A Red Box was a phreaking tool used to emulate touchtones on a phone. Withe a red box, you could transmit 0-9, *, and # on a phone that did not have an active number pad. Phlute is an homage to that tool, and why I painted mine red. With different fingerings, this two-neck instrument will transmit the Dual-Tone Multi-Frequency (DTMF) signals recognizable by your voicemail and automated call services.

I do not see a practical use for Phlute, but I taught myself about making a musical instrument with custom frequencies. There are inexpensive chips that detect DTMF signals, so it is possible to control an electronic circuit with the tones generated by Phlute.

Supplies

1/2" Schedule-40 PVC pipe (At least twelve inches)
3/4" 200-psi PVC pipe (At least six inches)
3/4" PVC T-coupler
Right-angle PVC coupler 3/4" to 1/2"

Step 1: Cut a Labium in Each Pipe Segment

Cut a 9" segment of the 3/4" PVC pipe
Cut a 6" segment of the 1/2" PVC pipe
Clean burrs and debris from the pipes
Drill a 1/4" hole in each pipe approximately 1" from the end
Carefully use a razor or scalpel to cut the hole into a 1/4" square
Use the blade to carve out a ramp into the hole to look like the picture

A labium is a wedge that oscillates moving air. Acoustic recorders and sports whistles use this method to generate a tone. They are rugged and relatively easy to make.

Step 2: Drill Tone Holes

For the 3/4" PVC pipe segment

Cut off the pipe 8" from the labium tip
Drill a 1/4" hole 2-1/8" from the labium tip
Drill a 13/64" hole 3-7/32" from the labium tip
Drill a 13/64" hole 5-3/32" from the labium tip
Drill a 13/64" hole 5-23/32" from the labium tip

I apologize for the finicky, perhaps overly precise measurements. We will tune the holes at the end, so do not fret about drilling them at the exact locations. If you do not have a 13/32" drill bit, use 3/16" instead.

For the 1/2" PVC pipe segment

Cut off the pipe 4-45/64" from the labium tip
Drill a 7/64" hole 1-11/16" from the labium tip
Drill a 7/64" hole 2-13/32" from the labium tip
Drill a 7/64" hole 3-5/64" from the labium tip
Drill a 7/64" hole 3-15/16" from the labium tip

If you do not have a 7/64" drill bit, use 3/32" instead.

Bevel the holes. I used a countersinking bit in my drill, but you can carve off thin layers with a razor. Do not cut so deep that the beveling widens the hole.

Step 3: Install a Block in Each Pipe

The block guides air across the labium to create oscillations. I made a block by stacking laser-cut semi-circles and bolting them together. I did not provide a file because the dimensions from one PVC pipe to another were different and the number of discs varies depending on how thick of a material you cut. For reference, I measured the interior diameter of my pipes, flattened off the top 1.5mm, and then added a bolt hole through the center.

The most common technique is to buy a wooden dowel and sand off 1/16" by hand.

Whichever approach you use, ensure your block sits flush with the labium window opening. It is all right if the block does not reach the tube opening. It should be snug in the pipe, but it does not have to be rock solid. I added a layer of masking tape to one of mine to make is fit more securely.

Step 4: Tune the Phlute

You tune Phlute by delicately widening the holes. I used a razor to carve them out. We want to generate four tones on the larger pipe: 697Hz, 770Hz, 852Hz, and 941Hz. I used a spectrum analyzer app for iOS, which highlighted the loudest frequency. A similar app should be freely available for all operating systems.

The above table is from Wikipedia's entry for DTMF Signaling.

For the 3/4" PVC pipe segment:

Tightly cover the three holes closest to the labium, and blow softly until you get a tone near 697Hz
[] (X) (X) (X) (O)
Widen the fourth hole until you are within 10Hz
Tightly cover the two holes closest to the labium, and blow softly until you get a tone near 770Hz
[] (X) (X) (O) (O)
Widen the third hole until you are within 10Hz
Tightly cover the hole closest to the labium and the farthest, and blow softly until you get a tone near 852Hz
[] (X) (O) (O) (X)
Widen the second hole until you are within 10Hz
Tightly cover the two holes farthest from the labium, and blow softly until you get a tone near 941Hz
[] (O) (O) (X) (X)
Widen the first hole until you are within 10Hz
Recover from all that exhaling and bandage any stabbed fingers

We want to generate three tones on the 1/2" PVC: 1209Hz, 1336Hz, and 1477Hz.

For the 1/2" PVC pipe segment:

Tightly cover the two holes closest to the labium, and blow softly until you get a tone near 1477Hz
[] (X) (X) (O) (O)
Widen the third and fourth holes until you are within 10Hz
Tightly cover the hole closest to the labium, and blow softly until you get a tone near 1336Hz
[] (X) (O) (O) (O)
Widen the second hole until you are within 10Hz
Uncover all the holes, and blow softly until you get a tone near 852Hz
[] (O) (O) (O) (O)
Widen the first hole until you are within 10Hz

I urge you to widen the holes gradually by making tiny cuts. I have seen people who twist scissors into the holes, and this does a remarkably good job. I used the razor because it made clean cuts on the plastic. If you accidentally widen a hole too much, starting over with a new pipe may be easiest.

Step 5: Assemble and Test

If you do not already have a 2" segment of 3/4" PVC cut one so you can mate the right angle coupler to the T-coupler. We want to be able to blow into the coupler and easily hold the tubes. Make sure you do not cover the labium window with the couplers.

Your Phlute should be functional! Test with a DTMF decoder app. Regrettably, I was not able to generate the fourth column which would have emitted technician tones, which do not appear on standard telephones.

You can paint your Phlute red, cover it in stickers, or red tape, or just show off the bare PVC.

Step 6: Additional Information

I have a few tips if you are interested in making custom tones not on the DTMF list.

Shorter pipes generate a higher frequency than longer ones
Smaller ID pipes generate a higher frequency than wider ones
Holes near the labium generate higher frequencies than ones near the far end
Larger holes generate a higher frequency than smaller ones
Beveling the holes leads to a more consistent sound
The size of the labium *may* affect the tone. I have not tested this hypothesis
Adding a closed cap to the end will significantly lower the overall frequency
Drilling a hole in the cap will raise the frequency
Adding an adjustable valve, like in the pictures, will let you experiment with different openings
How far into the tube your valve lies will affect the pipe's frequency

I blog incessantly at 24 Hour Engineer, and you can find my daily build logs for Phlute.