Ultrasonic Sensor Beam Width

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We have been using the EZ1 ultrasonic sensor to detect how far away we are from the wall during the autonomous mode, but we're running into problems with beam width. It is detecting other objects at a wide angle around it, so we are looking to narrow the beam width; however, we aren't sure how to do this. On the datasheet, it mentions something about dowel width on page one, but it was rather unclear about how to implement this. Do you know of any strategies for reducing the beam width? Thank you!

Datasheet: http://www.maxbotix.com/documents/MB1010_Datasheet.pdf