Low　acoustic　energy　conversion　efficiency　is　a　major　challenge　for　air-coupled　ultrasonic　technology.　In　the　determination　of　the　lift-off　distance　of　air-coupled　sensors,　there　is　a　balance　between　the　acoustic　energy　attenuation　and　the　difficulty　of　extracting　defect　information.　In　this　study,　an　air-coupled　local　defect　resonance　(LDR)　technique　with　coda　wave　analysis　is　proposed　for　the　nondestructive　evaluation　of　debonding　in　composites.　A　sensor　consisting　of　19　elements　was　used　to　simultaneously　excite　and　receive　ultrasonic　waves.　Air-coupled　LDR　experiments　were　conducted　on　the　two　types　of　composite　structures.　The　effects　of　sensor　lift-off　distance　and　coda　wave　analysis　on　the　performance　of　the　LDR　technique　were　investigated.　It　was　found　that　the　sensor　lift-off　distance　and　the　coda　wave　analysis　had　a　significant　effect　on　the　defect　detection　capability　of　the　LDR　technique.　For　composites,　the　optimal　sensor　lift-off　distance　was　found　to　be　between　3.5λ　and　5.5λ,　where　λ　is　the　wavelength.　Compared　to　multiple　reflection　echoes,　the　coda　waves　are　more　suitable　for　identifying　the　damage　in　composites.　The　proposed　non-contact　ultrasonic　technique　effectively　reduces　the　required　incident　acoustic　energy　and　can　be　used　for　efficient　detection　of　debonding　in　composites.　©　2024　IOP　Publishing　Ltd.　All　rights,　including　for　text　and　data　mining,　AI　training,　and　similar　technologies,　are　reserved.