Underwater acoustic communication uses sound waves and uses water as a propagation medium. This technology has been applied in various field, such as underwater wireless sensor network (UWSN), underwater monitoring system, and surveillance system. An underwater acoustic modem is required to be able to communicate between nodes. In this paper we have designed underwater acoustic modem with ultrasonic frequency using Frequency Shift Keying (FSK) Modulation. The designed system consist of FSK modem, microcontroller, amplifier and transducer. Waterproof ultrasonic sensors JSN-SR04T are used as transducers for designed modems. This sensor works well on ultrasonic frequencies, so the FSK modem frequency is adjusted to work properly and match with the transducer used. Therefore, the mark frequency of the modem is 40 kHz and the space frequency is 43 kHz. The test results, show that the FSK Modem can send text data through water media with good results at 1200bps. Result of BER measurement indicate that at distance 100cm and 130cm obtained BER value 0%. At a distance of 150cm obtained BER value 6.2%, and at a distance of 170cm got the biggest error that is 35% This indicates that the addition of the number of bits sent and the addition of distance gives an effect to the value of BER.
