Analogue Directional Speakers: How to Make and Test Different Types of Parabolic Reflectors? (TfCD)

This instructable explains how to make and test parabolic speakers.

We aimed at making an “analog” directional speaker. With a directional speaker it is possible to target audio at a specific area, so that people present inside this area can only hear it. These (parametric) speakers are quite expensive, so we tried to make parabolic speakers, which can be seen as an analog version of the directional speaker. A parabolic speaker reflects the sound of a sound source into one direction only, which makes it theoretically possible to focus the sound at a specific area as a directional speaker does. The parabolic speaker consists of a parabolic reflector and an existing speaker. (For more information about parabolic and directional speakers see the attached document.)

To find out what is the most effective parabolic speaker, which can be easily prototyped, we performed some tests with reflectors made out of different materials: cardboard, plastic and stainless steel. We tested them with a layer of aluminum inside and/or clay on the outside and without any layers. (See the attached document for theoretical knowledge on why to use aluminum and clay layers.)

In this instructable is being explained what steps are taken in order to make and test the parabolic reflectors. After testing the reflectors we concluded that the parabolic speaker did not work as well as desired. The reflectors did increase the volume of the sound, but they did not only reflect it into one specific area. The stainless steel reflector worked best, which was as expected due to the high reflective qualities of stainless steel and the smooth surface of the bowl. When applying the aluminum foil to the cardboard and plastic reflector a small improvement was measured according to direct the sound. When applying the clay to the stainless steel and the plastic reflector a small decrease of the volume was measured at the back of the reflector. The last step of the instructable contains some tips on how to improve the reflectors in order to create parabolic speakers that work better.

To make the cardboard parabolic reflector you need: cardboard A2, tape, a hobby knife, a ruler, a print-out of the template of the parabolic shape A2 (see document) and aluminum foil (for making the version with an aluminum layer inside).

Attach the template on the cardboard with tape. Cut the template out of the cardboard. Watch out: the lines are not straight, but slightly curved!

Fold the cardboard into a parabola and use tape to fixate the cardboard into the right shape.

For the version with an aluminum layer inside: place the aluminum foil inside the parabola. Try to keep the aluminum as straight as possible and let it follow the shape of the parabola. Fixate the foil with tape

To make the stainless steel parabolic reflector you need: a stainless steel bowl with a parabolic shape and clay (for making the version with a clay layer outside).

Make a plate of clay with an equal thickness at each place. (You can use a rolling pin or a wine bottle). Place this clay plate over the stainless steel bowl. Try to make a clay shell around the bowl with the same thickness at each place.

To make a plastic parabolic reflector you need: a plastic bowl with a parabolic shape (melamine), clay and aluminum foil (for making the version with an aluminum layer inside, a version with a clay layer outside and a version with a combination of an aluminum layer inside and a clay layer outside).

For the aluminum version: place aluminum foil inside the plastic bowl, as you did while making the cardboard parabolic reflector.

For the clay version: place clay on the outside of the bowl, as you did while making the stainless steel parabolic reflector.

For the version with a combination of aluminum and clay: use both steps above in combination.

If you also want to test these or other parabolic reflectors you need: a speaker or mobile phone, a continuous tone/music (see attached music file), something to place the speaker in the focal point of the parabolic reflector (this can be a piece of cardboard, for example), a decibel meter, a tapeline, a protractor and tape.

To test the parabolic reflectors you can make the set-up as can be seen above. The circled numbers indicate the places where to measure the sound. To make sure using a similar set up for every parabolic speaker you can use pieces of tape to mark all places where to measure the sound and to mark the place of the center of the speaker. The different set-ups as tested can be seen in the scheme below.

To perform the test you can use monotone white noise. (See attached music file.)

This monotone sound ensures the value on the decibel meter does not fluctuate too much at one measuring point. This makes it easier to read the decibel meter and to compare the results.

There are some factors, which can be adjusted to improve the parabolic reflector.

1. The shape of the reflector

The shape of the reflector needs to be a perfect parabola in order to reflect the sound in the desired way (into one direction only, instead of into multiple directions). This can be achieved, for example, by 3d printing the bowl.

2. The placement of the speaker

For an optimal effect of the parabolic speaker the speaker needs to be placed at the exact focal point of the parabola.

3. Surface quality

In order to reflect the sound waves optimally, the surface of the reflector needs to be as smooth as possible (so without the wrinkles in the aluminum foil as in this Instructable).

4. Thickness of the clay layer

When applying a thicker layer of clay the effect of the parabolic reflector will improve. Instead of clay another material with a high mass can be used as well.