Spherical HiFi Speaker

German language preferred? See step 14!

Eine deutsche Übersetzung dieses Instructables gibt es als Download im PDF-Format, siehe Schritt 14; Fotos und Zeichnungen sind in diesem Dokument jedoch in reduzierter Auflösung, die anderen Download-Dateien gar nicht enthalten.

Disclaimer: This instructable isn't exactly a construction manual. I rather intend to show you the problems I had and the solutions I found while building these loudspeakers. There will for sure be many other possible solutions, but as always, I tried to make the best use of the ideas, materials and tools I had at hand. Perhaps it will inspire one or two readers to try something similar.

And a 2nd disclaimer, right away: This is a 'dumb' loudspeaker. It doesn't have any smart bells and whistles - no built-in amplifier, no DSP, no bluetooth, neither internet nor network access, no WiFi, no USB port, no direct Spotify, TuneIn, Amazon Music, Tidal or SoundCloud access. If you want to listen to music through it, you do it the same old-fashioned way that we were used to in the 2nd half of the last century - by connecting it to an amplifier with loudspeaker cable.

I like speaker enclosures that differ from the usual 'box' design, such as pyramids, PE/PVC or even concrete tubes, or, as in the current example, spheres. Without going too deep into the details, from an acoustical point of view, spherical speaker enclosures only have advantages over standard, cuboidal boxes, the most important ones are:

1. Improved radiation characteristics, no 'baffle step' in the frequency response
2. Much less vibration of the curved sphere walls compared with flat panels
3. No internal standing waves at discrete frequencies because there are no parallel cabinet walls

But spherical enclosures are either difficult to produce, or, if you have a professional woodturner make them, they will become really, really expensive.

So, when some 11 years ago (!) I stumbled at Ikea's on hemispherical serving bowls made from bamboo wood, I immediately knew that I found something that could be used for upcycling (aka, diverting it from its intended use) and transforming it into loudspeaker enclosures. You might understand that, regarding the time it took me to finish this project, I didn't work on it all the time but needed several prolonged timeouts for thinking and finding solutions, and a lot of different, unrelated, other tasks.

And while you're asking: Yes, these bowls are still available from Ikea, although they look slightly different now. I shouldn't wonder if the dimensions might have changed slightly in the meantime, too.

Working with spherical surfaces is not exactly easy, and building a box-shaped speaker enclosure from chipboard or MDF is more straightforward, and much cheaper, at that. This of course is the reason for hardly finding any spherical speaker enclosures at your friendly neigbourhood HiFi dealer's.

And as opposed to several other loudspeaker instructables found here that, imoho, give you dubious information (such as enclosures built from cardboard or even tin cans, or damping with foam rubber), you can trust me – I know a bit about building DIY speaker systems, since I rather successfully do that since the 1970s ;-)

Materials for a stereo pair:

4 Ikea 'Blanda Matt' bamboo serving bowls, Ø 28 cm

2 17 cm (6½") mid-bass drivers, specified for a closed-box volume of about 8-9 litres

I used a pair of NOS Philips 'AD7066/W4 high-power woofers' with a nominal impedance of 4 Ω that lay waste in my junk box for many years. In the olden times, Philips loudspeakers had an excellent price/performance ratio and were preferred by many DIY speaker builders as well as HiFi manufacturers, such as Infinity, Ohm and Bang&Olufsen. They are no more available off the shelf since a long time, unfortunately, but sometimes they can still be found, either NOS or heavily used, in online auctions – but never really cheap.

Some additional comments are necessary here:

- The old Philips data books don't specify their speaker's efficiency as is standard today, so I tried an educated guess and established a value of approx. 88 dB/W/m; the respective pages from the 1976 (!) data book are attached below.

- The frequency response diagram given in the Philips data book seems rather awkward since they obviously did their measurements in a strange way. The weak bass response shown in the data book cannot be true and, as I said, seems to result from the measurement setup in their anechoic room. These drivers really have some bass, even in rather small enclosures!

2 Visaton 'CP13' 4 Ω tweeters with a 13 mm dome-shaped diaphragm

I selected them because they come in a tiny, (more or less) spherical enclosure. Their efficiency is given by the manufacturer as 86 dB/W/m; in a german magazine article (Hobby HiFi 2000-04) the efficiency is specified as 89 dB/W/m instead – so, regardless whom you believe, it is rather close to the one of the woofer. The manufacturer's data sheet is attached below.

4 M3x25 screws, washers and nuts

2 polyester (or similar) 4.4 µF capacitors (2 x 2.2 µF connected in parallel)

2 speaker terminal pairs, such as Monacor BP-500G. Dimension drawing attached below.

4 ring-tongue terminals for crimping

4 Faston terminals for crimping

Some red and black wire of sufficient thickness (about 2 mm²)

1½ packages of speaker damping material, such as 'Dämpfungsmaterial für HiFi-Lautsprecherboxen' by Visaton (seems to be some kind of BAF wadding)

2 pcs of foam rubber approx. 17x17x1 cm

16 pieces of M3 threaded rod

Eight of them are used during assembling and glueing the bowls. These need to be 27 cm long. The remaining eight pieces can be about 23 cm long. This means that you can cut two long and two shorter pieces from the 1 m long rods available at your hardware store, so you need to get four of them. All of them will be cut to measure later.

16 M3 rivet nuts

16 10 mm long, hex spacers with M3 thread

16 pcs of silicone tube, inner dia. 3 mm (optional, as experience showed later)

Wood glue (the white one)

Hot glue

UV-curing superglue, such as 'UHU LED-Light Booster'

Some scrap wood and misc. mounting hardware

For two 'feet':

2 circular bamboo chopping boards, 25 cm diameter

6 spherical rubber doorstops, 35 mm diameter, with a cut-off part of their plastic dowels

I used 'Burg Wächter TPF 2035'

6 M4 threaded dowels (aka Rampa inserts)

6 M4 countersunk-head screws (length depending on the thickness of the chopping boards - perhaps you need to crop them accordingly)

6 self-adhesive rubber bumpers

Tools

A large turning lathe (or a friend with a well-furnished workshop)

Drill press

(Cordless) power drill

Different drill bits

Miniature rotary power tool (e.g. Dremel or Proxxon) with a small cut-off wheel

Jigsaw

Hot glue gun

Different screwdrivers

Size 5.5 (mm) socket wrench

Crimping tool

Soldering iron

I fired up my vector graphics application and tried to sketch the cross-section of my intended speaker enclosure (see picture above). A spherical cap (aka spherical dome) of one of the bowls per enclosure must be cut off for installing the woofer – which isn't easy since the area supporting the woofer needs to be perfectly flat. But this exactly is what friends are for :-)

After that I also plotted a template of the woofer's outline, its mounting holes and the 'baffle' cutout (2nd picture above). I printed this template making sure that the printer didn't distort the exact dimensions, glued it on a piece of sturdy cardboard with all-purpose adhesive (perhaps a glue stick could have been used, too), then cut it out. I also cut out the mounting hole for the woofer. I made the eight holes with an office hole punch: I removed its plastic bottom cover, turned it upside down, slid the template in and could precisely aim the hole marking within the hole in the punch.

This template can be downloaded in PDF format here, but please note that, if you use a different woofer unit, you have to provide this template by yourself. For printing, make sure that your printer prints the exact size!

A friend owning a large workshop helped me making the opening for the woofer in two of the bowls. This isn't exactly easy; he first provided a piece of scrap wood with a circular furrow (1st picture) into which the rim of the serving bowls tightly fit. This scrap piece was fixed to the lathe's chuck, and the bowls were fixed to this piece with some insulated wire, taking care to not damaging the outer surface finish of the bowl. The first try, however, wasn't successful, see 2nd picture; some of the wood broke out in the last second. The next two tries went ok, see 3rd picture. The hole needed to be increased to 142 mm dia. for taking up the woofer – which of course also would have been easily possible using the lathe, but I didn't think of it at the time, so I had to use a jigsaw some time later.

And how to fix now the woofer in the front bowl? You can see from the first drawing in step 1 that the remaining material in the region of the driver's mounting holes is somewhat thin, and I was afraid that it might crack when using wood screws. One of the next ideas was pulling the driver towards the rear bowl using threaded rods, that btw also help to keep the two bowls together.

There is some drilling needed:

- Eight 3 mm holes in the front bowl; to mark them, I used the template from step 1 and took care that the hole positions corresponded with the wood grain pattern.

- Eight 7 mm holes at the adjacent positions in the rear bowl, for the woofer and the threaded rods; to mark these holes, the template from step 1 can be used as well while finding a good match between the holes and the wood grain. I recommend drilling these holes with wood drill bits using a drill press, so that they will all be parallel.

- Two holes of 6 mm dia. near the center of the rear bowl for the terminals.

- And for installing the tweeter you will drill 3 x 3 mm holes later, using a power drill.

Before proceeding with the spheres, I needed to think of some kind of feet or at least a rest for them so that they didn't roll around while I was working on them...

I found some black, spherical, rubber doorstops with almost the same diameter as the tweeter, and installed three of them each on a round bamboo chopping board. To fix them I cut away a part of each of the supplied dowels. These parts were then fixed with M4 Rampa inserts and M4 contersunk-head screws on the boards. The rubber doorstops could then be pressed on the screwed-on dowel parts. To avoid slipping of these supports I attached three self-adhesive rubber bumpers below them.

Before I could glue the two bowls together, their edges had to be free from varnish. I used a perfectly flat work surface and a piece of not-too-fine sand paper, not smaller than 30 x 30 cm; several smaller sand paper pieces could be used end-to-end on the work surface, fixed with thin, double-sided adhesive tape. Then I moved the edges of all the bowls around on the sand paper – so that the edge remained perfectly level – until the rounded edges were free from varnish on a width of about 5 mm.

I cut a template (that needs not be circular, btw) from a piece of scrap particle board (1st picture) and drilled eight 3.5 mm holes using the cardboard template from step 1 as a guide. It doesn't have to be as thick as my example, but this was what I had at hand.

I fixed a rivet nut at one end of eight pieces of the long threaded rods using superglue (2nd picture). The rods need to be long enough to reach from the holes in the rear bowl through the holes in the front bowl and the ones in the piece of particle board, plus an additional 2 to 3 cm. I fit the rods, the two bowls and the particle board piece together, leaving a gap between the bowls large enough for adding wood glue on the lower bowl's edge. I tried to use sufficient wood glue, but not too much, so that I got a nice and airtight fit of the bowls, but no glue squeezing out at the joint when pressing the bowls together. I placed a washer over the top end of each threaded rod and screwed M3 nuts on, in order to press the bowls together while the glue cured. Here, I used only moderate force (torque) on the nuts so that the bowls would not crack under the pressure.

Any excess wood glue squeezed out at the joint could be wiped off with a slightly damp towel as long as it hasn't already started to cure.

I let the glue cure over night; in fact it should be hard enough after about two hours, but better safe than sorry. After that I removed the hardware and the particle board template, and repeated the procedure on the second pair of bowls.

Once this was done, I removed the particle board template and the threaded rods, then used my glue gun to generously fill/cover the seam between the two bowls (on the inside, of course), for both air tightness and increased mechanical stability. As you can see from the pictures in the next two steps, the hot glue on the inside looks rather ugly. But fortunately this will be hidden after the final assembly.

Once the hot glue had cooled down, I installed the terminals in the rear bowl with additional, serrated lock washers both inside and outside the bowl to keep them from rotating when connecting the cable later. I used about 30 cm long cables with a generous cross section of 2 mm²; red for +, and black for –. For connecting the cables at the inside to the terminals I used crimp ring-tongue terminals on one end of the cables and tightly fixed them with the terminal's nuts and washers.

I used the glue gun again to generously drop hot glue over the terminals' threads for making the joints air-tight, and preventing them from rotating at the same time.

After this was done for both enclosures, I had then to shorten the threaded rods so that they protruded no more than about 5 mm over the woofer's mounting plane. I did that using the cardboard template plus two M3 nuts lightly tightened on the rod. I fired up my dremel with a small cut-off wheel to shorten the threaded rods close to the outer nut. Having a nut where a thread is cut off and removing it helps keeping the thread clean for screwing on a nut again later.

Some time during this process I realised that the threaded rods, when tightened, can act similar to the strings in a guitar or violin: they ring when excited which is definitely not good in a loudspeaker. I tried to damp this ringing by covering them with some matching silicone tube, but this wasn't enough. I thought it might not be a bad idea to also fix the rear end of the threaded rods from the inside of the sphere using a generous amount of hot glue, too – which, however, didn't reduce the ringing at all.

The idea of how to avoid this negative effect popped up some time later (and will be described a bit later, too).

As you can see from the manufacturer's data sheet, the spherical shape of the tweeter is not excactly a sphere, but has two flat sections at the rear. I wanted the tweeter's front somewhat recessed with respect to the front of the woofer, so that their acoustical centres are about the same distance away from the listener (some manufacturers claim to achieve more precise radiation using this trick). And since I wanted to have the tweeter's front in parallel with the one of the woofer, I went back to my original sketch in order to find out where to drill holes for mounting screws.

Using some 25 mm long M3 screws seemed to be a possible, passable solution. I ground their threaded part somewhat obliquely to get a flat surface, onto which I glued the rear of the tweeter housing. I drilled three holes, two of them for these screws, and one for the tweeter's fixed connecting cable, according to the sketch; see the picture above. The screws are inserted from inside the bowl and secured with matching M3 nuts at the outside – which is esthetically questionable, but seens mechanically ok, to me at least. The UV-curing 'UHU LED-Light Booster' superglue came in handy for fixing the tweeters' rear side to the tapered screws. I first just tacked them on with a small amount of glue. Once it was cured, I carefully added more glue so that the tweeters got sturdily fixed to the bowl, while taking care to not overdo it, so that no glue would drip off.

Once the tweeter was securely fixed, I dropped some hot glue over the screw heads and the feed-through hole for the tweeter's cable at the sphere's inside for sealing.

The crossover I suggest is the bare minimum possible - just as Albert Einstein allegedly said 'Make the things as simple as possible, but not simpler', see the diagram above.

The woofer's amplitude response drops nicely above about 5 kHz, and its impedance rises with increasing frequency at the same time, so a part of the crossover is contained in the woofer itself. A 4.4 µF polyester or polypropylene capacitor in series with the tweeter is sufficient to keep low frequencies below 4-5 kHz away from it. This is not a standard value but can easily made by combining two 2.2 µF capacitors in parallel. Please refrain from using electrolytic capacitors, even if they are cheaper – you don't want to compromise the quality here. To mechanically fix these capacitors I simply glued them together first and then to the woofer's magnet, using the hot glue gun.

All the negative terminals (woofer: no dot, tweeter and connection terminal: black wires) are connected. The woofer's positive terminal is marked with a red dot, and the positive connection terminal at the rear (to which the fat red wire is connected) features a red ring. Only the tweeter's positive wire, marked with a white line, goes to the capacitor's second terminal - in other words, it is also connected to the positive connection terminal, but via the capacitors. You can find a photograph of this assembly in step 11.

I admit that this crossover is a simple and cheap solution, but it has the big advantage that no inductor – with its inherent ohmic resistance – is in series with the woofer, and so the woofer's damping by the amplifier output is optimal. This results in a tightly controlled bass reproduction.

I used ¾ of a package of the specified damping material per enclosure, i.e. an amount for about 15 l. It was easiest to cut (or even tear) the large damping mats in several smaller parts for uniformly filling the enclosure, only leaving the room directly behind the woofer free.

To stop the threaded rods from ringing, I used a round (or octogonal) piece of 1 cm thick, heavy-duty packing foam rubber with about the same diameter as the woofer. I cut eight slots at its periphery, not too deeply. I inserted these pieces in the center of the enclosure, in such a way that the threaded rods locked home in the slots.

Before installing the woofer in the enclosure, it had to be connected to the cables coming from both terminals and going to the tweeter. Refer to the circuit diagram in step 9 and the picture above. Some of the connections (capacitor, tweeter) are soldered, and I crimped 6.3 mm Faston connectors to the cables coming from the terminals, then plugged them to the woofer's blade terminals.

Needless to repeat that (at least in this configuration) it is mandatory to connect all the negative ('–' or black) terminals together, then all the positive terminals ('+', or red; only the tweeter wire with the white mark goes to the capacitors), respectively – which, by the way, will not apply to every combination of speakers/crossover, but in this case it does.

The last thing to be done was installing the woofer in the enclosure. In the case of the vintage Philips woofers, there is not much room between the mounting holes and the upturned edge of the driver's sheet metal basket. It's not a problem for M3 nuts, but I wanted to use a socket wrench for tightening the nuts in a rather comfortable way, and there was just not enough space around the mounting holes for the socket wrench (this will be easier for most other woofer makes). So I needed rather long 'nuts' – and found some 10 mm long, M3 threaded hex spacers and abused them for this purpose. Using eight of them together with eight washers solved the problem.

Please note: NEVER use slotted-head screws for mounting a loudspeaker, unless you want to learn the hard way what damage a slipped-off flat-blade screwdriver can cause to your speaker! Always use at least Phillips screws; Allen or Torx screws are even better - or, as in my special case here, the long 'nuts' and a socket wrench that cannot slip off by design.

And not to forget: The circular cardboard templates I had made before were not lost – I painted them black and used them as spacers between the wood and the woofers, just to make the woofers (optically) circular rather than octogonal, and perhaps to use them as gaskets, too.

Before risking to connect my finished spherical speakers to an amplifier, I used an Ohmmeter to check for a short between the two terminals. It read 4.5 Ω for both speakers, so everything was ok.

So it looks nice – but how does it sound? This is somewhat difficult to describe, but I think my spheres perform rather ok. They are definitively not entitled to a 'high-end' tag, but I consider them a successful experiment nevertheless. Bass reproduction is effortless and free from obvious resonances, and the mid and treble ranges seem to be open and unintrusive – to me. Of course, such descriptions are always subjective, and you should never believe what the maker tells you :-)

I admit that my Studer active monitor speakers sound better and more neutral to me, and they go effortlessly deeper in the bass. But hey, they are at least twice the size, and they cost me almost 20 times the amount I invested in the spheres. On the other hand, the spheres look somewhat unusual and much nicer, just ask my better half – she would throw out the Studers without batting an eyelid! So at the end of the day (or rather, after eleven years) I'm quite satisfied with the result of this decade-long odyssey.

I hope that, with some luck, I will be able to measure their frequency response in the near future and will supply it, then, as an update (provided it's not too ugly).

Please let me mention that, firstly, after a short web search just out of curiosity, I was overwhelmed by the sheer amount of spherical loudspeakers already available on the market. Most of them are nicer to look at than my example, but are positioned in a high-end price range. Whether this also corresponds to a higher level of sound quality I cannot estimate.

And secondly, during the same web search I found out that several people already had the same idea of (ab-)using Ikea bamboo serving bowls as loudspeaker enclosures; some of them look nicer as my version here, but I cannot, of course, tell you whether they sound better, too. So I'm afraid I'm not the first one to reinvent the wheel...

Look e.g. here: https://ikeahackers.net/?s=blanda+matt.

Anyway, if you attempt something in this direction, please be aware of the fact that each make and type of low-frequency driver (aka woofer) requires its dedicated enclosure size (i.e., inner volume) for producing an optimum bass response, and not all woofers are happy in a closed volume!