Thiele/Small and Speaker Enclosures

Thiele/Small and Speaker Enclosures

We complete our tour through a typical car speaker specification list by looking at Thiele/Small Parameters.

These are important when looking to put speakers in a sealed or ported enclosure. They dictate the optimum size an enclosure should be in order to get the most out of a given speaker and were discovered and developed by two Australian engineers; A. Neville Thiele and Richard H. Small. Unexpectedly, the two engineers did not work together on their research. Richard Small published his research results some time after Thiele although relied very heavily on Thiele’s work.

In order to get the best out of any speaker, it is ideal if the air movement from the rear of the speaker does not affect that coming from the front. Traditionally, it was felt that an “infinite baffle” approach was perfect for this – my dad’s radiogram was a perfect illustration of this. Essentially, infinite baffle is where a speaker is mounted on a board or panel with no direct route for air movement from the rear to get to the front. This is also sometimes referred to as running “free air” in car audio terms. At higher frequencies, this works perfectly adequately in most instances. However, when it comes to bass frequencies, particularly in the challenging in-car environment, isolating air is much harder than it sounds. Putting a subwoofer in an enclosure gives a designer much more control and adding a port can actually help to enhance speaker performance.

In order to design an enclosure, several unusual physical and electrical properties of a given speaker come in to play in very, very complicated (for me!) mathematics. However, these parameters are often designed into a speaker in a process called synthesis. In this case, a speaker manufacturer will deliberately design in a set of parameters that allows a speaker to be optimised in a compact enclosure for instance. We have seen many of these hit the car audio market in recent years as space and the need to have bass solutions removable have displaced the urge to fill the entire luggage compartment with a giant box housing four 12″ drivers.

It is unusual these days for car audio designers to build custom enclosures for speakers other than subwoofers and so over time, Thiele/Small parameters have been removed. A good manufacturer will still list them as they add credibility to their products but may call them “electro/acoustic parameters” and are output as a result of finite element analysis often employed in speaker design.

There are a baffling number of parameters which can be used to differentiate one speaker from another however, the truly useful ones these days are grouped and labelled as “Large Signal Parameters”. These cover such attributes as cone excursion, magnet mass, cone area, impedance etc.

It would probably not be worth looking any further into these parameters unless you are looking for a subwoofer that would work in an existing enclosure or one for which an enclosure requires designing to fit in a specific area of absolute dimensions.

For most music lovers looking to improve the sound in a car, aesthetics probably have as much to do with product selection as Thiele/Small parameters. Fortunately, these days, a good number of subwoofers in optimised enclosures are available off the shelf. When it comes to size, the basic rules of thumb are that the smaller the enclosure, the less deep bass you will get. A ported enclosure will give much deeper bass but will be larger in size.

As always, your local FOUR MASTER can guide you through the often baffling list of strange runes, figures and symbols contained within a speaker specification list. There is however, no substitute for listening to a couple of examples and making up your own mind, based on how their performance moves you.

Some of our favourites come from the Audison Prima range. Neat and compact with enormous bass developed from minimal power. See an example here:

http://www.fourcaraudio.co.uk/brands/audison/audison-prima/audison-prima-apbx-8-r/ 

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