This Nicera ultrasonic acoustic transceiver combines transmit and receive into a single transducer. Its 10mm in diameter and functions as both an ultrasonic speaker and microphone. Ultrasonic acoustic detection is popular in sonar range-finding applications such as robotics for object detection and avoidance.

These sensors come housed in an plastic casing. Here are the specs:

• Sound Pressure Level: 122(min) @ 40 kHz
• Resonant Frequency: 40.0 kHz
• Capacitance: 2700 pF @ 1kHz
• Model: Nicera AT40-10PB3

This device is available locally in Akihabara however the datasheet is only in hardcopy form and in Japanese. I can't find the datasheet in soft copy form from the Nicera website.

Datasheet