Alto saxes and soprano saxes both come from the same woodwind sax family and are made of similar materials. The soprano sax is less common than the alto sax and has a significantly different timbre. Both of these instruments are capable of playing virtuosic material. Knowing how to tell the difference by shape, timbre, range and pitch are useful for audience members and composers.
The soprano sax looks significantly different than the alto sax. The alto sax has a crook that leads into the mouthpiece and a bell that curves around. In contrast, the soprano sax looks very similar to a clarinet and is straight with a small bell that flares out toward the end. Both instruments have brass exteriors and use wooden reeds to produce sound. One famous soloist who plays primarily soprano sax is Kenny G. Kenny G will occasionally play the alto sax as well.
The soprano sax has a much brighter sound that the alto sax. Alto sax commonly exists in jazz bands and wind ensembles, whereas the soprano sax is only occasionally brought out for solos. Often the alto sax player will double on the alto and soprano sax. Since there are usually two alto saxes in an ensemble and only one soprano, this usually doesn't pose a problem for overall ensemble sound.
The soprano sax can play about a sixth higher than the alto sax. This extended range gives the soprano sax a pitch advantage over the alto sax. However, the alto sax is able to play about a fourth lower than the soprano sax. This makes these two instruments perfect for use in a saxophone ensemble. Ensembles with a soprano, alto, tenor and baritone saxophone create a complete section that sounds homogeneous.
The final difference between the alto and soprano sax is their transposition. The soprano sax is in the key of Bb, which means that any note written will sound a major second lower. The alto sax is in the key of Eb, which makes the instrument sound a sixth lower than written. The reason for transpositions is to make it possible to keep the majority of the ensemble writing within the staff lines. Without transpositions, many pitches would have to appear outside staff lines.