How to reduce measuring angle of JSN SR04T?

I wanted an waterproof ultrasonic sensor. So I used JSN-SR04T. But the problem is it has huge measuring angle(75%) which results in measuring obstacles which are not required. How can I reduce the measuring angle of the sensor?

The wave properties of sound makes this very difficult to do. It would be easier to use several sensors with overlapping fields of view, and have the program decide between the responses.

The cone should be more like 60% according to the data sheet.

Nonetheless it's physics that get in the way of reducing the spread. Use a different sensor (e.g. IR or laser) to get a seriously more narrow field of vision.

Maxbotix has water-resistant ultrasonic sensors with very narrow beam spread - they claim as low as 11 degrees. https://www.maxbotix.com/