Page "Frame rate" Paragraph 2

When sound film was introduced in 1926, variations in film speed were no longer tolerated as the human ear is more sensitive to changes in audio frequency.

From 1927 to 1930, the rate of 24 FPS became standard for 35 mm sound film ; a speed of per second.

This allowed for simple two-blade shutters to give a projected series of images at 48 per second.

Many modern 35 mm film projectors use three-blade shutters to give 72 images per second — each frame flashed on screen three times.