Acoustic standing waves with an open far end

[anhhuyalex]

My textbook says

If the far end of the tube is not sealed, standing waves can still be established in the tube, because sound waves can be reflected from the open air.

I'm confused by this, because isn't a standing wave caused by reflection of the waves once it has hit the end of the tube and reflects backwards. How can sound waves be reflected from the open air? Thank you for taking the time to answer. :)

 

[Simon Bridge]

Welcome to PF;
Sound waves can be reflected off the open end because of the change in conditions there and the nature of the pressure wave.
Please see:
https://www.physicsforums.com/library.php?do=view_item&itemid=183