INFRASON : industrial boilers soot cleaning by infrasound, low frequency sound

How to create a standing wave in a resonance tube : (see scheme)

We will use the same way but the tube will not be infinite : its length will be ¼ of the wave length .

The wave length is the distance travelled by the wave for one cycle, mathematically, the sound velocity divided by the frequency.

Reviewing our example, we follow the rotation and consider the starting point at 90° :

At 90° : the slots are opened and the air is leaking to produce a wave front ( a localised high pressure)

From 90° to 180° : this wave is moving forward at the sound velocity towards the open side of the tube (we make a distinction here between the overall mass flow and the wave propagation , which is of course much faster).

At 180° : this positive wave is reflected at the open end as a negative wave (low pressure) l'onde de dépression " remonte " dans le tube à la vitesse du son.

From 180° to 270° : this negatives wave is moving backward to the closed end of the tube at the sound velocity.

At 270° : the negative wave is reflected on the closed side of the tube as a negative wave.

From 270° to 360° : the negative wave is moving forward to the open side of the tube.

At 360° : the negative wave is reflected on the open side of the tube as a positive wave.

From 360° to 450° (equivalent to 90°) : the positive wave is moving backward to the closed end of the tube.

At 450° : this positive wave is reflected as a positive wave on the closed end of the tube but is also now reinforced by a new pulse.

What is going on in the tube : (see scheme)

If the speed of the crankshaft is in phase with the wave a so called "standing wave" will appear in the tube.

The pressure in each point of the tube will not change and will be maximum close to the closed side of the tube and minimum close to the open side of the tube, while the amplitude of the pulsation of the gas will be minimum at the closed side and maximum at the open side.

This phenomena can be compared to the vibration of a spring hanging from a standing support at its top and free at its other end.

If an excitement is produce from the support, when at resonance, amplitude of the free end will be maximum.