A simple test for comparing the acoustic properties of potential box materials

A common question that speaker DIYers have is: what materials should I use for building a speaker box? MDF (medium density fibre board) is one very popular answer. Plywood is another option. Then there's hardwood, glass, concrete – just about anything with a good combination of high stiffness, high density, and rapid absorption of high frequency kinetic energy.

I once experimented with polypropylene and that seemed very promising. I found some published mechanical properties of a variety of plastics and polypropylene was a bit unusual as it had a very low yield strength relative to its ultimate tensile strength. That suggested that it would readily absorb mechanical vibrations and convert the energy into heat, which is exactly what I want a speaker box to do. The classic "tap test" produced a dull thud with no discernable ringing, but that's hardly comprehensive.

So, what to use?

Well, how about a music box? It's perfect! It is:

• Small and very portable,
• Reproduces a variety of frequencies upwards of around 1kHz, with lots of harmonic content,
• Does not require electricity.

Testing a material is as simple as pressing the music box against it and seeing how much of a difference it makes to the sound. Usually the difference is huge – the sound becomes dramatically louder. The apparent tone also varies depending on the size and shape of say, a block of wood.

Generally speaking, a suitable material for building the "box" part of a loudspeaker should be as acoustically inert as possible. We only want the speaker to produce sound, without various box panels vibrating and rattling in consonance.

If you'd like to recommend a good material(s) for building loudspeaker boxes, feel free to share your ideas here!

Mono subwoofer versus stereo woofers

This topic has been done lots of times by different people, but I think it deserves to be revisited as there are a couple of things that often seem to be overlooked.

SPL, maximum loudness and whatnot...

This angle is usually pretty well covered. If you want lots of bass, then you need to work out how much air a speaker can move. As a rough indication, multiply the surface area by the maximum linear displacement, usually referred to as Xmax. However, exactly what constitutes "linear" is debatable. One person's 10mm Xmax might be another's 2mm, depending on the amount of distortion they're willing to accept.

Here's a hypothetical example comparing the displacement volume of dual 25cm (10") woofers with a single 30cm (12") woofer:

A modest 0.5cm Xmax * 330cm² * 2 woofers = 330cm³

0.7cm * 540cm² * 1 woofer = 378cm³

However, as they say, "actual performance may vary". There are numerous other criteria to consider so it'll be far more accurate to do a simulation, especially since the peak displacement levels appear to be pretty similar at first glance.

Down-mixing a stereo signal to mono

This is where it gets tricky. Most music sources consist of more than just one channel, so if you're building an active crossover, or at least some kind of gain control, and you want to produce a mono signal for a subwoofer then you're likely to encounter a few unexpected issues. Contrary to popular belief, it's not as simple as taking a stereo signal (from a CD source or whatever) and summing the amplitudes. Why not adjust the gain by 0.707 so that the output powers are summed? Well that doesn't work either.

Well, what exactly is the problem? Take for example a pair of bass speakers placed 1.5m apart:

Initially only one of the speakers is plugged in, then the second one is connected and fed exactly the same signal. What happens then?

Well it depends on the frequency. At some frequencies the average acoustic power delivered to the room will double, while at extremely low frequencies the amplitude will be doubled (almost). The lowest frequency that can undergo strong cancellation is approximately 340/1.5/2 = 113.3Hz, where the 1.5m distance corresponds to a 180 degree phase difference. Below that frequency, the speakers will begin to undergo constructive interference that exceeds the average power doubling that occurs at higher frequencies.

This means that a pair of woofers are acoustically coupled together below a certain corner frequency, which produces a natural 3dB bass boost. The corner frequency is inversely proportional to the physical distance between the woofers. However, a mono subwoofer doesn't have that effect at all.

While there is no right or wrong answer, mono subwoofers aren't hot-swappable with large stereo woofers in full-range loudspeakers. Equalization and level-matching in active crossovers can be a tricky issue that is influenced by the satellite placement.

New loudspeaker plans... Part B

In an earlier post I started writing up some plans for a loudspeaker that I want to build...

To summarize, it's a 3-way design with a somewhat classic choice of driver sizes: 25cm (10") woofer for the bass, 12cm (4.5") midrange, and a fairly standard 25mm (1") dome tweeter. They will have sealed enclosures so I won't have to deal with issues like temperamental tuning and a peaky bass response. For the woofer I looked at the idea of "pressure loading" (or at least, that's what I'm calling it), whereby the box is small enough so that air resonances only occur at frequencies outside of the speaker's operating range.

The midrange

Before continuing with the box design for the bass, I need to start thinking about what to do for the midrange speaker. Firstly, I have a feeling that a pressure-loaded box won't be a good idea. Given an estimated cut-off frequency of 3~4kHz, the half-wavelength will be around 3~5cm, which is tiny! The air volume behind the midrange speaker would have to be less than one litre, so let's do some quick estimates and go from there:

Half wavelength at 5kHz:

λ/2 = 340/5000/2 = 34mm

Given a cone area of 50cm² (I'm using the Seas L12RCY/P for this example) the effective radius is:

r = √(50/π) = 40mm

A cylindrical volume that is slightly wider than the cone would be approximately:

[ π*(34mm + 40mm)² ]*34mm = 584914mm³ = 584.9cm³ = 0.58L

According to Subwoofer Simulator, when I specify 0.58L as the box volume for a Seas L12RCY/P, the system forms a mechanical high-pass filter with a cut-off at approximately 150Hz. That is decidedly annoying because it's fairly close to the 200~300Hz cut-off that I want. It means that if I want to electrically filter out some of the bass, then the combined filter response will have to be at least 3rd-order and its accuracy will be highly dependent on mechanical and environmental factors such as the speed of sound on a particular day.

Another thing that I'm not sure about is: what happens to the cone when it's cushioned by a relatively small pocket of air? Maybe it will flex and resonate at substantially lower frequencies than predicted in tests that use large air volumes? Maybe the air suspension will be sufficiently affected at low frequencies that it cause inter-modulation distortion when combined with other frequencies?

I think that the midrange needs a relatively large box instead, together with a highly effective means of absorbing resonances.

Anechoic wedges

What better way to describe my idea than to draw it?

What I originally had in mind was a sort of "reverse horn" where the sound waves gradually become more and more concentrated as they radiate away from the speaker. Eventually the energy becomes so concentrated that the surrounding enclosure turns into a heat sink. That idea evolved from a cumbersome spiral-shaped horn into an array of miniature wedges – it's basically still the same thing but in a different format.

Part C: enclosure materials, crossovers, and cohesion between bass and midrange.

New loudspeaker in the works... Part A

Well, I'm designing a new pair of loudspeakers. Basically, they'll be a pair of traditional 3-way loudspeakers that use 25cm (10") woofers housed in sealed boxes, 12cm (4.5") midrange speakers, and 25mm dome tweeters. (The original web page that I started about them can be found here.)

I don't intend to build these loudspeakers any earlier than 2008 – partly because I'm a great procrastinator and partly because I don't have anywhere to put them at the moment. So in the meantime I can do the fun part: explore various options, possibilities, and do calculations for the design...



I chose a classic 3-way style because:

1. It can use relatively benign crossover frequencies around 200~300Hz and 4~5kHz.
2. The ratio of driver sizes is quite evenly distributed, allowing a predictable off-axis response.
3. From past experience, a 25cm (10 inch) woofer in a sealed box would have quite well-balanced performance considering the effects of room gain at low frequencies and its ability to reproduce frequencies up to 200~300Hz.
4. Unlike many other designs such as 2-way and quasi 3-way loudspeakers with large midrange drivers and low crossover points, none of the drivers are pushed to their limits. This means lower distortion and higher power handling.

So if all goes well, the design might even turn out quite elegant due to the lack of any glaring weak points.

Drivers

As a starting point, a suitable woofer may be a Seas L26RFX/P. Midrange units: Seas L12RCY/P, Excel W12CY001, Visaton AL 130 8ohm, or Eton 4-300/25 Hex. The tweeters may be chosen at a later stage.

Box design

Tweeters

Practically all dome tweeters are sold with sealed chambers already built in – either behind the magnet or as a small pocket of air directly behind the dome – so designing a box for them is almost a non-issue. There are some things to remember however: mechanical vibrations and diffraction of the wave-front around nearby box corners. Those effects may produce resonances or adversely affect the off-axis response.

Woofers

The enclosures will be constructed from high grade plywood. One option that I'm interested in is the use of small pressure-loaded boxes.

Pressure-loaded, high Q enclosures:

Although the unequalized bass response may be less than optimal, there could also be several advantages over a classic design where the Qtc is tuned to 0.707:

• The idea is that if the box is sufficiently small, there won't be any resonant air modes throughout its entire operating range.
• A small box will be less reliant on the linearity of the speaker's spider and surround – the relatively high spring constant of the air will dominate.
  
• Extra space and convenience.
  

One way to work out whether or not such a design is feasible is to decide what the minimum resonant frequency should be and to calculate corresponding dimensions from that. For example, let's say that the lowest allowable resonant frequency is 500Hz. From that we can work out the wavelength, from the wavelength we can work out the length of a resonant column of air and therefore the maximum dimensions of a box.

Given a sound velocity of approximately 340m/s (it varies), the wavelength is:

λ = 340 / 500 = 0.68m

But, what fraction of the wavelength is relevant? A quarter? Half? A whole wavelength? And why? This is where a bit of nodal analysis comes in handy. (A refresher: nodes are regions of minimum velocity and maximum pressure oscillation, while anti-nodes are regions of maximum displacement and minimum pressure variation). A loudspeaker can be likened to a musical instrument where the speaker driver is like a resonator or an actuator of some sort, such as a reed, while the box forms a resonant column of air, similar to a stopped organ pipe.


My initial line of thinking was that there'll be an audible resonance when the length of an air column is one quarter of a wavelength. However, I looked around on the web and people's measurements suggested that the system would behave as though both ends are fixed, not just one end. At first I wasn't quite sure why that was the case.

I tried to visualize in my mind what happens and then I realised that when the speaker cone is at an anti-node, it directly controls the displacement of the air. It's not really a resonance at all if the cone directly controls the region of maximum displacement.

When it's at a node, there is no such control. It means that there's at least one anti-node floating inside the box, which could reach an enormous displacement amplitude if there's very little damping. The node next to the speaker cone therefore produces a large opposing force and that is what I want to avoid.

For a 500Hz frequency limit, the maximum allowable length is therefore:

L = λ/2 = 0.34m

Rough estimate for a cube shaped box:

V = 3.4dm³ = 39.3L

A slightly more accurate estimate would be to calculate the distance from the centre of one box face (where the speaker is positioned) to a corner on the opposite side:

B = √(A²/2)
C = √(A²/2 + A²) = √(1.5*A²) = 0.34m
A = √(0.34²/1.5) = 0.278m

V = 2.78dm³ = 21.4L

The resulting volume is a bit on the small side, however a cube isn't exactly the most efficient possible shape. How about a hemisphere? Using the formula for the volume of a sphere:

V = 4/3πr³

where r = 0.34m or 3.4dm, the volume of a hemisphere is:

V_h = 2/3πr³ = 82.3L

Obviously the calculations are just estimates that ignore lots of real-life variables, but they're accurate enough to give some ballpark values that could be used in a process of elimination. A box somewhere between 20L and 80L, with no resonances up to 500Hz certainly sounds appealing. The trick will be to devise an effective shape that doesn't clash with other requirements such as minimizing the distance between the woofer and the midrange driver.

Check back soon for Part B where I'll look at some other options: over-sized boxes, anechoic wedges, "reverse horns" and similar possibilities for the midrange enclosures.

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?

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.

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?