Introduction: Slap-O-Fone

About: I am an educator, technologist, maker, facilitator, scientist, and artist. I have a small streak of creativity for producing new things, but I think I really shine when I use my skills, knowledge, and determi…
After seeing this video on MAKEzine I decided that I was going to start collecting parts to build my own "Slap Tubes."  I did some research and discovered that the physics involved is fairly straightforward.  Most of my research came from this handy guide to building your own ESPAS (End Struck Plosive Aerophones - the technical name for this type of instrument.).

These instruments are extremely simple to design, build, and play.  They can also be made to be extremely cheap (free in my case) or very expensive (if you use PVC and make a huge set with many octaves.)  As an educator, I think these instruments could make for a great cross-curricular lesson involving music, physics, art, and just plain ol' fun because they are pretty simple, and have the potential to be very very cheap.

I will try to show/explain my steps in a couple different ways to accommodate for the various learning styles in the audience.  If you would like to know more about the theory behind my design please check out the guide linked above.

While I will try to be as clear as possible, bear in mind that while my steps are specific to my materials, the general process will be the same regardless of what you build your instrument out of.

Step 1: Step 1 - Materials and Tools

The Slap-O-Fone will work by slapping the end of a tube to create a sound.  As you change the length of the tubes, the pitch changes (longer tube = lower pitch, shorter tube = higher pitch).  Any tubes will work for your Slap-O-Fone.  I have read comments where people suggest 75mm diameter (3"), 12mm (1/2"), and everything in between.  I suggest using whatever you can find plenty of for cheap.

Materials I used because they were on-hand and free:
- 75mm diameter cardboard tubes (the kind that large rolls of paper are spooled onto for plotter printers) that are 92.1 cm long
- cardboard boxes (these were the boxes that held the paper before the rolls were emptied)
- pencils
- drink coasters  (cheap cardboard ones)

Tools I used
- cutting tool (i used a knife but a saw could work depending on your tube material)
- tape measure
- writing utensil
- elastic band
- hot glue
- duct tape
- Microsoft Excel (you could use a calculator, pencil, and paper instead)

Step 2: Step 2 - Calculations

As mentioned in the previous step, the Slap-O-Fone is essentially a series of different length tubes (think of a pan flute on a larger scale.)  I will only explain the science enough to explain the formula used to calculate how long each of your tubes should be.

As objects vibrate they produce a sound of a certain pitch.  The pitch is determined primarily by how long the object is.  The formula to calculate how long the object should be is:

Length = (Speed of Sound/Frequency) - diameter

The speed of sound is 343.8 m/s (for our purposes this is close enough).
The frequency is the musical note frequency you wish to produce.  A guide showing the musical notes and their frequency can be found here .
The diameter is the diameter of your tubes.  This part of the equation is a correction factor because open ended tubes have a different 'effective length' that solid objects.

For each tube, simply chose your note, find it's frequency, and calculate your tube length.  The note frequency can be found here.  I used an excel spreadsheet to find my tube lengths quickly and easily.  My spreadsheet can be found at the bottom of this step.

NOTE: I didn't know what the lowest note was that I could achieve with my tube lengths so I used excel to plot a spread of different notes to find the best range for my available materials.  If you build your Slap-O-Fones out of PVC you can get much longer tubes, and much lower notes.

Step 3: Step 3 - Measuring and Marking

Once of the tube lengths have been calculated, it is time to start measuring and marking the tubes.

Simply place the measuring tape onto the tube and mark the point where it will need to be cut in order to make the tube produce the preferred sound.

After marking the tube, write the note name and tube length near the marking on the side of the tube that will form the instrument.  This will make it easier to find the tube if you need to locate a specific note name.  It also makes it easier to locate the striking end after the tube is cut as it will be the end without the writing.

NOTE: You shouldn't cut the striking end of the tube.  You should always measure from an end that is factory cut and straight (striking end).  If you have a long enough tube to cut two lengths out, measure each length from the flat ends of the tubes.  This is not critical, but factory cut is typically better.

Step 4: Step 4 - Cutting the Tubes

Now that all of the tubes have been measured and marked, they are ready to be cut to length.  Before cutting each tube, put a rubber band around the cutting line.  The rubber band acts like a straight edge to help guide you while you cut.

With the rubber band in place, begin cutting the tube. (BE CAREFUL WITH YOUR CUTTING UTENSIL)

 If you want to test the sound of your tube, hold it in one hand and slap the open end with your other hand (or a flip-flop sandle.)

Repeat this step for all of the tubes.

Step 5: Step 5 - Creating the Instrument Base

Once all of the tubes have been cut, they are ready to be mounted into a stand.  For my stand, I used the cardboard box that the tubes came in (again, it was free and I'm trying to make this a cheap and easy project.)

Lay down the base of the stand and place the tube ends (the ones that were removed) onto the base in the arrangement best suited for the instrument (however you like).  Trace the tube outlines onto the base.

Cut out all of the holes on the base.  If using a cardboard box like me, make sure the holes on the top of the box match up with the holes on the bottom.  View the pictures to see how I managed to get the top holes to match the bottom holes.

Step 6: Step 6 - Assembling the Slap-O-Fone

All of the parts required to assemble the Slap-O-Fone should now be ready.  Begin by inserting the legs into the base. My base legs are full length cardboard tubes I had left over.

I shouldn't have cut the holes in the top corners of my box, I will cover them later when I finish and paint my Slap-O-Fone.

The tubes should be positioned/arranged in a manner that makes playing note sequences fairly easily.  I laid out my tubes so the first two rows made one chain of notes increasing in pitch (when the tubes are struck in a zig-zag pattern) and the third row increases in pitch from left to right.

Once all tubes are inserted into the base, use glue and/or tape to hold them in place.

Step 7: Step 7 - Creating Slap-Sticks (optional)

This step is optional, but highly recomended.

Find two pencils and two cardboard coasters.

Glue/tape the pencils to the coaster.

Hold the Slap Stick by the stick end, and strike the Slap-O-Fone tube with the bottom of the coaster.

Step 8: Step 8 - Playing the Slap-O-Phone and Reflections

To play the Slap-O-Fone, simply strike the open ends of the tubes with your hands or your Slap Sticks.

Happy Playing!


I had a great time building this instrument.  I was inspired by the video on MAKEzine and I also wanted to do a proof of concept instrument for an educational pilot program I will be running this year.  The end result is wonderful!  The whole project took me about 4 hours from start to finish and cost $0.  Schools could use wrapping paper tubes, or if they have a budget, PVC tubes.  I would use this instrument to teach kids/people about the physics of sound and basics of music.

I will definitely finish it with a sweet paint job and glue all of my connections better.

Next time (Slap-O-Fone 2.0) I will use small PVC tubes and a better base so I can have more flexibility with the tube layout.  Overall, a great build, cheap, fun, easy, potentially educational.

I welcome any feedback as long as it's constructive.