A collimated sound beam of frequency 340 Hz enters the open end of a U-shaped pipe, as shown in the figure. The experiment is done at room temperature. The two parallel parts of the U-shaped tube are separated by a distance of 15 cm and connected by a round turn. The parallel tubes have an initial length of 65 cm. The length could be extended by the same amount, l, on each side. The maximum extension is 65 cm. On the other open end, a sound level meter is used to detect the transmitted waves. Calculate the possible values of l for which the sound level meter picks up the highest level of sound.