What is the singular possessive form of bass?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: bass'

Add an apostrophe s ('s) after the existing s at the end of the Word: bass's

Examples:

The conductor was impressed with the bass' voice.

The conductor was impressed with the bass's voice.

OR:

The bass' size was a record for the fishing contest.

The bass's size was a record for the fishing contest.