"August Kundt, in 1866, demonstrated an acoustical standing wave by placing seeds of lycopodium or cork dust in a tube. When a sound was made in the tube, the material inside lined up in nodes and antinodes in line with the oscillation of the wave, creating a standing wave. Later that century, Behn showed that small flames could be used as sensitive indicators of pressure. Finally, in1905, using these two important discoveries, Heinrich Rubens, whom this experiment is named after, took a 4-metre-long tube and drilled 200 small holes into it at 2 centimeter intervals, and filled it with a flammable gas. After lighting the gas (whose flames all rose to near-equal heights), he noted that a sound produced at one end of the tube would create a standing wave, equivalent to thewavelength of the sound being made."
In many basic physics classes the Ruben's tube is used as a proof of the equation frequency = speed of sound/wavelength. Students can measure the wavelength of the standing wave (very carefully as not to burn themselves from flame maxima to maxima). Then, because the speed of sound at sea level in propane is 235 m/s, the students can attempt to guess the frequencybeing played into the Ruben's tube. The speed of sound in air is normally 344m/s, but because propane is almost twice as dense as air, the speed is lower in propane. (Well, not quite because it's twice as dense, but we'll leave it at that oversimplification for now.)
As the sine waves are played through the propane, they compress the propane in certain sections thereby increasing its density.This in turn increases the pressure, and because the area is at a high pressure (and air likes to flow from high to low pressure... thank you Le Chatelier) more air molecules are forced out these holes in an effort to reach equilibrium causing the flames rise higher. In areas where the propane is not as compressed, the area specific pressure is lower resulting in lower flame heights. Essentiallywhen you are visualizing the wave with fire you are seeing a standing wave. These areas of high pressure are known as pressure nodes, and contain the highest flames (most of the time but we'll get to that in a minute) whereas the areas of relatively lower pressure are thus antinodes and yield smaller flames.
Since we know that frequency (f) equals speed (v) divided by wavelength (lamba) we areable to quantitatively determine the frequencies running through the Rubens tube. (f=v/lambda) Now, when doing this, remember that "v" is not the velocity of sound in air, (roughly 344 m/s) but rather the velocity of sound in your gas ( in this case propane, so 235 m/s). Also keep in mind that elevation and temperature affect this, but 235 is a good estimate. So, measure your wavelength from peak topeak (flame maxima) or trough to trough (flame minima), plug into the equation, and you get your frequency.
standing wave, also known as a stationary wave, is a wave that remains in a constant position.
Two opposing waves combine to form a standing wave.
This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as aresult of interference between two waves traveling in opposite directions. In the second case, for waves of equal amplitude traveling in opposing directions, there is on average no net propagation of energy.Standing waves in resonators are one of the causes of the phenomenon known as resonance.
Standing waves result when two sinusoidal wave trains of the same frequency are moving in oppositedirections in the same space and interfere with each other. They occur when waves are reflected at a boundary, such as sound waves reflected from a wall or electromagnetic waves reflected from the end of a transmission line, and particularly when waves are confined in a resonator at resonance, bouncing back and forth between two boundaries, such as in an organ pipe or guitar string.