The Yamaha NS-SW40 is a compact 8-inch active powered subwoofer designed for small home theater systems, featuring 45W RMS output (100W dynamic) and Advanced YST II technology. It is a cube-shaped speaker often sold as part of the NS-P40 package that provides enhanced, rich, low-end sound with a frequency response of 28Hz-200Hz. 
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Key Features and Specifications
Design: Compact, front-firing, cube-shaped design, roughly 11.5" x 11.5".
Driver: 8-inch cone woofer.
Power Output: 45W RMS ( dynamic power).
Technology: Advanced Yamaha Active Servo Technology II (YST II) for clearer, deeper bass.
Connectivity: RCA line input/digital coaxial for easy integration with AV receivers.
Frequency Range:  - . 
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Common Usage
The NS-SW40 is generally designed as an entry-level subwoofer to add bass to small rooms or to complement bookshelf speakers in a budget 5.1-channel setup. It is often paired with the Yamaha NS-P40 system.