The longer the didg , the slower ones lips vibrate to match the change in the air pressure. At each length there is a different pressure required to kick start the vibrating drone of that particular didg. There is one other factor that can potentially make two didgs of different lengths have the same note. A didg with a substantial taper, compared with another didg the same length with an even diameter hole, will play a higher note than the even hole. The tapered didg takes less pressure to kickstart the didg and the lips vibrate quicker at a higher vibration. So in effect, length plus the hole configuration together, alter the pressure and the speed of the vibration, played by the speed of the lips vibrating. These two factors make for endless possible combinations and no two didgs the same.

Put another way Length is the main determinant,but secondly is the shape of the internal hole and the ultimate determinant which is both combined; is the ease for the air pressure to be released from one end to another. The more air volume for the sound to travel through plus the internal variations effecting ease, creates the lips to vibrate slower. So the quicker and easier for the air and sound to travel the higher the note.