Kumpoo JS-63 Sound Boom string review: thin, loud, and surprisingly durable

Overview

I recently picked up Kumpoo JS-63 Sonic Boom string. After a few sessions, here is my take. Thin gauge: 0.63 mm. Made in Japan — one of the few domestic-brand strings actually produced there. Construction: multi-strand fibres, braiding, core coating, side wire layer. Complicated craft for a thin string. "Sonic Boom" branding is bold; rebound and sound both score 10/10 on the packaging hype — and on court it mostly delivers. Strung at 25/27 lbs. Elasticity is very good in play. Sound is explosive — close to Yonex XB-63. At my tension I cannot say if 28+ lbs gets louder; coating on the outside improves wear resistance — four games, no fraying yet. Durability is solid. I once snapped BG-80 in one session; Raimei holds tension average but better than VBS-63. After a month, tension drop is obvious and sound dulls — I'll bump one lb next time. String migration is much better than VBS-63; barely need to realign the bed — similar to XB-63. Average price ~25 yuan; promos go lower. At ~20 yuan, value is strong for a thin, durable-feeling wire. If CB63 feels expensive and VBS-63 loses tension too fast on your lifts, JS-63 Sonic Boom is worth a shot.