Sunday, May 27, 2007

Simulation programs

Loudspeaker simulation/modelling programs are indispensable! One of the most important pieces of advice I could possibly give regarding loudspeaker simulators is: get one!

Calculators and formulae for working out box volumes are better than nothing, but they don't give you a 'feel' for what happens when you alter various parameters. For example: a Javascript based calculator might recommend an "optimum" box volume of 50L for a certain speaker, whereas a simulator might suggest that there's very little practical difference in the performance between 50L and 35L.

I've found programs such as Subwoofer Simulator and WinISD to be extremely useful, and not just for building a loudspeaker and getting the project "over and done with" more quickly. They help to create a better understanding of the mechanisms involved so you can make better decisions, choose more appropriate speakers for the job, consider variables that you might have overlooked and more. How does the cone excursion vary according to frequency? What's the maximum power that can be applied before the Xmax is exceeded?

Saturday, May 26, 2007

Acoustic labyrinth

As the name implies, a labyrinth design is intended to channel the speaker's back-wave through a series of tunnels/tubes/openings and so-forth (like a labyrinth) until practically all of the acoustic energy is absorbed. Very little sound is reflected back to the speaker and thus the system behaves like an infinite baffle.

It sounds nice in theory, but it's usually far from ideal in practice. At best, a knowledge of horn design principles can be used to design a box that absorbs energy in an effective manner. For example, one practical approach might be a spiral shaped column of air (think: sea-shell design). In the same way that horn loading increases a loudspeaker's efficiency, the same principle can be used to concentrate acoustic energy onto damping materials, thus making them more effective at absorbing energy. Anechoic chambers rely on this principle by using thin triangular wedges on the interior walls.

Vented vs. Sealed boxes

Vented/ported boxes are slightly different from sealed ones in that they have a hollow tube (or some other type of port or vent) connecting the internal air volume to the outside air.

At low frequencies, ported loudspeakers have:

  • Two masses – the speaker cone and the air mass.
  • Two springs – the speaker suspension and the air spring.
  • Two damping devices – electromechanical damping of the speaker, and a tube that opposes the velocity of the air flowing to/from the box.
A ported design has several advantages over a sealed design:
  • The two resonances can be tuned to provide a lower cut-off frequency than with a sealed design.
  • The excursion of the speaker cone at low frequencies can be much smaller for the same output, resulting in lower harmonic distortion and higher maximum output.
However, it also has disadvantages:
  • The system is more sensitive to variations in temperature, humidity and other environmental or design factors.
  • The system has 4 poles instead of 2. Thus, 'ringing' (boomy bass) is a common side effect due to the relatively high q-factor of one of the pole pairs.
  • Sounds at midrange frequencies may leak through the port.
  • At high amplitudes the port may produce audible hissing.

Sealed boxes

A basic sealed speaker box usually consists of a hollow box and a large circular hole where the speaker is mounted.

At low frequencies, the air and damping materials form an energy storage device – a mechanical spring – which produces an unavoidable resonance when combined with the spring constant and mass of the speaker cone. This resonance is usually exploited to optimize the low-frequency response of the loudspeaker.

At relatively high frequencies (e.g.: above 500Hz), the wavelength of the sound is short enough to produce various echoes and partial oscillations inside the box. Its purpose then is to absorb as much of that energy as possible. Damping materials such as pillow stuffing are often used, as well as design techniques such as an acoustic labyrinth.

A compromise has to be made between 'tuning' a box volume for good performance at low frequencies, or making it very large in order to reduce the amplitude of midrange resonances. Many designs make the enclosures relatively small because of space constraints. Furthermore, smaller boxes often have practical and structural advantages over relatively large boxes.

A sealed loudspeaker can be modelled as a second-order system at low frequencies. It has:

  • One mass – the speaker cone (in parallel with part of air mass inside the box).
  • One spring – the spring constant of the air, in parallel with the speaker suspension.
  • One damping device – electromechanical damping of the speaker (Qms in parallel with Qes).

Box design

Designing an enclosure is a fundamental part of overall loudspeaker design. It's nowhere near as simple as building a "strong, solid box". A speaker box performs a variety of functions and some of them are actually not so obvious.

Take for example: a speaker that's framed in a sealed hollow box, which has nothing but air inside it. What happens? How does it work? Why is the box necessary? Is it necessary?

In order to decide what kind of loudspeaker system you want to build, you'll also need some background knowledge and understanding of how various designs work. Anyway, check out some of my other posts regarding sealed, vented/ported, and labyrinth designs.

Friday, May 25, 2007

Comparison of cone materials

This seems to be a common question when choosing a speaker driver: what's the best cone material? Well, I don't think there's really any such thing as a "best" material – they all have strengths and weaknesses.

One difficulty that speaker manufacturers have to contend with when designing their speakers is that they have to operate across a wide range of frequencies. At low frequencies (such as 100Hz) the cone generally has to move as a stiff, cohesive unit, while at relatively high frequencies (such as 3kHz) it's sometimes preferable if only a small, low mass section of the cone vibrates and the rest remains still. So, what should they do?

There are two popular schools of thought regarding the design of speaker cones or diaphragms:

  1. The cone should be as stiff as possible and operate like a piston throughout its usable range.
  2. The cone should be stiff enough to operate like a piston at low frequencies, but also be flexible and have enough internal damping to behave in a well-controlled manner at high frequencies.
Both varieties have drawbacks.

Stiff cones:
  1. They tend to have very little internal damping and therefore ring prominently at their resonant frequencies.
  2. The relatively large radiating area introduces "beaming" at high frequencies, whereby most of the sound is projected in a forward direction along the cone's axis of vibration.
Flexible cones:
  1. They tend to introduce distortion due to hysteresis and intermodulation effects when the cone flexes and a variety of different frequencies are reproduced simultaneously.
  2. Minor resonances occur across a wide range of frequencies due to imperfect absorption of transverse ripples that travel across the cone's surface.
Stiff cones are usually constructed of materials such as: aluminium/magnesium alloys, titanium, ceramic, and sandwiched composites. Some of the manufacturers include: Seas, Visaton, Accuton, Alcone, Eton, Aurasound, and HiVi. Manufacturers of flexible cones tend to use materials like paper, polypropylene, and a variety of composites. Some manufacturers: Vifa, Seas, Audax, and Manger.

Of course, this isn't the "be all and end all" of speaker cones. It's just the beginning! Have a look around on the 'net and you'll find lots of cool variations, techniques and alternative technologies.

Deciding what you want to make

It's probably a good idea to do this step near the start – a loudspeaker project is one of those things where you won't get far unless you have a pretty clear idea of what you actually want to make. Look around on the Internet for sources of inspiration. Search through hi-fi journals, DIY forums, review magazines, other people's systems and what's available in shops...

How about 2-way speakers? 3-way speakers? Small "bookshelf" speakers that might literally fit on a bookshelf? Large floor-standing speakers? P.A. style party speakers with lots of grunt? A 5.1 HT system?

You should probably start off with a relatively small project and attempt a larger project later on when you have gained some experience. (Personally, I can be quite ambitious sometimes, but even I learnt the hard way that it's better to make mistakes on a reasonably small project rather than costlier ones on a larger project.)

As a starter option I'd recommend a two-way bookshelf system using a 25mm (1") tweeters and 12cm to 18cm (4.5 to 7") woofers, housed in a pair of 10 to 20 litre boxes. Getting this step out of the way makes it easier to cover the other bases such as driver selection, box design and so forth.

Intro <--- start here

This blog is all about designing and building your own loudspeakers. I don't consider myself a "self-styled expert" or anything like that – I just want to help and encourage people who are interested in the subject! So check out the posts, feel free to write comments and make suggestions and whatnot :-)

Anyway, I find that writing about the technical aspects of loudspeakers, reading what others have written, mulling things over and just thinking about how they work helps to improve my own understanding too.

I see "do it yourself" loudspeaker building as a pretty cool hobby. I could try to sell the virtues of DIY concepts and encourage others, but if it's not your 'thing' then there's probably not much I can do about it except perhaps to ask: why buy a shiny mass-produced "clone" of some sort, when you can build something unique?