Howling tube

If the howling tube is held at one end and the other end is rotated fast enough, a whistling or howling sound can be generated. However, if one end of the howling tube is closed, no sound is produced. This shows that the air passing over grooves on the interior of the tube causes the tube to vibrate. The vibration of the tube is transferred to the surrounding air, which is perceived as sound. The audio frequency of the open tube changes discretely with the rotational speed, since only integer multiples of the fundamental frequency of the tube can be produced.