English: Example showing music staff with three-quarters time signature. Made with Photoshop by adapting larger example: Image:Alternating_time_signatures2.gif
The suggested alt-text phrase is:

alt=Printed music staff with treble clef symbol, 3/4 time signature, and 3 quarter notes: A, F, and middle C

When the image is likely to be read by sight-impaired readers, then the alt-text can help to understand the contents of the image.