When the speaker produces a low-frequency sound, it may not be able to excite both tubes simultaneously due to their slightly different lengths. Resonance occurs when the natural frequency of an object matches the frequency of the sound waves being produced. In this case, the tubes have different lengths, so they will have different natural frequencies at which they resonate.
As the frequency of the sound waves gradually increases, it will eventually match the natural frequency of one of the tubes. At this point, that specific tube will start resonating and amplify the sound wave, resulting in a loud sound. However, the other tube, with a different length, will not resonate at this frequency, and the sound wave will not be amplified.
As the frequency continues to increase, there will come a point where the second tube's natural frequency aligns with the frequency of the sound waves produced by the speaker. At this point, both tubes will start resonating simultaneously and amplify the sound waves.
The reason there are high frequencies at which both tubes resonate, but no low frequencies, is due to the difference in the lengths of the tubes. The longer tube will have a lower natural frequency, while the shorter tube will have a higher natural frequency. Therefore, at low frequencies, only the longer tube will resonate, while at high frequencies, both tubes will resonate together.