Katydids sing by rubbing their wings together. The primary sound is made by two sclerotized (hardened) surfaces, one on each of the wings. These hardened areas resemble a zipper. The katydids rub the two surfaces together. They make sound the same way you can make a sound by running your fingernail across a zipper.
The sound quality is controlled by the movement of the wings and the qualities of the primary sound organ. A fast wing movement will create a short burst of sound with pulses of sound close together in time. A slower wing movement will create a burst of sound that lasts longer and the time between the individual pulses increases.
The attributes of the primary sound organ such as hardness of the surface and the thickness of the ridges will change the sound quality. Thicker ridges will produce a deeper pitch than a thinner area. The distance between the ridges in the structure and the speed of wing movement determine the rate of sound bursts. You can try this with a pocket comb that has a thick teeth at one end and thin teeth at the other end. If you pluck the tines of the comb with your fingernail, the thicker teeth will produce a lower pitch than the thin ones.
Next to the primary sound producer is an area of the wing that contains air that vibrates when the primary sound is produced. This “resonator” area of the wing amplifies the primary sound.
For attracting mates, the fidelity of the sound is important. Females select males that sing the appropriate song at a loud volume. If the pitch is too high or too low, or the rhythm is not correct, mating is less likely to occur. Thus, the males with the best songs will “get the girl” and the genes for the best song will live on in the next generation.