It is a single 15" subwoofer and mainly used for insufficient room of bass.
Detailed introduction to TA-115B Low-frequency loudspeaker system:
SINGLE15"SUB-BASS SPEAKER SYSTEM
Nominal power 400W(MAX/500W)
Frequency Range 30-500KHz(±3dB)
Nominal impedence 8Ω
Sensitivity 98dB/1W/1M
Drivet complement 15"
Dimensions (H x W x D)520x500x700(mm)
Box Material Plywood