These videos can help show this, too- we did ripple tank experiments in Physics class when I was in high school. it all applies to sound in our rooms, too. I would like the speed of the first video slower, so it would be easier to see the effects of the reflections/refractions (the 3rd video has slower motion)-
To show a way to measure the wavelength of frequencies, my Acoustics professor had a tube with a small speaker at the bottom, a frequency generator with small amplifier and cork wafers. He would set a frequency and when the SPL was high enough and then, he used a tweezer to grasp a wafer and insert it into the column of air above the tube- when he let go of the wafer, it would float in the air. He then added more wafers and they would find their own level because the wave is either providing positive pressure, negative pressure or a null point at the half wavelength distance.