how to control android with bluetooth?

Hi I want to make a sound on android when ultrasonic sensor detects something.

There are many projects about controlling arduino with smart phone but I can not find an opposition.

Can anyone give some advices???

It's basically the same: when the sensor connected to the Arduino detects something, the Arduino sends an order (via serial link) to the android device, that plays a sound. it can be a bluetooth link for instance.

Consider MIT App inventor to design the Android app.

Thank you for answering my question!