If you’re into sound systems, you’ll be well aware of the famed Thiele Small speaker parameters that are especially important when designing woofers and subwoofers. These parameters are the speaker specs that you plug into software (or an on-line calculator) to allow you to design the speaker box. That box design includes aspects such as internal volume, length and diameter of any ports, and so on.
Without the Thiele Small (abbreviated to TS) specs of the driver, you’re just guessing the box design – and the chances are overwhelming that your guess will be less than optimal!
So to design a good speaker enclosure, the TS specs are needed. Which is fine if you’re buying a new driver or one that is second-hand but still has specs available on it.
But what if you’ve sourced a speaker that is literally an unknown?
For example, a quality driver from a late model luxury car being sold off cheaply? Or even the speaker from a salvaged TV or surround sound system? (Don’t laugh: some of these consumer goods speakers are small and high quality – perfect for enclosures built into car doors or under seats. And people just throw these speakers away…)
Or what about a typical high quality aftermarket speaker designed to be mounted in a door or the rear deck? Generally, all that you’ll be told is the power handling and – perhaps – the resonant frequency.
So in all those cases, to get best results, the driver’s Thiele Small specs need to be measured.
Woofer Tester 2
Welcome to the Woofer Tester 2 – a complete speaker test unit. This incredible piece of hardware plugs into the USB port of a PC or laptop and connects straight to the speaker under test. Open the software, press a button and within literally minutes many of the TS specs are measured. Do some more testing (eg by weighting the cone by a known amount) and the rest of the important specs are there in front of you – it’s that easy!
Furthermore, the Woofer Tester 2 also includes a box design tool so you can also develop the enclosure without exiting the package.
And, and here’s a really tricky thing, once the box is built, you can use the Woofer Tester 2 to test the speaker mounted in its enclosure, seeing if the measured performance matches the prediction. (The Woofer Tester doesn’t include a microphone, so you cannot directly measure frequency response – but indirectly you can get a good idea of what is happening, eg by the impedance plot.)
There’s also nothing stopping you using the Woofer Tester 2 to measure the characteristics of existing, fully-built speaker systems.
At US$160, the Woofer Tester 2 is not dirt cheap – but if you buy second-hand speakers, you need to use it only a few times to save that amount.
(Of course, if you are using a new driver, you can also check to see if manufacturer’s specs for the individual speaker are in fact correct – there is often some variation in the build from driver to driver.)
At its most complex, the Woofer Tester 2 has plenty of in-depth capability. But at its simplest (as we’ll look at here), it’s easy to get up and running.
Measuring the specs for yourself?
If you do a search online, you’ll find plenty of DIY, low-budget techniques for measuring TS parameters.
You’ll need a precision resistor, an AC multimeter that measures over a wide frequency range, and a frequency generator. And, unfortunately, a lot of time spent doing very finicky measurements and plugging numbers into equations.
It’s certainly possible, but who wants to spend the time and effort doing all that? Especially if you’re sorting through a whole bunch of drivers to find one suitable for an application?
Well, with Woofer Tester 2, now you no longer need to do so!
Testing a driver
Woofer Tester 2 is used in the following manner. The software is installed and then the module calibrated using the provided test resistor. The provided alligator-clip leads are then used to connect the hardware to the speaker under test.
The speaker is placed on its back, ensuring that if it has a vented pole piece (ie vent hole in the magnet) this is not blocked.
In the software, the ‘WT control’ is opened from the View tab. The ‘Q, Fs’ test button is then pressed, and the test of the speaker starts automatically. The impedance (Re) of the speaker is measured and displayed, then the resonant frequency (Fs) and total Q (Qts) are ascertained. (Other factors are also measured, but I am trying to keep this simple!) This element of the test procedure takes a few minutes, during which you can see the impedance plot for the speaker developing on the screen in front of you.
So for example, the measured specs of a good 5-inch woofer might be:
- Re = 5.35 ohms
- Fs = 68.2Hz
- Qts = 0.48
The next step is to press the ‘Vas’ test button. A dialog box pops up that asks you to add a weight to the upwards-facing cone. I use two Australian $2 coins that have a mass of 6.6 grams each. The added weight is, in this case, therefore 13.2 grams. The effective diameter of the speaker cone is also manually measured (eg by callipers or a ruler) and then this figure is inputted.
The compliance (Vas) is then measured by the software, and the speaker sensitivity calculated. This takes about another 30 seconds.
This adds to our spec list for the 5-inch woofer:
- Vas = 11.9 litres
- Sens = 90.1 dB (at 1 watt, 1 metre)
Now to model a suitable enclosure.
Opening the ‘T/S Simulation’ from the View tab gives you the in-built Thiele Small speaker design software. The ‘Test – Sim’ button allows you to then import into the simulator the results you just got from the speaker test.
Opening the ‘Overlay’ allows you to graph the predicted results of your various enclosure designs.
These graphs change real time as you alter enclosure dimensions and port sizes. You can model vented, sealed, bandpass and passive radiator enclosure designs.
For example, using the tested specs described above for the 5-inch woofer, the software can be used to model an enclosure. In this case, a good result comes from using an 8-litre enclosure tuned to 57Hz using a port 25mm in internal diameter and 36.8mm long. The software can predict for this enclosure design the frequency response, impedance, phase – and a bunch of other stuff.
The next step is to build the suggested enclosure, then use the Woofer Tester ‘Box test’ function to test the completed speaker. You can then see how the tested box compares with the predicted performance, and make minor changes to the box design until the two line up perfectly.
If you wish, you can use Woofer Tester 2 to just measure the driver specs, and then use another software package to design the enclosure. For example, the data can be exported in file forms used by other packages (eg BassBox), or can simply be written down and then plugged manually into the other package.
(Note that programs like BassBox have a lot more functionality than the enclosure design software included in Woofer Tester 2, which in comparison is basic but effective.)
The enormous benefit of Woofer Tester 2 is that you can use it to measure the Thiele Small specs of drivers that are not provided with those specifications.
For example, very few – if any – 4-inch, 5-inch and 6-inch car sound speakers come with Thiele Small specifications. Thus, you have no real way of knowing what the results will be if you stick the speaker in a door (a sealed enclosure of – say – 50 litres) or in a dash factory speaker location (hmm, now maybe a sealed enclosure of 0.5 litres!).
In fact, one car sound speaker I tested with Woofer Tester had a very high Qts, giving a rapidly rising low frequency response in sealed enclosures of a very wide range of volumes – one-note bass, here we come!
And of course, without those TS specs, it’s impossible to design ported enclosures for these speakers, the approach that has the highest efficiency (ie loudest for a given power input) and a smoothly extended low frequency response.
But with Woofer Tester, you can now do what luxury car manufacturers have been doing for many years, and actually design small enclosures that can fit within doors and around the boot areas in hatchbacks. (Pictured here is the factory ported enclosure from a Lexus – it fits into a door.)
As described earlier, you can also make excellent use of salvaged and second-hand speakers. Brand-name surround-sound home sound systems often have small, high quality drivers – but they’re typically placed in poorly designed enclosures. Salvage some of these speakers, measure the driver’s specs, model a good enclosure to suit, build the enclosure and then test it – and you can have quality sound at a fraction of the price you’d otherwise pay.
And the disadvantages?
At US$160, Woofer Tester 2 is a not chicken-feed in price. However, if you share it around among a few mates (and you each put in say $40 to buy it) then it will pay for itself very quickly.
Another disadvantage is that you need to already know your way around speaker specs like Fs, Vas and Qts, and to have previously used some enclosure modelling software. I think that with no knowledge at all in these areas, the learning curve will be too great. The provided handbook is OK (but not wonderful) and some of the software actions are not intuitive and are therefore a bit clunky.
If you are interested in speaker design, this package is quite unbelievable in what it can achieve quickly and - once you get used to it - easily as well.