When a balloon provided with a light bamboo flute and filled with helium gas rises, a sound is created that travels through space: a special experience. You hear a light Doppler effect, as everyone knows from quickly approaching and disappearing cars on the road. 

These whistles are closed pipes, which means that the air can only escape through the so-called labium. The length of the pipe determines the pitch. But because the helium gas in the balloon mixes with ambient air in the whistle, the air in the pipe becomes thinner and the whistle changes while the balloon deflates, the tone makes a lazy upward glissando sound. Those two phenomena together give the special effect. 

When seventy of these helium-filled balloons are released in turns at the signal of the conductor, they travel through the air, away from the audience. Their pitches will be subject to additional beat frequencies, and difference tones arise between the loud penetrating flute tones. This sound effect is best heard in churchlike spaces with rich acoustics. 

You could also see this performance as a short celebration of life: rapid ascent, a prolonged phase if you‘re lucky, of hanging from the ceiling and then a slow descent. At the end of the performance, there is a battlefield of shrivelled balloons. It is an exciting and cheerful performance, appealing to everyone present. 

Hans van Koolwijk