This will effectively prevent horizontal motion of the air molecules. A sound wave vibrating between two walls, then, is analogous to a string vibrating between two solid supports. Remember, a longitudinal wave is difficult to draw, so we sketch it as if it were transverse. If we open up one end of a tube in which sound waves are vibrating, we get free motion of the sound wave, and it is analogous to the bathtub. Here are the harmonics for a tube open at one end, closed at the other.
Organ pipes are open at the end where the tongue is.
The other end may be open or closed. Find the wavelength of the third mode of vibration for a closed organ pipe 3.7 m long. Find the wavelength of the 2nd mode of oscillation for an open organ pipe 2.6 m long. Of course when we listen to an organ or a guitar, we are not too concerned with the wavelength of sound produced. We are more concerned with the frequency, because that is what we hear.