Loudspeaker Design - Designing Your Own Loudspeaker

Designing a loudspeaker without a good measurement system is a game of hit & miss. You might be lucky – your selection of drivers; cabinet type, size & shape; position of the drivers; crossover points; crossover slopes maybe just right – you might achieve the optimum balance of driver levels and spectral flatness, as well as minimal system resonances, low linear & non-linear distortion – your finished speakers might have great imaging and dispersion characteristics because of the excellent off-axis performance and minimal diffraction in your design...this however is unlikely - with all these variables to juggle, the chance of succeeding without the ability to measure what’s happening is very slim. As Lord Kelvin famously said “If you can't measure it, you can't improve it”.

You cannot trust your ears. Of course the final arbiter must be your ears, but during the loudspeaker design process, your ears cannot be trusted. The audio spectrum covers about 10 octaves - the human ear is not linear – for example it is far more sensitive (about 10dB) at 3000Hz than at 1000Hz; some types of distortion can be confused with ‘musicality’ or ‘detail’; 50% of what you hear in a typical listening room is reflected sound; one CD can sound good, another can sound awful; making a change can make a difference but is it better or worse?... There are so many variables in the system, and sound is so difficult for the ear-brain combination to ‘measure’ that the whole loudspeaker design exercise becomes a frustrating one of tail chasing.

Loudspeaker design priorities defined in engineering terms. When designing a loudspeaker the major design aims in order of importance are: -

1. Spectral flatness

2. Low energy storage

3. Minimal non-linear distortion

4. Minimal diffraction

By definition these cannot be measured by ear. Problems in any of these four areas may well be audible, but the ear alone will have difficulty pin-pointing the cause – but obvious in correct measurements.

Crossovers cannot be calculated. The heart of the speaker system is the crossover (unless you are using a single full range driver – although in reality these do not exist – most drivers can only cover about 3 to 4 octaves without generating considerable levels of distortion). Crossover design is critical to the loudspeaker’s performance – it has many jobs to do...

1. Pass the correct band of frequencies to the appropriate driver

2. Prevent the wrong frequencies from getting to drivers not designed to handle them (in particular stop low frequencies getting to the tweeter)

3. Obtain an acoustic response which is smooth and flat* across the audio spectrum

4. Ensure the load impedance presented to an amplifier is within design limits – i.e. a speaker with 8-ohms nominal impedance should not drop below 80% of this value (6.4 ohms) anywhere in the frequency spectrum.

5. Adhere to acoustic design parameters at the crossover points to ensure good transient performance – this means each of the low-pass, band-pass and high-pass filters must have the correct: Roll-off slope; Corner frequency; Summing point.

Note: * A perfectly flat response doesn’t guarantee a good sounding speaker – in some cases a response with a downward slope towards the HF end, or the so called ‘BBC dip’ in the mid range, may produce the best overall sound for a given loudspeaker -room combination.

The word ‘acoustic’ is underlined above - this is one reason why crossovers cannot be calculated – a filter can be designed electrically to be, for example, 12dB per octave with a Q of 0.5 but when measured acoustically can be something entirely different. It is true that electrical filters can be calculated and their behaviour is very predictable - this is the basis for the many calculators that can be found in books and on the internet. These formulae only work for a resistive load, i.e. loads that do not vary with frequency – a loudspeaker is nothing like a resistor which means all these calculators are fundamentally flawed. The best they can do is to provide you a starting point which will ensure you won’t damage your tweeter. To correctly design a crossover you need the following information: -

1. Impedance (magnitude & phase) data by frequency for each (actual*) driver mounted in the intended cabinet.

2. The distance between the intended listening position and the front of the cabinet

3. Anechoic frequency (magnitude & phase) data for each (actual*) driver mounted in the intended cabinet

4. Non-linear distortion (Energy-Time-Curves) data for each (actual*) driver mounted in the intended cabinet

What if you haven’t got a Loudspeaker Measurement System?

o If you are intending to design more than one speaker then maybe a measurement system could be a justifiable investment – Audio Components are suppliers of the Liberty Instruments PRAXIS system which is a very comprehensive analyser used by many commercial speaker manufacturers. Details of the PRAXIS are available on this website and Audio Components offer training in loudspeaker design and measurement using PRAXIS.

o If the purchase of a Measurement System is not feasible, Audio Components offer a loudspeaker design and service – this is how it works: -

a. You send one set of drivers to us for measurement

b. You construct a test box (this can be roughly made identical in size & shape etc to the intended cabinet) and ship it to us, or we can make a test box for you based on information you supply

c. We carry-out all the necessary measurements (as above) and supply both data files and printouts of results for you complete the design, or using the data we will design the crossover for you and supply you the schematic and layout (if required)

d. We ship the drivers and the test box if required) back to you

e. If you purchase the drivers and components from us, we will provide this service at half our standard price.

Don’t Spoil the Ship for a Ha’porth of Tar… If you are spending a lot of time and money on your cabinet and drivers don’t spoil your speaker because of a poor crossover. Allow your drivers to release their full potential – get the crossover right!

Note * It is always best to use the actual drivers because there can be performance differences due to production spreads etc. However drivers from highly regarded manufactures such as Scanspeak, SEAS, Volt, etc are likely to be very similar for a given manufacturing period.

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