Folded Horn Passive Phone Speaker

There's something very attractive about a piece of equipment that needs no electrical power, and the passive phone speaker fits into that category. And of course the challenge for the DIY'er is to build one him/herself.

I decided to build one based on the megaphone/horn principle, after building three prototypes based on three wholly different principles to compare their sound quality. Because the horn is folded, it is very space efficient.

The three designs I compared are shown in the picture. On the left is of course the folded horn, in the middle a solid plywood block where the sound is channelled from the phone's speaker through inside passages to two cone-shaped openings in front. To the right is a hollow design, with large plywood surfaces, the idea being that the surfaces will vibrate, thereby amplifying the sound and giving it more depth, almost like a violin or guitar.

My listening tests revealed the following:

The traditional design in the middle sounds the worst. It amplifies and directs the sound quite well, but the sound is still very tinny and flat.

The design to the right does indeed work as planned, although it's no Stradivarius. The vibrating surfaces do give the sound more depth, but at the cost of rather high colouration and weaker volume.

The clear winner is the folded horn design. It has the deepest sound (but of course still no real bass) and amplifies the sound two to three times, while still remaining quite compact. I have no doubt larger horns will give still better sound, but of course at the expense of size and complexity.

The video shows the improvement the loudspeaker makes to the sound of my Galaxy S2.

The first picture illustrates how the folded horn works. The sound enters the horn at the throat, the smallest part of the horn, and moves through the gradually widening horn until it exits at the mouth, the largest part of the horn. "Folded" of course just refers to the fact that space is saved by "folding" the horn to occupy the smallest space possible.

The second picture shows the hole which will be right behind the speaker phone to channel the sound into the throat of the horn.

No scientific calculations were done in designing this passive speaker. I arbitrarily chose a size which would suit my Samsung Galaxy S2 phone, en then fitted the horn in that size by trial and error.

All the necessary measurements are shown in the picture. I used cheap 3 mm plywood which turned out to be nearer to 4 mm. I recommend using a good quality plywood with a smooth surface, or hardboard, the latter which of course machines easily and neatly.

If you use plywood, remember to take measures to limit splintering of the edges to a minimum. This can include taping of cut lines with masking tape before cutting, or first scoring cut lines lightly with the saw before cutting it properly, and using sacrificial backing pieces behind the plywood when cutting. The width of all the pieces, except for the two side panels, is the same -- 70 mm -- as they all fit between the side panels. The 70 mm is based on the width of my Galaxy S2, fitted with a slim case. You'll have to adjust the width to you own phone.

The construction of the speaker is actually very simple, with no hardware used, only wood, and wood glue. I prefer the common PVA glue, usually white or cream in colour. It doesn't set too quickly, giving ample time to adjust the fitting of pieces, and it dries to a very strong binding material. It works especially well in applications like this one, where only butt joints are used. The more you apply, the stronger and sturdier the joint.

Best is to first cut all the necessary pieces to size, and then marking each one clearly with it's length (the width is the same for all the pieces, except for the two side panels). See the first picture.

As it is quite a few pieces that have to be glued together, I think it best not to try to glue too many together at the same time. Rather do it in simple steps, piece by piece. It will take longer, as you will have to allow the glue to dry after adding each piece, but it will keep the process simple and easy.

1. Begin by glueing the back panel at right angles to the right hand side panel. I used a simple jig consisting of little more than square block of wood to do this (second picture).

2. While the first two pieces are drying, you can also glue up the 30 mm and 65 mm pieces, again at right angles to each other (third picture). Take care to glue the edge of the 65 mm piece to 30 mm piece, and not vice versa (guess why I am so aware of that fact...). These two pieces will form the one side and bottom of the horn's throat.

3. As soon as the glue of the back and right hand side panel is dry, you can attach the 50 mm top piece, and the 74 mm bottom piece, as the back piece will help support them while drying. When glueing on these pieces and the ones that follow, keep the left hand side panel (which will be the last to be fitted) handy, and use it to test the fit of the panels, preventing any unwelcome surprises when this side panel is later fitted permanently. You can even use the side panel with a small weight on top to keep the inside parts in place while they are drying, as no more pressure than that is really necessary.

4. The 65 mm and 30 mm pieces you glued up earlier on, can now also be fitted in place, keeping in mind the distances from the sides as shown in the last picture above.

5. The last inside piece to be fitted is the 81 mm front panel. The bottom of the horn's throat must be 7 mm wide, and this is accomplished by positioning the bottom of the 81 mm piece correctly against the 30 mm piece. In my case it was necessary to sand the 81 mm piece shorter by a tiny whisker.

6. Glue the 11 mm, 14 mm and 16 mm pieces inside the appropriate corners, to hopefully help facilitate a smooth flow of sound. You can sand off the back edges of the pieces to make them fit neatly, and fill up any remaining gaps with wood filler or silicone if you so wish (I didn't bother).

7. Now you can glue on the left hand side panel.

This is what your project should look like now, except for the black primer and speaker hole I've added in the meanwhile.

The speaker hole must be drilled so that it is located directly behind the phone's speaker. In this case it is positioned for the Galaxy S2 phone, but the position of your phone's speaker will probably be somewhat different. As far as I know, most phones' speakers are located somewhere at the bottom on the back.

If the speaker is located higher up, you will of course still be able to drill a hole at the appropriate place on the 81 mm panel, I just don't know what the effect on the sound of the passive speaker will be.

Also note that upper front corners of the side panels have now been cut off in line with the front of the lip the phone will be resting on, and the top of the 81 mm front panel. I didn't have any trouble doing it at this stage, but it should be easier doing it before the glueing together of the speaker's part have started. If you prefer it that way, just make very sure of your measurements to prevent unwelcome surprises when fitting the inside panels later on.

Now the basic sound box is finished, and the final look is up to you. True to my lazy nature, I opted for two simple side pieces of 6 mm hardboard, which also form the feet of the speaker (second picture).

I gave the top and front edges of the side pieces a gentle curve with a router and a template (third photo) I've made years ago and have used for many projects since. The radius if the curve on the template is 500 mm. Lastly I rounded over the top and front edges with a rounding over router bit, and painted it with a can of spray paint. Then I just glued on the side pieces (last photo).

And there you are. Happy listening, if you decide to build one yourself.

PS: Since building the first one, I also built one for a Sony Xperia Go (second picture). Note the new position of the hole to suit the position of the Xperia's speaker.

Note: There seems to be interest in this design from owners whose phones have the speaker on the bottom of the phone, like the iPhone. I've added a step which shows a modification to accommodate those phones. Read on.

Place two blocks at each end to lift the phone a few millimeters off of the shelf it rests on. If you're paranoid, you can confine the sound more by using a longer block/blocks like in the second photo. Obviously the space between the blocks is where the phone's own speaker is situated.

To prevent the phone's sound from escaping to the front, and to reflect it backwards, add a small panel to the front of the shelf as shown in the last photo.

Drill the hole for the sound to enter the throat of the horn in the panel as shown by the arrow. Good luck!

In the picture you can see another version of the speaker I built to see if thicker wood and a smoother inside would benefit the quality of the sound. Strangely enough, it didn't, in fact the sound was more boxy than that of the original version. So I tried a modification I already had in mind earlier on.

I made a wedge of wood and fitted it inside the speaker as shown in the picture to decrease the volume of the horn's throat (but still keeping to the principle of the throat starting narrow and widening from there on).

And interestingly, it worked, improving the quality of sound almost to the level of the original version.

So you can experiment with this modification to see if it also works for you.