More details from B&C Speakers (www.bcspeakers.com):
The 21DS115 features a 30mm long, 4.5 inch diameter (116mm) Copper Clad Aluminum Wire voice coil. With a 1700watt AES power rating, 99db sensitivity, and over 16.5mm of Xvar, this high energy subwoofer is a significant step forward from similar models in the B&C range, even approaching the performance of our 6”voice coil alternative. Great performance, great value.