

PS by Peter Strassacker:
If you work according to our suggestions you will good results. A common problem when building speakers yourself: the designer wants to be creative, and therefore tries to design everything himself. Without the required tools the results are often not as good as expected.

If you do not own measuring tools we recommend to buy loudspeaker kits. You may still modify the finished speaker and the result will be excellent. For us it's important that you own loudspeaker that provide many years of listening enjoyment.