Good sound from old pipes!

Building speaker enclosures is good fun - and you can also save a lot of money over buying pre-built speakers. (That’s if you want quality sound anyway – poor quality speakers are available everywhere!) But while it initially seems straightforward, making good-looking, compact speaker enclosures requires excellent carpentry skills and usually a fair amount of home workshop equipment.

But here’s a different approach that needs only an electric jigsaw, and is quick and easy. And, if the enclosure is designed correctly for the driver being used, the sound quality can be excellent. How good then? Far better than the vast majority of commercial ‘surround sound’ speakers, and much better than many speakers used in compact and portable sound systems. (And a lot better than most speakers just screwed into a door panel.)

So what’s the trick? As you’ll have guessed from the photos, you use thick-walled plastic pipe to form the majority of the enclosure. The ends are constructed from plywood or particle board.

First steps

The first step in producing such a speaker is to source a driver appropriate for use in a small enclosure.

Electronics parts suppliers sell small speakers – for example, those around 3 inches (75mm) in diameter are a good fit for the type of small enclosures being covered here. However, in order to match the driver to the enclosure, you must have either:

1. A suggested enclosure design available for that driver from the manufacturer (for example, specific recommendations for enclosure volume and / or port dimensions), or

1. The Thiele Small specs of the driver that can be then plugged into a free on-line or commercial enclosure design software package that will calculate the required design, or

1. The ability to measure the Thiele Small parameters of the driver yourself, with these then able to be used in a free on-line or commercial enclosure design software package.

Having played with DIY speakers and speaker design for a long time, I recommend the last approach. I use Woofer Tester 2, a software/hardware package that I have found to be brilliant at rapidly measuring the Thiele Small specs of the driver, and then designing an enclosure to suit.

A huge benefit of measuring the specs yourself is that you can source drivers from anywhere – the ones used in this story were picked up very cheaply second-hand as home theatre surround sound speakers. In the original enclosures they sounded terrible; in the new custom designed enclosures, they sound excellent! (More on this later.)

Please note that you will simply not get good sound by grabbing a random speaker and putting it in an enclosure!

Doing it – first design

The first of the two enclosure designs I am going to cover uses 70mm drivers taken from ‘cube’ speakers - in fact, the rear speakers of a Sony surround sound system. Each original enclosure was only about 80 x 80 x 100mm – at about 0.6 litres, small indeed! A tiny rectangular port about 25 x 5mm was located on the rear. Four of these speakers were bought for AUD$10 second-hand – and that price was for the lot!

And how did I know these drivers would be of sufficient quality to work well in a better designed enclosure? I didn’t! But at that price and with the ability to easily measure the Thiele Small specs, it was worth taking a punt.

In fact, the measured specs were:

- Impedance (Re) = 2.9 ohms

- Resonant frequency (Fs) = 153Hz

- Total Q (Qts) = 1.36

- Compliance (Vas) = 0.54 litres

- Sensitivity = 81.7dB at 1 watt, 1 metre

These specs actually look pretty bad (high resonant frequency, low sensitivity and so on) but at 70mm diameter (and with an effective cone area of only 65mm), it’s a very small driver!

Some modelling with the Woofer Tester software showed that best results came from a 1.9 litre volume tuned to 132Hz with a 25mm internal diameter port that was 27mm long.

With the internal diameter of the pipe enclosure being 126mm, a 1.9 litre volume requires a length of 152mm (ie cross-sectional area of the pipe multiplied by length). Add 20mm for the thickness of the two endplates, and add a bit for the volume for the port and speaker, and I went with an overall length of 180mm.

The port was formed from 25mm internal diameter plastic electrical conduit – it needed to be only 27mm long, and again a building site offcut provided this.

The end plates were cut from medium density fibreboard (MDF) with an electric jigsaw, and the jigsaw was used to cut out the hole for the speaker.

The port was placed in the other end panel and the hole for this was formed by a hole-saw.

Use plenty of building adhesive to glue the end plates into place – the seal must be perfect. Get water clean-up glue and then wipe off the excess with a damp cloth.

Formed into a cylinder and then fed through the speaker opening was a piece of polyester quilt wadding (available from dressmaker supply shops). The rolled wadding springs open once inside the enclosure and so lines the interior pipe wall. It prevents reflections of sound off these internal walls and subsequently out of the port, or through the driver cone.

The driver can then be installed…

…and the speaker frame and surrounding panel sprayed black.

A car sound grille was then installed…

...and thin carpet added. For places where appearance doesn’t matter (eg in a workshop), the paint, grille and carpet steps can be skipped.

Results

So how does it sound?

For fun, I rigged up a changeover switch so I could make instant back-and-forth comparisons between one of the original Sony surround sound speakers and the new ‘pipe’ enclosure. After all, same driver - just different enclosures.

The difference was simply staggering.

The Sony surround sound speaker lacked any bass. Furthermore, the mid-range sound was distinctly coloured, presumably through enclosure panel vibrations and reflections. Switching to the new enclosure, the sound was immeasurably better. Singers’ voices no longer had unnatural timbres and the music was much more full-bodied.

Testing the new enclosure with Woofer Tester 2 showed good response down to 130Hz; the original could get down to only 190Hz!

Doing it – second design

The second speaker system was produced in much the same way – as it happens, using drivers again taken from a Sony speaker system. These speakers were also bought second-hand for chickenfeed, but this time the speakers looked like they’d been part of a small desk-top sound system and used ported, particle board enclosures.

Measuring the Thiele Small specs of the drivers and doing some modelling indicated that the original ported enclosure design was in fact quite wrong for these drivers! (So why did they sell them like this? Who knows… maybe the measuring and modelling software was simply not used – or not then available?)

Instead of the original ported boxes, a simple sealed enclosure of 2 litres gave good modelled results. To achieve this, more pipe-type enclosures were made, but this time without a port. In addition, a larger amount of quilt wadding was placed inside the enclosure – this causes the driver to ‘see’ a slightly bigger enclosure, which in a sealed design is beneficial. Finally, this time I used 10mm plywood for the end plates, rather than fibreboard.

The results? Speaker efficiency is lower – that’s because, all else being equal, more amplifier power is needed with sealed designs than with the ported speakers. But again, the sound ‘straight out of the box’ is absolutely fine. I actually built these enclosures as a weekend project with my 11-year-old son, and when the speakers were finished, he immediately wanted to run them in our home workshop.

I must say I was a bit doubtful (at 14 metres x 8 metres and with a 5 metre height, our shed is a very big one to try to fill with sound from such small enclosures) but with a bit of bass and treble boost provided by the amplifier, they were quite up to the task of providing a good quality background radio.

Plastic pipe enclosures of the sort covered here can also be easily hung in a vertical orientation from the roof – perfect in a workshop where you want floor clutter at a minimum.

Conclusion

Thick-walled plastic pipe makes excellent small speaker enclosures. Construction is also so fast and easy that making it yourself becomes a viable option. But best results come from measuring the driver specs (or getting these specs from the manufacturer) and then making enclosures to suit.

Why it works A speaker enclosure’s primary function is to ensure that the sound pressure waves coming from the back of the cone don’t cancel the waves being produced from the front of the cone. If the waves cancel each other, there will be no low frequency notes produced (i.e. no bass). A bare speaker being tested on the bench sounds very tinny because of this wave cancellation effect. A sealed enclosure completely separates these waves, with the rear waves dissipating inside the enclosure. On the other hand, a ported enclosure uses tricky acoustic design to alter the phase of the rear waves. This allows the pressure waves produced by the vibrating plug of air in the port to reinforce the front waves. A perfect material for making an enclosure would be stiff and acoustically ‘dead’. It would also be cheap, look good and be easily cut to size and joined. The nearest common material that approaches these requirements is veneered or plastic-coated particle board – and it is used in the vast majority of speakers. So how does plastic pipe compare? If the pipe that you use is thick-walled, believe it or not - it actually compares very well! Its shape gives it stiffness and the thick wall is quite acoustically dead. In the enclosures shown in this story, the pipe is 140mm in outside diameter and the wall thickness is 7mm. This pipe size is ideal for small drivers; for larger drivers, get larger pipe! However, note that thick-walled pipe like this is common in small diameters, but larger diameter pipe with proportionally thick walls is harder to find. The 140mm pipe used in this story was picked up for nothing as a discard at a building site. So the thick-walled plastic pipe is a great starting point. The next step in giving the enclosure stiffness is to use plywood or particle board discs, one at each end. These further stiffen the enclosure, as well as providing the end panels (one of which houses the driver). Finally, good quality building adhesive is used to glue the end panels into place inside the pipe. Let the glue dry and you have a very stiff, acoustically dead enclosure fit for the production of good sound!

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