A variable density track is a type of optical soundtrack. During the recording process sound waves cause fluctuations in a light source, which in turn causes variations in exposure on the printed track. Variable density tracks can be read with the same optical readers used for variable area tracks. They often have a higher level of noise than VA tracks, but the quality of audio can be the same or even better. VD tracks are no longer used for optical sound on prints but you may encounter them on older prints or newly struck prints of films that originally had a VA track. In some rare cases you may encounter prints that have BOTH types of tracks on the same print. Again, the optical readers used for both tracks are the same, but playback volume may need to be adjusted between the two.