Build a Parabolic Speaker

If you are looking for a temporary, unusual, and relatively inexpensive approach to audio-delivery, you have come to the right place. These speakers aren’t hi-fidelity, but they are fun and fairly simple to build. Since the domes help to direct the sound waves, you can use multiple speakers to create somewhat discrete, isolated listening experiences. Very soft sounds will be audible only from directly underneath the speaker, whereas louder sounds will be audible at a distance. These speakers are very lightweight, and you can create a pulley system to let listeners adjust speakers to their desired height.

Overview

In this project I have included a few different component substitutions so that you can build these with what you have on hand or can easily purchase. I first built these using a piezo disc—an electrical component with piezo crystals that convert mechanical vibrations to fluctuations in electrical current—as a surface transducer[1]. A surface transducer is a special kind of speaker without a speaker cone. A vibrating metal disc replaces the speaker cone; when this disc comes into contact with a resonant surface, the surface vibrates and amplifies the sound. Here, we use the Gard’n Dome as our resonant surface. Its unique shape allows us to direct the sound as desired.

[1] I subsequently built a speaker using a surface transducer component purchased from Sparkfun. Both versions work well.

Components

1. Gard’n Dome (or similar) and string
   • I used this product, purchased locally. This specific item appears to have been discontinued by the manufacturer, but hopefully other similar products are available.
2. Piezo disc, preamp and amp
   
   • You can buy prewired piezo discs from many online retailers, for example: http://contactmicrophones.com/
   • In a pinch, this radioshack amp will do: https://goo.gl/2BSLhs
   • I have had luck with this amp: https://goo.gl/fVKkx0
3. Or, surface transducer and amp
   
   • For example: https://goo.gl/FhTK6S and https://goo.gl/cqEbxg
4. Audio cable
5. 1/8th inch mono audio plug
6. String
7. Electrical tape and/or heat shrink
8. AC to DC adapter with variable power supply
   • For instance: https://goo.gl/i6uKLc
9. Alligator clips for testing audio
10. 1/8th inch male-to-male audio cable

Tools

1. Soldering iron & solder
2. Wire strippers
3. Hot glue gun & glue

Begin by hooking up your piezo speaker to make sure you have some sound.

To do so, first strip both ends of your audio wire to expose the ground and signal cables on both ends of the cable. Connect alligator clips to the signal and ground on one end of the exposed audio wire.

Connect the other end of the black alligator clip to the longer end of the 1/8th-inch plug, and the red clip to the short end of the plug.

Connect the piezo disc to the other end of the audio cable with two additional alligator clips.

If you are using a radioshack amp, connnect the 1/8th inch male-to-male audio cable from your sound source (computer or phone headphone jack) to the mic input of your radioshack amp[2]. Then, connect the 1/8th inch plug from the cable attached to the piezo to the output of your amp. Turn on your amp and play sound back from your computer or phone.

You should hear the sound playing softly through the piezo disc. If it sounds distorted, turn down the volume from your sound source and/or on your amp. Then, hold the piezo disc inside your garden dome, pressing it directly up against the black plastic piece in the center. (I have found that this spot provides the most amplification.)

[2] If you are using a separate amp, connect the line-level output from your audio source to the input of your amp and the output of your amp to your piezo disc or surface transducer.

Assuming you have sound, you are ready to solder and reinforce your connections. Remove the alligator clips and solder each of these connections. Use heat shrink or electrical tape to cover and isolate any exposed wires. I use audio cable itself to hang these speakers. Therefore, I reinforce soldered connections with hot glue and string.

I recommend testing your audio frequently throughout this process so that you find any errors/issues quickly. If you wait until the end and find that the sound is distorted or non-existent, you may have to take the entire speaker apart to test all connections.

You will need to solder this component.

1. On the left side: AIN is short for audio in, and will be the signal for your audio source. SHD is a special power-saving feature on this amp – don’t worry about it. 5V is where you will plug in the + side power source. Ground is where you will plug in the '– 'side of your power source. On the right side: Orient the blue mini pot with the numbers as shown in the diagram. This controls the volume. The two leads for the speaker will go to the speaker pins--polarity doesn’t matter unless you are making multiple speakers, in which case you want to make sure you are consistent in your wiring.

2. Next up, you’ll want to get power into your breadboard. Plug your variable power supply into the dc board mount jack, and into an AC power supply. Set it to 4.5V. Test with a multimeter to find the +/- pins and plug in your amp, + to 5V and – to GND.

3. Hook up your audio signal. This is a mono amp, so you will only be able to hook up one channel of audio. You will need to hook up the signal (tip) to AIN and the ground (sleeve) to GND.

4. Hook up your driver. You should have sound. Turn the pot to make it louder/softer.

As I mentioned, if you prefer, you can use a traditional surface transducer (w/magnet and electromagnet) and amp instead of a piezo disc + preamp + amp. For this version, I used the components from Sparkfun I mentioned previously. Though I haven't tested the latter, both the Sparkfun amp and the Modern Device amp should work fine with this surface transducer.

That's it! Hook this up to any sound source, (make sure your amp and/or preamp are on and powered), and play some sound.

If you give it a try, let me know how it went!