Speaker back cover and speaker

A speaker, comprising:

• a speaker driver, the speaker driver having a basket at its rear side;
• and
• a speaker back cover, the speaker back cover being sealed and fixed to the basket and comprising:
• a first casing part, the first casing part having a first channel structure, the first channel structure having an inner opening;
• and
• a second casing part, the second casing part having a second channel structure, the second channel structure having an outer opening, the first casing part and the second casing part being relatively movably connected and forming a resonance space at the rear side of the speaker driver, the inner opening being exposed in the resonance space, the outer opening being exposed on the speaker back cover, the first channel structure and the second channel structure being connected so that the resonance space communicates with an outside of the speaker back cover through the inner opening and the outer opening;
• wherein the first casing part and the second casing part is moved or rotated relative to each other to adjust a channel path length from the inner opening to the outer opening.