What is a “slap tube” and how does it work? “Slap tube” is the slang name for an instrument created by a series of wide pipes open at both ends. When the pipe is slapped, usually with a rubber mallet, a tone will sound. The technical name for a slap tube is “End Struck Plosive Aerophones,” most well known through the blue man group.

What is a “slap tube” and how does it work? Recall that sound waves are pressure waves. That is they push molecules together when they travel through them.

What is a “slap tube” and how does it work? Recall that for sound waves, which are longitudinal waves (left), one wavelength (l) is the distance between two compressions or two rarefactions. This is the same distances between two crests or two troughs in a transverse wave (right).

What is a “slap tube” and how does it work? When the pipe is struck on one end, a pressure wave is formed in the pipe, which travels down the pipe creating resonance. This wave, which bounces back and forth in the pipe, is called a standing wave.

What is a “slap tube” and how does it work? In the standing wave above, the areas labeled “node” are areas of minimum energy. The areas labeled “anti-nodes” are areas of maximum energy. In a slap tube, the anti-notes are always at the open ends of the pipe.

What is a “slap tube” and how does it work? The fundamental frequency, or simplest standing wave formed in a pipe open at both ends, has maximum energy at either end of the pipe. Alternating pressure concentrations create the sound.

What is a “slap tube” and how does it work? Look closely at the pipe on the left. Only ½ of a crest and ½ of a trough are inside the pipe. You can see the oscillation on the right. Notice that only ½ of the wave is actually inside the pipe. This means that the wave formula v=lxf changes to v=2lxf. When pipe length (L) is substituted for wavelength (l) the formula becomes L=V . 2f

What is a “slap tube” and how does it work? While pitch is related to frequency, timbre (“tam-ber”) is related to tone quality and is affected by the diameter of the pipe material. It is why an A4 note on a guitar sounds different than an A4 note on a piano, even if both notes are 440 Hz. Since we are using 2 inch diameter pipes we will correct the timber by adding to our formula: L=V – outer diameter of the pipe 2f