Coffee chain Dunkin’ Donuts has begun a brand new campaign in Korea that uses ultrasonic tags to provide coupons to smartphone holders, reports the The Register.

Users who’ve downloaded the app can browse the Dunkin’ Donuts and decide items they need, issuing a chit coupon with a three-hour expiration. Upon entering a participating Dunkin’ Donuts location, the user’s presence is recognized via an ultrasonic tone picked up by the smartphone’s microphone and the coupon is triggered.

The test-run for the recent system was done in Seoul, and the corporate plans to deploy it in 100 stores across South Korea.