The energy per unit area associated with a progressive sound wave will be doubled if:

A cylindrical tube, open at one end and closed at the other is in acoustic unison (resonance) with an external source of sound of single frequency held at the open end of the tube in its fundamental note. Then,

Select all the correct alternatives. Two identical straight wires are stretched so as to produce $6 \mathrm{beats} \mathrm{per} \mathrm{second}$ when vibrating simultaneously. On changing the tension slightly in one of them, the beat-frequency remains unchanged. Denoting by $T_1, T_2$ the higher and the lower initial tensions in the strings, then it could be said that while making the above changes in tension.